The evolution of radiative forcing bar-charts

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 (CO2, CH4, 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 CO2 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:

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  1. J. Hansen, M. Sato, R. Ruedy, A. Lacis, and V. Oinas, "Global warming in the twenty-first century: An alternative scenario", Proceedings of the National Academy of Sciences, vol. 97, pp. 9875-9880, 2000.