Figure 2-1 has been an effective way to portray the relative magnitudes of different radiative forcings, the associated scientific uncertainties, and an assessment of the current level of understanding. It has been used widely in the scientific and policy communities. However, it has some important limitations, including the following:

  • The figure does not provide information about the timescales over which each of the forcings is active. For example, the greenhouse gases in the first bar (CO2, CH4, N2O, and halocarbons) remain in the atmosphere for decades or longer, whereas the various aerosols persist for days to weeks.

  • The figure shows globally averaged forcings and therefore does not provide information about regional variation in forcing or vertical partitioning of forcing.

  • The figure does not provide information about other climate effects of each forcing agent, such as impacts on the hydrological cycle.

  • The figure gives the impression that one can simply sum the bars to determine an overall or net radiative forcing; however, such a calculation does not give a reasonable description of the cumulative effect of all the forcings.

  • The uncertainty ranges are generally estimated from the range of published values and cannot be readily combined to determine a cumulative uncertainty.

  • The figure does not consistently indicate the forcing associated with specific sources (e.g., coal, gas, agricultural practices).

  • The figure omits nonradiative forcings as discussed in this report.

Although it would be unrealistic to expect a single figure to fully portray all of these aspects of radiative forcings, there are clearly opportunities to improve upon Figure 2-1 and to introduce new figures that address these limitations in the next IPCC report.

WELL-MIXED GREENHOUSE GASES

The radiative forcing due to CO2, CH4, N2O, and various halocarbons is due to absorption of infrared (IR) radiation. It is well characterized and well understood. These gases remain in the atmosphere long enough to be well mixed; thus, their abundances are well known and have little spatial variability. Their concentrations have increased substantially since preindustrial times (see Table 2-1), and they are the greatest contributors to total anthropogenic radiative forcing. As shown in Figure 2-1, the IPCC estimate of the radiative forcing due to well-mixed greenhouse gases is +2.43 W m−2 from 1750 to 1998 (present), comprising CO2 (1.46 W m−2),



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