Description:
The radiative forcing from the increase in anthropogenic greenhouse gases since the pre-industrial era is positive (warming) with a small uncertainty range; that from the direct effects of aerosols is negative (cooling) and smaller; whereas the negative forcing from the indirect effects of aerosols (on clouds and the hydrologic cycle) might be large but is not well quantified. Key anthropogenic and natural factors causing a change in radiative forcing from year 1750 to year 2000 are shown in this figure, where wide, colored bars mark the factors whose radiative forcing can be quantified. Only some of the aerosol effects are estimated here and denoted as ranges. Other factors besides atmospheric constituents -- solar irradiance and land-use change -- are also shown. Stratospheric aerosols from large volcanic eruptions have led to important, but short-lived, negative forcings (particularly during the periods 1880-1920 and 1960-1994), which are not important over the time scale since the pre-industrial era and not shown. The sum of quantified factors in the figure is positive, but this does not include the potentially large, negative forcing from aerosol indirect effects.