Rising Atmospheric Greenhouse Gases
Greenhouse gases in the atmosphere trap the heat that radiates off the planet’s surfaces.
Therefore,adecreaseingreenhousegaslevelslowerstheaverageairtemperature. Anincrease
in greenhouse gases raises air temperature. Greenhouse gas levels have varied throughout
Earth history. For example, CO 2 been present in Earth’s atmosphere at concentrations less
than 200 parts per million (ppm) and more than 5,000 ppm. But for 650,000 years or more,
CO 2 has never risen above 300 ppm, during either glacial or interglacial periods. CO 2 levels
are higher during interglacial than glacial periods (Figure17.33).
Figure 17.33: CO 2 is high during interglacial periods, when temperatures are high. CO 2
is low during glacial periods, when temperatures are low. BP means years before present.
Glacial periods are shown in blue and interglacial periods are shown in yellow. Current
carbon dioxide levels are at 387 ppm, the highest level for the last 650,000 years. ( 4 )
Greenhouse gases are added to the atmosphere by natural processes, like volcanic eruptions,
and the decay or burning of organic matter. Greenhouse gases are also removed from the
atmosphere when CO 2 is absorbed by plant tissue. When plants die and are turned into
fossil fuels - coal, oil, natural gas - deep in the Earth, the CO 2 they hold is stored with
them. Storing CO 2 in the ground removes the greenhouse gas from the atmosphere, lowering
Earth’s average temperature.
Human activities are now releasing much of this stored CO 2 into the atmosphere. Although
peoplehavebeenburningwoodandcoaltomeettheirenergyneedsforhundredsofthousands
ofyears, fossil fuelusage hasincreased dramaticallyin the past200 years, sincethe Industrial
Revolution. Fossil fuel use has skyrocketed in the past few decades as population has grown,
and there are more and more cars, homes, and industries to power. Burning rainforests, to
clearlandfor agriculturepreventsthegrowingtreesfromremovingCO 2 fromtheatmosphere