2.9 Synthesis

This section begins by synthesizing the discussion of the RF concept. It presents summaries of the global mean RFs assessed in earlier sections and discusses time evolution and spatial patterns of RF. It also presents a brief synthesis of surface forcing diagnostics. It breaks down the analysis of RF in several ways to aid and advance the understanding of the drivers of climate change.

RFs are calculated in various ways depending on the agent: from changes in emissions and/or changes in concentrations; and from observations and other knowledge of climate change drivers. Current RF depends on present-day concentrations of a forcing agent, which in turn depend on the past history of emissions. Some climate response to these RFs is expected to have already occurred. Additionally, as RF is a comparative measure of equilibrium climate change and the Earth’s climate is not in an equilibrium state, additional climate change in the future is also expected from present-day RFs (see Sections 2.2 and 10.7). As previously stated in Section 2.2, RF alone is not a suitable metric for weighting emissions; for this purpose, the lifetime of the forcing agent also needs to be considered (see Sections 2.9.4 and 2.10).

RFs are considered external to the climate system (see Section 2.2). Aside from the natural RFs (solar, volcanoes), the other RFs are considered to be anthropogenic (i.e., directly attributable to human activities). For the LLGHGs it is assumed that all changes in their concentrations since pre-industrial times are human-induced (either directly through emissions or from land use changes); these concentration changes are used to calculate the RF. Likewise, stratospheric ozone changes are also taken from satellite observations and changes are primarily attributed to Montreal-Protocol controlled gases, although there may also be a climate feedback contribution to these trends (see Section 2.3.4). For the other RFs, anthropogenic emissions and/or human-induced land use changes are used in conjunction with CTMs and/or GCMs to estimate the anthropogenic RF.