New representations of clouds are making models more sensitive to carbon dioxide
National Center for Atmospheric Research/University Corporation for Atmospheric Research
As scientists work to determine why some of the latest climate models suggest the future could be warmer than previously thought, a new study indicates the reason is likely related to challenges simulating the formation and evolution of clouds.
The new research, published in Science Advances, gives an overview of 39 updated models that are part of a major international climate endeavor, the sixth phase of the Coupled Model Intercomparison Project (CMIP6). The models will also be analyzed for the upcoming sixth assessment report of the Intergovernmental Panel on Climate Change (IPCC).
Compared with older models, a subset of these updated models has shown a higher sensitivity to carbon dioxide – that is, more warming for a given concentration of the greenhouse gas -though a few showed lower sensitivity as well. The end result is a greater range of model responses than any preceding generation of models, dating back to the early 1990s. If the models on the high end are correct and Earth is truly more sensitive to carbon dioxide than scientists had thought, the future could also be much warmer than previously projected. But it’s also possible that the updates made to the models between the last intercomparison project and this one are causing or exposing errors in their results.
In the new paper, the authors sought to systematically compare the CMIP6 models with previous generations and to catalog the likely reasons for the expanded range of sensitivity.
“Many research groups have already published papers analyzing possible reasons why the climate sensitivity of their models changed when they were updated,” said Gerald Meehl, a senior scientist at the National Center for Atmospheric Research (NCAR) and lead author of the new study. “Our goal was to look for any themes that were emerging, especially with the high-sensitivity models. The thing that came up again and again is that cloud feedbacks in general, and the interaction between clouds and tiny particles called aerosols in particular, seem to be contributing to higher sensitivity.”
The research was funded in part by the National Science Foundation, which is NCAR’s sponsor. Other supporters include the U.S. Department of Energy, the Helmholtz Society, and Deutsches Klima Rechen Zentrum (Germany’s climate computing center).
Evaluating model sensitivity
Researchers have traditionally evaluated climate model sensitivity using two different metrics. The first, which has been in use since the late 1970s, is called equilibrium climate sensitivity (ECS). It measures the temperature increase after atmospheric carbon dioxide is instantaneously doubled from preindustrial levels and the model is allowed to run until the climate stabilizes.
Through the decades, the range of ECS values has stayed remarkably consistent – somewhere around 1.5 to 4.5 degrees Celsius (2.7 to 8.1 degrees Fahrenheit) – even as models have become significantly more complex. For example, the models included in the previous phase of CMIP last decade, known as CMIP5, had ECS values ranging from 2.1 to 4.7?C (3.6 to 8.5?F).
The CMIP6 models, however, have a range from 1.8 to 5.6?C (3.2 to 10?F), widening the spread from CMIP5 on both the low and high ends. The NCAR-based Community Earth System Model, version 2 (CESM2) is one of the higher-sensitivity models, with an ECS value of 5.2?C.
Model developers have been busy picking their models apart during the last year to understand why ECS has changed. For many groups, the answers appear to come down to clouds and aerosols. Cloud processes unfold on very fine scales, which has made them challenging to accurately simulate in global-scale models in the past. In CMIP6, however, many modeling groups added more complex representations of these processes.
The new cloud capabilities in some models have produced better simulations in certain ways. The clouds in CESM2, for example, look more realistic when compared to observations. But clouds have a complicated relationship with climate warming – certain types of clouds in some locations reflect more sunlight, cooling the surface, while others can have the opposite effect, trapping heat.
Aerosols, which can be emitted naturally from volcanoes and other sources as well as by human activity, also reflect sunlight and have a cooling effect. But they interact with clouds too, changing their formation and brightness and, therefore, their ability to heat or cool the surface.
Many modeling groups have determined that adding this new complexity into the latest version of their models is having an impact on ECS. Meehl said this isn’t surprising.
“When you put more detail into the models, there are more degrees of freedom and more possible different outcomes,” he said. “Earth system models today are quite complex, with many components interacting in ways that are sometimes unanticipated. When you run these models, you’re going to get behaviors you wouldn’t see in more simplified models.”
An unmeasurable quantity
ECS is meant to tell scientists something about how Earth will respond to increasing atmospheric carbon dioxide. The result, however, cannot be checked against the real world.
“ECS is an unmeasurable quantity,” Meehl said. “It’s a rudimentary metric, created when models were much simpler. It’s still useful, but it isn’t the only way to understand how much rising greenhouse gases will affect the climate.”
One reason scientists continue to use ECS is because it allows them to compare current models to the earliest climate models. But researchers have come up with other metrics for looking at climate sensitivity along the way, including a model’s transient climate response (TCR). To measure that, modelers increase carbon dioxide by 1% a year, compounded, until carbon dioxide is doubled. While this measure is also idealized, it may give a more realistic view of temperature response, at least on the shorter-term horizon of the next several decades.
In the new paper, Meehl and his colleagues also compared how TCR has changed over time since its first use in the 1990s. The CMIP5 models had a TCR range of 1.1 to 2.5?C, while the range of the CMIP6 models only increased slightly, from 1.3 to 3.0?C. Overall, the change in average TCR warming was nearly imperceptible, from 1.8 to 2.0?C (3.2 to 3.6?F).
The change in TCR range is more modest than with ECS, which could mean that the CMIP6 models may not perform that differently from CMIP5 models when simulating temperature over the next several decades.
But even with the larger range of ECS, the average value of that metric “did not increase a huge amount,” Meehl said, only rising from 3.2 to 3.7?C.
“The high end is higher but the low end is lower, so the average values haven’t shifted too significantly,” he said.
Meehl also noted that the increased range of ECS could have a positive effect on science by spurring more research into cloud processes and cloud-aerosol interactions, including field campaigns to collect better observations of how these interactions play out in the real world.
“Cloud-aerosol interactions are on the bleeding edge of our comprehension of how the climate system works, and it’s a challenge to model what we don’t understand,” Meehl said. “These modelers are pushing the boundaries of human understanding, and I am hopeful that this uncertainty will motivate new science.”
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About the article
Title: “Context for interpreting equilibrium climate sensitivity and transient climate response from the CMIP6 Earth system models”
Authors: Gerald A. Meehl, Catherine A. Senior, Veronika Eyring, Gregory Flato, Jean-Francois Lamarque, Ronald J. Stouffer, Karl E. Taylor, and Manuel Schlund
Journal: Science Advances
ECS is OK as a concept, for comparing models between themselves or with observations. It is not a real thing in any other way. It most certainly cannot be used to predict temperature changes given a change in CO2. It cannot be expected to be the only thing going on over millenia and multiple doubling. It can’t be used to explain the difference between some ridiculous global average temperature and some equally ridiculous claim of what that average would be given no atmosphere. The whole debate here is based on a false premise, or a number of false premises.
“We have introduced yet more uncertainty necessitating further study. Send more money”
“These modelers are pushing the boundaries of human understanding, and I am hopeful that this uncertainty will motivate new science.”
Heh. “new science”. They can’t even get the “old science” right. It’s like having a science based on the idea that heavier objects always fall faster than lighter ones. No matter how many models, and no matter how much tweaking and fiddling with them they do, they will never work right.
I stopped reading after this:
“The first, which has been in use since the late 1970s, is called equilibrium climate sensitivity (ECS). It measures the temperature increase after atmospheric carbon dioxide is instantaneously doubled from preindustrial levels and the model is allowed to run until the climate stabilizes”
That says it all folks.
Introduce 1 modern AI and the model will go blue screen of death. This is ridiculous. I want to know 2 things: 1. Is this the same model Mann created? (which would explain a LOT about him) and 2. Why is CO2 instantly doubled(?) how does that even equate to observations? And doubled from what reference point exactly? Nobody cared about CO2 until relevantly recently..and the one person that had was debunked wayyyyy before CO2 became the “bady” in this new fear mongering scam.
Scientific methodology anyone? No? Well then it’s not worth the 1’s and 0’s IMO.
Just Jenn,
The one thing common to all fossil fuel combustion is the release of CO2.
The eco-warriors don’t really give a damn about CO2 (per se) but they hate modern civilisation which relies for its very existence on coal and oil and gas.
There is *absolutely no way* they will persuade us to give up our modern lifestyle — indeed they can’t even persuade their own followers who turn up at protest camps in polluting old camper vans and leave behind tons of detritus, 90% of which is usually plastic!
HOWEVER … if they can persuade people — and especially governments — that there is global warming, that that global warming will irrevocably damage the planet and that CO2 in the atmosphere is the cause of that warming which can be stopped by putting an end to our use of hydrocarbons, then they get to have what they (think they) want and when the rest of us wake up what it really means in terms of lifestyles it will be too late!
That, in a nutshell, is what this farce is all about. “Scientists” make a lot of easy money, as do the people who get into “renewable energy”; control-freaks on the political hard left start to dream about a utopian one-world government; the rest of us are too busy trying to earn a living to concern ourselves with matters we don’t fully understand, and anyway, it’s about saving the environment, innit? So it must be “good”. Stands to reason.
And now at least you know why “nobody cared about CO2 until recently”! Because at any concentration below about 1,000 ppm it’s a complete irrelevance!
“Many research groups have already published papers analyzing possible reasons why the climate sensitivity of their models changed when they were updated …”
Are we to take it, then, that post-modern pseudo-science not only consists of playing computer “games” and pretending that the outcome is “evidence” of anything except the way the “games” were written and writing learned (?) papers to that effect but also writing equally learned (?) papers on why the results are different from the last “game” they invented.
And all this — natch — on the taxpayer’s dime. Only it isn’t dimes any more; it’s mega-dollars!
“But even with the larger range of ECS, the average value of that metric “did not increase a huge amount,” Meehl said, only rising from 3.2 to 3.7?C.”
Taking an average of wrong predictions will invariably give you a wrong answer. It is also a”chickens” way out of choosing the right one. Scientists need to be pushed to take a stand and choose one to advocate for. I suspect you would see a quick rush to change them to meet observational evidence.
The increase in ECS range is a tacit admission that either past models or the new ones are WRONG. There is simply no other conclusion you can reach.
It is not surprising that the scientists are struggling; as the problem goes back to an error of logic in the early days.
Initially as Radiative FORCING was defined which had units (in old money) of Lbs force. This was the magically? converted to an energy flux with units of foot Lbs/sec *ft^2 or Watts/m^2.**
This could only have been done if a sensitivity value had already been assumed based on the perceived climate behaviour at the time.
Thus, thereafter use of this flux value in subsequent models carried with it an intrinsic value of this sensitivity as a given; ignoring the fact that the climate must have inevitably changed since that flux was calculated.
As for the statement that “ECS is an immeasurable quantity” by Meehl : Is he not aware that at the evaporative phase change of water the Planck sensitivity coefficient is ZERO since the process occurs at constant temperature? Easily measured I think.
As this process is continuous and ubiquitous within the climate surely it deserves to be considered. Someone needs to start thinking outside the current Groupthink obsession with CO2. Perhaps they could start with looking at the basic thermodynamics of the Hydro Cycle.
** The electrical analogy here is where a Potential is defined but you can only convert this into a flux if the resistance of the circuit is known to enable a current and hence the flux to be calculated.
More Climate Hyperreality – the inability to tell the difference between reality and climate model simulations.
Perhaps they should build a Climate Change Disneyland or Potemkin Village.
We have had three warm days so far this year. I look forward to the non-stop heat wave we are supposed to experience.
— The headline baffles me.
— As does that extract.
1. Clouds do not cause warming in the climate system. They cause cooling. One empirical study after another shows that.
2. How are new models “new research“? Climate models have nothing to do with research. They are studies, but not research. Research implies someone investigated the world to discover something, or they invented something. Models are neither inventions nor discoveries. Models are more ‘computer games‘, or propaganda, or both. Not “research”.
3. “endeavor” indeed, but not climate endeavor.
This degeneration and misuse of language really annoys me.