Evidence of high climate sensitivity

I'm not going to say anything about this research because I've not read the paper, but it looks important. If someone out there writes something up I'll put a link here.

Here's the deal. Climate sensitivity is, very oversimplified, how much the surface of the planet heats up as we add CO2 and other greenhouse gasses to the atmosphere. More specifically, equilibrium climate sensitivity is the number of degrees C the atmosphere at face height and the sea surface heat up with a doubling of CO2 from pre-industrial levels.

If our atmosphere had just nitrogen and CO2 and that's it, the number would be fairly low, about 1.2 degrees C. But live would not exist here because there would be no water, so we would not be having this conversation. The fact that we are having this conversations suggests the existence of water vapor, which cranks up sensitivity quite a bit, because more CO2 means more heat means more water vapor. That is just one of a number of "positive" (read not good) feedbacks on climate sensitivity.

I've noted before that if you offer a group of informed climate scientist the chance to guess a single number for climate sensitivity, using the Free Beer method, is something like 3.0. Certainly not less than 2.0. But it could just possibly be much higher, like 6. The chances of climate sensitivity being 6 are small, and if it turned out to be, then we are truly Doomed. But here's the thing. The upper range of possible values for this important number is what is sometimes called a "fat tail." The chances are low, but not so low they can be ignored.

Here's a picture of a fat tail.

uncertainty_sensitivity

Even a value of 4 or 5 would be bad, and the chances are not vanishingly small that this would be the value.

So, about the latest research.

Title: Long-term cloud change imprinted in seasonal cloud variation: More evidence of high climate sensitivity

Authors: Chengxing Zhai, Jonathan H. Jiang, Hui Su

Abstract: The large spread of model equilibrium climate sensitivity (ECS) is mainly caused by the differences in the simulated marine boundary layer cloud (MBLC) radiative feedback. We examine the variations of MBLC fraction in response to the changes of sea surface temperature (SST) at seasonal and centennial time scales for 27 climate models that participated in the Coupled Model Intercomparison Project phase 3 and phase 5. We find that the intermodel spread in the seasonal variation of MBLC fraction with SST is strongly correlated with the intermodel spread in the centennial MBLC fraction change per degree of SST warming and that both are well correlated with ECS. Seven models that are consistent with the observed seasonal variation of MBLC fraction with SST at a rate −1.28 ± 0.56%/K all have ECS higher than the multimodel mean of 3.3 K yielding an ensemble-mean ECS of 3.9 K and a standard deviation of 0.45 K.

Potential meaning: Ruh roh.

These results are not particularly unexpected. But one would hope that more research would show a lower number, because we really don't want this to be a higher number.

See also: Future warming likely to be on high side of climate projections, analysis finds, which covers A Less Cloudy Future: The Role of Subtropical Subsidence in Climate Sensitivity, by John Fasullo and Kevin Trenberth. Science, 9 November 2012:

An observable constraint on climate sensitivity, based on variations in mid-tropospheric relative humidity (RH) and their impact on clouds, is proposed. We show that the tropics and subtropics are linked by teleconnections that induce seasonal RH variations that relate strongly to albedo (via clouds), and that this covariability is mimicked in a warming climate. A present-day analog for future trends is thus identified whereby the intensity of subtropical dry zones in models associated with the boreal monsoon is strongly linked to projected cloud trends, reflected solar radiation, and model sensitivity. Many models, particularly those with low climate sensitivity, fail to adequately resolve these teleconnections and hence are identifiably biased. Improving model fidelity in matching observed variations provides a viable path forward for better predicting future climate.

See also: A bit more sensitive, which discusses "Spread in model climate sensitivity traced to atmospheric convective mixing" by Stgeven Sherwood, Sandrine Bony, and Jean-Louis Dufrense, in Nature, January 2 2014.

Equilibrium climate sensitivity refers to the ultimate change in global mean temperature in response to a change in external forcing. Despite decades of research attempting to narrow uncertainties, equilibrium climate sensitivity estimates from climate models still span roughly 1.5 to 5 degrees Celsius for a doubling of atmospheric carbon dioxide concentration, precluding accurate projections of future climate. The spread arises largely from differences in the feedback from low clouds, for reasons not yet understood. Here we show that differences in the simulated strength of convective mixing between the lower and middle tropical troposphere explain about half of the variance in climate sensitivity estimated by 43 climate models. The apparent mechanism is that such mixing dehydrates the low-cloud layer at a rate that increases as the climate warms, and this rate of increase depends on the initial mixing strength, linking the mixing to cloud feedback. The mixing inferred from observations appears to be sufficiently strong to imply a climate sensitivity of more than 3 degrees for a doubling of carbon dioxide. This is significantly higher than the currently accepted lower bound of 1.5 degrees, thereby constraining model projections towards relatively severe future warming.

See also: Overlooked evidence - global warming may proceed faster than expected

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high climate sensitivity

I guess this issue comes up because of the uncertainty of the forcings we've generated (sulphate aerosols in particular). If the only forcings were those that changed predictably then we might be able to estimate future temperature rise by just extrapolating from the 0.7℃ or so rise over the past 40 years and adjusting for the change in the rate of increase of high certainty forcings.

That expected temperature rise is bad enough (something in excess of 0.2℃/decade in the near future) but the effect of variations in the very uncertain forcings is seriously worrying.

By Chris O'Neill (not verified) on 03 Nov 2015 #permalink

Chris, we could do that, but one problem is the lag from increased concentration of GHG to the realization of an equilibrium. The factors that determine that lag overlap in effect (but are different from each other) with the factors that determine the equilibrium value. So we are trying to predict where the trend line in surface temperatures will stop if the key independent variable (GHG/CO2) stopped changing, and when that would happen. I think that is mathematically impossible without knowing the effects of positive and negative feedbacks.

There's been a lot of back and forth about the effects of aerosols, but I'm thinking that this is being nailed down pretty well, and it is long term probably not that much of a factors. Aerosols' effects are short term, and over decades, probably even out even if there is a lot of variability.

Thus the focus on vapor and clouds.

I am shocked, just shocked to find uncertainty in here.

Of course,, there is still argument over the force of gravity:
The search for Newton’s constant.
In Fig 1, many of the error bars don't even overlap.

How can we not know g exactly by now? Further research is needed, and until then, we cannot do anything involving gravity, and jumping off bridges must be safe.

By John Mashey (not verified) on 04 Nov 2015 #permalink

John, "jumping off bridges is safe" -- if you are a right-wing science denier, that is.

By Brainstorms (not verified) on 04 Nov 2015 #permalink

Depends how high the bridges are.

In this metaphorical case , that would be *very* high and deadly indeed.

This is a really worrying development with dire consequences that go so much further than I think most folks really appreciate.

In other related news :

http://www.abc.net.au/news/2015-11-04/kennedy-space-centre-sea-level/68…

Among some very many other places. Everywhere really. Globally anyhow.

"But live would not exist here because there would be no water...."

Life, too. :-)

That's the problem: if the uncertainty was much smaller, scientists and policy would have a better understanding on what to do and when to do it. As it is, the greater the uncertainty, the greater the need to act as soon as possible.

By Desertphile (not verified) on 05 Nov 2015 #permalink

That’s the problem: if the uncertainty was much smaller, scientists and policy would have a better understanding on what to do and when to do it. As it is, the greater the uncertainty, the greater the need to act as soon as possible.

I have found over many years that when I ask new students "what do you think statistics is about?" the common answer is "studying averages" - they are shocked when most of the discussions center on explaining reason and sources for variation.

Dean: Yes
I used to give a a computer performance analysis talk called "Summarizing Perfomrance is No Mean Feat" for exactly that reason.
It focused on use of lognormals for performance ratio distributions, in part with idea of getting meaningful 2nd, 3rd and 4th moments.

By John Mashey (not verified) on 05 Nov 2015 #permalink

The Cartesian philosopher, François Poullain de La Barre, observed in On the Equality of the Two Sexes, a Physical and Moral Discourse in which We see the Importance of Ridding Oneself of Prejudices ( 1673) that ;

'The idea of reality being naturally attached to that of science, man tends to assume as true that which is proposed by those who have the reputation of being wise.

And as the number of those who are but reputed wise so greatly exceeds the number of those who are so in fact, the average man, who simply counts the number of opinions, cleaves to the wisdom of the group, and the more willingly embraces their opinion when it more closely corresponds to that which he already believes.

In the case of poets and orators, who aim but to please and persuade, the appearance of truth is sufficient for most people. Thus, as exaggeration and hyperbole serve well their purpose, by willfully exaggerating their ideas , they cause things to appear good or bad, big or small, as they please. The ornamental style of their discourse marvelously serves to create the belief of those who are not vigilant. They speak with facility and grace, using attractive, agreeable and uncommon manners of expression to dazzle the mind and prevent it from perceiving the truth.'

In the case of poets and orators, who aim but to please and persuade, the appearance of truth is sufficient for most people

but unlike most people, scientists obtain most of their knowledge and understanding through reading, a much more intellectual act than merely listening.

By Chris O'Neill (not verified) on 07 Nov 2015 #permalink

In reply to by Russell (not verified)

Russell: "The Cartesian philosopher, François Poullain de La Barre, observed in On the ....."

Did he or she write anything about climate sensitivity?

By Desertphile (not verified) on 08 Nov 2015 #permalink

In reply to by Russell (not verified)

Russell is entertaining us. He's invalidating the act reasoning via appeal to authority... by appealing to authority.

Is this but a case of one pot claiming to be shinier than the other pot, while the kettle looks on and laughs?

By Brainstorms (not verified) on 07 Nov 2015 #permalink

He was too busy "writing in the ornamental style of discourse marvelously" to be bothered with such empirical matters.

By Brainstorms (not verified) on 08 Nov 2015 #permalink

Russell

The ornamental style of their discourse marvelously serves to create the belief of those who are not vigilant. They speak with facility and grace, using attractive, agreeable and uncommon manners of expression to dazzle the mind and prevent it from perceiving the truth.

For the opposite - unadorned scientific exposition of the data - see Rohling et al. (2012).

Low sensitivity (less that ~2C per doubling) is effectively incompatible with palaeoclimate behaviour. The long-established ~3C per doubling best estimate provides a better fit and the actual value could be higher.

Even if we take the (unwarranted) step of assuming S is right at the low end of the range there's *still* no policy wriggle-room if we want to avoid the risk of serious impacts.

You sound as though you are proposing a conspiracy theory of some kind. Are you a conspiracy theorist?

The Polar philosopher, Jean-Philippe Soutien de La Droiture, observed in On the False Equality of the Two Views of Global Warming, a Physical and Moral Discourse in which We see the Importance of Ridding Oneself of Anti-Science Prejudices (2013) that:

‘The idea of reality being naturally detached to that of science, the Conservative-leaning man tends to assume as true that which is proposed by those who share similar political agendas, attributing to them a reputation of being wise.

‘And as the number of those who are but so reputed wise so greatly exceeds the number of those who are so in fact, the average Conservative man, who simply counts the number of opinions which soothe his fears, cleaves to the perceived wisdom of any right-wing faction, and the more willingly embraces their opinion when it more closely corresponds to that which he already believes.

‘In the case of anti-science bloggers, who aim but to please and persuade such persons, the appearance of truth is sufficient for most of their disciples. Thus, as exaggeration and hyperbole serve well their purpose, by willfully exaggerating their ideas, they cause climate science to appear questionable, its revealed implications small, as they please. The ornamental style of their discourse marvelously serves to create the belief of those who are not vigilant. They speak with rabid facility and gracelessness, using attractive, yet disagreeable and uncouth manners of expression to dazzle the mind and prevent it from perceiving the truth.’

By Brainstorms (not verified) on 09 Nov 2015 #permalink

Brainstorm is confusing Solutien de La Droiture 's treatise with the nearly indistinguisable executive summary of Ourseblanc de Quiberon's On the Hazards of satirizing the Propaganda of the Jacobins without reference to the Existence of one's Own.

Russell

BBD, the subject is not palaeoclimate

If you want to understand climate sensitivity, you look at palaeoclimate behaviour. Obviously.

What you just did at #18 was deny the scientific evidence, which converges on a best estimate of about 3C per doubling. With a fat tail in the uncertainty over higher values, of course.

Incidentally, do I have the honour of addressing Russell Seitz? Judging from your links I do, but assumptions are dangerous.

Converges?

You call that a convergent sequence of values ?

Please see what I wrote in Foreign Affairs five years ago.-- policy analysts are stiill waiting for one significant digit.

Russell

You can deny that a best estimate for ECS exists or that it is ~3C but it does exist and it is ~3C. And it has been for decades.