For scientists modeling climate, the behavior of both natural and human-made airborne particles, called aerosols, is notoriously difficult to capture. Ranging in size from a few nanometers to tens of micrometers—the latter the width of a human hair—specks of soot, sulfur dioxide, nitrates, and black carbon form a constantly shifting mixture. As they drift through the atmosphere, they alter clouds’ albedo, the proportion of sunlight that gets reflected away from Earth.
Generally, scientists have found that aerosols increase albedo, masking the warming effects of increased greenhouse gases to some extent. But such studies have typically looked at just one cloud type: warm, low clouds whose parallel, planar shape and round, liquid cloud droplets are fairly straightforward to simulate in computer models and retrieve in satellite observations. That leaves out a great diversity of clouds made of both water and ice, such as the massive, heaped thunderheads known as cumulonimbus clouds.
To address that gap, Christensen et al. collected and analyzed data from NASA’s “A-Train,” a constellation of six satellites that flies in formation, crossing the equator every day at 1:30 p.m. local time. They analyzed data on temperature, water vapor, and rainfall, as well as three-dimensional views of clouds and airborne particles and aerosols between 2006 and 2010. For the first time, the researchers were able to estimate the extent to which aerosols in different cloud types—such as puffy, cotton ball–shaped cumulus clouds and the icy filaments of high-altitude cirrus clouds—affect albedo and thermal infrared radiation.
As the atmosphere became more polluted, the low, relatively flat “boundary clouds” that cover approximately 40% of the globe’s oceans became brighter, or, in other words, more reflective. As the tops of the clouds got colder and icier, however, they dimmed, likely because the growing ice crystals fell as precipitation, the team reports. Convective clouds—the rising, dome- and tower-shaped clouds formed by rising air currents, which cover about 14% of global oceans—also reflected less heat than the warmer, flatter clouds. As air pollution increased, so did the rate of convection, causing the cloud tops to rise and block some of the outgoing thermal emission of radiation to space.
Overall, aerosols’ effect on cold boundary layer clouds and convective clouds has a net warming effect on global temperatures, reducing the cooling caused by reflected radiation by about half compared to estimates using only the low, relatively flat, warm boundary layer clouds, the team found. The findings suggest that cloud changes in response to increased aerosol pollution may not provide as much of a buffer against global warming as previously thought. (Journal of Geophysical Research: Atmospheres, doi:10.1002/2016JD025245, 2016)
—Emily Underwood, Freelance Writer