4.1.2.1 Greenhouse gases

The main radiative forcings that have affected the Earth's climate can be grouped into different categories. This has classically been done to estimate both the anthropogenic and natural forcings compared to preindustrial conditions corresponding typically to 1750 (Fig. 4.2, see also section 5.5.3). Over the last 250 years, the changes in greenhouse gas concentrations have played a dominant role (note that this also seems to be valid in a more remote past, see section 5.3). The largest contribution comes from the modification of the atmospheric CO₂ concentration, with a radiative forcing of about 1.7 Wm^-2 between 1750 and 2005. However concentrations of CH₄, N₂O and the halocarbons also have to be taken into account.

Estimating the radiative forcing $\Delta$Q associated with the changes in the concentrations of these gases requires a comprehensive radiative transfer model. However, relatively good approximations can be obtained for CO₂ from a simple formula:

where [CO₂] and [CO₂]_r are the CO₂ concentrations in ppm for the period being investigated and for a reference period, respectively.

Similar approximations can be made for CH₄ and N₂O:

where the same notation is used as in Eq. 4.1, the concentrations being in ppb.

For halocarbons, a linear expression appears valid. When evaluating the radiative forcing since 1750, reference values for this period are classically: CO₂ (278ppm), CH₄ (715 ppb), N₂O (270 ppb) (see in Forster et al 2007).

CO₂, CH₄, N₂O and halocarbons are long-lived gases that remain in the atmosphere for decades if not centuries. Their geographical distribution is thus quite homogenous, with only small differences between the two hemispheres. Other greenhouse gases such as O₃ (ozone) have a shorter life. As a consequence, their concentration, and thus the associated radiative forcing, tend to be higher close to areas where there are produced, and lower near areas where they are destroyed. Tropospheric ozone is mainly formed through photochemical reactions driven by the emission of various nitrous oxides, carbon monoxide and some volatile organic compounds. Globally, the impact of the increase in its concentration is estimated to induce a radiative forcing around 0.35 Wm^-2. However, the forcing is higher close to industrial regions, where the gases leading to ozone production are released. By contrast, stratospheric ozone has decreased since pre-industrial time, leading to a globally average radiative forcing of -0.05 Wm^-2. The stratospheric ozone changes are particularly large in polar regions, as the reactions responsible for the destruction of ozone in the presence of some chemical compounds (such as chlorofluorocarbons) are more efficient at low temperatures. The largest decrease is observed over the high latitudes of the Southern Hemisphere. There, the famous ozone hole, discovered in the mid 1980's, is a large region of the stratosphere where about half the ozone disappears in spring. Because of the Montreal Protocol which bans the use of chlorofluorocarbons, the concentration of these gases in the atmosphere is no longer increasing, and may even be decreasing slowly. However ozone recovery in the stratosphere is not yet clear.

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