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Natural Variations of Radiative Forcing
We should ask how this
0.55 Wm-2decade-1 compares with natural fluctuations in
radiative forcing (Figure 10).
For instance, the luminosity of the sun fluctuates, which causes variations
in the amount of energy the earth receives from the sun. It would be
useful to compare measurements of this variation with present and projected
future contributions due to enhanced greenhouse effects.
Other natural variations are due to orbital changes of the earth around the sun, changes in the radius of the sun, and effects of volcanoes. Large explosive volcanic eruptions produce copious amounts of SO2 that combine with H2O to form H2SO4 (sulfuric acid), which condenses rapidly in the stratosphere to form fine sulfate particles that remain suspended in the atmosphere for 1-2 years (Figure 14) before being removed by natural processes.
Variability of orbital parameters of the earth's revolution about the sun is reduced by internal feedback mechanisms, but its effect is estimated to be about 0.035 Wm-2decade-1, which is much less than the 0.55 Wm-2decade-1 due to anthropogenic greenhouse gases. Variability of total output of the sun over short periods (e.g., a few years) is about 1.4 Wm-2decade-1 (see Figure 11), but this variability tends to be cyclical over longer periods. Averaged over the last 100 years, variability of total output of the sun has been about 0.1 Wm-2decade-1, again far less than 0.55 Wm-2decade-1. The rate of heat loss due to heat flow from the center of the earth is 0.06 W m-2.
Changes in radius of the sun and changes in sunspot activity also lead to natural variability of about 0.1 Wm-2decade-1. A major volcano releases large amounts of sulfate and other particles that may persist in the stratosphere for 1 to 3 years and lead to global cooling due to reflection of incoming solar radiation. This can change radiative forcing by 0.2 to 0.4 Wm-2decade-1 for the period of time that the dust lingers in the stratosphere. Because such major eruptions occur only once per decade or so, their average effect is far less than 0.55 Wm-2decade-1.
NEXT: Increase in Radiative Forcing due to Humans
