What is the climate system? The climate system includes all of the components of the Earth system, and all of the interactions among those components, that determine how climate varies in space and time. The main components of the climate system are the atmosphere, hydrosphere, cryosphere, lithosphere, and biosphere. Each component plays a role in the climate system that depends on its ability to store and transport mass and energy.
What is the greenhouse effect? When Earth’s surface is warmed by the Sun, it radiates heat back into the atmosphere. Carbon dioxide and other greenhouse gases absorb some of this infrared radiation and reradiate it in all directions, including downward to Earth’s surface. This radiation maintains the atmosphere at a warmer temperature than it would be if there were no greenhouse gases, similar to the warmer air temperature maintained in a greenhouse.
How has Earth’s climate changed over time? Natural variations in climate occur on a wide range of scales in both time and space. Some variations result from factors outside the climate system, such as solar forcing and changes in the distribution of land and sea surfaces caused by continental drift. Others result from variations within the climate system itself. Short-term regional climate variations include the El Niño–Southern Oscillation. Long-term global climate variations are exemplified by the Pleistocene glacial cycles, during which average surface temperatures changed by as much as 6°C to 8°C.
What are ice ages, and what causes them? Studies of the geologic ages of glacial deposits on land and in marine sediments show that continental ice sheets advanced and retreated many times during the Pliocene and Pleistocene epochs. Each ice age involved a massive transfer of water from the hydrosphere to the cryosphere, resulting in expansion of glaciers and a lowering of sea level. The favored explanation is that these glacial cycles are being driven by Milankovitch cycles, small periodic variations in Earth’s movement through the solar system that alter the amount of solar radiation received at Earth’s surface. These variations have been amplified by positive feedbacks involving atmospheric concentrations of greenhouse gases. The global cooling that initiated the Pleistocene glacial cycles may have resulted from continental movements that changed oceanic circulation patterns.
What are geochemical cycles? Geochemical cycles are fluxes of chemicals from one component of the Earth system to another. The atmosphere, hydrosphere, cryosphere, lithosphere, and biosphere act as geochemical reservoirs and are linked by processes that transport chemicals among them. If a reservoir is at a steady state, inflow balances outflow, and the residence time of the chemical can be calculated as the total amount of the chemical in the reservoir divided by the inflow.
What is the carbon cycle? The carbon cycle is the flux of carbon among its four principal reservoirs: the atmosphere, lithosphere, oceans, and terrestrial biosphere. Major fluxes of carbon between these reservoirs include gas exchange between the atmosphere and the ocean surface; the movement of carbon dioxide between the biosphere and the atmosphere through photosynthesis, respiration, and direct oxidation; the transport of dissolved organic carbon in surface waters to the ocean; and the weathering and precipitation of calcium carbonate.
What are the effects of anthropogenic carbon emissions? Human emissions of carbon are enhancing the greenhouse effect by increasing the concentration of carbon dioxide in the atmosphere. Some of this carbon dioxide dissolves in the oceans, where it combines with water to form carbonic acid. The resulting ocean acidification acts to increase the concentration of bicarbonate ions at the expense of carbonate ions, making it harder for marine organisms to precipitate shells and skeletons of calcium carbonate.
Was the twentieth-century warming caused by human activities? The observed increase of about 0.6°C in Earth’s average annual surface temperature during the twentieth century is correlated with a significant rise in atmospheric concentrations of CO2 and other greenhouse gases. The changing isotope ratios of atmospheric carbon show that much of it is being produced by fossil-fuel burning. Most experts on Earth’s climate are now convinced that the twentieth-century warming was human-induced and that the warming will continue into the twenty-first century as atmospheric concentrations of greenhouse gases continue to rise.
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