FIGURE 1-1 The climate system, consisting of the atmosphere, oceans, land, and cryosphere. Important state variables for each sphere of the climate system are listed in the boxes. For the purposes of this report, the Sun, volcanic emissions, and human-caused emissions of greenhouse gases and changes to the land surface are considered external to the climate system.
There is an emphasis that CO2 is a particularly serious climate forcing, in contrast to aerosols, in that its residence time in the atmosphere is so long. While there are some who disagree with such a long length, the observation that show it is well-mixed in the atmosphere supports the conclusion of a long residence time.
The other climate forcings are assumed to be quickly purged. This is not a correct, however.
The claim that CO2 is unique in its long residence time is due to the atmospheric centric view of the climate system. The climate system, however, also includes the oceans, the cryosphere and the land. Other human climate forcings with at least as long (or longer residence times) include , for example, nitrogen deposition, soot (black carbon deposition), dust, and land use/land cover change. Information on these four examples is presented below.
The take home message is that there are a number of human climate forcings, in addition to CO2, that have a long term effect on the climate system.