When you were young, you probably used to lie on the grass and gaze at the clouds. They formed rabbits, hats, elephants, faces, dragons, and giant towers. But you probably didn’t notice that they also form giant mirrors that reflect incoming radiation. In fact, the reflectiveness of clouds might be a key factor in climate change.
Before we discuss the climactic effects of clouds, let’s go over some background. The climate of the earth- temperatures, storms, rainfall, etc.- is controlled by the amount of radiation entering the earth’s atmosphere and the amount of radiation leaving it. The incoming radiation comes from the sun, while the outgoing radiation is either reflected solar energy or geothermal energy.
Aerosols, or particles suspended in air, determine how much radiation enters and how much leaves. Aerosols act like tiny two-way mirrors- they reflect some incoming radiation and absorb some outgoing radiation. They are like an atmospheric gate, only allowing a certain amount of heat to pass through.
Clouds, then, are really just conglomerations of aerosols. Because clouds cover 50% of the earth’s atmospheric surface, they reflect quite a bit of radiation back into space. You can feel this phenomenon when you are outside on a sunny day- the moment a cloud drifts over you, the temperature decreases by a few degrees. But they also keep quite a bit of radiation inside the atmosphere, causing temperatures to rise.
This deceptively simple behavior hides some complex implications. Rising temperatures will produce more water vapor and thus more clouds. But will these clouds reflect more heat into space than they reflect back to earth, or vice versa? And will increased cloud formation raise or lower the levels of “free” aerosols in the air?
The answers to these questions most likely lie in pollution. Pollutants are one of the major sources for aerosols and thus significantly affect atmospheric temperatures. In Frans Olofson’s landmark 2008 dissertation on cloud and climate relationships, he noted that pollutants have a “strong impact on the properties of the urban aerosol” and thus on the reflective properties of clouds. So, in the future, clouds in pollution-rich areas might reflect more heat back to earth than “normal” clouds.
The connections between clouds and climate change are major concerns in climatology. According to the UN Climate Panel, clouds are “the single greatest uncertainty when estimating the sensitivity of the climate”. They may seem innocuous at first, but the white shape-shifters floating above your head have a serious effect on the future of our planet.
Bohren, C. F. and Huffman, D. R., Absorption and scattering of light by small particles,
Wiley Science Paperback Series, New York, 1998
Olofson, Frans. "Lidar Studies of Tropospheric Aerosols and Clouds" (Doctoral Thesis). 2008.