Increased warming in latest generation of climate models likely caused by clouds


"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."

https://www.eurekalert.org/pub_releases/2020-06/ncfa-iwi061820.php

Hands up all those who thought scientists tried to close the uncertainty gap?

Ajax wrote on Jul 11^th, 2020 at 1:56pm:




Climate scientists can't model the climate with 100% certainty therefore let's just go with some guy on a blog who says it's the su--

Oh, sorry, I got you confused with one of these other nuts. You prefer the "secret weather machine" theory of climate change, right?

lee wrote on Jul 12^th, 2020 at 2:58pm:


The quote that every poseur keeps in his handbag for those occasions when he finds himself completely out of his depth, yet desires to maintain the pretense that he has even the faintest clue.