Changes in cloud altitude have minimal impact on climate sensitivity

Although climate change causes some cloud types to change altitude, the effects on climate sensitivity cancel each other out.

Clouds of different structures and at different altitudes over the blue ocean.

Clouds of different structures and at different altitudes over the blue ocean.

To the point

• Important for the climate: Clouds influence climate sensitivity, i.e., how much the Earth warms for a given increase in atmospheric carbon dioxide.
• Changes due to climate change: If the effect of cloud altitude is studied in isolation, climate change does cause cloud altitude to change, but this has only a minor influence on climate sensitivity.
• Reference value: These findings provide a baseline for investigating how other cloud properties—such as changes in cloud brightness and extent—influence climate sensitivity.

Clouds are important for the Earth's energy balance because they interact with radiation in different ways: On one hand, low clouds reflect incoming solar radiation and thus cool the 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 investigated the radiative effect of clouds at constant albedo using a simple, idealized model.

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.

Temperature profile varying with altitude (shown as pressure). There are light blue bars at three levels, labelled "fixed height", "freezing level", and "convective top"

Temperature profile varying with altitude (shown as pressure). There are light blue bars at three levels, labelled "fixed height", "freezing level", and "convective top"

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 degree Celsius 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 degree Celsius. 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 degree Celsius. 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 Max Planck Institute for Meteorology.