Use of too hot climate models exaggerates the effects of global warming

(ORDO NEWS) — According to one study, Arctic precipitation will become dominant in the 2060s, decades earlier than expected.

Another claims that air pollution from wildfires in the western United States could triple by 2100. The third says that an oceanic mass extinction could happen in just a few centuries.

All three studies published last year rely on future projections made by some of the world’s next-generation climate models. But even model makers admit that many of these models have a serious problem: they predict a future that gets too hot too quickly.

While model makers are adapting to this reality, researchers using model predictions to assess the impacts of climate change have yet to follow suit. This has led to a parade of “faster than expected” results that some researchers fear threaten to undermine the credibility of climate science.

Scientists need to be more choosy about how they use model results, a team of climate scientists argue in a commentary published today in the journal Nature.

Researchers should no longer simply use the average of all climate model projections, which could result in global temperatures 0.7°C warmer by 2100 than the Intergovernmental Panel on Climate Change (IPCC) estimates.

“We need to take a slightly different approach,” says Zeke Hausfather, head of climate research at payment services company Stripe and lead author of the commentary. “We must move away from the naive idea of ​​model democracy.” Instead, he and his colleagues call for a meritocracy of models, sometimes favoring the results of models,

All in all, climate models remain incredibly successful research tools, and nothing in this “too hot” generation takes away climate science from its foundations, says Keith Marvel, a climatologist at NASA’s Goddard Institute for Space Studies and co-author of the commentary.

The greenhouse effect continues to warm the planet. Ice is melting, seas are rising, and droughts are becoming more frequent in some areas. But the models aren’t perfect, says Marvel. “They are not crystal balls.”

The issue of models that are too hot emerged in 2019 as part of the Model Mapping Project (CMIP), which brings together the results of global models in anticipation of the IPCC keynote reports every 7 or 8 years.

In previous rounds of CMIP, most models predicted “climate sensitivity” – the warming expected from a doubling of atmospheric carbon dioxide compared to pre-industrial times – from 2°C to 4.5°C. But for the 2019 CMIP6 round, the sensitivity of 10 of the 55 models exceeded 5°C, which is a sharp deviation.

The results are also at odds with a landmark study that did not take into account the results of global modeling, but instead used paleoclimate and observational data to determine the sensitivity of the Earth’s climate.

It showed that the value is somewhere between 2.6°C and 3.9°C. The discrepancy in sensitivity estimates is “a sobering example of the complexity of the climate system,” says Christopher Field, a climatologist at Stanford University who specializes in impact studies.

Since then, researchers have tracked the causes of overheated models, which include models developed by the National Center for Atmospheric Research, the US Department of Energy, the UK Metropolitan Office and Environment and Climate Change Canada. These are often related to how models render clouds; one result of this has been excessive projected warming in the tropics.

However, many of these models represent the world better than their predecessors, and the centers that created them have been openly diagnosing the problem, says Marvel. “This is commendable.” But it will be years before the centers can produce new forecasts for widespread use.

The IPCC attempted to offset this problem last year when it published its first working group report on the physical basis of climate change. The IPCC evaluated the models for their ability to reflect past historical temperatures.

She then used sophisticated models to generate official “warming estimate” projections for various fossil fuel emission scenarios. When it came to studying future changes on Earth, the IPCC presented the results of all models depending on the degree of warming: 1.5°C, 2°C, 3°C. This made it possible to use useful information from hot models even if they reached these thresholds too quickly.

While the IPCC was up to the task, it didn’t do a very good job of telling everyone about the real issue, says Hausvater, who is himself an IPCC co-author. “A lot of our colleagues had no idea that the IPCC was doing this,” he says.

And since then, dozens of published studies have used predictions based on the raw mean of all CMIP6 models.

The results, they note, are often “worse” than the IPCC projections, and this has drawn the attention of those who are unaware of the problems underlying these models. “It’s not because someone is acting in bad faith,” says Marvel. “It’s simply because there is no leadership.”

According to Hausfather and his co-authors, climate impact researchers should emulate the steps taken by the IPCC. First, they should avoid dubious time-based scenarios and instead emphasize the effects of specific levels of global warming, regardless of when those levels are reached.

They should also use the IPCC’s own “warming estimate” projections of when these levels of warming might occur.

And for studies where details of the warming trajectory are important, they can use separate models that reflect warming with relative accuracy, such as those developed by NASA and the National Oceanic and Atmospheric Administration, among many others.

“I agree with almost everything the authors say and suggest,” says Claudia Tebaldi, a climatologist at the Lawrence Berkeley National Laboratory and co-leader of CMIP’s climate prediction scenarios.

However, she says the recommendations may underestimate the desire of politicians for time-bound information, which, in her experience, is almost always requested. And some climate impacts, such as rising sea levels, vary with the time it takes to reach warming levels, not just with the absolute amount of warming.

Researchers should consider going even further and examining whether certain models have large regional skews, for example, says Reto Knutti, a climate scientist at ETH Zürich who has been calling for a “model meritocracy” for more than a decade.

As more urban planners and outside scientists turn to these projections, they should first consult with a climate model expert. “Given that these results drive climate adaptation and billions of dollars of investment, this seems like a worthy effort,” says Knutty.

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