They may be mere swirls of dust and mist, but clouds have proven to be a hefty challenge for climate projections. We know that humans are making the planet hotter, but it is difficult to predict exactly how bad things will get; climate scientists think temperatures could climb by anywhere between a relatively mild 1.5 degrees Celsius and a devastating 4.5 degrees by the end of the century. Clouds are about 50 percent of the reason that range is so large, because scientists aren’t sure how clouds will behave on a hotter planet — if climate change decreases cloud cover, more sunlight will reach the earth’s surface and temperatures will rise higher. Conversely, if cloud cover increases, they could block sunlight and help to alleviate rising temperatures.
A new project, led by Gregory Cesana from the Center for Climate Systems Research at Columbia University’s Earth Institute, will unravel the mysteries of how low clouds (less than 3 kilometers from the ground) respond to climate change, to help narrow the range of how much warming we can expect as CO2 continues to rise.
Low clouds, particularly those in the tropics, are one of the main culprits behind the cloud-related uncertainties in climate projections. This is due to the fact that the tropics (the areas between 30 degrees north and south of the equator) receive the largest amount of solar radiation, and because the low clouds cover a large portion of these areas, said Cesana. His work will project the future low cloud cover not only in the tropics, but also the extratropics, which extend nearly to the polar regions.
Most climate models predict that low cloud cover will decrease as the planet warms, but it has been difficult to say for sure because scientists haven’t had enough information to evaluate the models. That’s beginning to change. Using data from two NASA satellites, CloudSat and CALIPSO, Cesana plans to evaluate how two main types of low clouds — shallow cumulus and stratocumulus — are behaving today, and use this information to estimate how they’ll evolve in the future.
Shallow cumulus clouds tend to be small, cottony clouds that often have a flat base. Stratocumulus clouds form more of a blanket over the sky. Both types of clouds have different impacts on the climate and could respond differently to climate change. However, their behaviors have been hard to sort out because they have been difficult to monitor from space on a global scale. Since they’re so low in the sky, traditional satellites often can’t see them because they are masked by overlapping higher clouds. In addition, older satellites couldn’t tell apart the different types of clouds accurately. “Now this has become possible because the particular satellites I’m using are able to observe a whole transect, a whole profile of atmosphere from the top to the base,” said Cesana. “You can see what’s happening in the atmosphere at every level with a better vertical and horizontal resolution than previous satellites. We are now able to tell the stratocumulus and shallow cumulus clouds apart, and we can use this to evaluate their behavior in the models.”
Cesana will use the satellite data to study how the low clouds are responding to surface temperature and the stability of the lower troposphere, the two main factors that control low cloud formation. Understanding this relationship in the present day will make it possible to test how well the models simulate present-day low cloud behavior. It will also make the future of low clouds much clearer, assuming that the models simulate the correct change of temperature and stability.
“If you know how the surface temperature and stability will evolve in the future, then you can tell how the low clouds are going to evolve in the future,” Cesana explained.
At the end of the three-year project, he plans to compare his findings to simulations from two of the most widely used climate model experiments (CMIP5 and CMIP6). If the models match his data, then they’re doing a good job of modeling the clouds’ response to global warming. If not, it would mean the models are probably not accurately representing the relationships between surface temperature, stability, and low cloud behavior, and would need to be refined.
Cesana’s project recently received funding through NOAA’s Modeling, Analysis, Predictions, and Projections program. He expects to have some preliminary results next year, and hopes that the work will eventually help to reduce uncertainty around low cloud feedbacks in climate models, so that projections for future warming will be more accurate.
Every degree or even half of a degree of global warming can have widespread and devastating impacts, said Cesana, “so it’s very important to be able to narrow this down.”
