This is a story that is scaring some people. New research that is suggesting that perhaps the climate is more sensitive to CO2 than previously thought - but this is way jumping the gun. We get many results like this sometimes running too hot, sometimes too cold. Only the ones that go too hot hit the headlines. But the IPCC will look at all the evidence with their next high level review in 2021. Meanwhile we should use the values for the high level review in 2018 which remains our best estimate to date.

This is another article I'm writing to support people we help in the Facebook Doomsday Debunked group, that find us because they get scared, sometimes to the point of feeling suicidal about it, by such stories. Do share this with your friends if you find it useful, as they may be panicking too.

Do not adjust your set. Transmission will resume on this topic once the scientists have worked it all out in high level reviews in 2021.

The message that used to show on TV screens when there was a problem with transmission - and the name of an early TV comedy series, pre-cursor to Monty Python. Do Not Adjust Your Set - Wikipedia

Here are some examples of the scary headlines:

But it’s not quite as it seems. The models that run the hottest are less likely to be right, as are the ones that run the coldest (which don’t hit the news in the same way).

Michael Mann is quoted (in an otherwise rather scary article) as saying:

"Some earlier versions of the models appeared to underestimate climate sensitivity somewhat, and I suspect that some of these more recent versions are actually over-estimating it a bit."

"I suspect, when all is said and done, we're probably looking at something in the range of 3-4°C and no higher, at least for near-term warming. If we allow the warm[ing] to persist for centuries, then other long-term positive feedbacks (vicious cycles) could kick in, giving us substantially more warming."

'Terrifying' New Climate Models Warn of 6-7°C of Warming by 2100 If Emissions Not Slashed

The Equivalent Climate Sensitivity is very hard to work out because it is about the equilibrium temperature after thousands of years after all the ice that will melt melt. From the last ice age to pre-industrial there has been enough time for it to reach equilibrium. But from pre-industrial to the present there is nothing like enough time to equilibrate.

If we stop all CO2 emissions, as we hope to do by 2050 for the 1.5 C goal, the easiest way of getting there - then not just the CO2 levels stop rising, they start to decline right away.

With the declining CO2 levels, our temperature also levels off immediately and then starts to decline. That is what happens with the bottom dark blue line in this graph, which is what we hope to achieve with the Paris agreement as we ramp up the pledges.

This shows the CO2 levels for, from bottom to top, the 1.5°C path, the 2.4°C path, the 3°C path and the 4.9°C path (most people are more familiar with the expected temperature rise than the RCP numbers though they are more technically accurate).

There 4.9°C is “business as usual here shown as continuing fossil fuel burning as for now right the way through to 2200 with no CO2 reductions at all, and it then levels off because we have run out of fossil fuels).

However, the ECS is about what happens if you keep the atmospheric levels of CO2 constant.

That’s actually the idea for all the pathways except the lowest, the 1.5°C one.

For all the others, once the target CO2 level is reached a small amount of anthropogenic emissions continues, just enough to keep the CO2 levels at that constant level. That is the ECS situation.

This shows what happens to the temperature with these four scenarios

This shows temperatures as measured from 2000, not from pre-industrial - from pre-industrial it is a degree higher.

The temperature for the dark blue line, which we are targeting, immediately starts to fall as soon s we reach zero around the middle of the century - that’s because of the CO2 gradually being absorbed by the land, soil, and sea.

For the others with constant CO2 then as soon as the constant level is reach the temperature continues to rise. Over thousands, and tens of thousands of years it can rise significantly.

As an example it takes a thousand years for all the Greenland ice to melt at "business as usual" continuing CO2 emisssions as they are now all the way through to 2200 - which is not a realistic sceanrio. On the other scenarios it takes far longer.

This shows the ice sheet for the year 3000 under three scenarios - I’ve added extra labels with the projected temperatures by 2100 since that’s much better known than the RCP numbers:

Contribution of the Greenland Ice Sheet to sea level over the next millennium

There the amount of ice lost and the sea level rise by 2300 are

  • 1.5°C:
    8 to 25% lost by 3000, contributing 0.59 to 1.88 m
  • 2.4°C:
    26 to 57% lost by 3000, contributing 1.97 to 4.16 m
  • 4.9°C:
    72 to 100% lost by 3000, contributing 5.38–7.28 m

For more on this see my How long does it take for the Greenland ice to melt completely?

So, to work out the ECS correctly involves considering what happens over thousands of years into the future. That is the only way we can make a fair comparison between the effects of the CO2 we are adding now, and the effects of the increase in CO2 since the ice age.

We have had a doubling since the ice age. If we double again then the eventual temperature rise should be around the same (temperature depends on CO2 logarithmically - a doubling of CO2 has the same temperature rise) but only over thousands to tens of thousands of years. Of course there are also other differences apart from the CO2 such as the Earth getting more absorbing of sunlight when the ice covering northerly latitudes melts etc.

You can see how it would be tricky to work it out.

So, the ECS is a techy thing that goes up and down all the time yet over 25 years the range of values remains the same.

This video shows how these ECS values go up and down all the time.

(click to watch on Youtube)

This latest research is an “up” but they do not know why. It could be an issue with the models rather than our understanding of ECS in the last global assessment. It wouldn't be first time.

This shows how the estimates vary depending on the approach you use, this is from last year, June 2018:

Shows the range of high, low and the black dot shows the best estimate for each method.

From: Explainer: How scientists estimate climate sensitivity (Carbon Brief)

So what’s happened is that since then, the first one, the models, are now trending much higher than before. Which conflicts with the others.

There's evidence from other methods not the models that the ECS is about right at 3 C or so (perhaps 2.8 C) - with a range of around f 2.2–3.4 degrees Celsius for the most likely ECS. May well get another batch of models that make it too low, it just goes up and down.

Weather Undeground published a comment on this by the climate researcher Peter Cox of the University of Exeter. a researcher into Equilibrium Climate Sensitivity:

“It does indeed look like many of the latest models will have ECS values higher than the IPCC ‘likely range’ of 1.5-4.5°C. It seems that the new models with high ECS have more low-level cloud that tends to burn off under climate change, producing an amplifying feedback on warming.”

However he goes on to say:

“It is worth noting that observational constraints from both the temperature trend and temperature variability still suggest ECS of around 3°C. So climate science has a conundrum to solve here.”

New Models Point to More Global Warming Than We Expected

He is the author of this paper from January 2018:

Here we present a new emergent constraint on ECS that yields a central estimate of 2.8 degrees Celsius with 66 per cent confidence limits (equivalent to the IPCC ‘likely’ range) of 2.2–3.4 degrees Celsius.

Emergent constraint on equilibrium climate sensitivity from global temperature variability

There may be something these new models are leaving out or exaggerating, when you extend them to the entire world.

It’s natural that their own research is convincing to these researchers that their models mean that the ECS has to change.

However, others will have other researches that lead in other ways. This is how it works in science, research like this is part of a long term dialog over many years and decades.

This is why we have the IPCC. Individual research groups are too close to their work to have the overview the IPCC has. The IPCC is not any individual scientist with an overview. It is the result of hundreds of scientists coming together and with the help of the co-chairs, working together on a consensus understanding of the entire field - that is including the full range of well supported views in the field. I.e. not a consensus view, but a consensus understanding of the range of views and which are the best supported views and what the level of support is.

It is also certainly true these new models are more detailed because of improvements in computer power and algorithms. They give much better predictions for local regions like Europe.

However, what is not clear is how well they work as models for the world as a whole, especially when other research contradicts what they say, as always happens in this topic of Equilibrium Climate Sensitivity (ECS).

We have to wait until 2021 for the next high level overview. Meanwhile we should use the 2018 report.


This is not a new problem, it’s been in the climate change news for most of this year. Some of the background is explained in this story:

They found that their models were modeling El Nino / La Nina wonderfully well but they were running hot on the large scale.

To get an idea of how tricky it is to get these models working right they describe an incident where for an entire year one model was running so cold it gave no warming at all for the twentieth century. Was working fine until you added data about aerosols from natural processes and industy. These make a difference to cloud formation and also can reflect sunlight. Adding this in, a small tweak, lead to no warming at all. They eventually sorted it out but it took a year to figure out why this was happening.

With this background you can see that we shouldn’t take things so seriously until they have had time to work on these models and gain a lot more experience with them.

This is part of a new batch of models that they are working on in the lead up to the next IPCC review in 2021. They will be doing many more than the usual 5 simulations, an extra 23 modeling requirements about such things as cloud feedbacks. They will also have to document their code more rigorously.

The next IPCC report may not lean on the models as heavily as previous ones:

In assessing how fast climate may change, the next IPCC report probably won’t lean as heavily on models as past reports did, says Thorsten Mauritsen, a climate scientist at Stockholm University and an IPCC author. It will look to other evidence as well, in particular a large study in preparation that will use ancient climates and observations of recent climate change to constrain sensitivity. IPCC is also not likely to give projections from all the models equal weight, Fyfe adds, instead weighing results by each model’s credibility.

New climate models predict a warming surge

This is how Kate Marvel responded to the models back in April:


You might at first think that surely it is obvious it is better?

If a model works betterfor predicting the climate in Europe and other local regions - isn’t it bound to be a better model for the whole world?

Well, not really, doesn’t follow logically that it has to be. Because they are different questions.

The whole point in these new models is that they are very high resolution. That they work better for Europe is no surprise - that is why they were built for after all.

But does modeling all the details a bit better really give you a better picture of the whole?

It is like an artist who wants to paint a portrait. But they do not step back and look at the whole face. They look just at the nose, and do a wondefully detailed portrait of the sitter’s nose as a nose. They then look at the left eye and do that in wonderful detail. Then at the mouth. Then the left ear, right ear and so on.

Those may all be perfect representations of someone but they may not come together to give the whole face quite.

Here is an example, by George Condo

George Condo Princess

Some of the details being reasonably exact, like the artist’s depiction of the princess’s left eye - does not mean the whole is right as in an exact photorealistic representation of the princess.

The details being exact does NOT mean the whole is right. And for climate models, unlike painting, the whole has to be an exact representation of what the world will be like in 2100 (say).

Here that it works wonderfully well for the La Nina / El Nino cycles, or if you focus on Europe and ignore the rest of the world, it does not necessarily mean the same model is going to work so well when it is ran as a whole world simulation.

That is what needs to be investigated.

Here is how Kate Marvel puts it in her Ted talk:

Computers hate clouds because they are simultaneously very large and very small. To accurately model clouds we need to track the behaviour of every dust grain or water molecule … We can zoom in and get the details right and have no idea what is going on worldwide. Or we could sacrifice realism at small scales in order to see the bigger picture.

(click to watch on Youtube)

Meanwhile, we should continue to use the 2018 value for ECS.

It is far too soon to change it. This needs more research, high level review, and in 2021 it could go either way - or if you step back and look at this with perspective of history - the most likely outcome is that it remains roughly the same, around 3 C.

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