Climate models

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

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As scientists scramble to determine why some of the latest climate models suggest the future may be hotter than previously thought, a new study says the reason is likely related to challenges simulating training and cloud evolution.

The new research, published in Scientists progress, provides an overview of 39 updated models that are part of a major international climate effort, the sixth phase of the Coupled Model Intercomparison Project (CMIP6). The models will also be analyzed for the next sixth assessment report of the Intergovernmental Panel on Climate Change (IPCC).

Compared to older models, a subset of these updated models showed higher sensitivity to carbon dioxide, i.e. more warming for a given concentration of greenhouse gases, although that a few also showed lower sensitivity. The end result is a greater range of model response than any previous generation of models, dating back to the early 1990s. If high end models are okay and Earth is truly more sensitive to carbon dioxide than scientists are. thought, the future could also be much hotter than expected. But it is also possible that updates to models between the last intercomparison project and this one cause or expose errors in their results.

In the new paper, the authors sought to systematically compare CMIP6 models with previous generations and catalog the likely reasons for the increased sensitivity range.

“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, senior scientist at the National Center for Atmospheric Research (NCAR) and lead author of the new study. “Our goal was to research any themes that emerged, especially with the high sensitivity models. contribute to higher sensitivity.

The research was funded in part by the National Science Foundation, which is the sponsor of NCAR. Other supporters include the US Department of Energy, the Helmholtz Society, and the Deutsches Klima Rechen Zentrum (Germany’s Climate Computing Center).

Model sensitivity assessment

Researchers have traditionally assessed the sensitivity of climate models using two different measures. The first, in use since the late 1970s, is called equilibrium climate sensitivity (SCE). It measures the increase in temperature after atmospheric carbon dioxide instantly doubles from pre-industrial levels and the model is allowed to run until the climate stabilizes.

Over the decades, the range of DHW values ​​has remained remarkably consistent – somewhere around 1.5 to 4.5 degrees Celsius (2.7 to 8.1 degrees Fahrenheit) – even though the models have become considerably more complex. For example, models included in the previous phase of CMIP in the last decade, known as CMIP5, had ECS values ​​ranging from 2.1 to 4.7 C (3.6 to 8.5 F).

CMIP6 models, however, have a range of 1.8 to 5.6 C (3.2 to 10 F), widening the gap to CMIP5 on the low and high end. The NCAR-based Community Land System Model, Version 2 (CESM2) is one of the higher sensitive models, with an ECS value of 5.2 C.

Model developers have been busy separating their models over the past year to understand why ECS has changed. For many groups, the answers seem to boil down to clouds and aerosols. Cloud processes take place at very fine scales, which has made them difficult to accurately simulate in models on a global scale in the past. In CMIP6, however, many modeling groups added more complex representations of these processes.

New cloud capabilities in some models have produced better simulations in some ways. Clouds in CESM2, for example, appear more realistic compared to observations. But clouds have a complicated relationship with global warming: Certain types of clouds in certain places reflect more sunlight, cooling the surface, while others can have the opposite effect, trapping heat.

Aerosols, which can be released naturally from volcanoes and other sources as well as human activity, also reflect sunlight and have a cooling effect. But they also interact with clouds, changing their formation and luminosity and, consequently, their ability to heat or cool the surface.

Many modeling groups have determined that the addition of this new complexity in the latest version of their models has an impact on ECS. Meehl said it was not surprising.

“When you put more detail into the models, there are more degrees of freedom and more different outcomes possible,” he said. “Models of the Earth system today are quite complex, with many components interacting in sometimes unexpected ways. When you run these models, you will get behaviors that you wouldn’t see in more simplified models. “

An unmeasurable quantity

ECS aims to tell scientists how the Earth will react to increased atmospheric carbon dioxide. The result, however, cannot be verified against the real world.

“ECS is an immeasurable amount,” Meehl said. “It’s a rudimentary metric, created when the models were much simpler. It’s still useful, but it’s not the only way to understand how increasing greenhouse gases will affect the climate. “

One of the reasons scientists continue to use ECS is that it allows them to compare current models to early climate models. But researchers have developed other metrics to examine climate sensitivity along the way, including a model’s transient climate response (TCR). To measure this, modelers increase carbon dioxide by 1% per year, compound, until carbon dioxide is doubled. While this measure is also idealized, it can give a more realistic view of the temperature response, at least over the short-term horizon of the coming decades.

In the new article, Meehl and colleagues also compared how the TCR has changed over time since its first use in the 1990s. CMIP5 models had a TCR range of 1.1 to 2.5 C, while the range of CMIP6 models increased only slightly, from 1.3 to 3.0 C. Overall, the change in average TCR warming was almost imperceptible, from 1.8 to 2.0 C (3 , 2 to 3.6F).

The change in TCR range is more modest than with ECS, which could mean that CMIP6 models might not perform differently from CMIP5 models when simulating temperature over the next several decades.

But even with the larger range of DHW, the average value of this metric “didn’t increase dramatically,” Meehl said, dropping only from 3.2 to 3.7 C.

“The high end is higher but the low end is lower, so the average values ​​haven’t changed too much,” he said.

Meehl also noted that the increased range of ECS could have a positive effect on science by spurring more research into cloudy processes and cloud-aerosol interactions, including field campaigns to gather better observations on how whose interactions take place in the real world.

“Cloud-aerosol interactions are at the forefront of our understanding 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 hope this uncertainty will motivate new science.”

Some of the latest climate models provide unrealistic projections of future warming

More information:
Context of interpretation of the climate sensitivity to equilibrium and the transient climate response from models of the CMIP6 Earth system. Scientists progress (2020). DOI: 10.1126 / sciadv.aba1981

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