If you have any affinity with climate science, this should interest you – probably a lot:

Piers Forster, James Hansen, Gavin Schmidt, Alan Robock, Michael Mann, Ken Caldeira, Stefan Rahmstorf, Chris Forest, Gabriele Hegerl, Hans Joachim Schellnhuber, Jonathan Gregory, Drew Shindell and Andrei Sokolov share their thoughts, and gut feelings, on climate sensitivity.

[Update! Shortly after publishing this piece we’ve received the answer of three other leading climate sensitivity experts, Mark Zelinka, Trude Storelvmo and Knutti Reto. You can find their thoughts on ECS value in the update at the bottom of this article.]

How high is the water Mama? 3 feet high and rising. How high is the water Papa? She said it’s 3 feet high and rising! (The range of climate sensitivity estimates follows a normal distribution – with a ‘fat tail’. Time to make better sense of it…)

Yes, the science is settled – and yes, of course there are also uncertainties. Climate sensitivity is perhaps the most important one…

We’ve known since the 19th century that (extra) CO2 warms the climate. We can judge by observed global temperatures this is already taking place – and increasingly so.

What is still a matter of some real scientific debate is the extent of the change – the amount of warming we can expect from rising atmospheric CO2 once the thermal inertia of the climate system has more or less settled.

But let’s make it clear this puzzle is not a game. A suiting way to rephrase the question is ‘are we in for bad (the lukewarmers) – for really bad (let’s say NASA) – or devastating‘ (Yale) – or are we for ever caught in a wide margin of uncertainty (IPCC)?

It’s called ‘equilibrium climate sensitivity’ (ECS) and it’s the most well-known metric for projecting global temperature rise. How much will the temperature go up (in degrees Celsius), if we double atmospheric CO2?

Asking sharp climate minds not what they think, but what they suspect

Now of course we can sift through the piles of proper academic research that is already performed in this field. And we do. For breakfast, daily. That is also what the rest of our ‘real’ global temperature trend series is about.

But there is something else we strongly believe in. And that’s asking the people who simply know best. The true and most established climate sensitivity researchers. The most renowned climate scientists. The masterminds – and then asking these sharp minds what they ‘suspect’ – something in between what they know, and what they feel: their “scientific gut-feelings”.

And yes – they replied. 13 [update: 16!] leading climatologists and climate sensitivity experts. (Who happen to also be very friendly folks!) Short summary:

It’s good to know all seem to more or less agree. The most likely value for ‘equilibrium climate sensitivity’ is ~3 degrees Celsius – something close or perhaps just over 3 degrees, closer than the wide range of IPCC estimates suggests. They also agree about the limitations of the definition and even the metric of climate sensitivity. There are feedbacks, reactions, disturbances – and inevitably there is even warming, beyond climate sensitivity. Whether you call it Earth System Sensitivity or ‘ocean heat content change’ or biosphere CO2 & deep sea methane.

Short hommage to climate scientists (in general) before we start

We would like to thank (in order of reply) Piers Forster, James Hansen, Gavin Schmidt, Alan Robock, Michael Mann, Ken Caldeira, Stefan Rahmstorf, Chris Forest, Gabriele Hegerl, Hans Joachim Schellnhuber, Jonathan Gregory, Drew Shindell and Andrei Sokolov – for sharing their insights! [Update: and Mark Zelinka and Trude Storelvmo!]

…and given that chance we would like to thank the global climate science community in general – the many thousands of exceptionally clever and hard-working men and women at universities and climatological institutions across the world to whom we, the rest of humanity, owe the fact that there is a light shining on the otherwise completely darkened road that lies ahead of us. Thanks to these people we can see ahead. And thanks to these people – we can also steer, steer away in a direction so that we can survive.

Please take a moment to think about it.

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Now here goes, starting off with the first climate expert, Piers Forster, answering our impossible question, opening up to “scientific gut feeling” – if you would have to pick one number (degrees Celsius) for ECS – what would be your best guess?

1. Piers Forster, Professor of Physical Climate Change at the University of Leeds:

“It has changed over the years but my current thinking puts it just shy of 3C for a doubling of carbon dioxide.”

2. James Hansen, climatologist at Columbia University, former head of NASA GISS

“It depends on what you mean by ECS, specifically E, what is equilibrium? The usual “fast feedback” sensitivity is pretty tightly constrained to 3C + or – 0.5C for a 4 W/m2 doubled CO2 forcing.

However to get that response you have to wait several centuries, by which time slow feedbacks, such as ice sheet disintegration, will have come into play (see our paper).

The transient climate sensitivity is much more complicated. It includes effects such as the cooling around Antarctica and SE of Greenland due to ice melt (see).”

3. Gavin Schmidt, climate modeler, head of NASA GISS

“Somewhere between 2.5 and 3 deg C for a doubling of CO2.”

4. Alan Robock, climatologist, Professor Environmental Studies Rutgers University

“3 K for doubling CO2”

5. Michael Mann, climatologist, head of Earth System Science Center at Pennsylvania State University

“While the canonical (“most likely”) estimate of the “fast feedback” ECS is around 3C, I feel that more recent evidence suggests it might very well be higher than that, between 3C and 4C (say, 3.5C).

The long feedback “Earth System Sensitivity” (ESS) is almost certainly higher, closer to 5C.”

6. Ken Caldeira, climate scientist at Carnegie Institution for Science

My ‘gut’ feeling, i.e., not informed by any careful analysis is something on the order of 3 C per CO2-doubling.

When I was a grad student nearly 30 years ago, I would have said 2 C per CO2-doubling, so I have gone up a degree in my estimation over the past 30 years.

My estimate is probably based largely on CMIP5 model results. That is the area of my work where I come in most contact with estimates of climate sensitivity.

Ken Caldeira also added some interesting additional thoughts about political bias:

“It would be interesting to do a study of climate sensitivity estimates and correlate them with political viewpoints. Marty Hoffert has hypothesized that there is a correlation with people on the right estimating a lower climate sensitivity than people on the left. This would be an interesting study in sociology of science.”

…and on the ‘fat tail’ of very high climate sensitivity estimates:

“I do think there is high uncertainty. I see you have this post [cloud feedback – climate sensitivity 5-5.3 degrees] – my gut feeling is that if you really had this high a climate sensitivity, then aerosol forcing would have to be very strongly negative to explain the historical record.”

(Here are two other studies suggesting possible fat tail climate sensitivity – one suggesting high aerosol reflectivity – and one suggesting high aerosol dimming.)

About the metric of ECS, its usefulness for climate policy and possible underestimation of warming due to ignored carbon feedbacks, Ken answered:

“I interpreted ‘equilibrium climate sensitivity’ to be “Charney sensitivity” — allowing for feedbacks from clouds, snow and sea ice but not slower responses like the large ice sheets. Further it does not include biogeochemical feedbacks.”

“Earth System sensitivity is a bit more complicated as it might make more sense to think about sensitivity to an emissions rather than a concentration. The idea of doubling or quadrupling CO2 and then holding it constant for many thousands of years is a bit artificial, especially if you are considering biogeochemical feedbacks to be included in your definition of climate sensitivity.”

“Earth System Sensitivity is a different question. If you are including carbon cycle feedbacks, does it make sense to ask what is the sensitivity to a CO2 concentration with carbon cycle feedbacks? Isn’t the point of carbon cycle feedbacks that it can change CO2 concentrations?

That is why I suggest that it may be better to think about Earth System Sensitivity in terms of a response to an emission rather than a response to a concentration change.

I think 3C is “good enough” when it comes to policy. We know we need to transform our energy system into one that does not use the sky as a waste dump and we need to do this as quickly as possible. That conclusion is robust to variation in climate sensitivity estimates within reasonable ranges. That was more-or-less the conclusion of our 2003 paper in Science. (Also please look at our recent paper on Antarctica which gives some indication of some long-term responses to emissions in the Genie model.)”

Clever people are always right..!

7. Stefan Rahmstorf, Professor of Ocean Physics at Potsdam University

“Short answer: 3 °C.”

To which he added:

“Lots of lines of reasoning (including our own work on deriving sensitivity from paleoclimate data) consistently point to about 3 °C, while those studies that created a stir with higher or lower numbers invariably turned out to be methodologically questionable. I even think the IPCC uncertainty range of 1.5 – 4.5 °C is too wide, I personally say 2 – 4 °C.

8. Chris Forest, Associate Professor of Climate Dynamics at Pennsylvania State University

“3.3C – Chris.”

…and when asked for his thoughts about the ‘usefulnes’ (for climate policy) of the metric of ECS, he replied:

“While ECS has been useful for climate science, the transient climate response (TCR) and TCRE are much more relevant for policy, in general.

As a scientific community, we have needed diagnostics such as ECS to improve our understanding of the earth system response to anthropogenic and natural factors. But it has become clear that we will never live to see the earth reach an equilibrium.”

9. Gabriele Hegerl, Professor Climate System Science at the University of Edingburgh

“I suspect it’s under 3 and over 2. My gut range is 2.2 to 2.8. (And as my work is about certainties, I really can’t give a single guess!)

10. Hans Joachim Schellnhuber, Director of the Potsdam Institute for Climate Impact Research

“Ignoring many complexities involved, my best guess for the ECS is 3°C + small delta.

As for the full Earth-system response including all feedbacks, we do not know enough to provide any reasonable number. This is highest-priority research terrain.”

Interpretation of ‘3C + small delta’ – that is a real scientist saying “just over 3 degrees”, so Hans Joachim Schellnhuber’s remark is in line with for instance Chris Forest and Michael Mann (and of course all others suggesting ECS values close to 3 degrees – the clear majority of leading climate experts).

11. Jonathan Gregory, climate modeller National Centre for Atmospheric Science and Met Office Hadley Centre

“It’s a good question but I don’t place any confidence in gut feelings, so my answer would be the likely range of the AR5. I found Kahneman’s discussion convincing in “Thinking, fast and slow” of the ways in which intuition misleads us, and in particular that experts are overconfident.”

12. Drew Shindell, Professor Climate Sciences at Duke University

“The thing is, my gut feeling is that the starlight I see when I look up at night can’t possibly have been emitted long before I was born, and that quantum mechanical entanglement of two particles at a distance can’t happen, but I know these feelings are wrong. So I trust the research and not my feeling on climate too and hence go with the studies’ results that from multiple lines of evidence point to 2-4.5C for the most credible papers.”

Wow. Beautiful.

And what an ending to our climate senstivity expert survey. But it’s not. Not yet – almost. Because we have one more to go – the honor to end this inquiry falls to the last of our climate sensitivity experts, Andrei Sokolov:

13. Andrei Sokolov, climate sensitivity specialist at MIT

“I’ll say it is 3.5C per CO2 doubling.”

And that’s another very clear and straightforward reply.

[Update: following two expert replies were received after publication of this article] 

14. Mark Zelinka, climate researcher and cloud specialist at Lawrence Livermore National Laboratory

“As I actually read your article yesterday, my gut feeling is likely now unduly influenced by the opinions of the luminaries in the field. So take it with a grain of salt:  I find the Physics Today article by Stevens and Bony (2013) in which they derive something of a null hypothesis ECS of 2.7K based on robust, relatively well-understood feedbacks, to be pretty convincing.  This, coupled with an ever-growing body of evidence that (1) an overall negative cloud feedback is unlikely, and that (2) the overall net negative feedback weakens in strength as equilibrium is approached, make me doubt that ECS is under 3.”

Wonderful reply. It is very nice to hear this article is actually worth reading for the specialists themselves. We add another voice to the expert list stating ECS could well be >3 degrees Celsius.

15. Trude Storelvmo, atmospheric scientist, Associate Professor at Yale University

“I would vote for an ECS somewhere between 3 and 4 K. How is that for a short and simple answer:)”

It is straightforward, easy to comprehend, and it adds to the majority of consulted specialists that think climate sensitivity could be higher than 3 degrees.

16. Reto Knutti, Climate Professor at Institute for Atmospheric and Climate Science, ETH Zürich

“If I had to give a number I’d say 3°C, with a tendency for slightly above 3°C. If you want a range, I’d say 2.5-4°C. But note that this is an estimate of the effective climate sensitivity, i.e. essentially the warming we would expect if the feedbacks stayed similar as they are today. The true equilibrium sensitivity might be quite a bit higher, first because there is growing evidence for the feedbacks not to be constant (even the ones in the models) and second because of Earth system feedbacks that the models don’t have. Whether that number actually matters is another question. For most decisions a transient response is more important.”

Very interesting, because we have yet another expert leaning towards a 3 + something, as an estimated value for equilibrium climate sensitivity, and yet another expert stating ‘Earth system sensitivity’ might be higher still.

(“Regarding gut feeling, I don’t think I have a gut feeling any more about this. Having done IPCC assessments on climate sensitivity more than once, I think the above is more of an informal continuous assessment based on papers, discussions, our own results, and other people’s arguments I trust. Many of these people are on your list.”)

Yes, we can imagine Reto. Therefore we think by now it is time to call this list complete :) Thank you Reto – and once more, thank you to all the other climate experts. It was a joy to have the correspondence and to do this piece – and it really helped our own general understanding of climate sensitivity, and it’s estimates(!) Now let’s try a conclusion, shall we?

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Shall we dare a conclusion? “Climate sensitivity is…”

Quick & dirty: +3 degrees Celsius for a doubling of CO2. Or something close. Probably closer than the IPCC range.

A more decent summary: If you see value in “scientific gut-feelings” [we do!] then the most likely value for equilibrium climate sensitivity starts with a 3. (4 [update: 7(!)] experts say it’s likely a bit higher (Michael Mann, Chris Forest, Hans Joachim Schellnhuber, Andrei Sokolov [update: and Mark Zelinka, Trude Storelvmo, Knutti Reto]; 3 experts say it’s perhaps just a tad under (Piers Forster, Gavin Schmidt, Gabriele Hegerl), and 4 experts name exactly the number (James Hansen, Alan Robock, Ken Caldeira, Stefan Rahmstorf) – 3 degrees Celsius.)

If you don’t see value in scientific gut-feelings, you can go with the two remaining experts (Gregory and Shindell) who prefer to just stick with the big pile of established research, which suggest (90% interval) 2-4.5 degrees, with most likely value at 3 – so that would get you to almost exactly the same spot as the experts’ gut shots.

(Let’s ignore the lukewarmers (~1.5), is what all real climate experts seem to agree on.)

Weak point: You can’t add carbon feedbacks to CO2 climate sensitivity. By definition. This leads to possible underestimation of warming

Yet another thing there seems wide consensus on, is that you simply don’t know, once the ball starts rolling, how far it will roll, nor for how long. There may very well be ‘warming beyond ECS’, thanks to positive carbon feedbacks. But you can’t include carbon, when you assume fixed carbon, which is the used definition (a doubling of CO2, as Caldeira explains).

Weak point: The word equilibrium makes it theoretical – the climate will keep changing, for as long as our children will live

Another weakness of the metric, is that equilibrium. We’ll never see another equilibrium climate, many experts note – not in this century at least. The one change triggers the other. That ball is already rolling.

From ECS to TCR – we need something to draw on, something that’s good enough

But many experts still say ECS (together with transient climate respons, TCR) is a useful metric, albeit a scientific one. It helps to establish policy, as it enables the creation of 21st century warming scenarios – therefore of required emissions reduction paths. That’s important. That is the stuff that shines a light through the darkness ahead of us.

Earth System Sensitivity. There’s more to climate change then global temperature

But of course it’s also not all about net global temperature rise, it is about many other changes to system Earth. Ice sheets, to name one. Ocean currents, are easy to add. That’s where it gets more complicated still. And on the other hand also very practical. Two degrees is not just two degrees of extra atmospheric warmth. It is also something in the order of 12-32 metres sea level rise. Sorry. ‘Equilibrium sea level rise’.

Well, that was it. We thank all the experts for being so kind to share their thoughts on this particular matter and we thank all the climate scientists across the globe, whom we can all learn so much from. And of course we thank you for reading.

Now let’s all agree to start removing fossil fuels from the global economy. Because the necessity to reduce emissions, drastically, globally – that is far beyond these scientific uncertainties, beyond any doubt, whether it be mind, heart …or gut.

© Rolf Schuttenhelm | www.bitsofscience.org