Prior to each Intergovernmental Panel on Climate Change (IPCC) report, the Global Climate Modeling Centers produce a central database of standardized simulations. Over the past year, an interesting trend has emerged in the most recent round of this effort: the newer and better versions of these models are, on average, more sensitive to CO2, warming up more in response to it than previous iterations. So what is behind this behavior and what does it tell us about the real world?
Climate sensitivity is one of the most discussed numbers in climate science. Its most common formulation is the amount of warming that occurs when the concentration of CO2 in the atmosphere is doubled and the planet takes a century or two to reach a new equilibrium. It’s an easy way to get an idea of what our shows are likely to end up doing.
In climate models, this number is not chosen in advance; it is apparent from all the physics and chemistry of the model. This means that as the modeled processes are updated to improve their realism, the overall climate sensitivity of the model may change. Over the results of the latest generation of models, their average climate sensitivity has increased significantly. A new study conducted by Marc Zelinka from the Lawrence Livermore National Laboratory analyzes these new model simulations, comparing their behavior to the previous generation.
The analysis focused on the simulations of 19 of the 35 modeling groups, because not everything has happened yet. The climate sensitivity of these models ranged from 1.8 to 5.6°C for double CO2, with an average of 3.9°C. The last generation fell between 2.1 and 4.7°C, with an average of 3.3°C. So there is a slight increase in sensitivity in the latest generation of models. (For context, the best global estimate of real-world climate sensitivity for decades has been 1.5-4.5°C, centered on 3°C.)
The models work by dividing the atmosphere into pieces called grid cells and then simulating the atmospheric physics in each of them. To find out what caused the higher sensitivities, the researchers used a tool to break down the model’s responses into each grid cell. They focused on feedbacks, processes that amplify or dampen warming. Differences in the various feedbacks are usually responsible for the range of overall sensitivity values.
In this analysis, most of the feedbacks have basically the same strength as in the previous generation of models. Low clouds, however, behave differently. This is quite important because low clouds can have a huge impact if they are fluffy and reflective, shading the planet. If the amount of cloud shading changes as temperatures rise, which it does, you have feedback.
The behavior of low clouds in models is a particularly lively subject of study. Previous research has shown that, for example, models with the most realistic clouds (to some extent) tend to have higher climate sensitivities. And a lot of work is needed to do New measurements of cloud physics and chemistry, with modelers then trying to ensure that their simulations match this behavior.
In the new study, the researchers found that the weak cloud feedback in the new generation of models appears to have changed outside the tropics, particularly in the southern hemisphere at mid-latitudes. The average feedback between the models is a bit more positive, amplifying the warming. This would mean that as the Earth warms, the low cloud cover in this region decreases a bit and reflects less sunlight back to space.
The researchers dug a little deeper to see what was behind this change and discovered that it was likely related to some changes in cloud physics equations. Comparisons with satellite data have shown that the models should allow more liquid droplets to remain liquid at cold temperatures, and at least some models have been modified accordingly. That could be enough to alter the degree of cloud change as temperatures rise.
In short, the new models don’t seem to be doing anything unrealistic to provoke their higher sensitivities. But that doesn’t necessarily mean they’re right. It’s going to take a lot more work to sift through these models and see which one has clouds that best matches the real world. And because Earth’s climate system is so interconnected, it’s even possible that another factor, like an ocean temperature pattern, is partly shaping cloud behavior.
Models aren’t the only way scientists can estimate Earth’s climate sensitivity. This is also done by studying historical records, for example, and past climate changes recorded by things like ice cores. Assessing the new generation of climate models against these events could also provide insight.
This situation will certainly represent a challenge for the authors of the next IPCC report, as they will only be able to include studies published by September 2020. It’s a safe bet that we’ll see more research on this topic in the coming months – and hopefully a lot, for the good of the IPCC authors.
Geophysical Research Letters, 2020. DOI: 10.1029/2019GL085782 (About DOIs).