Radiative Forcing & Effective Radiative Forcing due to Anthropogenic Aerosols
Figure 7.19 | (a) GCM studies and studies involving satellite estimates of RFari (red), ERFaci (green) and ERFari+aci (blue in grey-shaded box). Each symbol represents the best estimate per model and paper (see Table 7.3 for references). The values for RFari are obtained from the CMIP5 models. ERFaci and ERFari+aci studies from GCMs on liquid phase stratiform clouds are divided into those published prior to and included in AR4 (labelled AR4, triangles up), studies published after AR4 (labelled AR5, triangles down) and from the CMIP5/ACCMIP models (filled circles). GCM estimates that include adjustments beyond aerosol–cloud interactions in liquid phase stratiform clouds are divided into those including aerosol–cloud interactions in mixed-phase clouds (stars) and those including aerosol–cloud interactions in convective clouds (diamonds). Studies that take satellite data into account are labelled as ‘satellites’. Studies highlighted in black are considered for our expert judgement of ERFari+aci. (b) Whisker boxes from GCM studies and studies involving satellite data of RFari, ERFaci and ERFari+aci. They are grouped into RFari from CMIP5/ACCMIP GCMs (labelled CMIP5 in red), ERFaci from GCMs (labelled AR4, AR5 in green), all estimates of ERFari+aci shown in the upper panel (labelled ‘All’ in blue), ERFari+aci from GCMs highlighted in the upper panel (labelled ‘Highlighted GCMs’ in blue), ERFari+aci from satellites highlighted in the upper panel (labelled ‘Highlighted Satellites’ in blue), and our expert judgement based on estimates of ERFari+aci from these GCM and satellite studies (labelled ‘Expert Judgement’ in blue). Displayed are the averages (cross sign), median values (middle line), 17th and 83th percentiles (likely range shown as box boundaries) and 5th and 95th percentiles (whiskers). References for the individual estimates are provided in Table 7.3. Table 7.4 includes the values of the GCM and satellite studies considered for the expert judgement of ERFari+aci that are highlighted in black.
In AR5, aerosol radiative forcing is assessed as radiative forcing due to aerosol-radiation interaction (RFari) only. This corresponds to the direct aerosol forcing in AR4. Then, deviated from AR4, AR5 combines the aerosol-cloud interaction (RFaci) – namely cloud albedo effect or first indirect aerosol forcing in AR4, with other rapid cloud adjustments to formulate the effective radiative forcing due to aerosol-cloud interactions (ERFaci). A third important aerosol forcing assessment is effective radiative forcing due to aerosol-radiation interaction (ERFari). The effective radiative forcing (ERF) is a new concept formulated by AR5. It is the radiative forcing from a forcing agent after taking into account of rapid adjustments (or response), such as changes in height, thickness, or lifetime of cloud, changes in lapse rate, and aerosol microphysical effects on mixed-phase, ice, or convective clouds.
In AR5, the three above-mentioned forcing (RFari, ERFari, & ERFaci) are evaluated between 1750 and 2010 using AeroCom II models. The reference year 1750 (some studies using 1850) is used as a divider from pre-industrial era. And therefore, the changes in forcing are regarded as anthropogenic aerosol forcing. To be consistent with AR4, ranges in anthropogenic aerosol forcing assessment represent a 5 to 95% uncertainty range.
As we know, radiative forcing uncertainties due to anthropogenic aerosols and its precursors, as well as due to aerosol-cloud interactions contribute to the largest uncertainties in current estimate of the anthropogenic radiative forcing (Figure SPM.5). In the meantime, the anthropogenic radiative forcing is an important index if one wants to evaluate the impact of human activities on climate change. Therefore, parameterization of radiative forcing from anthropogenic aerosols will affect the near-term or long-term climate projections by climate models. This section is closely related with IPCC AR5 WGI Chapter 8 “Anthropogenic and Natural Radiative Forcing”. Purpose for this chapter is to assess the significance of radiative forcing due to anthropogenic aerosols.
1. Aerosol Radiative Forcing
In IPCC assessments, aerosols is treated as
a mixture of seven major components – namely
sulfate, black carbon (BC) from fossil fuel and
biofuel, organic aerosol (OA) from fossil fuel and
biofuel, BC an OA combined from biomass burning
(BB), nitrate, secondary organic aerosol (SOA)
and mineral dust (or sea salt). Figure 7.18 (right)
illustrates the RFari from individual aerosol
components and the total RFari between 1750
and 2010, with hatched whisker boxes showing
median and extreme values with 95% percentile
range, and solid boxes representing the AR5 best
estimates with 90% uncertainty range. Table 8.4
(above) from Chapter 8 summarizes the assess-
ments of individual and total RFari from most
recent four IPCC assessments (2nd SAR to 5th AR5).
In AR5, RFari is estimated to be -0.35 (-0.85
to +0.15) W m^-2; RFari due to black carbon
is assessed to be +0.4 (+0.05 to +0.8) W m^-2.
2. Aerosol Effective Radiative Forcing
As mentioned above, ERF is derived from RF after rapid
adjustments. Figure 7.19 (below) compiles the ERFaci and ERFari+aci assessments from various studies since AR4. Due to large information contained by this figure, refer to the figure caption for further details.
In summary, AR5 best estimate of ERFari+aci (excluding the effect of absorbing aerosol on snow and ice) is -0.9 (-1.9 to -0.1) W m^-2; ERFari to be -0.45 (-0.95 to +0.05) W m^-2.
3. Radiative Forcing due to Anthropogenic Aerosols
Figure 7.17 (right) shows the annual zonal
mean TOA radiative forcing due to
anthropogenic aerosols. In general, the
anthropogenic RFari is negative, expect for
the polar and northern hemisphere high
latitude regions. The minimum occurs
around 30°N.
