Future temperature rises underestimated by current climate projections


In a new study, researchers from the National Center for Atmospheric Research (NCAR) found that the climate models which have been able to best model current relative humidity conditions are also the ones that predict the highest temperature increases in the future.

"There is a striking relationship between how well climate models simulate relative humidity in key areas and how much warming they show in response to increasing carbon dioxide," said John Fasullo, , according to Science Daily. Fasullo is a project scientist with NCAR's Climate Analysis Section (CAS) and one of the study's co-authors. "Given how fundamental these processes are to clouds and the overall global climate, our findings indicate that warming is likely to be on the high side of current projections."

Fasullo and Kevin Trenberth, the head of the CAS, ran simulations on 16 of the best climate models in use — the same ones used in generating the United Nations Intergovernmental Panel on Climate Change (IPCC) report — to see which ones most accurately modeled current observations of changes in tropical and subtropical relative humidity. They focused their attention primarily on the subtropics, because — as shown by Hadley cell circulation — the warm, dry sinking air at those latitudes creates the conditions that formed most of the world's major deserts, and shifts in the location of this sinking air can show general trends in temperature rise, as well as which areas will be vulnerable to future droughts and heat waves.

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"The dry subtropics are a critical element in our future climate," said Fasullo, according to Science Daily. "If we can better represent these regions in models, we can improve our predictions and provide society with a better sense of the impacts to expect in a warming world."

Comparison of climate models uses a measure known as 'equilibrium climate sensitivity' (ECS). This is the temperature increase that the models show, over time, when levels of carbon dioxide in the atmosphere reach double what they were before the industrial revolution. Some models show as little as a 1.5 degree C increase, others show as much as a 4.5 degree C increase, with the leading models showing an average increase of about 3 degrees C. It has been difficult to narrow down this range before now, because it was difficult to factor certain climate parameters into the models, such as cloud cover.

Fasullo and Trenberth concentrated on relative humidity in their models because there are more reliable records of relative humidity than there are of cloud cover, and relative humidity is easier to model. The relative humidity in the subtropics is usually around 15-20%, however most climate models tend to overestimate it at around 30% or more. Some of them do much better, though, and Fasullo and Trenberth found that these better-performing models where the ones that had the higher ECS values, showing future temperature rises of 4 degrees C and higher.

"Because we have more reliable observations for humidity than for clouds, we can use the humidity patterns that change seasonally to evaluate climate models," said Trenberth. "When examining the impact of future increases in heat-trapping gases, we find that the simulations with the best fidelity come from models that produce more warming."

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According to their paper, titled A Less Cloudy Future: The Role of Subtropical Subsidence in Climate Sensitivity and published in this week's issue of Science: "In addition to providing a path forward and focus for improving models, results strongly suggest that the more sensitive models perform better, and indeed the less sensitive models are not adequate in replicating vital aspects of today's climate."