The evolution of radiative forcing bar-charts

These updates added land use/land cover changes to albedo, decadal trends in volcanoes, and (in 2000) made the subtle point that the greenhouse effect from CFCs was offset a little by the impact CFCs were having on the ozone layer. An analogous diagram was very prominent in the 2001 IPCC Third Assessment report (TAR):

As with the SAR version, the confidence levels are present, there has been a switch from 1850 as a baseline in the SAR version, to 1750 in order to capture the beginning of the industrial rise in the GHGs, and again additional items were included: some aerosol related (sulphates, mineral dust, biomass burning, carbonaceous aerosols (incl. black carbon)), and two associated with aviation (via contrails and enhanced cirrus cloud formation). Concurrently, the Hansen et al (2001) version:

included even more details – the effect of black carbon on snow, nitrate aerosols, and an enhancement of the solar effect via ozone changes.

In the 2007 AR4 SPM, the main innovation was to rotate the axes by 90ยบ and to add a bit more colour:

Though stratospheric water vapour makes a comeback, and the indirect effect of black carbon on snow makes an entrance for IPCC. In the AR5 SPM though, something more interesting happened…

The effects are now grouped by emissions, rather than by concentrations. This too has it’s antecedents, Fig 2.21 in the AR4 full report did the same thing, but was little noticed. In turn, that figure was drawn from work by Shindell et al (2009). This allows many of the indirect effects to be seen clearly. A particular point of interest is that the forcing by emission for CH4 is twice as large than its forcing by concentration, because of the important indirect effects on ozone and aerosols. The inclusion of CO, VOCs and NOx – normally considered as air quality issues – which affect climate via their indirect effects on ozone etc, is a salient reminder that the two issues are very much connected.


The most obvious change over time is that the visual styling of the graphs has improved over time. The latest version is far more comprehensive – including more effects, more connections, more error bars – and is, arguably, more useful. This follows from the fact that it is emissions that can be potentially moderated, and the latest iteration shows explicitly what the key emissions are (as opposed to what their consequences are after atmospheric chemistry has done it’s thing).

A key change over time is of course the increasing forcing from CO2. In 1993 it was 1.24 W/m2, in 2001, 1.4 W/m2, to today’s 1.7 W/m2.

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  1. D.T. Shindell, G. Faluvegi, D.M. Koch, G.A. Schmidt, N. Unger, and S.E. Bauer, "Improved Attribution of Climate Forcing to Emissions", Science, vol. 326, pp. 716-718, 2009.