Stephen Hawking celebrated his seventy fifth birthday on the 2nd July. We can all be delighted that he lived so long, and continues in robust health, after he was diagnosed to survive only three years. May he continue to "live long and prosper" as the fictional Vulcans say in Star Trek (a UK man can expect to live another 12 years if he reaches his 75th birthday, other things being equal). He's a brilliant physicist, everyone agrees, noted for his work in General Relativity and related areas. However he has never done any research in climate change AFAIK. The BBC interviewed him on his birthday, asking what his views are on Trump's withdrawal, and as they say in their article:

 "Prof Hawking said the action could put Earth onto a path that turns it into a hothouse planet like Venus."

You can check for yourself, what he said, listen to the interview here

"We are close to the tipping point where global warming becomes irreversible. Trump's action could push the Earth over the brink, to become like Venus, with a temperature of 250 degrees, and raining sulfuric acid. Climate change is one of the great dangers we face, and it's one we can prevent if we act now."

He is so famous, that whatever he says just gets reported straight, "as is". Not only by the BBC, by top papers too. Some famous people respond to fame like this, by making pronouncements in areas that are way outside their sphere of expertise, on topics that they have never studied academically. Stephen Hawking often does this. The Japanese scientist Michio Kaku is another famous scientist who has a reputation for doing this. 

Stephen Hawking experiencing zero g in the "vomit comet". He is keen on human spaceflight, and hopes to fly suborbital with Virgin Galactica. He has often made statements saying that he thinks we are at risk of extinction on Earth and need to go into space for a "backup".

Surely he didn't just make this up however? Where did he get the idea? First, let's see how hot the Earth could get if we did burn all the coal, oil and gas.

HOW HOT CAN THE EARTH GET?

We can get an idea from the “Palaeocene-Eocene Thermal Maximum”, or PETM for short, 55 million years ago. The Earth briefly got so warm that the average temperatures of the sea was up to 10 °C at the polescompared to -2 °C today.

As summarized on the BBC page, How hot could the Earth get?

“The PETM shows clear parallels to today’s world. In particular, the pulse of greenhouse gases that set it in motion seems to have been roughly equivalent to what humans could release if we burnt all recoverable fossil fuels. Those gases warmed the planet by at least 5 °C and maybe as much as 8 °C, probably over a few thousand years”

The first primates evolved in the early Eocene

Plesiadapis - an early primate. This creature was alive last time the Earth went through an extreme warming event. The Earth got 8 °C warmer in about a thousand years, but though many species went extinct, it remained very habitable and modern primates and then humans evolved from the creatures that survived it.

Modern primates, our hoofed animals such as deer, sheep, cows etc, rhinos, and many other species owe their origins to creatures that first evolved in the PETM event. So we actually owe a lot of our modern ecology to this last greenhouse spike. See Robert Kunzig's "World without Ice".

So - yes it could potentially have a dramatic effect - ten degrees centigrade at the poles! That's a future to avoid, especially to get there quickly.

But runaway warming turning Earth into a second Venus? Could that happen?

COULD IT BE POSSIBLE TO TURN EARTH INTO A SECOND VENUS?

This idea stems from a book from 2009, “Storms of my Grandchildren” by James Hansen where he wrote

“If we burn all reserves of oil, gas, and coal, there is a substantial chance we will initiate the runaway greenhouse. If we also burn the tar sands and tar shale, I believe the Venus syndrome is a dead certainty."

However other scientists were already skeptical at the time, for instance, as quoted by National Geographic, Raymond Pierrehumbert of the University of Chicago said

"If we were going to run away, we'd probably have done it during the PETM."

See: Will Earth's Ocean Boil Away?

Colin Goldblatt of the University of Victoria in British Columbia decided to follow this up by actually running the numbers, with a paper in Nature Geosciences, published in 2013. It's called "Low simulated radiation limit for runaway greenhouse climates". You can read the article in full if you click through the link from the National Geographic page Will Earth's Ocean Boil Away?

The abstract ends:

“A runaway greenhouse could in theory be triggered by increased greenhouse forcing, but anthropogenic emissions are probably insufficient”

The paper itself says it more clearly later on, making a similar comment to the one by Raymond Pierrehumbert just quoted:

“The so called “hothouse” climate of the Eocene is the most useful constraint for anthropogenic change. With the solar constant 1% less than today and a few thousand ppmv CO2, the mean temperature was ~ 10 °K warmer than today. With CO2 and temperature both higher than we expect in the foreseeable future, this implies that anthropogenic runaway greenhouse is unlikely.”

He also talks about this in the National Geographic interview

“What my results show is that if you put about ten times as much carbon dioxide in the atmosphere as you would get from burning all the coal, oil, and gas—about 30,000 parts per million—then you could cause a runaway greenhouse today. So burning all the fossil fuels won't give us a runaway greenhouse. However, the consequences will still be dire. It won't sterilize the planet, but it might topple Western civilization. There are no theoretical obstacles to that.”

So, he says, 30,000 ppm, or about 3% of carbon dioxide could do it, according to his model. The current level is about 0.04% of carbon dioxide. 3% is a lot.

His model was a simplification, a 1D model (so it doesn't take account of things like atmospheric circulation from the equator towards the sub tropical regions and the poles), which also didn’t take account of clouds, as he explains:

"But your model does not consider the moderating effect of clouds."

"That's correct. You start off with the simplest model you can, and then you build in complexity. We've calculated the maximum amount of sunlight Earth will absorb and the maximum amount of thermal radiation it will emit. So the next step will be to do some modeling with clouds in, which will probably modify the answers."

"Clouds reflect sunlight, but if you put them high enough in the atmosphere, they'll also have a greenhouse effect. On Earth today, the reflection effect dominates—clouds overall have a cooling effect."

Matej Malik and Colin Goldblatt investigate it further in this later paper in 2016. They find that the clouds cool the Earth down by reflecting away the heat, more than they warm it up by blocking the escape of infrared into space. The end of this article has a useful summary of research in the topic up to the date of publication.

They mention another more recent paper which takes account of clouds in a detailed 3D model. This paper calculated that to have a runaway greenhouse effect, then the Earth would need to have 375 Watts per square meter from sunshine. They conclude

“With this new estimation, the inner edge of the habitable zone for Earth like planet in the Solar System is pushed inward to ~0.95 AUs which means that the Earth should not enter a runaway greenhouse state before at least 1 billion years”.

The main thing preventing a runaway greenhouse in this model was not the clouds, as it turned out, but rather, atmospheric dynamics of the water vapour. As a parcel of moist air is heated, with no other source of water to replenish it, it becomes drier, because warmer air can hold more water vapour. As warm air rises in the tropics, it is warmed further and dried out in this way. Then the hot air is compressed as it descends in the sub tropical regions. This creates regions of drier air in the atmosphere, which, as they explain, play the role of “radiative fins” to cool down the Earth.

“As they stabilize Earth tropics today, such dynamically unsaturated regions where water vapor greenhouse effect is reduced stabilize climate against runaway greenhouse by playing the role of "radiative fins" where the emission can exceed the maximum emission for a saturated atmosphere”

What’s more, these "radiative fins" get drier and more extensive as the sun gets warmer. The stratosphere also gets cooler (page 6).

Other papers come to similar results, for instance this one by Kasting et al from 2013, says that the inner edge is 0.99 au, so 99% of Earth's orbit

"Our revised model predicts that the moist-greenhouse limit for our Sun, which defines the inner edge of the HZ, is at 0.99 AU. The OHZ, where gaseous CO2 produces its maximum greenhouse effect, is at 1.70 AU. Although it appears that Earth is perilously close to the inner HZ edge, in reality, cloud feedback and low upper tropospheric relative humidity act to stabilize Earth's climate."

This paper, also from 2013 defines it as 95% of Earth's distance from the sun. This may seem precarious. But there's nothing that's going to change our orbit even by that tiny amount, so we are safe from a runaway greenhouse for hundreds of millions of years into the future.

We seem to be close to the edge of the habitable zone, but we haven't reached its inner edge yet, and won't do so until the sun warms up some more.

The debate continues, as to when in the future the sun will get warm enough for a runaway greenhouse. Maybe half a billion years from now? Maybe a billion years from now? However they are generally agreed that it’s nowhere near warm enough for this to happen at present.

It needs some science fiction scenario where we import oil and gas, from Titan say (which has oceans of methane and ethane), and even then, it's not sure we could trigger it. We would have to be very stupid to do that. At any rate it's not something to worry about at present.

In the distant future, billions of years from now, the Earth will become too hot for humans as a natural effect of the sun getting hotter. But you are talking about timescales here, long enough for humans to evolve a second time all the way from the smallest microscopic multicellular creatures.

On such huge timescales - whatever civilization there is around then may well be able to something to prevent it.

They could

  • Shade the sun with shades orbiting Earth (probably easiest) - vast sheets of mylar or similar.
  • Or physically move the Earth. Don Korycansky suggests using a 100 km diameter asteroid as a gravity tug that does repeated flybys of Jupiter and Earth. It's not so wasteful as the idea of doing many individual asteroid flybys., as you just need to move one asteroid. And because it is so large,  it only needs to do a flyby of Earth every 6,000 years to do the trick. So - every few thousand years as it approaches Earth or Jupiter you have to do some careful fine tweaking of its trajectory. But most of the time you can just forget it. For more on this, see Planet Earth on the move
  • Or for some other ideas for a method to move the Earth slowly outwards without having to do continual fine tweaking of asteroids with a risk of hitting Earth if you get it wrong, see David Brin's "Let's Lift the Earth"

Those ideas, and probably many more, may feasible with near future megatechnology. We could start projects like those, even maybe a few decades or centuries from now, never mind millions of years into the future.

But right now, for hundreds of millions of years and perhaps as much as a billion years into the future, we don’t need such heroic measures.

INTERMEDIATE STATE - A STABLE HOT MOIST GREENHOUSE 

Popp et al, in a paper published on 2015, found another stable state that an Earth like warm water world could flip into, if the sun was 3% warmer than it is today. (Total Solar Irradiance 1.03 that of today). In the second half of the paper they show that it could also get into this state with 770 ppm of CO2 in the place of a brighter sun. See Transition to a Moist Greenhouse with CO2 and solar forcing.

This is a 3D model of a pure aqua planet with no land. They find a new steady state with a global average sea surface temperature of 330 °K or about 57 °C. Though not a Venus type greenhouse, it would be too hot for humans to survive, without technology - and with so much water in the atmosphere, right up to the upper atmosphere where the Sun can dissociate it into hydrogen and oxygen, it would rapidly lose all its water to space.

You can't transfer this directly into our Earth's case, because of the mix of land and sea. In the second half they discuss what levels of CO2 could flip an  Earth like planet with our mix of land and sea into a state like this, coming up with a figure of several thousand ppm.

It's a possible future for Earth hundreds of millions of years into the future, or for exoplanets. It doesn't seem that we could tip Earth into this state quite yet, even if we burnt all our coal, oil, gas etc. But it's rather closer to our present state than the Venus hot greenhouse.

Do say if you know of any other more recent research on this topic. I've written this section based on reading the best articles I can find on the topic.

SO WHAT ABOUT STEPHEN HAWKING'S STATEMENT?

So where does this leave us with professor Hawking's statement? Well if it's based on Hansen's idea, in a way he is correct,  but he is a few years out of date. It was hunch by James Hansen, which he put into his 2009 book on climate change. There was enough of substance in this for climate modelers to look at it carefully. They found that Earth is closer to a runaway greenhouse than they previously thought, leading to estimates that it could happen some time in the next half billion to a billion years. Right now though, all the research points to the conclusion but his hypothesis is false by climate modelers and experts as they looked into the matter carefully and rigorously.

He doesn't say what his remark is based on. It can't be his own research as he doesn't research into climate change AFAIK.

We do risk something like the PETM if we burn all the oil, coal, gas, shale oil, tar sands etc.  That's bad enough. But it's not Venus.

You might say 

"What's the harm in saying this? It's helping to galvanize people to action about climate change". 

Well I think the truth is very important myself. I don't agree at all with this idea some have that it is okay to repeat and publicize and propagate exaggerated statements, justifying this by saying that "look, the effects are to push people to action in the right direction." I think people act more sensibly and wisely if they are presented with the truth just as it is, and not presented with a bias to attempt to push them towards some favoured course of action.

I suggest that this may be part of what gives climate change a bad name and makes it easy for climate skeptics to deny it. This way that such exaggerated claims about the effects often get publicized. They are more "click worthy" and get shared more than the sober assessments by the IPCC and climate scientists. So, even though statements like this are in the minority, they get shared much more widely than the careful statements by the mainstream climate scientists.

This also scares vulnerable people. I'm the author of the kindle book Doomsday Debunked, also often write articles for my Debunking Doomsday quora blog, and I'm the founder and one of the admins of the Doomsday Debunked facebook group. I often spend hours a week reassuring scared people who come to me afraid the world is going to end soon, or in the near future. Many get really scared that the world will end. They often are scared day and night, and every time they wake up are scared the world will end. I find that many of them are getting treatment from doctors for anxiety about imminent Doomsday, and sometimes say that they have felt suicidal at times about it. They are sometimes scared of ideas that are scientific nonsense, like the idea of Nibiru. This is a totally fictional planet, what Brian Cox colourfully calls this "Imaginary Bullshit Planet", supposedly going to hit us in the next few months. (I used his phrase in the title of one of my articles about it also made into a kindle booklet). According to its followers, this mythical BS Nibiru has been about to hit Earth or fly past us some time in the next few months or years, for over a decade now, ever since the idea was first invented by Nancy Lieder in 2003.

That idea is an astronomical and scientific absurdity. But they also get scared of these scientific news stories, and often share those with me too, asking me to look at them and debunk them. These are also just as false, if not so easy to see as such.

We have a big problem, I think, of "Fake Doomsday News". It's no less "fake news" if a famous person like Stephen Hawking says it and sincerely believes it to be true. I think scientists have a special responsibility to check Doomsday stories before sharing them with the public, and especially so if they are famous and the things they say get reported uncritically by the press. And science reporters have this responsibility too. I don't know if I can make a difference by saying this but I'll try.

Please, Stephen Hawking, and Michio Kaku and the others of you that publicize doomsday scenarios - do check them carefully before sharing! 

They would not present their own research in this way. They'd check it carefully, and make sure they are up to date on everything. 

It's sometimes a matter of omission too. Every year on June 30th, then astronomers involved in asteroid detection hold events to raise awareness of the risks from asteroids. And every year you get stories in the papers quoting these scientists, saying that humans risk extinction from giant asteroids hitting Earth. The scientists know full well that we run no such risk. Hundred kilometer asteroids can't hit us at all, from the cratering record. The last such impacts were over three billion years ago, on Mars, Mercury, the Moon and what we know of the history of Earth. Our solar system has settled down since then, none of the asteroid belt asteroids that size can hit us, and the evidence is clear, that Jupiter protects us from the larger comets that come in from the outer solar system.

Now, Chicxulub sized 10 km asteroids can hit us - yes long term - but we already have a 100% complete survey of all the Potentially Hazardous Objects (PHA's) that could hit Earth of that size, and none of them are going to hit us in the next few centuries. I understand why they talk about these eye catching events as a way of drawing attention to the risk. They then go on to talk about smaller asteroids like the Chelyabinsk one, and others that would be larger and threaten cities or even small countries - rare, but not impossible. But in the process, they often forget to mention, or emphasize, that the larger 10 km ones that they mentioned in the beginning are not a risk at present, not for centuries. Then the reporters of course focus on the dramatic ideas, and give those even more prominence until it ends up as a story in a tabloid saying we risk extinction of humans from giant asteroid impacts in the near future, as in the next few decades,which is false.

These fake or false, or misleading doomsday stories also turn people away from the things that really could happen. Scared people are likely to give up, and end up paralyzed doing nothing. We need hope, and the truth gives us this, while these over exaggerated stories, though they catch the headlines, tend to propagate helplessness rather than hope.

We are approaching several tipping points, and may have reached them already. But how many of us know clearly what these identified risks are? I think scientists and scientific journalists have a responsibility to be careful in checking stories like this.

OTHER THINGS THAT STEPHEN HAWKING HAS SAID MAY MAKE US EXTINCT

Stephen Hawking often talks about natural events that could make humans extinct. Asteroid impacts. Supernovae. Gamma ray bursts. Climate change. Or human caused, nuclear war. He has an agenda here, because he wants us to go into space. He thinks the future of humanity is in space, and he thinks this is how we can escape from disasters that could make Earth uninhabitable. But you don't have to motivate humans in space in this way. You can motivate it just as well as a way to move heavy industry into space, to protect Earth from asteroids, for knowledge, for resources from the Moon and asteroids, for the search for biology based on different principles from DNA, for adventure and tourism, in many other ways.

If you follow through these examples he gives, they won't make us extinct either.

For details see my

I think it's a good general guideline, from my own careful reading the literature on extinction risks, that if a scientific news story predicts human extinction, it's almost certainly false. Indeed, I've looked carefully at all the scientific extinction scenarios I know of, for my Doomsday Debunked book, and not one of them actually would lead to extinction of humans.

  • There is no possibility of an asteroid large enough to do it - we can see from the cratering record that the time of asteroids large enough to boil the oceans, melt the land surface etc is long gone, over three billion years ago. There are plenty of comets and asteroids large enough, but they are either in stable orbits over timescales of millions of years, or beyond Jupiter. Jupiter protects us, as we can see from studying the craters on the Moon, Mars, Mercury and what we have of the history of Earth.

    We know for sure that there are no asteroids that big headed our way for several centuries as we know all the Near Earth Asteroids that do flybys of Earth of 10 km or larger (and more than 90% of those of 1 km and larger).

    As for comets, only 1 in 147 flybys at present is by a comet, making the chance of a 10 km comet hitting Earth currently less than 1 in 100 million per century. It's well worth working on asteroid defense, to defend from the smaller asteroids, of tens to hundreds of meters in diameter.

    We can deflect the smaller asteroids, and even these large ones, easily given enough warning time. With a decade or more of warning the delta v needed goes down to centimeters per second, or even less if it goes through a "gravitational keyhole" in a previous flyby before impact (the usual situation). Or in worst case if we spot them too late, we will know the time of impact to the minute and can evacuate the impact zone and prepare for it. Spotting asteroids early is the key here. This is the one natural disaster we can predict to the minute, and also what's more, prevent too. But we don't need to worry about an asteroid making humans extinct.
  • A nearby supernova or gamma ray burst, again would not make us extinct. It would damage the ozone layer, and lead to increased UV radiation. Our atmosphere, ten tons per square meter of protection over our heads, would protect us from the radiation itself. Though it might make many species extinct, this again would not make humans extinct - we could use sun cream to protect against the UV.

    And anyway - we now know our stellar neighbourhood very well, far better than we did even a decade ago. There are no stars able to go supernova that are close enough to harm us. Betelgeuse is an example of several that could go supernova eventually - but it is far too far away to be more than a very bright star in the sky. There is one potential gamma ray burst candidate - but it's tilted away from Earth, they now know, so if it does do a gamma ray burst, it will miss us. None of the others are facing our way at all.
  • A global nuclear war would not make us extinct either. It would cause massive hardship of course, and tip Earth into a nuclear autumn, not a nuclear winter, based on new models since the Kuwaiti fires proved that the original models predicting a nuclear winter were false. The entire southern hemisphere is a nuclear free zone, and in the northern hemisphere, most of the radiation effects are over especially after airbursts, within weeks, leaving hot spots (especially from ground strikes) that we would need to keep clear of for longer periods of time of decades or more.
  • Also humans won't go extinct easily. We are a very resilient species with even the minimum of technology. We are omnivores, can eat anything, even insects and shellfish, which were a staple of early hominids in the colder parts of the world. We can make clothing, boats, build houses, use fire. A mass extinction that made nearly all species extinct would not make us extinct. On the resilience of human beings, and indeed our basic understanding of science, to mass destruction, see my Why Resilient Humans Would Survive Giant Asteroid Impact - Even With Over 90% Of Species Extinct.

NOT GOING TO RUN OUT OF SPACE TO GROW FOOD

This is another science fiction scenario, that you may worry about. The idea that Earth won't be able to feed its population when it reaches 10 billion. Our population increase is slowing down and has stopped in many places, and will probably stabilize at around 10 or 11 billion people in 2100. So could we feed them all?

Well yes, so long as we can cultivate crops, then we have plenty of resources to feed everyone. We have a food surplus because of the often forgotten green revolution of the 1930s through to the 1960s. Without that, we'd surely have billions of people starving. That's not a problem for us now. We have a food surplus and have starvation only through economic and distribution reasons. But we haven't taken this process to its limits, not nearly. There are many ways we could increase the amount of food we grow many times over, if we needed to.

With conventional farming, you need about an acre of land to grow food for one person, around 4,000 square meters. 

By using good gardening practices and by careful choice of crops you can grow all the food for one person in 4,000 square feet, about 372 square meters, or less than a tenth of an acre. That's the approach of biointensive mini gardening.

But you can do much better than that. The easiest way to grow plants for food in space is to use soilless gardening with hydroponic solutions or with aeroponics where plants are grown with roots suspended in a fine mist (uses much less water). You would use rapidly growing crops, for instance dwarf wheat can crop in 30 days, and there are many other plants that can crop over similar timescales.  This leads to huge savings in the precious area you need to grow crops. 

In the BIOS-3 experiments, in Russia, back in the 1960s through to 1970s, the Russians showed that you can grow all your food with only 30 square meters of growing area per person. The crops were grown on a culture conveyor with 2 to 10 plantings of different ages simultaneously. They grew wheat, sedge-nut, beet, carrots, and other crops, ten crops in total. With those 30 square meters per person, they produced 95% of the daily requirement for oxygen, water, food and everything else (by weight). The remaining 5% consisted of animal products, salt for the humans, nutrients for plants and personal hygiene supplies. They could produce 45% of the food and nearly all the oxygen and water with only 13 square meters.

For details, see my An astronaut gardener on the Moon - summits of sunlight and vast lunar caves in low gravity, which some sites have linked to as "The Complete Guide to Lunar Gardening" :).

Looked at this way, with the eye of a "space colonist", then much of Earth itself is ripe for colonization, including its deserts and the oceans. With the technology proposed for the Moon or Mars, we could feed the entire world from only 2.5% of the Sahara desert. Our oceans are four times the surface area of the land, in effect giving us four new "ocean world" planets which we barely use except for transport and fishing. If we use space habitat technology for the seas as well, we could feed the population of those four extra "ocean world" planets, with four times the population of Earth, from only 0.5% of the Pacific ocean. We are talking here about minimal impact sea steading, in tethered floating sea cities. that rely on just sea water, the air, and minimal imports from the rest of Earth.

Artist's impression of a water planet with two moons. Our oceans, with just the sea water, air and sunlight, could support four times the population of Earth from only 0.5% of the surface area of the Pacific, using much lower levels of technology than we'd need to set up space colonies.

They would have no impact on sea life as they wouldn't need to fish and the land area is too small to shade out significant amounts of light if carefully situated.

For details, and the calculations, see my An astronaut gardener on the Moon - summits of sunlight and vast lunar caves in low gravity again. There's no need to go into space to find places to colonize. Earth, right here, is ripe for high tech colonization of this sort, and is far far more habitable than the Moon or Mars. This is intermediate in technology between ordinary agriculture and space colonization. No need to filter the atmosphere, no need to scrub CO2 if it builds up, no need for radiation shielding, no need to hold in the atmosphere of your habitat against ten tons per square meter of outwards pressure. Just open a window to get all of that for free, and the Earth's atmosphere itself protects us against cosmic and solar radiation with the equivalent of ten tons per square meter of radiation shielding  overhead.

A lower tech thing we can do in the near future is to build seawater greenhouses to grow crops in the deserts - this is already being done, in Australia, Technologies - Sahara Forest Project

BACK TO GLOBAL WARMING

So, back to global warming. Earth is actually going through a cold spell at present - one that lasted for tens of millions of years. Earth has still got ice at both poles. Usually there's no ice at all, except at the tops of the highest mountains. So geologically speaking we are still in the middle of an ice age. Geologists call our current climate an "interglacial" - a warmer period during an ice age between periods of extensive glaciation.

So far actually, global warming has made Earth more fertile, by increasing the amount of carbon dioxide in the atmosphere, and warming it up. It will continue to get more fertile for some crops, such as rice and wheat. But it's reached the limit for maize, which has a more efficient way of extracting carbon dioxide, and won't benefit from any further increases. The increased droughts and heat spells will reduce crop yields and combining those effects our global temperature is now optimal.

But as the Earth continues to warm, then areas that are now permanently frozen in Canada and Siberia and so on become fertile. Meanwhile some areas that are very hot will become as hot as death valley and we'll get more areas of the planet that are uninhabitable by humans without permanent air conditioning. Just too hot for humans to survive. It's a case of swings and roundabouts as they say. Some parts more habitable, some less so. But as a whole, the Earth will remain very habitable to humans all the way through to complete melting of the Antarctic and Greenland ice sheets and beyond.

We also don't need to worry about running out of oxygen. Even if we were to destroy all the green vegetation in the world - well most of the oxygen comes from green algae. If we destroyed all the algae too, the air would still remain just as breathable as it is now. That's because oxygen persists in the atmosphere - it's residence time is 3,000 to 10,000 years.  Residence times of some atmospheric gases. So even if there was nothing to create oxygen, the air would stay breathable for humans for thousands of years. Plenty of time for the trees to grow back, green algae to spread and the atmosphere oxygen cycle to return to normal again.

Nor are we at any risk of CO2 poisoning. If we burnt all the available fossil fuels, (which hopefully we will have enough sense not to do) including shale gas, tar sands and other unconventional reserves, one estimate puts the amount of CO2 it would generate at 1400 ppm. That’s well below the 5,000 ppm level which would start to impact on our health.

Actually 70 to 80% of our oxygen comes from green algae The Most Important Organism? | Ecology Global Network

But even if we destroyed all those too in some unimaginable eco-catastrophe we'd still be able to breath the air due to the oxygen that is still in it, leaving centuries for the Earth to recover and start creating oxygen again.

There are many reasons to keep our trees. 

  • Are the homes of about a million hunter gatherers in tropical regions, and a hundred times as many around the edges of the forests depending on their products
  • Gene pool and sources of new drugs (tropical species rich rainforests)
  • Preventing soil erosion (not everywhere, where I live, West coast of scotland then when you cut down trees all you get is peat bog because it is so cold and wet)
  • Local effects on climate - planting forests can bring rainfall to desert regions and we can replant trees also to reverse desertification
  • CO2 sink. If you destroy forests this releases CO2 into the atmosphere which contributes to global warming. Grow more trees and you can counteract CO2 emissions.
  • Source of timber and other forest products
  • Wood fuel
  • Water retention
  • Home for wildlife
  • Places for spiritual renewal and refreshment.
But we don't need them as "lungs" - a strange metaphor anyway as they produce oxygen, and lungs consume it. Anyway we don't need them for that. They are very valuable, but let's not confuse and scare people by suggesting we need forests for oxygen. For more about this, see my Why Forests are not the "Lungs of the planet"

Dwellings of an uncontacted tribe in the Amazon rain forest - the place is their home. Tropical forests are home to about a million indigenous people, and about a hundred million people around the edges of the forests depend on their products.

Forests are of great value to us. They are carbon dioxide sinks. They are sources of genetic diversity, and have many benefits. But they are not needed for oxygen, as oxygen has a residence of time of thousands of years. Even if we destroyed all the vegetation and all the green algae on Earth, we could still breathe just fine for thousands of years. 

So we don't need to worry about apocalyptic scenarios like that. But there are real things that we do need to be concerned about. 

REAL TIPPING POINTS

So what are the real tipping points and the real future problems we face as a result of climate change?

Well first of all, the way that carbon dioxide acts as a greenhouse gas is itself a kind of tipping point mechanism. It puts us into a series of tipping points one after another. The problem is that when you add a small amount of carbon dioxide, this warms the Earth up slightly. If that was all, it would be not a big deal, the effect is minor and the carbon dioxide would be removed again with a half life of about 50 years.

The problem is though, that when you add carbon dioxide, it warms the oceans up slightly. Carbon dioxide, as it happens, is more soluble in a colder ocean. So as the ocean warms up, it can no longer absorb as much carbon dioxide. Meanwhile, a warmer atmosphere is drier for the same amount of moisture content - that's how tumble driers are able to dry clothes. This means that when the air is warmer, it can also take up more water vapour.

A cat called Polo in a tumble drier. Tumble driers work by warming the air which makes it drier without removing any of its water content, because warmer air can absorb more water vapour. As we add carbon dioxide to the atmosphere then this warms the atmosphere slightly. This dries out the air, just as a tumble drier does, so makes it able to carry more water vapour. The dry air takes up more water from the oceans, and because water vapour is a greenhouse gas, this then warms up the Earth further. This is the main way that the Earth is warming due to human created carbon dioxide. 

Water vapour is the real greenhouse gas - it's much more effective than carbon dioxide. But water of course just rains out of the warm air and clouds in our atmosphere within hours, if the air cools down. So water vapour by itself can't keep the Earth warmer than a certain amount. The reason such tiny amounts of carbon dioxide, 0.04%, can make such a difference is because by warming the oceans slightly, they keep more water vapour in the atmosphere, which warms the oceans up a bit more, makes them less able to absorb carbon dioxide, and so keeps the Earth warmer long term.

However this is a process that works more strongly in one direction than the other. It's true that if you remove carbon dioxide, it cools down the Earth, and so there's less warming from water vapour. But the more the carbon dioxide warms the Earth, the warmer the oceans, and so the harder it is for the carbon dioxide to be removed from the atmosphere again. So, unusually, there's a preferred direction here, easier to warm the Earth than to cool it again.

The end result of this is that when you add carbon dioxide to the atmosphere, some of it will come out of it within 50 years, but some of it will stay in the atmosphere for an astonishingly long time.

VERY LONG TERM EFFECTS OF CO2

The average lifetime of a CO2 molecule is around 4 years in the atmosphere. But that’s for the more rapid processes such as plant growth, and absorption in the surface water of the ocean. Much of that gets circulated back into the atmosphere again.

After adding a pulse of CO2 to the atmosphere, half is removed in 50 years. You might think, half would be removed again in the next 50 years, but no, it takes 250 years to get down to a quarter of the original concentration. Finally, around 15 percent of the original pulse remains in the atmosphere pretty much permanently, for thousands of years. CLIMATE CHANGE - the IPCC scientific assessment

So of the roughly 32 billion tons of CO2 humans added to the atmosphere in 2010

  • 16 billion tons will still be there 50 years later, in 2060,
  • 8 billion tons will still be there 250 years later in 2260
  • 4.8 billion tons will still be there indefinitely, for thousands of years.

From the Climate Change 2014 Synthesis Report Summary for Policymakers

That’s why it is important not just to stop the increase in CO2 emissions, but to dramatically reduce emissions if we want to stay within 2 °C rise by 2100, and even more so to stay within 1.5 °C. If the climate modelers are right, we need to act quickly, before 2020. We are not going to hit 1.5 °C by 2020 if we continue business as usual. But if we continue to emit 32 gigatons of CO2 a year up to 2020, we will have added enough of it to the atmosphere already so that in most scenarios we will no longer remain within the target of 1.5 °C by 2100.

This is from the 2014 IPCC report Climate change threatens irreversible and dangerous impacts, but options exist to limit its effects - UN and Climate Change

Once we achieve zero emissions, the temperature of the atmosphere will stabilize quickly but the world will remain warmer, and will not cool down back to 1950 levels. Instead, temperatures will stay at the same higher temperatures for many centuries. 

That is, unless we remove large amounts of CO2 from the atmosphere over a sustained period after we achieve zero emissions. There are ways of doing this. But it would need a major sustained effort, for years. Remember 15% of the carbon dioxide we added to the atmosphere is still there thousands of years later, and 25% is there 250 years later. Think how much industry we have, adding carbon dioxide to the atmosphere every year? Now - how much industry would we need to remove a quarter of those quantities of carbon dioxide, every year, for the same amount of time? We'd need a huge global industry sustained for decades or centuries, to remove the carbon dioxide we are so busily adding today.

There are many longer term effects also.

The ocean warms only slowly, influenced by the warm air above it. The ice in Antarctica melts even more slowly, and those also contribute to sea level rises (any ice floating on the sea, such as the ice in the Arctic, makes no difference to the sea level when it melts, but ice on land such as the Greenland and Antarctic ice sheets will raise the sea levels when the melt). Over the long term, a thousand years into the future, the Greenland ice sheet is almost certain to vanish completely eventually if we reach 4 °C, and may vanish even at only a 1 °C rise, leading to a sea level rise of seven meters.

So the sea levels will continue to rise for a long time to come again no matter what we do. But by acting quickly we can reduce the amount of the total future rise.

Ocean acidification will also continue for centuries, because of the elevated carbon dioxide levels in the atmosphere, and strongly affect marine ecosystems.

Other risks include the possibility of abrupt and irreversible regional-scale change in the composition, structure and function of marine, terrestrial and freshwater ecosystems, including wetlands

However we shouldn’t go away with the view that this means that the Earth is going to become uninhabitable!

WORLD GETS MORE HABITABLE IF ANYTHING, NOT LESS SO - IT’S THE CHANGE THAT’S THE ISSUE

In the short term the worst case scenario is that the world gets 4 °C hotter by 2100, and the Paris climate change agreement measures so far will reduce the rice to 3.4 °C - all temperature rises here are relative to pre-industrial levels.

So that’s obviously not going to make it too hot for us. The main issues are due to the speed with which the climate is changing, not the final climate which may be more habitable in some ways. Earth is actually unusually cold at present. At times in the geologically recent past the world has been so hot that there were palm trees as far north as the Arctic circle, no ice at either pole and typical polar temperatures 10 °C.

Phanerozoic Climate Change

500 million years of climate change. As you can see, on the timescale of millions of years. Earth has never been this cold for the last 450 million years. In this diagram, one part per thousand of oxygen 18 corresponds to around 1.5 - 2 °C

Most of the time Earth has no ice at all at its poles, no permanent ice at all except at the top of high mountains. Compared to that, the Earth is unusually cold at present. We are in the middle of an interglacial but geologists would say we are in the middle of an ice age still, technically, since we have permanent ice at the poles.

In the worst case scenario, the world stabilizes at 7 °C hotter than pre-industrial levels many centuries into the future - assuming we have stopped creating CO2 by then.

This will be uneven though. The tropics see smaller temperature changes than the Arctic. However if you are a frog living in the tropics then you are used to temperatures that hardly change from one year to another, and even a few degrees increase in temperature could make you extinct, unless you can find a cooler place to migrate to.

SO WHY DID SO MANY NATIONS SIGN THE PARIS AGREEMENT?

The reason that so many countries signed the climate change agreement in Paris is not to protect humans from extinction, which was never a risk. Nor is it to prevent Earth’s climate changing, which it does slowly all the time anyway.

It’s a case of paying a bit more now, or even maybe not paying much, just planning now, to avoid much larger costs a few decades into the future and at the same time to protect fragile environments too, which are at risk because of the speed of the change.

Even the speed of change isn’t that unusual - our climate has been relatively stable for 10,000 years so it is a fast change compared to the last few thousand years - but during the ice age between 18,000 and 180,000 years ago then it fluctuated rapidly even within a few decades. Abrupt Climate Change During the Last Ice Age

But for us, living at a time of relatively slow climate change, this is something we are not used to

COLLAPSE OF THE WESTERN ANTARCTIC ICE SHELF

Eventually, with "business as usual", the Greenland ice sheet is likely to melt completely leading to a sea level rise of 7 meters average, and much higher in some parts of the world. The threshold for this to happen is probably less than 4 °C relative to pre-industrial and it may happen even with a 1 °C rise. But this is likely to take a thousand years (it takes a long time to melt all that ice). What about the nearer future, in the 22nd century? 

In these new studies, a team of glaciologists using satellite and air measurements say that the ice in Western Antarctica has already started a process that is probably impossible to stop. With ice penetrating satellite radar mapping of the terrain beneath the ice (using the EU Sentinel 1 satellites), they say that here are no mountains or hills significant enough to slow the collapse. The fastest melting glacier, Smith glacier, is losing 70 meters thickness of ice a year. It's grounding line - the point at which it starts to float on the sea - is retreating two kilometers a year and has been doing that since 2011, is continuing unabated.

There are six glaciers that will collapse, enough to raise the sea level another four feet. But these may collapse other glacier leading to a rise of sea levels triple that. A separate team studying just one of the glaciers, Thwaite glacier, came to the same conclusion that collapse is inevitable. That is, will happen anyway, based on the CO2 emissions so far.

If so then this would cause a 10 foot rise in sea level. This would cause issues for coastal cities like New York and low lying countries like the Netherlands and Bangladesh which is the area in the world likely to be most affected by sea level rise since much of the country is not far above sea level.

So, when the IPCC talks about tipping points, it doesn't mean in the sense of a  tipping point to a Venus type climate, but rather, tipping point to e.g. a world that's 2 °C warmer, to a world 4 C warmer, to a world with most of the coral lost to acid oceans, to e.g. 1 meter rise in sea level, that sort of thing.

A MORE HABITABLE WORLD

The world will be more habitable if anything in a warmer world, e.g. Siberia, Arctic, even Antarctica eventually habitable. Meanwhile some of the hottest areas of the world, in Africa and the middle East, could become as hot as Death Valley in California and no longer be survivable by human beings without air conditioners. So parts of the world would become less habitable, but other large areas would become more habitable.

The carbon dioxide in the atmosphere actually has a fertilizing effect on most plants, such as rice, wheat etc. It doesn't help maize, or sorghum, millet, or sugar cane as those are C4 plants that already are super efficient in how they fixate carbon dioxide. Right up to 700 ppm, then rice, wheat etc will benefit to some degree from the increases in carbon dioxide (though they may of course also suffer from other effects such as drought, heat waves, etc).

So, it’s not about a greenhouse runaway disaster, that can’t happen. Nor is it about the Earth getting less habitable. Nor is it about the whole of Antarctica melting. The temperature rises even with no restraint on global warming aren’t nearly enough to melt Antarctica in the next few centuries (though it may melt over thousands of years). Indeed at the moment ice is still accumulating in the center of Antarctica. It has continued to accumulate there all the way through the past ice ages and interglacials including times warmer than today. That's why we can have uninterrupted ice cores from Antarctica going back so far into the past. Some ice sheets in Antarctica and Greenland melt. But the prospect of an Earth with no permanent ice at sea level anywhere is a long way into the future.

Also we can certainly survive even the worst case scenarios of global warming. Put enough money into it, for flood defenses, growing new crops that farmers aren’t used to in their region, relocating people, building new foundations to replace the ones destroyed by melting permafrost in Alaska and Canada, building houses to withstand flood damage, more severe hurricanes, better disaster relief and so on - we can cope with it. But it would be expensive.

Rather, the idea of the climate change agreement is that it is far more expensive to respond to climate change like that - than it is to prevent it from happening in the first place which we can do mainly through policy changes. 

The Paris agreements doesn't necessarily even cost us anything. It may have economic benefits, e.g. transition to clean energy leading to new industries. The nations that work hardest on mitigating climate change, e.g. Germany, and for that matter China which is also putting a lot of finance into clean energy, are building up an industry that the rest of the world will want. They may well profit, indeed are already profiting, from the clean energy they have developed.

Also if we just let the temperatures rise and mitigate the effects, it has environmental effects too which can't be reversed, e.g. 99% of all corals affected by a 2 °C rise compared to 90% for 1.5 C.

If we act later, it probably will impact on quality of living and more than that, many people will need to relocate and it will impact on the environment in various ways. If we act now, then none of that happens, and we also benefit right now as well. It's a win win win situation.

There’s a summary here: 

“For example, an extra 0.5C could see global sea levels rise 10cm more by 2100, water shortages in the Mediterranean double and tropical heatwaves last up to a month longer. The difference between 2C and 1.5C is also “likely to be decisive for the future of coral reefs”, with virtually all coral reefs at high risk of bleaching with 2C warming.
Scientists compare climate change impacts at 1.5C and 2C | Carbon Brief

There’s a graphical summary here

Note that with 2 °C rise, instead of 1.5 °C, it’s wheat down by 16% instead of 9%. Maize down 6% instead of 3%.

Rice production however goes up 8% instead of 6% so some crops benefit from a warmer world.

This is about the Paris agreement, pledges so far should keep temperatures by 2100 to between 2.9 and 3.4 degrees C.

Paris climate deal enters force as focus shifts to action - BBC News

The sea hasn’t risen much but in the tropical oceans some islands are really low and have submerged.

Five Pacific islands lost to rising seas as climate change hits

None inhabited yet but others have had to retreat. I think the thing is that coral reefs build up to just the sea level exactly, so a small rise over that, if the island is based on a coral reef, will flood it.

There is no risk to life at all, so long as we are willing to pay up the costs to relocate people, flood defenses, grow more crops etc. None of the scenarios mean that. But they could be very expensive, or if we don’t pay to compensate for the things that go wrong, then humanitarian disasters can occur. And they can make vulnerable species extinct as some can only tolerate tiny changes in temperatures - while humans can tolerate huge temperature changes. Even with "business as usual" then there won't be much of the world that's so hot that it is totally uninhabitable for humans, even without air conditioners.

EFFECTS ON CORAL

This is one of the most dramatic effects of climate change - that we will almost certainly lose all except 10% of our coral reefs through bleaching, by 2100. That's with the 1.5 °C rise in temperatures. We can save the remaining 10%, but if it increases to 2 °C from pre-industrial we can only save 1% of the coral reefs we have today.

So, that's a dramatic change. But - it's not making the oceans less habitable. Just different. Sometimes the oceans are acidic, sometimes they are alkali. Right now they are alkali and this favours corals with their skeletons made of calcium carbonate. When the oceans are more acidic, this favours sponges and other creatures with a silica skeleton.

Back in the Jurassic period, then there were vast reefs of sponges. They don't exist any more and until the late 1980s, were thought to be completely vanished. That's until scientists discovered a "living fossil" sponge reef in Canada.

Perhaps in the future with a more acidic ocean, instead of corals, we'll have the living wonders of sponge reefs, or similar. It's not the acidity of the ocean that's the problem. It's the transition. From a present day with complex ecosystems based around coral reefs, in future, over timescales of thousands and then millions of years, we may have equally complex ecosystems based around sponge reefs or similar. But the transition will lead to bleaching events and loss of corals world wide.

LONGER TERM HABITABILITY FOR HUMANS

Longer term, if we did continue "Business as usual" and burn all the recoverable reserves of coal, oil, gas etc, then we might well get temperature rises as high as for the PETM event. Though that leaves a world that some mammals could survive in, it would make large areas of the Earth uninhabitable for humans. Many areas in the warmer parts of the world are already close to the limit of habitability for humans without technology to keep them cool. They are just within it, by enough to make pretty much the entire Earth still habitable.

This paper from 2010 looks at the long term effects of small increases in temperature on these habitable regions, coming to the conclusion that the effects would dwarf the results of flooding, if the temperature increases reach 7 °C or higher.

"We conclude that a global-mean warming of roughly 7?°C would create small zones where metabolic heat dissipation would for the first time become impossible, calling into question their suitability for human habitation. A warming of 11–12?°C would expand these zones to encompass most of today’s human population. This likely overestimates what could practically be tolerated: Our limit applies to a person out of the sun, in gale-force winds, doused with water, wearing no clothing, and not working. A global-mean warming of only 3–4?°C would in some locations halve the margin of safety (difference between TW max and 35?°C) that now leaves room for additional burdens or limitations to cooling. Considering the impacts of heat stress that occur already, this would certainly be unpleasant and costly if not debilitating. More detailed heat stress studies incorporating physiological response characteristics and adaptations would be necessary to investigate this.

"If warmings of 10?°C were really to occur in next three centuries, the area of land likely rendered uninhabitable by heat stress would dwarf that affected by rising sea level. Heat stress thus deserves more attention as a climate-change impact.

"The onset of TW max > 35 °C represents a well-defined reference point where devastating impacts on society seem assured even with adaptation efforts. This reference point contrasts with assumptions now used in integrated assessment models. Warmings of 10?°C and above already occur in these models for some realizations of the future (33). The damages caused by 10?°C of warming are typically reckoned at 10–30% of world GDP (33, 34), roughly equivalent to a recession to economic conditions of roughly two decades earlier in time. While undesirable, this is hardly on par with a likely near-halving of habitable land, indicating that current assessments are underestimating the seriousness of climate change."

By comparison the Paris agreement is based on keeping the temperature increase below 2 °C, ideally below 1.5° C from pre-industrial, and the pledges so far should keep it below 3.4 °C by 2100.

The Persian gulf region is particularly vulnerable because of shallow seas, and intense sun, leading to hot damp conditions - the most difficult conditions for humans to survive in. Elahir and Pal, reporting in Nature, found that many major cites could reach a tipping point later this century, where for the first time, conditions exceed the limits for human survival, with wet bulb temperatures exceeding 35 °C every few decades. Most of these are high technology cities where the inhabitants could just stay indoors and use air conditioners to stay cool, but it's pretty major even so, especially as the hottest days they get would now become their new normal.

Quoting from the MIT press release:

" The models show that by the latter part of this century, major cities such as Doha, Qatar, Abu Dhabi, and Dubai in the United Arab Emirates, and Bandar Abbas, Iran, could exceed the 35 °C threshold several times over a 30-year period. What's more, Eltahir says, hot summer conditions that now occur once every 20 days or so "will characterize the usual summer day in the future."0

"While the other side of the Arabian Peninsula, adjacent to the Red Sea, would see less extreme heat, the projections show that dangerous extremes are also likely there, reaching wet-bulb temperatures of 32 to 34 °C. This could be a particular concern, the authors note, because the annual Hajj, or annual Islamic pilgrimage to Mecca — when as many as 2 million pilgrims take part in rituals that include standing outdoors for a full day of prayer — sometimes occurs during these hot months. While many in the Persian Gulf’s wealthier states might be able to adapt to new climate extremes, poorer areas, such as Yemen, might be less able to cope with such extremes, the authors say."

Video summary here

In conditions like this, it's not just the elderly and infirm that are affected. Nobody can survive such high temperatures with the air as moist as that without technology. This century, for the first time in human history, we may see conditions arise from time to time in highly populated areas of the world which make it impossible for humans to survive unaided by modern technology.

EFFECTS ON THE US

You can look up the predicted effects of climate change on your country, also globally. Once you do that you may understand why so many nations signed the Paris Climate Change agreement. I'll focus on the US here. In the US the main changes projected include:

(quoting from the US Environmental Protection Agency

report Future of Climate Change) -

  • “Summertime temperatures in the United States that ranked among the hottest 5% in 1950-1979 will occur at least 70% of the time by 2035-2064“
  • “Northern areas are projected to become wetter, especially in the winter and spring. Southern areas, especially the Southwest, are projected to become drier“
  • “Heavy precipitation events will likely be more frequent, even in areas where total precipitation is projected to decrease.“
  • “Heavy downpours that currently occur about once every 20 years are projected to occur between twice and five times as frequently by 2100, depending on location“
  • “The intensity of Atlantic hurricanes is likely to increase as the ocean warms.“
  • “Cold-season storm tracks are expected to continue to shift northward. The strongest cold-season storms are projected to become stronger and more frequent“
  • “Permafrost is expected to continue to thaw in northern latitudes, damaging buildings, infrastructure, and ecosystems in Alaska.“

You can also look up the effects for individual states, e.g. this climate change report for Nebraska

RISING SEA LEVELS

The US, along with China, Japan, Bangladesh, and several others, is one of the countries most impacted by rising sea levels in the climate change projections. Even if we stopped all carbon dioxide emissions, the oceans continue to warm and ice sheets to melt, from the carbon dioxide in the atmosphere already, so the effects are most severe in the long term.

The IPCC has already said that the sea levels will rise by up to one meter by 2100 if we don't use stronger methods to limit greenhouse gases. These graphs are from their summary for policy makers from 2014.

There the RCPs are the Representative Concentration Pathways considered by the IPCC. RCP 8.5 is business as usual. RCP 2.6 has emissions peak before 2020 with an immediate and rapid decrease in emissions, roughly what the Paris agreement is attempting to achieve. RCP 5.6 has a peak around 2040 and RCP 6.0 has a peak at 2080.

This shows the temperature changes for the business as usual and the rapid reductions scenarios.

  • RCP 8.5 is "business as usual" with emissions continuing to rise. Temperature rise by 2100: 2.6°C to 4.8°C, sea rise 0.45 to 0.82 m
  • RCP 6.0 has emissions peak at 2080, temperature rise 1.4°C to 3.1°C
  • RCP 4.5 has a peak around 2040, temperature rise 1.1°C to 2.6°C
  • RCP 2.6 has them peak before 2020, temperatures: 0.3°C to 1.7°C. Sea level rise 0.2 to 0.6m

The temperature increases there are measured relative to the average for 1986–2005

There are many uncertainties there as you can see by the range of values with 2.2 °C difference between the projections for 2100. It's also possible that the temperatures are above or below those numbers. The same applies even more for the rising oceans. You might think that a sea level rise has to happen uniformly over the entire world, as after all the oceans are interconnected. But no, they don't rise uniformly. As an example, New York experiences a much higher sea level rise than normal while London will have a much lower sea level rise than normal for the same average sea level rise.

There are several effects here

  • Parts of the Earth are still rising as a result of the ice sheets withdrawal at the end of the last ice age
  • With a warmer ocean, the distribution of water across the globe changes
  • The mass of ice in Greenland and Antarctica exerts a gravitational pull on the water closest to their coasts, raising the sea level there .As the ice melts then this gravitational pull is reduced, so the water previously piled up around Greenland and Antarctica gets redistributed over the globe.

Florida is the US state most affected by flooding, because of its location, near the sea, and because it's geology, based on porous limestone, making flood defenses impossible. Here are some maps to show what could happen there:

Florida at current sea level (click to go to interactive map) - The areas shaded are not mapped, so ignore them.

Here it is at 3 feet

Florida at 3 feet - a level likely to be reached by 2100 with "business as usual" - shaded areas are not mapped

Florida at six feet, a level likely to be reached in the 22nd century if the West Antarctic ice continues to melt. Some think we may reach that level sooner, even by 2100, with "business as usual" Again, shaded areas not mapped.

Florida before and after a 3 meter sea level rise due to melting ice from the Antarctic and Greenland ice sheets. Image made using the Eustatic Sea Level Change tool from Virtual Earth System Laboratory (VESL).

Florida is already getting affected more than usual by hurricanes because of the one foot the sea has risen by so far. See Goodbye Miami for an article in Rolling Stone magazine about these issues.

However, remember that Florida is in an area that is much more affected by the global sea level rise than the average, so it may be flooded much more than this. The Bahamas are also impacted. Just a 1.5 °C or 2 °C rise in temperature will already have a significant impact there. 5% of the population of the Bahamas would be displaced if the sea level rose by one meter.

SURGING SEAS RESULTS

Shows the effects on Japan of a 4 °C rise (left) and 2 °C rise (right), image from Climate Central

One paper suggested a 6 meter rise, though this is not the rise by 2100, but the eventual rise which might take anything between centuries and thousands of years. It did it by comparing the present with past warming episodes in previous interglacials.

With this study, cities in Japan would also be severely affected. According to one study, 18 million people in Tokyo and Osakai would find their homes are underwater with a rise in temperature of 2 °C and 34 million people with a 4 °C rise. Similarly, 25 million in the United States, 20 million in the Philippines, 19 million Egypt and 16 million in Brazil would find their homes underwater eventually with a 4 °C rise.

The worst affected cities in the US include Miami, New Orleans and New York.

145 million people in China live in cities that would be underwater with a 4 C rise in temperature.

Tokyo, Osaka, other mega-cities will be swamped by surging sea levels, even at 2 degree rise: study | The Japan Times

The big question is how long this would take. If it’s a thousand years, then that’s time to move the population of the cities or respond in other ways. If it’s a century or two, it’s a major upheaval. It is difficult though to model this exactly- as it depends on the behaviour of both the Greenland and Antarctic ice sheets, and the local effects, e.g. how much a particular country is affected, will depend on the gravitational effects of the remaining ice, which depends on which of those melts the most, as well as other effects such as the distribution of the sea over the globe changing as its temperature changes.

I’ve used material from different sources in my articles. They don’t always say the same thing and as you’ll see there is a fair bit of uncertainty in the projections. That’s normal for cutting edge science which is what this is. The general trend is the same for all the sources, but they differ in detail - how much each ice sheet would be affected, and now much etc.

For a detailed near term prediction, let's look at New York city.

NEW YORK PROJECTED SEA RISE EFFECTS

This is the projection for New York city which experiences higher sea level rises than the global average:

  • Projections for sea level rise in New York City are 11 to 21 inches by the 2050s, 18 to 39 inches by the 2080s, and the highest estimates (shown as + in the diagram above) could reach as high as 6 feet by 2100.

New York City Panel on Climate Change 2015 Report Executive Summary

Here is their detailed map of possible sea flooding zones for New York city by various dates from the 2020s to 2100.

SEA LEVEL MAPS

You can have an explore of the world to get a rough first idea of the effects of local sea level rises with this interactive map

Global sea level rise map. This map was created by Alex Tingle using Google maps and NASA elevation data.

This shows Florida, the Bahamas and Cuba after a 9 meter rise, which isn’t going to happen any time soon, like in the next century or two, but may be the very long term effect of all that ice melting.

It uses NASA altitude data which is approximate. It also just maps a new sea level against the topography of google maps. Places that are inland below sea level of course will not experience sea level rises even if shown flooded in the diagram (you need to trace out to see if there is a connection with the sea). But in the case of Florida, since the underlying rock is porous, then the map probably pretty much shows what the effect would be no matter what flood defenses are used.

Also this map doesn't take account of the uneven nature of sea level rise around the world. So you can’t just dial in the projected global sea level rise. You need to dial in your local sea level rise if you know what it is projected to be.

SURGING SEAS MAP

For a more accurately done map - but one that is slower to navigate unless you have a fast internet connection, see the Surging Seas. It takes account of local differences when working out sea level effects and also takes account of areas contiguous to the sea. To find out when your area will be affected by the various sea level rises, then click on Projections on the map and then choose the relevant scenario and it will show the date by which that sea level rise is projected to happen, if it happens before 2200.

Use this to get a better idea of what the effects would be. But for sea level rise especially, its estimates for when you will reach a particular sea level for the various scenarios may be an underestimate, as the IPCC projections are regarded as quite conservative.

NOAA MAP

The maps I did for Florida used the NOAA map, which is detailed and accurate but is only available for the USA. You can find it here, Sea Level Rise Viewer.

WHAT ABOUT TRUMP?

Going back to the original question BBC asked Stephen Hawking, what's the effect of Trump taking the US out of the Paris agreement? Well first, it's a long process. They remain formally within it for several years yet. Meanwhile, his successor, or Trump himself, can rejoin it again at any time. He doesn't have to renegotiate anything to do that, as the whole thing is voluntary. The whole idea of his suggestion that we should "renegotiate the Paris climate change agreement" so that the US could return to it was puzzling, and doesn't make much sense. He can commit the US to as little or as much as he likes, and then rejoin the agreement. He can withdraw from any particular promises that president Obama made for the US that he likes. He can safeguard the coal industry too, one of his campaign promises, by working on carbon capture and storage technology. That's a way to make coal fired power stations climate friendly. 

He does often seem to "turn on a dime" in his changes of policy, so I think that's not beyond possibility, that he could return to the agreement in the future. Not right now, it would be politically damaging to do it now. But some day. Some emotionally heart tugging event that suddenly makes him aware of climate change, as something that has real world consequences for people he cares about, and he might change overnight, I think.

As a result of his announcement, the US joins a select group of three countries that haven’t signed the agreement: Nicaragua, Syria and now the United States of America.

The US is even more isolated than that suggests. Nicaragua’s reason for not joining the Paris agreement is as a symbolic protest because they don’t think it is strong enough. It’s going to do tough reductions of emissions but not within the agreement.

Meanwhile, in the case of Syria then as the Washington Post put it: “Given the nature of the conflict during Paris negotiations, the Assad government was in no position to commit to limiting Syria's climate emissions.”

So really the US are now  in a club of one. It's also only in the US that this is a political debate between Left and Right. Elsewhere, nearly all political parties support the Paris agreement (with some exceptions like the fundamentalist Democratic Unionist Party in Northern Ireland). 

I'm from the UK, and the agreement was signed by our own right wing Tory government - now headed by Theresa May, previously by David Cameron. It has support from governments of all political persuasions, unless you think Saudi Arabia, and Iran have left wing governments! You'd have to say that the US is the only right wing government in the world, to try to make it a polarization between left and right wing governments.

It also has support from the most capitalist countries as well as the most communist ones. This is the 2015 list of the most "economically free" countries. in the Economic Freedom of the World (EFW) annual survey by the Fraser Institute

They say

"Hong Kong and Singapore, once again, occupy the top two positions. The other nations in the top 10 are New Zealand, Switzerland, United Arab Emirates, Mauritius, Jordan, Ireland, Canada, and the United Kingdom and Chile, tied for 10th."

All of those have now ratified the agreement.

It's all rather puzzling too, especially for outsiders watching these debates from other countries like the UK, as the US is one of the countries in the world that's quite severely economically affected by climate change. Here is a map of the predicted economic damage per county as percentage of the county's GDP in the US. Some of the worst affected may lose 30% of their GDP from climate change, and many of them lose at least 5%. The US as a whole could lose 6% of its GDP if the Earth warmed by 6 °C.

He said in his speech that he was acting for Pittsburgh rather than Paris. But Pittsburgh is one of many US states and cities that are involved in their own climate initiatives which they have the ability to do under the US constitution no matter what Trump says US as the whole is going to do. They aim to eliminate 1.3 million tons of CO2 emissions by 2023. It seems rather ironical that he would choose such a city as his named city in the speech, and say he is representing them by withdrawing from the Paris agreement. It’s no wonder that the Mayor of Pittsburgh tweeted his protest to that.

Meanwhile, I think myself that Trump withdrawing from the Paris agreement seems to have galvanized everyone else to even more action, including many in the US. Also, in the US then new green technologies like solar power are leading it towards cleaner energy too as they become more competitive. So, is probably too early to see what the effects will be of him withdrawing, but so far, many of them outside the US have been actually rather positive.

Outside the US, and even within in many states, cities, and companies, it seems to have galvanized many to action. Long term - well the US will need to fall into line at some point, to keep us below e.g. 2 °C or ideally 1.5 °C but meanwhile other countries are taking up the slack. Also, as a high technology country, the US emissions may well go down through clean energy costing less and the actions of the many people, including city mayors, CEO's of major companies, and many states as state policy, who do care in the US. Will need to watch the spot. But I think short term, lots of positive has actually from it already

GREEN CLIMATE FUND IS VOLUNTARY

The only financial commitment that the US had was to a fund to help poorer countries to develop their green energy solutions. But that also is a voluntary fund too. The US made the largest pledge, $3 billion of which it has paid around $1 billion so far. The UK, a much smaller country, pledged $1.2 billion and Japan pledged 1.5 billion, France and Germany also both pledged $1 billion - and the UK and Japan have both paid significant amounts into the fund so far.

The highest per capita pledge for this fund is from Sweden, pledging $60 per person and the UK has pledged £15 per person.

To see how voluntary it is - neither India nor China have put forward anything into it yet. The US could just say that they aren't going to put anything more into it, so joining China and India, and remain within the Paris agreement. None of this was imposed on the US by anyone else, indeed, the whole Paris agreement was instigated mainly by the US as one of the key players who got the ball rolling.

Reality Check: What do countries spend on climate fund? - BBC News

TAKE HOME MESSAGE

My central point here is that we need to focus on the real world scenarios. Let's not scare people with imagined futures that can't happen. There is plenty of research pointing to the need for urgent action based on sober careful science.

We can solve this, and there is much that we can do to avoid the worst effects of climate change. Trump withdrawing from the Paris agreement seems to have galvanized other countries, and even possibly the US itself, in the direction of doing something about climate change. The key to this, I think, is to make a careful evaluation of our present day situation and future prospects and report on the research carefully with due diligence.

The research so far says that we don't risk Earth tipping into a hot Venus, even if we follow business as usual and burn all the extractable hydrocarbons. We would need to burn at least ten times the amount available. The next step down is the moist hot greenhouse too hot for human habitability, and again, it seems we don't have enough hydrocarbons to burn to get to that state. However, if we followed "business as usual", it does seem possible that we could end up with a world with the wet bulb temperatures above 35 °C over much of the Earth, leaving only the higher latitudes habitable for humans without technology. That would happen at a temperature increase of 7 - 10 °C above pre-industrial.

However we aren't following "business as usual". Even with the US out of the Paris agreement, still many in the US are doing their bit to reduce greenhouse gas emissions. Outside the US, then the commitment is stronger than ever to do something about it. The pledges to the Paris agreement so far should keep temperatures within 3.4 °C by 2100. That's plenty of time for many new administrations in the US. We can move to a carbon neutral world by 2100, hopefully well before then.

This still leaves us with many challenges. To save 10% of the coral reefs, and the worst effects of climate change, we need to keep the temperature increase to below 1.5 °C, and that is a major challenge. We could be headed for a world where parts of the Persian gulf require technology for humans to continue to survive, even if it is needed only once every few decades. There may be widespread sea flooding,in places like Bangladesh, China, Japan, Florida, and maor cities along the western coast of the US amongst other places. Agriculture is likely to be impacted negatively. There's no chance of a world where we don't have the ability to produce enough food, as our technology could produce far more food from the available land than it does at present, but food could become more expensive, harder to grow.

So there are plenty of challenges and many good reasons to really crack on and do our bit to work towards a carbon neutral future. But all the research I've seen says we don't need to worry about Earth turning into Venus.

(I've fixed some errors in this article in response to comments on facebook by John Byatt. Of course remaining mistakes are all my own responsibility, and there may well be more to fix, but I'd like to thank him for putting me straight on a number of points here).

If I have made any mistakes in this, however small, or you spot any omissions, please be sure to say. What do you think?

Parts of this are from my articles

(These two articles explain why climate scientists are confident in the ability of their models to make predictions about the general climate of Earth decades in advance).

and my quora article and answers

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