Increased anthropogenic activities since the onset of Industrial Revolution have caused a positive total net anthropogenic radiative forcing (Figure 14.6) due primarily to the modification of atmospheric composition. The increase in the amounts of greenhouse gases causes positive radiation forcing (IPCC 2007). Increases in atmospheric carbon dioxide contribute to the radiative forcing in the greatest extent. CO2 is responsible for a radiative forcing of +1.66 ± 0.17 W m–2. Contribution to radiative forcing of other long-lived greenhouse gases, such as methane, nitrous oxide and halocarbons were +0.48 ± 0.05 W m–2, +0.16 ± 0.02 W m–2 and +0.32 ± 0.03 W m–2, respectively, between 1750 and 2005. Radiative forcing of other industrial fluorinated gases, like hydrofluorocarbons (HFCs), perfluorocarbons (PFCs) or sulphur hexafluoride (SF6) are relatively small (+0.017 W m–2) but are increasing rapidly. Estimated radiative forcing caused by tropospheric ozone is +0.35 [+0.25 to +0.65] W m–2. However, changes in stratospheric ozone have caused a small negative radiative forcing (–0.05 ± 0.10 W m–2).
Direct emission of water vapour by human activities has a negligible contribution to radiative forcing. However, increasing methane concentration could increase stratospheric water vapour due to oxidation of CH4, which can cause an estimated positive radiative forcing (+0.07 ± 0.05 W m–2)
The effect of the increasing amount of aerosol particles on the radiative forcing is very complex and not yet fully known. The direct effect of aerosols is the scattering of a part of the incoming solar radiation back into space. This effect causes a negative radiative forcing, which that may partly, and locally even completely, compensates the enhanced greenhouse effect. However, due to their short atmospheric lifetime, the radiative forcing of aerosols is very inhomogeneous in space and in time. On the other hand, some aerosols, such as soot particles, absorb the solar radiation directly, leading to local heating of the atmosphere, or absorb and emit infrared radiation, adding to the enhanced greenhouse effect. Aerosols may also affect the number, density and size of cloud droplets. This may change the amount and optical properties of clouds, and hence their reflection and absorption.
Against the greenhouse gases, increasing amount of anthropogenic aerosols, in total, have led to a net negative radiative forcing, with a greater magnitude in the Northern Hemisphere than in the Southern Hemisphere. A total direct aerosol radiative forcing considering all aerosol types is estimated as –0.5 ± 0.4 W m–2. Anthropogenic aerosols effects on water clouds cause an indirect cloud albedo effect causing a radiative forcing of –0.7 [–0.3 to –1.8] W m–2.
Net radiative forcing is also affected by some other anthropogenic factors, such as persistent linear contrails from global aviation, or human-induced changes in surface albedo (Figure 14.6)
The estimated direct natural radiation forcing due to the changes in solar irradiance between 1750 and 2005 is +0.12 [+0.06 to +0.3] W m–2, which is much smaller than the net anthropogenic contribution to the global average radiative forcing over the industrial period.
Total net anthropogenic radiation forcing has a potential impact on regional and global climate. Based on direct measurements from 1880, the global average surface temperature has increased, especially since about 1950 (Figure 14.7). Observations and model simulations also indicate that positive radiative forcing affect the surface moisture budget. Some forcing agents (particularly aerosols) may have more strongly influenced the hydrological cycle, than other compounds.