Climate models

Clouds could be the missing link in climate models


Scientists have been trying to figure out why the latest generation of climate models predict a much hotter future than previously thought. The main culprit could be clouds, according to a new article published on June 24 in Scientists progress (1).

Every few years, a more advanced generation of climate models emerges. In the latest series still reaching us from leading climate researchers around the world, scientists have already noticed something surprising: a subset of the updated models show much higher climate sensitivity. In other words, no more heating for a given concentration of CO2.

Equilibrium Climate Sensitivity (ECS) is the theoretical point at which the climate comes to equilibrium after CO2 levels have doubled, and leading metric climatologists use it to estimate future global warming. If the Earth is indeed more sensitive to CO2, we could face in the years to come a much hotter world than expected.

Unfortunately, this also means a much smaller carbon budget (the cumulative CO2 emissions allowed over a period of time to keep temperatures below a certain threshold to avoid catastrophic climate change).

What causes this increased sensitivity?

The reasons for the increase in 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 Scientists progress paper and head of the climate change research section at the National Center for Atmospheric Research (NCAR) explained, “Our goal was to research all emerging themes, especially with high sensitivity models.”

“What keeps coming up is that feedbacks from clouds in general, and the interaction between clouds and tiny particles called aerosols in particular, seem to contribute to higher sensitivity.”

To build realistic computer models, scientists often make simplifying assumptions that allow them to simulate large-scale climate processes within the limits of processing power and computing time. Clouds are incredibly difficult to model as there are many factors involved in the complex process of cloud formation, from air temperature, humidity and wind conditions, to aerosols, to tiny particles. in the atmosphere that greatly influence how quickly clouds form, their size and duration. to stay.

New climate models have changed the way clouds are represented in the atmosphere, as new aircraft observations improve our understanding. While simulating these subtleties can improve the accuracy of climate models, they also introduce more uncertainty and could lead to conflicting results.

Clouds can have a cooling or warming effect by trapping heat or reflecting sunlight away from the earth’s surface, depending on how much water or ice they contain. In addition, clouds and aerosols may not be the only factors causing increased warming.

Nevertheless, 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 at the forefront of our understanding 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 hope 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 initiative, and will contribute to the upcoming Sixth Assessment Report of the Intergovernmental Panel on Evolution climate (IPCC).

(1) Meehl, GA et al. Context of interpretation of the climate sensitivity to equilibrium and the transient climate response from models of the CMIP6 Earth system. Scientific advances (2020). DOI: 10.1126 / sciadv.aba1981