Clouds are important components of Earth's life-sustaining greenhouse effect. Alterations in cloud density, prevalence, or altitude caused by global temperature increases may have far-reaching consequences for Earth's temperature equilibrium.

Clouds have a profound influence on local and global climate. Scientists have known for many years that clouds are a major component of the greenhouse effect, which makes life possible on Earth. What is not well understood is what effect clouds may have on global warming and cooling trends. Of the many variables that factor into global warming, clouds present the greatest uncertainty in predicting climate change.

Clouds are composed of droplets or ice crystals that form around aerosols in the atmosphere. These droplets accumulate until they become visible. The many shapes and sizes of clouds are divided into three basic types: cirriform (wispy and transparent), stratiform (layered), and cumuliform (mounded and fluffy). Three altitude divisions are also used for classification: high (more than 5,000 meters above sea level), middle (between 2,000 and 5,000 meters above sea level), and low (less than 2,000 meters above sea level). The terms are combined, with the upper and middle altitudes given the prefixes cirro- and alto-; a midlevel stratus cloud is called altostratus, for example.

Clouds reflect solar radiation back to space, and they reflect Earth's radiation back to the surface. Thus, all other factors being equal, nights are warmer and days are cooler when clouds are present. The amount of reflection back to space depends on cloud thickness, with a low of approximately 20 percent for cirrus clouds and a high of 90 percent for cumulonimbus clouds. Clouds contribute significantly to global radiative balance, or radiative equilibrium, an overall balance between solar radiation received by the Earth and heat reflected back to space. If the Earth absorbed all the radiation it receives, it would be much too hot to sustain life.

However, the Earth does not absorb all the solar energy it receives, but instead reflects visible radiation and emits infrared radiation. Without an atmosphere, the total return of energy to space would make the Earth much colder than it is--the average surface temperature would be approximately -18œ Celsius. Clouds and other greenhouse gases (GHGs) absorb some of the energy, trapping it and sending some of it back to the Earth's surface. This is the greenhouse effect.

"Feedback" is a term that applies to any multipart system in which a change in one part produces a change in the other part, which then affects the original part, and so on. A simple example is a thermostat, which responds to a drop in temperature by turning on a furnace. The furnace raises the air temperature, which then causes the thermostat to turn off the furnace. Feedback systems can affect changes in cloud thickness and prevalence as a response to climate changes, particularly temperature. The greenhouse effect creates a state of equilibrium, and cloud feedback disturbs that equilibrium.

Cloud feedback can be either positive or negative. Positive feedback increases the enhanced greenhouse effect, while negative feedback lowers global temperatures. Most clouds provide both positive feedback (transmitting energy down toward the Earth) and negative feedback (transmitting solar radiation back to space). Determining whether the net feedback is positive or negative is a complicated process.

Climate change can increase both positive and negative feedback. Studies have shown that cold water produced by the melting polar ice cap causes phytoplankton to release chemicals that produce more and brighter clouds, thereby increasing negative feedback. Aerosols may produce either positive or negative feedback, depending on the source of the aerosols. Volcanic eruptions and pollution from technologically advanced countries, consisting of sulfates and nitrates, generate clouds that produce negative feedback. However, the developing world produces pollution that contains these substances as well as large amounts of black carbon, the by-product of incomplete combustion of carbon-based fuels. Black carbon aerosols generate positive feedback. The net feedback of pollution is very difficult to determine.

A long-held belief is that if the Earth's climate warms, water vapor amounts in the atmosphere will increase, creating more low-level thick clouds that will generate negative feedback. That belief is being called into question by recent studies that have shown that turbulence created by rising warm air currents will actually lead to fewer clouds being formed overall.

The impact of clouds on climate change is extremely difficult to model. Most of the important climate modeling systems represent clouds with a small number of variables, masking the subtleties of cloud dynamics. This is due to the extremely complicated mathematics required to model clouds realistically. While analyzing the results of some large-scale climate studies, scientists have concluded that neither the magnitude nor the sign of cloud feedback can be relied upon.

When weather data are entered into climate models, the resulting pictures of cloud cover and thickness often do not match actual conditions. Further complicating matters, studies done with live data collection also show contradictory results. For example, some studies indicate that decreasing cloud cover over China (resulting from large amounts of pollution) may be responsible for increasing temperatures there. However, other studies performed in other parts of the world indicate that temperatures have increased as cloud cover has increased.

The impact of clouds on global climate cannot be overstated. Clouds are extremely sensitive to fluctuations in solar radiation, Earth's surface temperature, and many other environmental factors, including pollution levels. It is this sensitivity, combined with nearly infinite variations in cloud size, make-up, and altitude, that make predicting cloud feedback so difficult. Since clouds may either mitigate or increase global warming, it is imperative that scientists intensify their efforts to produce better predictive climate modeling systems, as well as promote studies that analyze data collected in the field. Throughout the world, decisions are being made using climate predictions that may be flawed as a result of the uncertainty presented by cloud behavior. As these decisions will have a considerable impact both economically and sociologically, minimizing the uncertainty presented by clouds in a warming climate may become a priority in environmental agendas.

Bibliography:

1)         Combs, Peter. "Clouds and Climate Change." Focus 46, no. 1 (Spring, 2000): 35-36.

2)         Mlynczak, M. G., et al. "A Detailed Evaluation of the Statospheric Heat Budget and Global Radiation Balance and Diabatic Circulations." Journal of Geophysics Research 104 (1999): 6039-6066.

3)         Soden, Brian J., and Issac M. Held. "An Assessment of Climate Feedbacks in Coupled Ocean-Amospheric Models." Journal of Climate 19 (2006): 3354-3360.

4)         Stephens, G. L. "Cloud Feedbacks in the Climate System: A Critical Review." Journal of Climate 18 (2005): 237-273.

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