Radiative Feedbacks

While most of the feedbacks at play in the Earth’s ecosystem are subject to the closed system of matter, radiative feedbacks are those that deal with the open system in which the sun’s energy is transferred to the Earth and then absorbed or reflected back, to varying degrees. Because the amount of absorbed or reflected solar energy is the principal component in the planet’s temperature (the other factors then determine what happens to that heat, but the solar energy provides the initial quantity), these feedbacks are key to understanding and modeling global climate. Feedbacks are relationships found in complex systems, in which the output (or result) of the system is returned to the input. For instance, as summers get hotter, people run their air conditioners longer, releasing more chlorofluorocarbons, accelerating global warming and causing hotter summers. That is a simple example of a positive feedback often used in schools. The radiative feedbacks in question involve principally clouds, ice, and water vapor.

A critical feedback in global warming is the albedo-ice feedback. Though the melting of the polar ice caps is often associated with rising sea levels, it is only one of the important effects, and simply the most vivid to illustrate. The more complicated effect is in the change it enacts on how much solar energy is retained by the Earth. Ice is far more reflective than land or liquid water, and as the polar ice melts, the area of the surface that it occupied is replaced by one of those two things. As a result, more sunlight is absorbed by the surface instead of being reflected, which then warms the poles further, melting more ice.

Clouds are also critical. Clouds act as a sort of imperfect barrier, absorbing a limited amount of heat moving in either direction, heat emanating from the Earth into space, and sunlight shining on the Earth. Models disagree on whether their overall feedback effect on global warming will prove positive or negative. On the one hand, the evaporative feedback in general accelerates global warming: as the temperature increases, the capacity of the air to hold moisture increases exponentially, so as water evaporates into the warm air, it is able to stay there. Since water vapor is a greenhouse gas, as it accumulates, it causes the temperature to get warmer and warmer. 

On the other hand, sufficiently great cloud cover shields the Earth from solar radiation, and cloudy days aren’t as hot. High-enough clouds can reflect sunlight back down on the Earth, however, more than balancing out the sunlight they have blocked. Water vapor also retains heat better than the atmosphere does; a wetter atmosphere loses heat more slowly. As more and more water evaporates, more and more heat from the sun can be retained, which makes for a warmer Earth and greater evaporation.

It is possible that the effects of positive radiative feedbacks on global warming have been muted until recently by global dimming, the gradual reduction of the Earth’s irradiance (emitted radiation, including heat and light) in the second half of the 20th century, which has reversed in the 21st century. Global dimming is probably caused by the coalescing of water vapor around anthropogenic particles in the air, the product of pollution and other industrial activities. The resulting water droplets form a little differently than they would otherwise, and make more highly reflective clouds. The Clean Air Act and other antipollution efforts are presumed to be responsible for ending global dimming. The apparent recent increase in global warming may be because that dimming had been reducing global warming’s effects by limiting the amount of sunlight entering the system.