Statistically derived human influences

I came across a paper today that I thought I might highlight here. It’s in Nature Geosciences, the authors are Francisco Estrada, Pierre Perron & Benjamin Martinez-Lopez, and the paper is called Statistically derived contributions of diverse human influences to twentieth-century temperature changes. The goal of the paper seems to be to try and actually attribute the observed warming to human activities. The abstract ends with

Our statistical analysis suggests that the reduction in the emissions of ozone-depleting substances under the Montreal Protocol, as well as a reduction in methane emissions, contributed to the lower rate of warming since the 1990s. Furthermore, we identify a contribution from the two world wars and the Great Depression to the documented cooling in the mid-twentieth century, through lower carbon dioxide emissions. We conclude that reductions in greenhouse gas emissions are effective in slowing the rate of warming in the short term.

I haven’t read the paper in great detail, so was hoping that some regular commenters might have some thoughts. The main figure seems to be the one below which shows the surface temperature anomalies from NASA (left-hand panel) and HADCRUT4 (right-hand panel) with various fits to the temperature records based on different forcing approximations (RFGHG – greenhouse gases only; TRF* – all anthropogenic forcings; TRF – TRF* plus solar forcings).

What they claim is that there are various breaks in the temperature record (HADCRUT4 : Global and NH – 1956 & 1966; SH – 1909 & 1976. NASA : Global – 1956; NH – 1968; SH – 1923 & 1955). The paper then seems to go on to consider the evolution of various anthropogenic forcings, shown below. The various different anthropogenic emissions are CFCs, methane, carbon dioxide and anthropogenic aerosols. They then find various breaks in the anthropogenic forcings that seem similar to, but not quite the same as, some of the breaks in the surface temperature dataset. So, as far as I can tell, the claim is that these breaks in the anthropogenic forcings are statistically consistent with being associated with the breaks in the surface temperature dataset. It does talk about other factors, such as the Atlantic Meridional Oscillation, so I don’t think the paper is claiming that changes in the anthropogenic forcings, alone, were responsible for changes in the surface temperature dataset.

The paper also discusses the recent slowdown and says

The causes of a slow-down in warming since the mid-1990s have been a subject of interest. Some proposed the joint effect of increased short-lived sulphur emissions, La Nina events and the eleven-year solar cycle as offsetting the effect of rising greenhouse gas concentrations. We show that the effects of the Montreal Protocol and of changes in agricultural practices in Asia have been large enough to change the long-run path of radiative forcing. Tropospheric aerosols contributed to making this slowdown more pronounced.

So, one of the conclusions of the paper seems to be that this shows that our policies can have an measurable impact on warming, even on relatively short timescales. Additionally, the final comment of the paper is

Paradoxically the recent decrease in warming, presented by global warming sceptics as proof that humankind cannot affect the climate system, is shown to have a direct human origin.

So, I can’t really tell if this is an interesting and valuable piece of work, a bit of statistical trickery that’s trying to identify anthropogenic influences in what is clearly a quite complicated dataset, a combination of the two, or something else entirely. I’m not even sure if I’ve summarised it particularly well, so any comments from those who know more than me would be most welcome. Given that it seems to be suggesting an important role for anthropogenic aerosols in producing the recent slowdown, I thought (hoped :-)) Karsten might have some thoughts.