Scientists have been trying to figure out why the latest generation of climate models are predicting a much warmer future than previously thought. The main culprit might be clouds, according to a new paper published on 24 June in Science Advances (1).
Every few years, a more advanced generation of climate models emerge. In the latest series still trickling in from leading climate researchers around the world, scientists have already noticed something surprising: a subset of the updated models shows much higher climate sensitivity. In other words, more warming for a given concentration of CO2.
Equilibrium climate sensitivity (ECS) is the theoretical point at which the climate comes into equilibrium after CO2 levels have doubled, and the main metric climate scientists use to estimate the future global warming. If the Earth is indeed more sensitive to CO2, we could be facing a much warmer world in years to come than previously predicted.
Unfortunately, this also means a much smaller carbon budget (the cumulative CO2 emissions permitted over a given period of time to keep temperatures below a certain threshold to prevent catastrophic climate change).
What is causing this increased sensitivity?
The reasons for the increased ECS are still unclear. One possibility is the more realistic representation of clouds and aerosols in climate models. In a statement, Dr Gerald Meehl, lead author of the Science Advances paper and head of the Climate Change Research Section at the National Center for Atmospheric Research (NCAR) explained: “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”.
To build realistic computer models, scientists often making simplifying assumptions which allow them to simulate large-scale climate processes within the constraints of processing power and computation time. Clouds are incredibly difficult to model since numerous factors are involved in the complex cloud formation process from air temperature, humidity and wind conditions to aerosols, tiny particles in the atmosphere that greatly influence how quickly clouds form, how big they get, and how long they stay.
Newer climate models have changed the way clouds are represented in the atmosphere, as new observations from aircraft increase our understanding. While simulating these intricacies can improve the accuracy of climate models, they also introduce more uncertainty and could lead to contradictory results.
Clouds can have a cooling or warming effect by either trapping heat or reflecting sunlight away from the earth’s surface, depending on how much water or ice they contain. Moreover, clouds and aerosols might not be the only factors driving increased warming.
Nonetheless, clouds are a crucial component of climate systems and scientists need more robust observational programs and better satellite measurements to further improve the simulation of cloud processes.
Meehl said: “Cloud-aerosol interactions are on the bleeding edge of our comprehension of how the climate system works, and it‘s a challenge to model what we don‘t understand”.
“These modelers are pushing the boundaries of human understanding, and I am hopeful that this uncertainty will motivate new science.”
The updated climate models are part of the sixth phase of the Coupled Model Intercomparison Project (CMIP6), a major international climate endeavour, and will contribute the upcoming sixth assessment report of the Intergovernmental Panel on Climate Change (IPCC).
(1) Meehl, G.A. et al. Context for interpreting equilibrium climate sensitivity and transient climate response from the CMIP6 Earth system models. Science Advances (2020). DOI: 10.1126/sciadv.aba1981