The Greenhouse Effect

THE ‘‘GREENHOUSE EFFECT’’ REFERS TO THE PROCESS by which infrared radiation-absorbing gases in Earth’s atmosphere trap heat and thus influence climate. This article gives an overview of the anthropogenic loading of greenhouse gases into the atmosphere and associated effects on recent and future climate change, summarizes feedback effects, and describes potential and current impacts of climate change on biodiversity.
Earth’s climate, from daily weather events to glacial and interglacial cycles, is driven by the amount of radiation received from the sun and how that radiation is distributed throughout the global Earth–atmosphere system. The atmospheric greenhouse effect acts as an important factor in establishing a temperature that is hospitable for life. The basic mechanism is simple and was first detailed by the Swedish physicist Svante August Arrhenius in 1896. Light from the sun largely penetrates the atmosphere and is absorbed at the planetary surface. There, it is converted from energy in the form of light to energy in the form of heat (longwave infrared radiation). As the surface temperature rises because of this heat, Earth radiates more and more heat back out to space, thereby maintaining an energy balance. Several gases in the atmosphere, referred to as ‘‘greenhouse gases,’’ absorb some of the heat emitted from Earth’s surface and reradiate it back toward the surface, causing the temperature to rise. Without this naturally occurring greenhouse effect, Earth’s average surface temperature would be _19_C, about 33_C colder than it is today. The term ‘‘greenhouse effect,’’ though popular, is a misnomer because the warming effect of glass greenhouses is due primarily to suppression of convection, not trapping of infrared radiation.
The most important naturally occurring greenhouse gases are water vapor, carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), and ozone (O3). Although water vapor plays the biggest role in generating the natural greenhouse effect, anthropogenic emission of the other gases, along with artificially produced chlorofluorocarbons (CFCs), are most important in generating an enhanced greenhouse effect. Detailed instrument data show that concentrations of these gases have been increasing since preindustrial times (_1750) (Table I), particularly in recent decades, largely due to human industrial, agricultural, and urbanization activities. As the concentrations of greenhouse gases increase in the atmosphere, they continue to trap and reradiate more and more heat, resulting in rising surface temperature and other climatic changes. Increases in CO2 account for about 65% of the current direct positive radiative forcing due to anthropogenic loading of greenhouse gases (Fig. 1). The atmospheric CO2 concentration has increased 30% since preindustrial times, as a result of increasing emissions from fossil fuel combustion, land conversion, and cement production, and is continuing to increase by 0.4% per year (Fig. 2). If future emissions of CO2 are maintained at 1994 levels, its atmospheric concentration will be close to double the preindustrial level by the end of the twenty-first century. In the absence of strong emissions controls, given increasing global energy and resource consumption, CO2 concentrations may double by 2040 and will continue to increase dramatically Concentrations of other greenhouse gases, particularly methane and nitrous oxide, are also expected to rise, resulting in an earlier doubling of the equivalent CO2 concentration. Greenhouse gases tend to remain in the atmosphere for many years (see Table I) and consequently are well mixed. They continue to affect the climate long after initial emissions and later stabilization of atmospheric concentrations.
Aerosols can alter the climate by changing atmospheric albedo. These fine particles absorb and reflect solar radiation and alter cloud properties. Sulfate aerosols from fossil fuel emissions and smelting tend to have a negative effect on radiative forcing and thus cool the climate. Current estimates of direct radiative forcing are _0.5 W m_2 due to aerosols compared to 2.45 W m_2 due to greenhouse gases. Unlike greenhouse gases, aerosols are very short-lived in the atmosphere and therefore are not well mixed and respond rapidly to changes in emissions.