Climate Change

Global Nature of Climate Change and the Greenhouse Effect

The presence of greenhouse gases (particularly water vapor but also carbon dioxide, ozone, and methane) in the atmosphere leads to an Earth temperature significantly warmer than it would otherwise have been without their greenhouse effect. As human activities add more or new greenhouse gases (chlorofluorocarbons, for instance), the Earth's average temperature increases.

The Earth is naturally warmed by solar (shortwave, SW) radiation absorption and cooled by emission of infrared (longwave, LW) radiation. It is the balance between the warming from SW absorption and the cooling from LW emission that governs the Earth's temperature. When this balance is altered, the Earth's temperature adjusts to the new equilibrium. Such perturbation of the Earth's energy balance is called a radiative forcing, and the greenhouse effect is the main perturbation whereby the presence of greenhouse gases in the atmosphere blocks the emission of LW radiation to space, therefore reducing the cooling this escaping radiation normally causes. This process leads to a warming of the Earth.

Of the CO2 emitted by human activities, nearly half is quickly taken up by land and ocean, and the remainder is transported by atmospheric motion around the globe. This CO2 remains in the atmosphere for hundreds of years before being taken up by chemical weathering on land and sediments deposition in the ocean over geological time scales. CO2 can therefore be considered as having a long lifetime, and the location where it is added to the atmosphere has essentially no influence over where the greenhouse effect will occur. The greenhouse effect occurs globally.

Regionalization of the Warming

Surface observations averaged over the entire globe show a general temperature increase (∼0.8°C) during the last century as is expected from the greenhouse effect. A map of surface temperature changes during the last several decades, however, displays some spatial variations in the warming (Figure 1). In particular, the high latitudes of the Northern Hemisphere display a warming two to three times larger than the globally averaged one. These spatial variations do not result from local greenhouse or other radiative forcing but are the consequence of a regional feedback (a self-reinforcing reaction to a forcing that either enhances or reduces the original forcing) process that tends to regionalize the warming. The so-called snow/ice albedo feedback effect is responsible for the large temperature increase in Northern Hemisphere high latitudes. In regions covered by snow or ice, a large part of the radiation from the sun is reflected (the surface is said to have a high albedo), and little radiation penetrates into the surface. As the surface temperature increases from the greenhouse effect, some snow or ice melts, exposing the surface (whether land or ocean) and allowing solar radiation to penetrate the upper layer where the radiation can be absorbed and warm it. This induces an additional melting of the adjacent snow/ice covered areas, giving rise to more solar radiation absorption. This self-enhancing process is called a positive feedback: Under an initial fluctuation (the warming from increased CO2 in this case), a domino effect gets established that expands the original melting. The result is a larger temperature increase in these regions than in surrounding regions not covered by snow or ice.

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