A new study suggests that at certain carbon dioxide (CO2) levels the Earth could reach a tipping point at which dense low-lying cooling cloud layers could become unstable and break up into smaller, more scattered clouds. This could significantly affect global warming. According to the simulations, this would lead to a dramatic increase in surface warming ― up to 8 degrees Celcius ― especially in the subtropics. The findings were published on 25 February in Nature Geoscience (1).
The so-called stratocumulus clouds form large protective sheets that provide an important cooling effect by shading large areas the Earth’s surface from sunlight and reflecting it back into space. Since they cover more than 20% of the tropical oceans, this could critically affect the Earth’s energy balance, the authors write. Clouds are important components involved in regulating the surface temperature of the Earth and reducing global warming. But the air movements and heat exchange processes that maintain these clouds are not considered in current climate models.
Thus, to date, interactions between greenhouse gases and clouds have remained unknown because climate models are unable to deal with clouds ― the climate processes driven by clouds are on scales too small for even highly sophisticated climate models to resolve. To overcome this, cloud processes are typically simplified or ignored altogether. And while the impact of cloud formations can be roughly estimated using variables like temperature and humidity, this method typically underestimates their influence on global temperatures.
So this time, scientists, led by Prof Tapio Schneider, a climate scientist at the California Institute of Technology, focused on the cloud scale and instead, simplified the large-scale interactions, such as the heat and energy between the ocean and atmosphere. The entire simulation was then performed on the cloud scale. To achieve this, the researchers used machine learning to train the existing climate models to more accurately represent clouds. Algorithms were taught using both real-world data and simulations of certain small-scale processes in order to make the cloud-scale processes as accurate as possible.
The lowest levels of CO2 at which the instability occurred in the simulations was 1300 ppm. The finding was based on a high emissions (business as usual) scenario ― assuming fossil fuels continue being burned at the current rate. As the atmosphere warms, it seems the dense layers cloud start to break into smaller, puffier clouds at this critical value, which is only around three times higher than the current atmospheric CO2 level.
According to the prediction, fewer clouds could mean up to 8ºC of additional global warming within the next century if current emissions levels continue. Furthermore, once these clouds disappear, they might never form again ― in the simulations, clouds did not reappear until CO2 levels decrease to well below the level at which the instability occurred.
The new model is by no means perfect and there are still a number of uncertainties. However, the results do suggest that many models, including those used by the Intergovernmental Panel on Climate Change (IPCC), may be underestimating potential future climate change impacts.
(1) Schneider, T., Kaul, C.M., and Pressel, K.G. Possible climate transitions from breakup of stratocumulus decks under greenhouse warming. Nature Geoscience (2019). DOI: