Short summary: This is a first paper in what they hope to be a long term study. It’s an interesting project, but it should not be used to predict what will happen to Earth in the future. Instead we should rely on projections by the scientists who are studying the effects of climate change, our population growth, predicting food security and looking at the intricate details of how our world works. The UN population division says as its middle of range projection that we level off at 11 billion people by 2100 and experts say we can feed everyone too, if we continue on target with the things we are doing to make sure it happens. It requires work, it's not going to happen automatically, but we can do it, and with food security as well. And with plenty of power too.

Their research is at far too early a stage to be relevant to any of that. It makes many simplifying assumptions that do not apply to Earth as it is now. But their aim wasn't to predict what would happen to Earth. It was to try to get a general picture of what can happen to all possible exoplanet civilizations. A huge project that they are just making a start on.

I am publishing this article because I've been contacted by several people who got scared by the news reports about this study thinking it means we are likely to go extinct, and if not, something very horrible will happen to us, probably ending our civilization. Not it doesn't mean that at all. That's just journalists sensationalizing it.

It is a very simple model, with a civilization with only two resources, and no social decisions or social effects except the possibility of deciding to change from the non renewable resource (assumed harmful to the environment) and the renewable resource (assumed to have no harm), both assumed to have infinite reserves.

First, let's look at some of its simplifying assumptions - none of which apply to our Earth (no surprise)

  • One of its assumptions is that populations grow to carrying capacity (via the Logistic equation, more on that later) - actually our populations are leveling off in many places and in Japan is already declining rapidly, only Africa grows quickly still and is expected to level off by around 2100 in middle of the range projections now. This is because of social changes, not resource scarcity. Indeed the opposite. As people become more secure, and have better health care for their children,, they have less need for large families. Most are content to have two children and once you take account of the childless and single people, the average number of children drops below replacement. Having plenty of resources is leading to a reduction rather than an increase of population, paradoxically. We reached close to peak child a decade or so ago and the population is now growing mainly because people are living longer with each year.
  • They do however assume that renewables can sustain the same population as the non green power supply (e.g. fossil fuels). That does apply to Earth. There is no problem supplying as much power from green energy as from coal / oil / gas, as we transition to clean energy with the help also of carbon capture and storage and nuclear power for the time being. However, arguably long term the potential power supply from renewables is vaster than from fossil fuels. A small fraction of the Sahara desert can provide enough solar power to power the entire world at its current levels. Some people object to this idea based on energy return on energy invested (EROEI), well, solar power has proved to be superb as have some other renewables.
  • It also assumes that the non renewable power source can make the planet uninhabitable in a runaway effect through a "heating term" . Maybe this is inspired by the idea of a runaway greenhouse? If so that’s debunked, see Will Earth's Ocean Boil Away?. No amount of degradation of Earth’s environment would make Earth uninhabitable for humans. Indeed, it is becoming more habitable if anything, as we’ll see, it’s the transition that’s the problem. But in their model they assume infinite reserves of the non renewable environment damaging resource (this might seem strange but it is a common simplification if you don't want to have to deal with the complexities of a finite item of fixed size - just assume it is infinite and leave the complication of a finite resource to a later study).
    Of course if there were infinite reserves of oil or coal, eventually as you burn enough then you would get a runaway greenhouse for sure. If extra terrestrials have more fossil fuel than us or a more sensitive environment, it might apply to them.

So, what is actually going to happen to Earth?

  • As we are now, we are on target to feed everyone by 2100 using conventional agriculture, with the population predicted to level off at 11 billion according to the UN population division. There are challenges but it is well within our capability. With higher standards of living, and better medical care, most families are content to have two children. Once you take account of the childless and single, that leads to populations that drop below replacement through social changes, in a situation of resource abundance rather than resource scarcity. Japan already have a rapidly declining population.

    We can feed everyone too, according to experts, so long as we continue with the measures we are doing already such as starting a new green revolution in Africa that got missed out in the 50s through to 70s. It's not going to happen automatically, but we can do it, with food security, and we can do it while ramping up power levels too and bringing everyone up to the quality of life we enjoy in the more developed countries. This is not pie in the sky, it is the view of leading scientists and hard nosed politicians, as we'll see as I get into the details.
  • We can ramp up power as well, while keeping everything headed towards a carbon neutral eventual future. That is not “pie in the sky”. It is feasible, it's what the Paris Agreement is all about, and hard nosed politicians have signed up for it in every country worldwide except the US, which also was signed up for it before the current president withdrew. Actually technically the US is still in it to the end of his current term. So, yes, we can have a decent standard of living for everyone too.

What could happen even longer term?

  • The carrying capacity of Earth is rather “soft” - using the technology proposed for space habitats, we could have the equivalent of the population of four new planets from floating sea cities covering just 0.5% of the Pacific, growing all their food and getting all their water just using sea water and the air and not exploiting anything else. We could do that for far less technology per person than a space colony.
  • We also have the possibility of vast amounts of power in the future, either from nuclear fusion (if we manage to develop it by then, and it is easy to use) or space based solar power (which Japan is exploring) - or maybe some other power source (including possibly ideas we haven't thought of yet). It could increase by orders of magnitude.

They never claimed any direct relevance to Earth and expect future studies to be more detailed, exploring the range of futures for extra terrestrial civilizations.

As they do so, perhaps they may find trajectories resembling the one we ourselves are on, but at present it is just too simple a model to apply to Earth.


First, here are two sensationalist versions of it

Here is a rather more sober version but not that critical

So first I’ll summarize the research,, and say briefly how the simplifying assumptions don’t apply to Earth as is. Then talk in more detail about why their assumptions don’t fit Earth’s current situation.


The paper itself is here The Anthropocene Generalized: Evolution of Exo-Civilizations and Their Planetary Feedback. This summary is based on what they say they did in the paper itself rather than any of the summaries elsewhere. .

So first, they say they think of it as a simplification similar to James Lovelock's famous "Daisy world" where he simplified a planet's ecosystem to just two types of daisy as the only vegetation, and showed how with basic assumptions the planet could maintain its temperature as the sun warmed up continuing through the main sequence. This was never meant as a serious model of Gaia, but an illustration, to show the basic idea of how the feedbacks in Gaia work.

They say a full simulation would be far more complex, more like the complexity of the global circulation models used for climate simulations. They are using a one dimensional population dynamics model.

They base it on a model used for Easter Island. This was a rather simple situation, a civilization in a remote island in the middle of the Pacific. They used up their resources, cut down all the trees for instance, and the civilization collapsed. It attracts population dynamicissts as it is something they can hope to model. They use insights from this case to motivate their model.

The population dynamics are based on the Logistics equation according to which populations continue to grow until they reach a point where they are running out of resources when they level out gradually at the carrying capacity due to members of the population competing for resources. For Easter island they modify this by adding an extra term that expresses how as humans reach the carrying capacity of the island, they don't just reach a steady state, but they gradually degrade the environment, more and more the longer they stay at that level of population.

Techy details indented:

They go on to develop a more complex equation that has many parameters including the state of the environment, its carrying capacity when it is non degraded, environment recovery rate, how the resources benefit population growth and how they harm the environment, the natural growth rate of the species, and a critical environment state at which point it is so degraded that it can't support anyone at all (that's their table 1).

So first they look at the case where there is only one resource, and that harms the environment. The population then rises rapidly, starts to level out like the Logistic curve does, but especially if damage to the environment is delayed, it then rapidly dies back to a much smaller population. They give an example, setting the environment recovery rate to 1% of the natural population growth rate, with a population that falls to 2.7% of its peak value.

Their second model allows the civilization to start using a green energy source. For simplicity they assumed it has the same benefit as the damaging source (so that if they used it from the start they would have just had a normal logistic curve growth). Then they look at what happens depending on when the civilization decides to start using the green energy. They have a parameter that looks at how much their environment has to degrade before they start to use the green resource, and then, the speed at which the switch is made. Depending on values of those two terms it can end up rising to a steady state, rising to a lower steady state with a permanently partly degraded Earth, and rising to a maximum and declining due to degradation of resources continuing so much they can't sustain the maximum value (see their figure 6).

Their third model adds in an extra heating term that can lead to a runaway greenhouse effect. This is what makes total extinction possible in the scenarios. Since this is not possible for Earth, this means their extinction scenarios are not relevant to us. But could be relevant to extra terrestrials on planets that have large quantities of fossil fuel or more sensitive environments than we have, including a biology that is more sensitive to temperature changes.

With all these assumptions, they come up with four scenarios, the three given + a fourth oscillating one. Quoting from the paper’s discussion and conclusion the four states are (with the terms used for them in the journalist articles in square brackets):

  • Sustainability: … The population rises smoothly to a steady-state value. The planetary environment is [somewhat damaged, but ] reaches a new steady state that can support a large population [“soft landing”]
  • Die-off: … The population overshoots the environment's carrying capacity, reaches a peak, and is forced to decline as the environment reaches its new steady state. [“wither away”]
  • Collapse: … the population experiences a rapid decline after reaching its peak value. It is noteworthy that collapse can occur even though the population has begun leveling off due to the civilization's switching from high-impact to low-impact energy modalities. [“full scale collapse”]
  • Oscillation: In this class, a stable limit cycle exists rather than an equilibrium. The population and the planetary environment cycle between high and low values - they got this in their third class of models in its simpler form with one resource and a warming parameter. Once they had two resources and a warming parameter then this one dropped out, if I understand right.

You can see examples of  various scenarios in their figure 13.

There the red line shows the warming effect - and as you see, if the red line goes high, the population drops to 0 in their runaway greenhouse effect.

They say at the end that, as should be obvious by now, it is a highly simplified representation and intended as a demonstration of a methodology that may help us learn about exocivilizations.

“We first note that our study was specifically intended as a demonstration of the methodology. Our models are a highly simplified representation of the true complexity inherent in the interactions between a civilization (human or otherwise) and the host planetary systems. Thus the different classes of trajectories observed in the models represent an initial exploration of the richness to be expected as we begin building representations that capture higher degrees of veracity in the coupled dynamics of civilizations and their planetary system.”

As they do that they will probably come to include a richer class of models including ones more closely resembling the possible paths that lie in Earth’s future.

So now, let's turn to what we know about Earth itself rather than a simplified model, first in the near future through to 2100, and then I'll take a larger view on it.


Our population growth is leveling off, towards a population of perhaps ten or eleven billion by the middle to the end of the century, well before the peak of what our environment can sustain.

We have already reached peak child. Most areas of the world now have fertility levels at or below replacement, and feature a slowly growing, steady, or declining population. Our world population continues to grow only because with better health world wide, people on average live longer each year.

The middle of the range projections for the year 2100 have the Earth's population trending towards 11 billion, while lower projections have it level off at ten billion or even start to decline towards the end of the century.

Whatever happens, we aren't headed for Malthusian type exponential growth because we have reached peak child already. In the graph above, the red dotted lines show the upper and lower limits for the 95% prediction interval. The blue lines are for +- 0.5 children per couple average. You can look up the data here, the graphs page for the UN population division.

Though we may not reach peak population worldwide quite this century, most parts of the world have a good chance of stabilizing before then, especially the more developed countries. Some have already.

The least developed countries are the ones that would get most population growth. In these projections, the most rapid growth is in Africa You can see a break down for each region of the world here,

Older figures from 2014. The least developed countries are the ones that grow most rapidly, and the projected continued population growth in Africa is a reflection of the slower pace of development in Africa

Note how Asia particularly has a peak in 2050 and then declines. Some are declining already, Japan particularly:

“Japan's total population was 127.09 million according to the Population Census in 2015. This was a decrease by 962,607 people as compared to the previous Census (2010), indicating the first population decline since the initiation of the Population Census in 1920. In 2016, it was 126.93 million, down by 162,000 from the year before.”

Statistical Handbook of Japan 2017

That makes the population in 2010 127.09+0.96 = 128.05 million. It is now 126.70 million. It has shrunk by 1.35 million since 2010.

“A government institute projected in April 2017 that the population would fall below 100 million in 2053 and drop to 88.08 million by 2065, when people 65 or over will account for 38.4 percent of the total.”

Japan's population shrinks for seventh consecutive year as it falls to 126.70 million | The Japan Times

It’s due to shrink from 128.05 million in 2010 to 100 million in 2050. This is nothing to do with carrying capacity or energy availability. It is just social change and a declining birth rate. It might even be an issue for Japan as, what with increasing longevity, they will eventually by 2050 have to support 38.4 percent of the population at retirement age.


We are surrounded by an abundance of clean energy from the sun, if we but had a way to use it. That alone would solve all our power issues, combined with a way of storing the power overnight and transmitting it over long distances. Add power from solar panels or thin film mirrors in space and the energy available from the sun alone is almost limitless.

But let’s look first at what we are doing right now. There’s no chance of running out of power for our civilization any time soon, and we can reduce the impact on our planet by various methods that have no effect at all on the total power available. Not only that, we can continue to generate more and more power, bring everyone gradually up to the living standards of the wealthier countries, have plenty of power, and yet at the same time reduce the impact of global warming as much as we can.

This is the very idea behind the Paris agreement. To prevent global warming from being excessive while still keeping countries on a trajectory towards modern living standards of the wealthier countries.

The agreement lets countries set their own targets and methods. You can find the NDC's (Nationally Determined Contributions) here. Several countries are aiming to go carbon neutral by 2050, or are working towards it. Or even earlier, Sweden commits to go carbon neutral by 2045 - by increasing use of biofuels and electric cars for public transport reducing its emissions by 85% and then planting trees and investing in projects abroad for the remaining 15%.

It's not just whether a country can go carbon neutral by 2050. What matters more are the emissions leading up to it. To stay within 1.5 °C by 2100 we have to start reducing CO2 emissions significantly worldwide in the next few years, or aim for significant carbon capture from the atmosphere in the future, because carbon dioxide is unusually persistent in the atmosphere (though most is gone in decades, 15% of a pulse of CO2 added to the atmosphere is still there thousands of years later).

Morocco is on track for 1.5 °C increase by 2100 in comparison with its pre-industrial emissions (it's one of the countries most affected by climate change), and India is on track for 2 °C, already, may be able to aim for 1.5 °C. The way the Paris agreement works is that the countries are expected to increase their commitments as it continues. More countries will need to follow these good examples.

Some recent news stories suggested that China is going to wreck the Paris agreement. Despite an increase in the non renewables mix for just one year, bucking its trend, it is still strongly promoting renewables and is well on target to meet its National Determined Contribution and to peak by 2030. China is crucial if we want to stay within 2 °C and has to peak before then to achieve this, and improve on its initial NDC. But experts think it may well actually peak between 2020 and the mid 2020s. If so we can achieve 2 °C rise, maybe even 1.5 °C.

Prof. Niklas Höhne is a top expert, lead author of several IPCC assessments . Here he talks about how China is still on track for its NDC and how experts think it may well peak before 2030.

(Click to watch on YouTube)

He is also optimistic of the US Paris commitment, quoted as saying:

"If developments on renewables continue as positively as in the past, and new commitments by US states, cities and businesses are implemented, the US could still meet its Paris commitment,"

For more on this see also this recent article in Scientific American: How the World Is Coping 1 Year after Trump Abandoned Paris Climate Pact

However, if we don't achieve 2 °C, we already have policies in place that will keep us within 3.6 °C temperature rise, by 2100. That’s the “New Policies scenario”.

The IAEA has developed one way ahead that could keep us within 1.5 - 2 °C called the sustainable development scenario. It involves use of renewables, efficiency improvements, carbon capture and storage and nuclear power, and some other methods:

In this approach, then natural gas is kept for a fair while into the future - the reason being that it is easy to ramp power stations that use natural gas up and down in power to compensate to fluctuations in renewables, so until we have other ways to do that in place, we can continue to use natural gas power stations. Here’s a road map for solving 3 of the world’s biggest problems

Not only do we have enough power for the near future though, through to 2100, which I don’t think anyone really doubts, we have potential vast amounts of green energy into the indefinite future for millennia and millions of years.

  • Solar power

At the moment solar power is an increasing part of the mix of green energy solutions. But we are very far from realizing its potential.

We could power the whole world with solar energy, many times over, from the Sahara desert alone. It has eighteen times the surface area needed to power the entire world. Of course you don’t want to completely cover it with solar panels, there are areas of great natural and environmental interest, but there are also areas that aren’t of much interest and nobody would mind covering them in solar panels.

Yes there are issues with providing solar power at night, but we have ways around that including molten salt which can store power for weeks at end. Yes we have less solar power at high latitudes in winter especially, but we could deal with that by using long distance power transmission. (e.g. the Sahara desert

This was a plan by Desertec - Wikipedia to power all Europe right up to Iceland from the Sahara desert. It wasn’t using solar panels, but rather, mirrors to concentrate heat, a practical way to do a large scale power station.

The squares show the area of the Sahara desert that need to be covered in solar panels or solar power concentrating mirrors to power the entire world, Europe, and Germany respectively at 2005 power levels. Solar power capacity - Wikimedia Commons

For power distribution it would use high voltage direct current, a proven method that is used in China, for instance for very long distance transmission of clean energy. It’s used in other places too. For instance Boston uses a lot of power from Quebec, 1,000 miles away, losing maybe 2% over that distance (see Should we solar panel the Sahara?). The longest HVDC line under construction, actually UHVDC (Ultra High Voltage Direct Current) is in China, 3000 km, 12 GW capacity and at a voltage of 1100 kV. That’s not far off the distance from Morocco to Iceland.

Desertec actually planned to use existing long distance power transmission lines between countries linking them together, and then add to them to give more efficient transmission further and further afield. Eventually it could get as far as Iceland. Here is how it might develop in the middle term:

DLR studies of existing and hypothetical HVDC transmission lines

It’s certainly technically possible to power most of the world from solar power in our deserts. They didn’t manage that big scheme, but they have developed a big solar power plant in Morocco.

(Click to watch on YouTube)

  • Solar power from space

Some people poo-poo the idea of solar power from space. They argue that it is far more expensive to build solar panels in space than on Earth and that it can never be economic.

But that’s to miss the entire rationale for space based solar power. In space you don’t need the big heavy panels we need on Earth, made robust to deal with our weather, winds, rain etc. They are like solar sails. Square kilometers of gossamer thin mirrors that would not stand up to the slightest breeze on Earth but can be extended in space and then reflect sunlight onto a solar furnace or solar panels. Or in other plans, still gossamer thin, but they are now solar panels on one side, and microwave beaming antennas on the other side - because we have got to the point where this is technologically feasible.

Japan is especially interested in this as it imports a lot of energy and it doesn’t have much by way of natural resources to exploit. Japan is also very high tech and space capable with the JAXA space agency.

This shows how it works, sunshine falls on the thin solar panels from left, they then generate microwaves headed for Earth on the right via a phased array - a technique that became feasible a decade or two ago for microwave beaming using lots and lots of tiny microscopic microwave emitters that don't have dishes but use the microwave phase alone to focus the beam.

This is how it is received.

So -it is very safe because it’s not a single beam. Loads of tiny feeble beams and they are guided by a pilot beam from the receiving antenna on Earth. If that beam fails then they will just broadcast randomly into space with no other way to focus anywhere. You only encounter microwaves at all if you go actually onto the receiving antenna. But as well, they are low power, don’t think it is like a microwave oven. These are microwaves that you can safely walk through, birds can fly through etc. Very low intensity microwaves, but still enough intensity to transmit the power needed to Earth over a large receiving antenna - shown here in a lake in Japan.

Their simpler version just has solar panels that face straight upwards, in geostationary orbit, so the amount of solar power varies during the day and night, highest at midday, zero at midnight. The more complex system uses a couple of extra angled mirrors to get high levels of sunlight all the time. They are actively working on this and hope to have a commercial system some time in the future. SSPS : Space Solar Power System

There’s vast promise in space. If we can produce the current world energy from a small patch in the desert, what can we do if we have vast gossamer thin film mirrors or solar panels in space generating solar power? And this time we don’t need power storage for the night, we have solar power 24/7.

  • We can actually continue to use coal and oil if we can capture the CO2. This is the idea of carbon storage and capture. The easiest way to do that is at the power plant itself. The UK was exploring that quite vigorously but the current government pulled the plugs on finance for the scheme, opting for nuclear power instead. But there are several other projects worldwide and is likely to become an important part of the mix. This is a finite resource, yes, but there is much more oil and coal, shale gas etc. than we used to think and it could power us for a long time into the future if we had a clean way to burn it.

There are many other possibilities in the mix. This is a summary, details with links and cites in a comment on Quora.

  • Nuclear power (fission) 11% of world energy is supplied using uranium, and we have enough for 100 years.

The main problem is how to deal with nuclear waste. Short lived waste can be stored until it is harmless, including burying. Long lived waste needs more thought, either very stable geological formations deep underground (you need a lot of confidence in predictions of geology) or another idea is to burn the plutonium 239 in fast breeder reactors. The UK has enough plutonium stored at Sellafield, extracted from spent fuel already, to supply it with power for 500 years if burnt that way, which you might think is the obvious way to use our plutonium - but there is more to it than one might think and they are looking into all the options at present.

There are significant amounts of uranium naturally in sea water which we could extract. If we do that, there’s enough there to last us for thousands of years. If we use a fast breeder reactor to convert that uranium to plutonium, and then burn the plutonium, you are talking about a potential multi-million year supply from sea water uranium at current energy levels. A civilization with a lot of confidence in its nuclear power handling capabilities has almost limitless power from this resource, for thousands of years if it has similar levels of uranium in its sea water.

Then- there are many other renewables. At present actually wind power produces a much higher percentage of the world power than solar. Tidal currents like the gulf stream involve enough energy to power the world many times over. Of course we can’t use it all and stop the currents even if it was technologically possible, but a tiny fraction can supply a lot of power locally. The ice melting in Greenland in summer produce vastly more power than Greenland needs, enough to power much of western Europe (all of it in a paper studying this in 1974). The UK has enough wind, wave and tidal power so that if it was developed we could export power to the rest of Europe. Another source of power is the jet stream, which could be harnessed using kite type devices.

I’ve added a note about some of these to a comment.

  • Fusion power - we don’t have this yet, but some time in the future, we may have fusion power. There are many lines of development, not just the big fusion plant of ITER but also the Polywell and laser fusion and many other ideas. One or other of those may pan out in the next few decades.

Although we don’t have it yet, one or other of those methods may pan out in the near future. A civilization with easy and abundant fusion power would have a clean energy source that actually gives it far more energy available than we have now. It would be hugely energy abundant - so long as the fusion is reasonably easy to achieve.

We may well have this some time before 2100, and if so it would change everything.

But we don’t really need any of these, because solar power alone is enough to power the whole world anyway as we transition to more use of renewables. The rest just add to the mix, make the transition easier, or in the case of easy fusion power would be a complete game changer.


Actually we have already gone far beyond the carrying capacity predicted in the early part of the twentieth century. Billions would have died if it weren’t for the green revolution in the 1950s through to the 70s.

Then, it may surprise you to know that we actually produce more than enough food to feed the world. We have starvation for political reasons at present. It's an income and distribution problem.

As an example, the world had a food surplus of 510 kcal / cap / day in 2010 increased from 310 kcal / cap / day in 1965, even though the population in the same time more than doubled from around 3.33. billion in 1965 to around 6.92 billion in 2010. All the indications are that we should be able to feed 11 billion people in 2100.

One of the areas where we can make a big difference most easily is in Africa. The “Green Revolution” which revolutionized yields world wide rather passed Africa by, and Africa is one of the places that still has rapidly growing populations. If we can do the same for Africa that we have already done in other places like India this will make a big difference for food security. This involves scientific study of crops, breeding new crop varieties with special capabilities working out ways to cultivate them to improve yields and so on. Step by step things but done with the full weight of science and using experimental methods can make a big difference. We need to do this with the crops grown in Africa and for the conditions met in Africa. That’s the motivation behind the Alliance for a Green Revolution in Africa (AGRA). See also AGRA


Summary: 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 rise to 3.3 °C - and the US pullout increases that only to 3.6 °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 45 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.

In theory we could make the world uninhabitable by triggering a runaway greenhouse effect, but to do that we would have to burn ten times the total reserves of oil, gas and coal in the world. So in practice we can't, short of doing something really dumb like importing vast amounts of methane and ethane from Saturn's moon Titan and burning that.

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 from ever changing, which it does slowly all the time anyway.

It's because we are used to a relatively stable climate, and because the speed of the change is going to have effects on our environment, and it’s also going to be expensive to deal with the issues later. If we act now, it doesn’t even need to impact on our standard of living. It’s a case of policy change mainly. Promoting clean energy and measures to reduce carbon dioxide. If we act later, it probably will impact on quality of living and more than that, many people will need to relocate, others need expensive mitigation measures, and it will impact on the environment in various ways.

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

Debunked: Climate change will make the world too hot for humans by Robert Walker on Debunking Doomsday


So far, global warming has made the Earth a better place for agriculture. And increasing CO2 does increase crop yields if nothing else is considered, especially for C3 plants, both through CO2 fertilization and reduced respiration as the pores close at higher temperatures. But that's only part of the picture as it also changes the climate. The effects of CO2 fertilization are highly uncertain so they publish figures both with and without it. In their summary in this table, the results are with CO2 fertilization, and the uncertainty range is shown after the figures in square brackets:

Figure from: Differential climate impacts for policy-relevant limits to global warming: the case of 1.5 °C and 2 °C

As you see, the projections show an increase in productivity through to a 1.5 °C rise. But for a 2 °C rise just about all those benefits cancel out and we are actually worse off for some crops than we are today.

Only rice of the four major crops studied, wheat, soy, maize and rice see a major benefit still at 2 °C with a 7% increase, the same as for 1.5 °C.

Soy sees a modest 1% increase at 1 °C compared to 7% at 2 °C.

Maize sees a major decrease at 2 C by -6% compared to -1% at 1.5 °C.

Wheat has a modest increase at 1.5 °C but this is wiped out at 2 °C with a 0% increase.

The changes are far larger on a region by region basis. Some regions have large increases. Northern Europe particularly has large increases for all the crops studied, e.g. for Soy, an increase of 82% at 2 C, and still increasing. That’s middle of the range, it could be much higher, several hundred percent at the 66% confidence level.

North America has modest increases for most of the crops through to 2 °C.

The tropical regions are hit particularly badly as you’d expect in a warming world that is predicted to get hotter than optimal even for traditional tropical crops. But they manage fine for rice, not so well for the other three crops.

There is considerable uncertainty in these figures. At 2 °C for instance at the 66% confidence level global productivity of maize could see anything between a 38% decrease and a modest 2% increase, and wheat anything between a 42% decrease or a 14% increase.


It is not taking account of, for instance, the possibility of eventually opening up currently frozen or otherwise non agricultural regions in Canada or Siberia to crops that used to grow at lower latitudes or of reversing desertification in a major way.

Potentially by using an increased area for agriculture then we can offset some of the effects. Particularly, reversing desertification is something we can do.

It does however take account of growing new crops in regions that are already used for agriculture that used to be too cold for them.


There are three major types of photosynthesis, C3 which is used by nearly all our crops, the C4 method which is much more able to use carbon dioxide than C3, more efficient at all temperatures, and then there is the CAM system mainly used by succulents and not used for many food crops except pineapples.

Although the C4 method is better at converting CO2 into plant mass for the same levels of sunlight, the C3 crops we have tend to have a higher percentage of the plant as crop, which rather offsets that benefit of C4 synthesis.

The CAM system works better in dry conditions (e.g. cactuses).

There’s interest in the idea of modifying plants genetically so that C3 crops use C4 synthesis which could give us the best of both approaches. For a short summary, see Can Plant Researchers Adapt Plants to Cope with Climate Change?

Most of our crops use C3 synthesis. That includes most of the cereals wheat, soy, rice etc, also fruit and vegetables. C3 plants benefit from CO2 fertilization in a major way. C4 plants like maize and sugarcane are hardly affected at all.

But the climate as it changes reduces crop yields as it gets too hot for good yields for plants, especially in tropical areas. If you look at the projections on a regional basis, tropical regions in Africa, India, southern Asia, and Central America are particularly badly affected already at 2 C temperature rise. That’s part of the reason why ideally we want to stay within 1.5 C which still has major effects on crop yields


It doesn’t take account of new crops being developed that weren’t used before, or were not major crops previously. For instance new crops might cope well in hot areas just as rice does. There are plenty of crops used in a small scale in the tropics and maybe with a new green revolution some of them can be major crops worldwide in a warmer world.

In short, there’s no reason at all why a warmer world can’t support a larger population if any, even using conventional agriculture with the right crop species and methods.

There are plenty of reasons to follow the Paris agreement. There are going to be major issues already with the transition, old crops not working as they would, yields reduced, having to grow crops farmers have never grown before, effects on the environment- - forests especially can’t migrate very quickly, effects on coral reefs etc. Climate refugees from low lying places like Bangladesh, storm surges and need for protection of low lying coastal cities, or else millions have to move. More hurricanes and heavy precipitation events, more wild fires, longer droughts in summer, and protecting other species and the habitats we share our world with.

There are many excellent reasons to sign up for the Paris agreement. But none of it is about preventing human extinction or the end of civilization. Rather it’s about spending small amounts now, mainly just changing our practices and shifting to new types of technology, to save billions, even trillions later on.

Also, to leave the next generation and the generation after that with a world not that different from the one we have today that continues to have the unique and varied habitats that we enjoy in our current world, and the wide range of creatures we share our planet with.

(uses material from Robert Walker's answer to What is the impact of global warming on agriculture? )

Not only can we feed everyone, we will be way under capacity for agricultural land if we use more efficient methods of agriculture.


All this is far from the capacity of a technological civilization.

Remember, our population is leveling off at 11 billion. We can feed that much with conventional agriculture. We don’t need to do any more than that to feed everyone - new green revolution in Africa, better agricultural practices, and we can do it.

But I want to debunk this idea that we have a “carrying capacity”. This is irrespective of how much Earth’s environment is degraded. There are many reasons to not want the environment to degrade. In practical terms it makes things more expensive. It means climate refugees. We lose unique species and entire habitats, that may also be of value to us in other ways too. And most people would be sad to see some of the wonderful environments in our planet disappear.

But it won’t make it impossible for us to survive. So long as we keep our crops, and no reason we would lose those, we can feed everyone actually many times over, ten, even up to a hundred times over from the same land area.

First, ordinary soil based gardening can also be used with the methods of biointensive mini gardening. 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. Using these methods theoretically we can feed 110 billion people, or feed those 11 billion people from a tenth of the agricultural area currently used.

Grow biointensive - sustainable mini farming - this method needs only 372 square meters of growing area per person.

If we use ideas developed for space colonies, then we can grow food in a very small area. 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).

This leads to huge savings in the precious area you need to grow crops. Instead of one acre of farmland needed per person for conventional agriculture (4000 square meters approximately), you can grow 95% of the food, water and oxygen for an astronaut from just 30 square meters, with a conveyor belt system, of rapidly growing crops such as wheat, sedge-nut, beet, carrots, etc. For details see Sending humans to Mars for flyby or orbital missions - comparison of biologically closed systems with ISS type mechanical recycling (also relevant to long duration lunar missions).

We can get an idea of how efficient these methods are by working out the total land area needed to feed the world on a vegetarian diet by all the methods. With a million square meters to a square kilometer, then we just need to multiply the numbers by 7,500 to get the area in square kilometers needed to feed a population of 7.5 billion. We get

By comparison, the Sahara desert is 9.2 million km². With the BIOS-3 system, we would need only 2.5% of the Sahara desert to feed the world. The total land area of the Earth is 148 million km². But of course much of that is desert, mountains, ice etc, some is uncultivated and animals require more land area than plants.


Although I am very keen on humans in space, we don’t need to be multi-planetary to feed everyone. Indeed - I think we need to be very careful about sending humans to Mars at all for the reasons explained here:

I think we should focus on the Moon as our first destination for humans in space - and it is not needed to escape from Earth. Earth is just fine as a place for humans to live. Indeed nowhere else in the solar system is even a patch on it. Even the harshest desert on Earth is an absolute paradise compared to Mars.

These are the same methods as would be used for a Mars colony - but of course it is far far easier to use them here, where we have much more sunlight, no dust storms, no need for radiation shielding from cosmic radiation and solar storms, no need to build our habitats and greenhouses so strong they can hold tons per square meter of outwards pressure - and most of all - we can just breathe the air. Walk outside without a very expensive spacesuit like a mini spaceship, clumsy, and needs hours of maintenance between EVA’s.

No, if you need to feed more people, it’s easiest to do it here. Longer term we could build huge city domes and vast spinning habitats in space - and they may be as easy to live in as Earth itself - but that’s a vast investment to build them originally.

If you want “another planet” to emigrate too, you can start by using our deserts and oceans. Vast areas that are hardly used at all.


Most of the Earth is desert or ocean. We can reverse desertification and make more efficient use of what is there.

We are actually doing this already in a small way, with the salt water greenhouses, so I think you can say that not only should we do it, but we already are. This is an Australian desert project. The sea water is used to make water through the sunlight in the desert, and cool down the greenhouses.

(Click to watch on YouTube)

These ideas could be used to reverse desertification in the Sahara desert and other deserts. This is how it works:

Diagrams by Raffa be from wikipedia

It not only lets you grow crops in the greenhouses - it can also help make the surrounding areas more habitable, so you’d get trees and crops growing in an area around the greenhouses as well. Doesn’t extract anything from desert aquifers, rather, it adds to them.

Sundrop farms have a large area set out for greenhouses like this now, in the middle of a desert, so this is taking off in a big way in Australia. Early days yet though.

This video just shows the greenhouses, and when they go inside in the video there is nothing growing there yet, not sure why, maybe it is a new installation, but it shows how it’s quite big in Australia.

(Click to watch on YouTube)

There are many countries working on reversing desertification Israel does a lot of reversing of desertification.

One of the worst areas of encroaching desertification is the southern edge of the Sahara desert. The first priority there is to stop the spreading desertification - then to reverse it. Many African countries are collaborating in the Great Green Wall project to plant a forest along the southern edge of the desert.

Then, there is a similar project underway there now to the Australian Sundrop farms, using seawater greenhouses.

Technologies - Sahara Forest Project

This gives far more food and living space for the same amount of cost, compared with the billions of dollars to set up a few people in a space habitat. It's also far far easier to build a greenhouse in a desert on a planet with abundant sea water, and breathable air, than to do it on Mars. And as we just saw, with the BIOS-3 system, we would need only 2.5% of the Sahara desert to feed the world. So if we grew plants on Earth in as small an area as we could do for space habitats, we could feed the entire world easily with minimal impact, at least on the mainly vegetarian diet we would need for space colonies.


If we fill all the deserts, or you just don't have a handy desert in your country that's suitable for building on, you can build on the sea,

The Seasteading Institute | Opening humanity's next frontier

We could have sea cities covering much of the seas if we really need more space for people to live.

By a sea colony here, I mean one that only uses the sea water and the air, with a few imports from Earth - as that would be the equivalent of a Mars habitat. There'd be no need for fishing or anything else, just air, and sea water, and the materials to build the original city, and some imports, and if advocates are right about Mars colonies, there would be little by way of those too.

Four fifths of the surface of our planet is ocean, so if we could live on the sea, in more or less self contained habitats, as with the ideas for Mars, that's be like finding four new planets to live on.


The surface area of the Pacific is 165.2 million km². Four times the population of Earth would need about 0.5% of the surface area of the Pacific to grow all of its own food using space colony type technology.

That's actually the aim of the seasteading project too. See their section 5. Sustainability and ecology

"After the concept design is finished, the next challenge is to find the appropriate adaptation strategy – a strategy that creates a safe and livable urban environment on the sea, while minimizing impact on the ecosystems and making efficient use of the available resources. In this section, we explain the Blue Revolution concept and apply it to the seasteading concept"

They explain it in detail there, With its use of aquaponics and aeroponics, it resembles ideas for space habitats.

A sea city would have minimal impact on sea life if done in the same way as for a space colony, growing all their own food inside the habitats. Perhaps this could be one outcome of space settlement, that by learning how to live in space, with such a high priority on efficient recycling, we can also learn to live on Earth as well, with minimal impact. Perhaps both approaches will influence each other.


This is something you might well ask if you have watched “cowspiracy”. And yes, it takes a lot more land to keep animals than to grow vegetables. So, yes, it will make it easier to feed everyone if we have less meat consumption per head. The figure of 0.5% of the Pacific to feed four times the Earth’s population assumes space colony type technology, so a basically vegan diet, with supplements, but most of the diet has to come from vegetables.

If most people eat large quantities of meat, then that needs more land, of course. However if you’ve watched “cowspiracy” then it exaggerates the situation. For a criticism of the film by the union of concerned scientists: Movie Review: There’s a Vast Cowspiracy about Climate Change.

Livestock produce 8-18% of greenhouse gas emissions according to the paper they cited - that is for all livestock world wide, not just beef. It's still a lot. But there is no conspiracy to hide this. It seems a very active area of research with many papers. These are the google scholar search results for 2016.

Farmers often keep sheep and cows on land that is not used for agriculture. I live on the Isle of Mull in Scotland and much of it is used for sheep. If the sheep were removed, then the land wouldn't be used for agriculture. In principle it could, the ground is peat bog mainly, actually potentially good for agriculture if it was drained - but there are far easier and less expensive ways to grow food than to cultivate mountain slopes, so in practice it wouldn't. It would be labour intensive also and it would be hard to find anyone wanting to do all that hard work to grow food.

Now, if they did stop rearing sheep here, it could return to forest, but only if they culled all the deer, as those would take the place of the sheep. And even if it returned to forest, that's a CO2 sink for as long as the forest grows, but not after that and anyway the grass also and peat bogs are CO2 sinks too, the land here is covered in large areas of peat that take up CO2 from the atmosphere and store large amounts of it.

So - yes if you replace good crop growing land with pasture and keep cows and sheep instead, then it is contributing to global warming short term from the methane, and they use water that may be in short supply, and land that could be used for growing crops. But you can't say that as a blanket statement e.g. to say to the Sami people that they have to stop keeping reindeer - they aren't going to grow crops there instead.

Sami woman with white reindeer

(this includes material from my MOON FIRST Why Humans on Mars Right Now Are Bad for Science )


I don’t think we risk extinction or even the end of civilization myself, pretty much no matter what we do. Our actions this century will not make us extinct even if we damage our environment hugely. Our world remains tremendously habitable to humans, an incredibly adaptable and versatile species, one of the least at risk of extinction, and able to live almost anywhere, an omnivore, able to survive on nuts, fruit, modified grasses (such as wheat or rice), shellfish, insects, meat, roots, with minimal technology able to live anywhere from the Kalahari to the high Arctic. 

There are many reasons to prevent global warming. But they are most of them to do with the many issues involved in the transition, the bad weather, drought, wildfires, climate migrants, flooding of coastal cities, environments not able to adapt fast enough etc. Nothing to do with the end state which remains very habitable for humans and indeed for most other creatures that share our planet.

There is one main exception, the corals, with a slightly acid ocean they would eventually be replaced by sponge reefs, as has happened many times in our planet’s past. It often flips between slightly acid with sponge reefs and slightly alkaline with coral reefs. There are some other examples like that, polar bears in the Arctic for instance But for most other habitats the main issue is that plants, and especially trees, can't migrate easily, and tropical rainforests are especially vulnerable to small temperature changes. 

As for our civilization - if you try to compare it with other civilizations like various empires that flourished and vanished - well ours is a distributed civilization. We have widespread literacy. Just about every country and group of people except uncontacted tribes in the rainforests are now in the situation where most people can read.  With our high literacy levels worldwide, and a robust distributed civilization - like the internet - we could lose the internet but we won’t lose all our libraries, all centers of learning, all teachers, we will always have examples of our technology to use, knowledge of basic ideas that would save decades. Even if we got knocked back to the nineteenth century, we’d be back to the present again probably in a few decades.

But we are being sensible, and following the path we are following, it’s not going to be a “withering away” at all, but a sustainable future. The main thing is to make sure that we do it with minimum of suffering, use a bit of foresight to avoid hugely expensive fixes later on, and leave the world in the best possible condition for the next generation.


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