Clouds play an important role in Earth’s global climate, both by reflecting the Sun’s radiation back out into space and by holding in thermal radiation. The extent to which clouds affect climate, however, depends on the amount, height, and opacity of the clouds.
Generally, low-level clouds reflect a lot of solar radiation but trap almost no thermal radiation, so they cool the climate. Higher-up, wispy cirrus clouds trap more thermal radiation than whatever solar radiation they reflect back out, whereas thicker clouds reflect at least as much solar radiation as whatever thermal radiation they hold in, such that high-level clouds can either warm or cool the climate system.
Cloud properties change in response to changes in the environment, such as warming air or sea surface temperature, creating a feedback loop. Cloud feedbacks vary greatly among climate models—in fact, this variation is the dominant reason why models disagree about how much global warming will occur in response to elevated carbon dioxide. So it is crucial for scientists to make progress constraining it.
In a recent study Zhou et al. examined how clouds respond to warming temperatures in different regions in order to better understand how and why the strength of global cloud feedback is influenced by the spatial pattern of surface warming. The researchers used the Community Earth System Model (CESM) to conduct an extensive series of experiments, in which sea surface temperatures in individual patches of the ocean experience warming. From there, they calculated how the climate responds to these changes.
They found that when warming is applied to the warmest regions of the tropics where air ascends, low-level clouds increase in remote regions where air descends, thereby cooling Earth. On the other hand, when warming is applied to colder regions, low-level clouds decrease locally, and the planet is heated. The overall cloud feedback strength thus depends sensitively on how temperature changes in each region.
This study provides a unique, new approach to understand how clouds respond to regional surface temperature changes, both locally and remotely. The approach yields key insights into how and why the global cloud feedback depends on warming patterns. (Journal of Advances in Modeling Earth Systems (JAMES), https://doi.org/10.1002/2017MS001096, 2017)
—Sarah Witman, Freelance Writer