Climate

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Climate

Climate is the general, cumulative pattern of regional or global weather patterns. The most apparent aspects of climate are trends in air temperature and humidity, wind, and precipitation. These observable phenomena occur as the atmosphere surrounding the earth continually redistributes, via wind and evaporating and condensing water vapor, the energy that the earth receives from the sun.

Although the climate remains fairly stable on the human time scale of decades or centuries, it fluctuates continuously over thousands or millions of years. A great number of variables simultaneously act and react to create stability or fluctuation in this very complex system. Some of these variables are atmospheric composition, rates of solar energy input, albedo (the earth's reflectivity), and terrestrial geography. Extensive research helps explain and predict the behavior of individual climate variables, but the way these variables control and respond to each other remains poorly understood. Climate behavior is often likened to "chaos," changes and movements so complex that patterns cannot be perceived in them, even though patterns may exist. Nevertheless, studies indicate that human activity may be disturbing larger climate trends, notably by causing global warming. This prospect raises serious concern because rapid anthropogenic climate change could severely stress ecosystems and species around the world.

Solar energy and climate

Solar energy is the driving force in the earth's climate. Incoming radiation from the sun warms the atmosphere and raises air temperatures, warms the earth's surface, and evaporates water, which then becomes humidity, rain, and snow. The earth's surface reflects or re-emits energy back into the atmosphere, further warming the air. Warming air expands and rises, creating convection patterns in the atmosphere that reach over several degrees of latitude. In these convection cells, low pressure zones develop under rising air, and high pressure zones develop where that air returns downward toward the earth's surface. Such differences in atmospheric pressure force air masses to move, from high to low pressure regions. Movement of air masses creates wind on the earth's surface. When these air masses carry evaporated water, they may create precipitation when they move to cooler regions.

The sun's energy comes to the earth in a spectrum of long and short radiation wavelengths. The shortest wavelengths are microwaves and infrared waves. Infrared radiation is felt as heat. A small range of medium wavelength radiation makes up the spectrum of visible light. Longer wavelengths include ultraviolet (UV) radiation and radio waves. These longer wavelengths cannot be sensed, but UV radiation can cause damage as organic tissues (such as skin) absorb them. The difference in wavelengths is important because long and short wavelengths react differently when they encounter the earth and its atmosphere.

Solar energy approaching the earth encounters filters , reflectors, and absorbers in the form of atmospheric gases, clouds, and the earth's surface. Atmospheric gases filter incoming energy, selectively blocking some wavelengths and allowing other wavelengths to pass through. Blocked wavelengths are either absorbed and heat the air or scattered and reflected back into space. Clouds, composed of atmospheric water vapor, likewise reflect or absorb energy but allow some wavelengths to pass through. Some energy reaching the earth's surface is reflected; a great deal is absorbed in heating the ground, evaporating water, and conducting photosynthesis . Most energy that the earth absorbs is re-emitted in the form of short, infrared wavelengths, which are sensed as heat. Some of this heat energy circulates in the atmosphere for a time, but eventually it all escapes. If this heat did not escape, the earth would overheat and become uninhabitable.

Variables in the climate system

Climate responds to conditions of the earth's energy filters, reflectors, and absorbers. As long as the atmosphere's filtering effect remains constant, the earth's reflective and absorptive capacities do not change, and the amount of incoming energy does not vary, climate conditions should stay constant. Most of the time, though, some or all of these elements fluctuate. The earth's reflectivity changes as the shapes, surface features, and locations of continents change. The atmosphere's composition changes from time to time, so that different wavelengths are reflected or pass through. The amount of energy the earth receives also shifts over time.

During the course of a decade the rate of solar energy input varies by a few watts per square meter. Changes in energy input can be much greater over several millennia. Energy intensity also varies with the shape of the earth's orbit around the sun. In a period of 100 million years the earth's elliptical orbit becomes longer and narrower, bringing the earth closer to the sun at certain times of year, then rounder again, putting the earth at a more uniform distance from the sun. When the earth receives relatively intense energy, heating and evaporation increase. Extreme heating can set up exaggerated convection currents in the atmosphere, with extreme low pressure areas receiving intensified rains and high pressure areas experiencing extreme drought .

The earth's albedo depends upon surface conditions. Extensive dark forests absorb a great deal of energy in heating, evaporation of water, and photosynthesis. Light, colored surfaces, such as desert or snow, tend to absorb less energy and reflect more. If highly reflective continents are large or are located near the equator, where energy input is great, then they could reflect a great deal of energy back into the atmosphere and contribute to atmospheric heating. However, if those continents are heavily vegetated, their reflective capacity might be lowered.

Other features of terrestrial geography that can influence climate conditions are mountains and glaciers. Both rise and fall over time and can be high enough to interrupt wind and precipitation patterns. For instance, the growth of the Rocky Mountains probably disturbed the path of upper atmospheric winds known as the jet stream. In southern Asia, the Himalayas block humid air masses flowing from the south. Intense precipitation results on the windward side of these mountains, while the downwind side remains one of the driest areas on Erth.

Atmospheric composition is a climate variable that began to receive increased attention during the 1980s. Each type of gas molecule in the atmosphere absorbs a particular range of energy wavelengths. As the mix of gases changes, the range of wavelengths passing through the filter shifts. For instance, the gas ozone (O₃) selectively blocks long wave UV radiation. A drop in upper atmospheric ozone levels discovered in the late 1980s has caused alarm because harmful UV rays are no longer being intercepted as effectively before they reach the earth's surface. Water vapor and solid particulates (dust) in the upper atmosphere also block incoming energy. Atmospheric dust associated with ancient meteor impacts is widely thought responsible for climatic cooling that may have killed the earth's dinosaurs 65 million years ago. Climate cooling could occur today if bombs from a nuclear war threw high levels of dust into the atmosphere. With enough radiation blockage, global temperatures could fall by several degrees, a scenario known as nuclear winter .

A human impact on climate that is more likely than nuclear winter is global warming caused by increased levels of carbon dioxide (CO₂) in the upper atmosphere. Most solar energy enters the atmospheric system as long wavelengths and is reflected back into space in the form of short wavelength (heat) energy. Carbon dioxide blocks these short, warm wavelengths as they leave the earth's surface. Unable to escape, this heat energy remains in the atmosphere and keeps the earth warm enough for life to continue. However, many studies suggest that the burning of fossil fuels and biomass have raised atmospheric carbon dioxide levels. Rising CO₂ levels could trap excessive amounts of heat and raise global air temperatures to dangerous levels. This scenario is popularly known as the greenhouse effect . Extreme amounts of trapped heat could disturb precipitation patterns. Ecosystems could overheat, killing plant and animal species. Polar ice caps could melt, raising global ocean levels and threatening human settlements.

Increased anthropogenic production of other gases such as methane (CH₄) also contributes to atmospheric warming, but carbon dioxide has been a focus of concern because it is emitted in much greater volume.

No one knows how seriously human activity may be affecting the large and turbulent patterns of climate. Sometimes a very subtle event can have magnified repercussions in larger wind, precipitation, and pressure systems, disturbing major climate patterns for decades. In many cases the climate appears to have a self-stabilizing capacity—an ability to initiate internal reactions to a destabilizing event that return it to equilibrium. For example, extreme greenhouse heating should cause increased evaporation of water. Resulting clouds could block incoming sunlight, producing an overall cooling effect to counteract heating.

Furthermore, human influences work on climate within a context of continually changing natural conditions and events. On a geologic time scale, temperatures, precipitation, and ocean levels have fluctuated enormously. A long series of ice ages and warmer interglacial periods began 2.5 million years ago and may still be going on. The last glacial maximum, with low sea levels because of extreme ice volumes, ended only 18,000 years ago—an instant in the earth's climate history.

Natural fluctuations occur on a more human time scale, as well. A summer of extreme drought and high temperatures in the United States in 1988 brought threats of global warming to the public's attention, but the drought itself resulted from a temporary aberration in high altitude wind patterns that centered an unusually stable high pressure zone over the Midwest. This temporary departure from normal conditions was simply part of the continual fluctuation within the chaotic climate system. Terrestrial events, such as the 1991 eruption of Mount Pinatubo in the Philippines, also cause large, natural disturbances in climate. Dust from its eruption reached the upper atmosphere and was distributed around the globe, blocking enough incoming solar radiation to temporarily cool global temperatures by about 1.8°F (1°C).

No one can yet predict with precision how climate variables will respond to human activity. The earth's climate is so complex that human alterations to the atmosphere (such as those caused by carbon dioxide emission ) amount to an "experiment" having an unknown—and possibly life-threat ening—outcome.

[Mary Ann Cunningham Ph.D. ]