Insolation is a word derived from INcoming SOLar radiATION. It is a measure of the amount of the sun's energy that is reaching a given area of Earth's atmosphere or surface in a given period of time.
Insolation is an important influence in determining the type and extent of vegetation that can grow, which in turn is important in determining the type of environment that develops. For example, an equatorial region of high insolation and low rainfall is more apt to become a vegetation-sparse desert than a forested more northern region of lower insolation and more extensive rainfall.
Although insolation variations have occurred naturally through recorded time, the changing composition of Earth's atmosphere that has resulted from human activities is affecting insolation. Overall, more of the sun's energy is being retained. Put another way, the fraction of the insolation that is being reflected back to space has decreased, which is resulting in the warming of the atmosphere.
Historical Background and Scientific Foundations
Because Earth is a sphere and its axis is tilted with respect to the sun, the intensity of the sun's rays that contact the atmosphere—the amount of solar radiation per given area of the atmosphere—differs with different locations over the surface of Earth. For example, when the northern polar region is tilted farther away from the sun than the southern polar region during Earth's rotation about the sun, the northern region will receive sunlight over a greater area than the southern region. As a result, the insolation of the northern region will be less than the southern region. In more tropical regions, where the sun tends to be higher overhead than at the poles, the sunlight per given area is more intense, producing higher insolation.
Changes in insolation can markedly alter climate. For example, the ice ages that have occurred through time have been due to variations in insolation over Earth's surface. Specifically, the amount of sunlight reaching northern regions was less, which reduced temperatures by several degrees, which was sufficient to produce the growth and southward movement of glaciers. This is still relevant today, serving as an example of how slight atmospheric temperature changes can result in drastic changes in climate.
The climate variations that produce ice ages, and which have occurred naturally in cycles of approximately 100,000 years, were caused by insolation variations that are, in turn, a consequence of the altered movement of Earth in its orbit about the sun. The planet's rotation about its axis changes over time, as does the tilt of the axis relative to the sun. Both these changes affect the amount of sunlight that reaches the surface.
These cycles of orbital change and related insolation variation are known as Milankovich cycles, after the Serbian engineer and mathematician Milutin Milankovitch (1879–1958), who proposed the link between planet movements and climate.
Insolation is also indirectly influenced by human activities. Although the amount of sunlight reaching the atmosphere has not changed over the recent three centuries, the quantities of sunlight-absorbing compounds in the atmosphere have increased. According to the 2007 Assessment Report of the Intergovernmental Panel on Climate Change (IPCC), the increases in carbon dioxide, methane, and nitrous oxide have been largely due to human industrial and agricultural activities. This is supported by analyses done on ice cores, which demonstrate higher levels of these compounds coinciding with the expansion of industrial activities.
Impacts and Issues
The changing insolation pattern of Earth's atmosphere that began in the mid-eighteenth century has been a driving force in the altered climate of the planet. The accelerated heating of equatorial regions has changed the global air circulation pattern. One result is the increased warming of the northern polar region.
The temperature increase is spurring increased melting of polar ice. According to the IPCC, the average global sea level has risen by .12 in (3.1 mm) per year since 1993, with the melting of polar ice being a contributor. The rising ocean could alter the coastlines of maritime regions within the coming century. Moreover, the addition of large volumes of melted freshwater to northern regions of the Atlantic is already producing changes in the circulation pattern of ocean currents such as the Gulf Stream. Although the changes thus far have been temporary, more long-term change in ocean currents— which are the main determinant of global climate— have been suggested in studies that have modeled the effect of continued polar ice melting on the oceans.
WORDS TO KNOW
FOOD INSECURITY: Unreliable access by a person or group to adequate food. Persons who do not have adequate food and who are without prospects to find adequate nutritional sources.
ICE AGE: Period of glacial advance.
MILANKOVITCH CYCLES: Regularly repeating variations in Earth's climate caused by shifts in its orbit around the sun and its orientation (i.e., tilt) with respect to the sun. Named after Serbian scientist Milutin Milankovitch (1879–1958), though he was not the first to propose such cycles.
During the twentieth century and afterward, changes in global weather have been documented. These changes, which include more frequent and severe tropical storms and increased occurrence of droughts in certain regions, are related to insolation. Drought has been particularly harsh for equatorial regions of undeveloped nations such as Africa, resulting in agricultural failures and food insecurity.
DiMento, Joseph F.C., and Pamela M. Doughman. Climate Change: What It Means for Us, Our Children, and Our Grandchildren. Boston: MIT Press, 2007.
Gore, Al. An Inconvenient Truth: The Planetary Emergency of Global Warming and What We Can Do About It. New York: Rodale Books, 2006.
Hillman, Mayer, Tina Fawcett, and Sudhir Chella Rajan. The Suicidal Planet: How to Prevent Global Climate Catastrophe. New York: Thomas Dunne Books, 2007.
Lovelock, James. The Revenge of Gaia: Earth's Climate Crisis and the Fate of Humanity. New York: Perseus Books, 2007.