Tackling “Too Hot” Climate Models

May 7, 2022

Climate science does not rely on models. But as the saying goes, :all models are wrong, but some are useful”.
And some useful models might not be the best to apply in every situation.

An all-star team including Gavin Schmidt, Zeke Hausfather, and Kate Marvel has looked at a number of climate models that they describe as running “too hot”. It’s important to recognize that we are in big trouble, but also not to overstate what we know.


We are climate modellers and analysts who develop, distribute and use these projections. We know scientists must treat them with great care. Users beware: a subset of the newest generation of models are ‘too hot’2 and project climate warming in response to carbon dioxide emissions that might be larger than that supported by other evidence37. Some suggest that doubling atmospheric CO2 concentrations from pre-industrial levels will result in warming above 5 °C, for example. This was not the case in previous generations of simpler models.

Earth is a complicated system of interconnected oceans, land, ice and atmosphere, and no computer model could ever simulate every aspect of it exactly. Models vary in their complexity, and each makes different assumptions about and approximations of processes that happen on small scales, such as cloud formation.

The Intergovernmental Panel on Climate Change (IPCC), to its credit, has recognized this ‘hot model’ problem. Scientists contributing to the main sections of its Sixth Assessment Report (AR6; published over the past few months) reconciled the newest climate models with key observational constraints on global mean warming, sea-level rise and ocean heat content, and other analyses. They applied statistics to determine the most reasonable projections, consistent with many lines of evidence, which they call ‘assessed warming’.

Unfortunately, little guidance was made available for scientists wishing to study projections in other contexts. We are concerned that in the absence of such guidance, much of the scientific literature is at risk of reporting projections that are inconsistent with the approach taken by the IPCC, and that are overly influenced by the hot models.

Studies that cover monthly or daily extremes or regional climate impacts, for example, are instead left to use the full set of CMIP6 models. And simply taking an average of those leads to higher projections of warming than the IPCC’s assessed-warming averages. As a result, some studies have reported projections that might be inconsistent with AR6 assessments. Findings that show projected climate change will be ‘worse than we thought’ are often attributable to the hot models in CMIP6.
It is important to emphasize that, whereas unduly hot outcomes might be unlikely, this does not mean that global warming is not a serious threat. Multiple lines of evidence establish that the planet is more than 1 °C warmer than it was before the Industrial Revolution, and that further warming poses severe risks to society and the natural world. There are many aspects of climate change we do not yet understand, hence the continued necessity of climate science. But there is no serious disagreement that continued emissions will lead to dangerous levels of warming.


Overall, climate models remain incredibly successful research tools, and nothing about this “too hot” generation invalidates the tenets of climate science, says Kate Marvel, a climate scientist at NASA’s Goddard Institute for Space Studies and co-author of the commentary. The greenhouse effect is still warming the planet. Ice is melting, seas are rising, and droughts are becoming more frequent in some areas. But the models are not perfect, Marvel says. “They’re not crystal balls.”

The problem of the too-hot models arose in 2019 from the Coupled Model Intercomparison Project (CMIP), which combines the results of the world’s models in advance of the major IPCC reports that come out every 7 or 8 years. In previous rounds of CMIP, most models projected a “climate sensitivity”—the warming expected when atmospheric carbon dioxide is doubled over preindustrial times—of between 2°C and 4.5°C. But for the 2019 CMIP6 round, 10 out of 55 of the models had sensitivities higher than 5°C—a stark departure. The results were also at odds with a landmark study that eschewed global modeling results and instead relied on paleoclimate and observational records to identify Earth’s climate sensitivity. It found that the value sits somewhere between 2.6°C and 3.9°C. The divergence in sensitivity estimates is a “sobering example of the complexity of the climate system,” says Christopher Field, a Stanford University climate scientist who focuses on impacts.

Researchers have since tracked down the causes of the too-hot models, which include those produced by the National Center for Atmospheric Research, the U.S. Department of Energy, the United Kingdom’s Met Office, and Environment and Climate Change Canada. They often relate to the way models render clouds; one result has been excessive predicted warming in the tropics.

Still, many of these models render the world better than their predecessors, and the centers that produced them have been open about diagnosing the problem, Marvel says. “They’re to be commended.” But it will take years before the centers can produce new projections for broad use.

IPCC tried to compensate for this problem last year when it published its first working group report, which covers the physical basis of climate change. IPCC rated models on their skill at capturing past historical temperatures. Then, it used the skillful models to produce its official “assessed warming” projections for different fossil fuel emissions scenarios. When it came to studying the future changes to Earth, IPCC reported results from all the models based on degree of warming: 1.5°C, 2°C, 3°C. That allowed useful information from the hot models to be used, even if they reach those thresholds too fast.

Although IPCC rose to the challenge, it didn’t do a great job telling everyone about the actual problem, says Hausfather, himself an IPCC co-author. “A large number of our colleagues had no idea that the IPCC did this,” he says. And since then, dozens of published studies have used projections based on the raw average of all CMIP6 models. The outcomes, they note, are often “worse” than the IPCC projections—and that has drawn attention from those unaware of the underlying problems with the models. “It’s not because anybody is acting in bad faith,” Marvel says. “It’s just because there’s no guidance

3 Responses to “Tackling “Too Hot” Climate Models”

  1. indy222 Says:

    From the research I’ve read on first-hand paleo data from Friedrich et al 2016, the review work of von der Heydt later, the cloud work of Steve Sherwood, and others, the “too hot” modelling is not to be discounted so easily as this above would have you believe. Models which rely on 19th and 20th century warming as determining ECS are forgetting that ECS is clearly state-dependent.

    The point is not what WAS ECS before we started tipping the Amazon to carbon source, and the permafrost thaw feedback, and the Arctic Ocean from bright reflector to dark absorber, the point is what is the ECS most relevant for calculating our future starting now.

    The IPCC policy statements are bent towards preserving the monetary powers that control the UN, not towards the future. Keep that in mind.

    • rhymeswithgoalie Says:

      I like to remind everybody that the ‘I’ in IPCC stands for intergovernmental.

  2. rhymeswithgoalie Says:

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