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Clouds are important for Earth's energy balance because they interact with radiation in different ways. On one hand, low clouds reflect incoming solar radiation and thus cool Earth through a property known as albedo. On the other hand, clouds mainly at high altitudes prevent thermal radiation from escaping into space, which has a warming effect. Overall, the cooling effect currently dominates.

If global warming causes clouds to change their extent or brightness, this could increase or decrease their albedo and thus their cooling effect. The warming effect of clouds could be altered if they moved to different altitudes as temperatures rise. Both would have an impact on climate sensitivity, which measures how much Earth's surface warms for a doubling of carbon dioxide and is a key measure for climate projections.

To isolate the aspect of changing cloud altitude, a team led by Lukas Kluft from the Max Planck Institute for Meteorology (MPI-M) investigated the radiative effect of clouds at constant albedo using a simple, idealized model. The paper is in the journal Atmospheric Chemistry and Â鶹ÒùÔºics.

How warming impacts cloud altitude

In the model, the atmosphere is represented as an air column with a typical temperature and pressure profile. The researchers introduced clouds in three layers: low clouds near the surface, mid-level clouds forming at freezing level, and high ice clouds. The experiment assumed that the albedo of these clouds did not change with increasing global temperatures, but allowed them to rise or sink.

"The beauty of such simplified models lies in their proximity to our conceptual understanding. In their simplicity, they are clearly defined, allowing us to experiment in a targeted manner," says Kluft.

As the simplified model does not allow to predict the cloud properties, the researchers simulated a large number of plausible cloud combinations consistent with current satellite measurements. The climate sensitivity calculated for these approximately 500 configurations with a fixed albedo was 2.2°C for a doubling of carbon dioxide levels, just below the theoretical value for a completely cloud-free atmosphere in the absence of any surface changes.

When the researchers increased the temperature in their model, low clouds remained at the same pressure level and became warmer. Mid-level and high clouds moved to higher altitudes and largely maintained their temperature. Although the unchanged temperature leads to more warming, the higher altitude also dampens the greenhouse effect of carbon dioxide. These two effects cancel each other out, so that the overall climate sensitivity remains nearly unchanged.

Important reference value

"It was important for us to establish the baseline implied by theoretical understanding: We can understand a climate sensitivity of 2.2°C. If our more complex climate models deliver different values, we can investigate the origin of possible deviations," explains Kluft.

For example, simply accounting for the retreat of surface snow and ice with warming increases the expected climate sensitivity to the more commonly quoted values near 3°C. The researchers have thus established an important reference value that can form the basis for further studies, for example, of potential changes in cloud albedo using the new generation of Earth system models being developed at the institute.