As part of the IPCC WG1 SPM (pdf) released last Friday, there was a subtle, but important, change in one of the key figures – the radiative forcing bar-chart (Fig. SPM.4). The concept for this figure has been a mainstay of summaries of climate change science for decades, and the evolution over time is a good example of how thinking and understanding has progressed over the years while the big picture has not shifted much.



The Radiative-Forcing bar chart: AR5 version





The earliest version of a bar-chart that shows radiative forcing is this chart from one of Jim Hansen’s papers (Hansen et al, 1981):

In it, they demonstrate the relative importance – cooling or warming – of a number of relevant changes in radiatively important components (CO 2 , CH 4 , the sun, aerosols etc.). While the y-axis is the no-feedback surface temperature response, and the changes aren’t with reference to the pre-industrial, this might qualify as the ‘ur’-figure – the one from which all the others below are derived. (Note, if you know of an earlier version, please let me know and I’ll update the post accordingly).

I can’t find any examples for a decade or so, and in the First Assessment Report (FAR) (1990) there wasn’t such a figure either, even in the main text. (Again, please let me know if I’ve missed one). However, in the early 1990s, the figure appears in a form much closer to what we’ve come to expect. For instance, in Hansen et al (1993), the forcings in 1990 with respect to 1850 are given:

The transition to W/m2 as the unit has now been made, different greenhouse gases are separated, and an acknowledgement of more complicated issues associated with ozone and stratospheric water vapor is included. The main conclusion is that CO 2 had been historically the most important forcing (around 1.24 W/m2). Shortly thereafter, the 1995 IPCC Second Assessment Report (pdf) added a couple of innovations:

Namely, an assessment of confidence, and the addition of aerosol forcings, while lumping the well-mixed gases all together. There is also the addition of the non-anthropogenic solar term. The figure was updated in 1998 and 2000 by Hansen and colleagues:

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 90º 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 CH 4 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 NO x – 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.

Summary

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 CO 2 . In 1993 it was 1.24 W/m2, in 2001, 1.4 W/m2, to today’s 1.7 W/m2.

The treatment of aerosols – and particularly the difference between absorbing (i.e. black carbon) and scattering (sulphates, nitrates) – has varied a lot. This is partly because of new information (on sources, concentrations, effects), but also because the aerosol issue has been reframed many times. The situation of black carbon is the most complicated. BC on it’s own is strongly warming, and it’s additional indirect effects on snow albedo amplify that. However, BC is almost always emitted in combination with organic carbonaceous aerosols (and/or secondary organic aerosol precursors), and so with respect to the emission-producing activity, the net effect on temperature is partially compensated (see the TAR version for instance). BC is chiefly associated with incomplete combustion of fossil fuel, or alternatively with biomass burning (through deforestation, land clearance or naturally occurring forest fires), and these two classes of sources have sometimes been grouped (2007), and sometimes separated (2001). The AR5 version groups all the aerosol factors into one bar with each of the separate constituents delineated. A further breakdown of this into contributions by activity would be useful, but as I understand it, this was considered not within the scope of WG1.

One final example is also worth noting. In all of the pre-AR5 figures (except Hansen in 2000), tropospheric and stratospheric ozone were considered separately. But while there are two separate effects going on (ozone precursors increasing in the lower atmosphere, and ozone depletion due to CFCs above), there is not a clean separation between changes in the troposphere and stratosphere. Thus the AR5 version correctly shows the ozone changes as indirect effects of the different emissions without delineating where the changes in ozone are occurring. This is a definite conceptual improvement among many.

References