Climate Change Science
Cloud Cover and Atmospheric Feedbacks
The two fundamental elements of climate change science—greenhouse gases, primarily carbon dioxide, and global average temperatures—are relatively easy to track and correlate. Although CO2 is the principal agent of climate change, this is mostly as a trigger, one that raises atmospheric temperatures sufficiently to vaporize the most powerful greenhouse agent, water. A rise in temperature would, it is argued, result in higher rates of ocean evaporation and cloud formation that would, in turn, trap even more heat. The predicted operation of the greenhouse effect is based on such feedbacks accentuating the heat-trapping properties of carbon dioxide and other greenhouse gases. However, increased cloud cover would also result in a greater albedo, or amount of solar radiation reflected by clouds back into space without penetrating the lower atmosphere. Clouds therefore have potentially positive (warming) and negative (cooling) feedback effects. Since the atmosphere is such a complex and variable system, it is challenging to observe and measure the operation of such atmospheric feedback effects. The role of clouds and atmospheric moisture in particular have been at the center of recent controversy over climate change, and remain the least understood of all the possibly significant feedback mechanisms. The most highly regarded critic of the IPCC consensus statements, Richard Lindzen of MIT’s Earth, Atmospheric and Planetary Sciences Department, has undertaken hydrological research to understand how clouds regulate the temperature of the earth’s atmosphere. In a model he has advanced since 1989, Lindzen and colleagues argue that high-level tropical clouds over the Pacific operate as a sort of enormous heat valve, allowing the release of heat into space and so bringing temperatures to equilibrium. He further argues that thermal equilibrium is achieved primarily through the heat loss accompanying atmospheric convection and the transport of moisture from warmer to cooler latitudes, rather than through infrared radiation of the sort trapped by greenhouse gases. Mainstream climate researchers, however, point to evidence contradicting Lindzen’s convection model—“satellite and balloon observations showing that water in the upper troposphere increases, not decreases, whenever and wherever the lower troposphere is warmer.” They also argue that, although Lindzen is the only scientist to develop a full-blown, alternative model of climate systems, the bulk of circumstantial evidence still points towards the probability of positive climate change.