A paper published in Nature Geoscience last month received a lot of media attention. And rightly so. It showed that the Black Carbon (BC) component of soot is responsible for up to 60% as much warming as CO2. That is significant for many reasons, only some of which were covered in the newspapers.

The Guardian’s account is fairly typical:

Scientists warn of soot effect on climate

· Coal and wood ‘more damaging than thought’

· Black carbon harms environment and health

Most reports also mentioned that BC-induced warming is more amenable to mitigation than that caused by CO2. This is because BC persists in the atmosphere for periods of days rather than the decades that CO2 does, so reductions in BC output will take more immediate effect, and because BC and the so-called white aerosols such as sulphates, which have a cooling effect, have only partially overlapping sources, providing the potential to decouple white and black aerosol production. So far, so interesting. But what didn’t get mentioned is even more so.

First, there are the implications of the research for the climate models. It hardly needs pointing out that the identification of a factor that causes 60% as much warming as CO2 is going to require something of a re-adjustment of the models. The graph that usually gets wheeled out on such occasions is this one, which shows how the models juggle what are thought to be the five major forcing factors to come up with a line that kind of agrees with observed temperature variation over the last century:

Black carbon doesn’t even feature. In its latest round of reports, the IPCC assigns BC a warming effect of 0.2-0.4 Wm^-2 (a consensus figure based on 20-30 modelling studies), in contrast to the Nature Geoscience paper’s estimate of 0.9 Wm^-2 (the result of a review of the models combined with new empirical data from satellites, as well as aerial and terrestrial measurements of “brown clouds” over the Indian Ocean and Arabian Sea).

More generally, the findings reveal how little is understood about the role of aerosols (regarded as having a net cooling effect) on climate dynamics. Which is especially interesting because aerosols are absolutely central to the standard way of explaining away a thorny problem for global warmers – the period of cooling (~1944-1974), which occurred in defiance of rising CO2 concentrations (see graph above). The argument goes that the temperature slump is the result of white aerosols – released from coal and oil burning – masking the warming effect of anthropogenic greenhouse gases, until various clean air acts in the US and Europe allowed the anthropogenic warming signal to re-emerge.

Indeed, this is one of those items of ‘settled science’ flagged up in an open letter to Martin Durkin’s Wag TV, makers of the infamous The Great Global Warming Swindle, organised by Bob Ward, former Senior Manager for Policy Communication at the Royal Society and now Director of Global Science Networks at risk analysis firm RMS and signed by 37 scientists. The letter demanded that Wag TV correct “five major misrepresentations of the scientific evidence” before distributing the DVD version of the program. One of those major misrepresentations concerned the post-war temperature slump:

However, the DVD version of the programme does not make any mention of the impact of atmospheric aerosols on the record of global average temperature. The producer of the programme, Martin Durkin has attempted to justify this by suggesting that if aerosols caused the cooling between 1945 and 1975, then global average temperatures should be lower today, because he believes that atmospheric concentrations of aerosols should be even higher today than they were during that period. But the IPCC Fourth Assessment Report pointed out that “[g]lobal sulphur emissions (and thus sulphate aerosol forcing) appear to have decreased after 1980”.

However, according to the authors of the Nature Geoscience paper, it is nothing like so clear cut. First up, University of Iowa atmospheric chemist Greg Carmichael:

Climate Resistance: Are we now not so certain that the post-war cooling is due to aerosols? 

Greg Carmichael: This is an added complication. But it’s also an added level of understanding. And as we get better measurements of the present, and better models that can drive these simulations for the last 50 years, or so, we’ll see that we’ve improved our understanding and that the aerosol effect is as important as we’ve indicated.

CR: But we don’t actually know that yet?

GC: We still have a way to go before understand how the heating-cooling push-pull really plays out.

UC San Diego atmospheric physicist Veerabhadran Ramanathan is more candid:

Climate Resistance: What are the implications of this work for the idea that the post-war temperature decline is the result of sulphate aerosols masking the warming effect of CO2 emissions? 

Veerabhadran Ramanathan: After the 1970s, when the West was cleaning up pollution, there was a rise in temperatures. We stopped burning coal in cities etc, and coal puts out a lot of sulphates, and sulphates mask global warming. At the same time, in the tropics, China and India, they were growing fast and putting a lot more Black Carbon.

CR: So the sulphate component must have been reduced more than the Black Carbon component for the aerosol masking theory to hold? We now need empirical data to compare the effect of black and white aerosols during the post-war temperature slump?

VR: Exactly.

CR: Do we have that empirical data?

VR: No. The data we have is for 2002-2003. We don’t know what happened in the ’50s, ’60s and ’70s. The implication of this study is that we have to understand what is the relative change in the sulphur emissions versus the Black Carbon emissions – and we don’t know that.

CR: So what is the empirical evidence that, 50 years ago, white aerosols were masking GW due to CO2?

VR: It’s pretty flimsy. The main information we have […] is our understanding of the SO2 emissions by coal combustion, and oil. But we need to know not so much how much SO2 we put out, but how much was converted to sulphates, how much was removed [etc]

CR: So you don’t even know the life cycle of the SO2 and sulphates?

VR: No. All the information we have is from models… It could still be true [that white aerosols account for the post-war temperature slump]

CR: But it could not be true?

VR: Yes. The picture is complicated. But this paper is not saying it is wrong […]

CR: So we now have a better idea of what is happening aerosol-wise in the present, but what was going on in the 1950s/’60s is still elusive?

VR: Yes, There’s a lot of research needs to be done on that – what happened in the ’50s and ’60s, and then why the rapid ramp up [from the ’70s]. I’m not saying our current understanding is wrong, just that it is a more complicated picture. I would say it’s uncertain.

All of which tells a rather different story about the state of knowledge than Bob Ward’s letter would have us believe. It continues:

Just like Bob Ward failing to mention that the empirical evidence that aerosols account for the period of slight cooling between 1945 is “pretty flimsy”, in fact – which is perhaps why Durkin didn’t mention it. And just as Ward slights Durkin for bolstering his case by omitting ‘inconvenient’ facts, there is little difference between what he accuses Durkin of, and the way he and his fellow accusers carried on.