Who'd've Discredited It?

‘Case against climate change discredited by study’ shrieked the Independent yesterday. That must be one hell of a study. Except that it isn’t:

A difference in the way British and American ships measured the temperature of the ocean during the 1940s may explain why the world appeared to undergo a period of sudden cooling immediately after the Second World War.

Scientists believe they can now explain an anomaly in the global temperature record for the twentieth century, which has been used by climate change sceptics to undermine the link between rising temperatures and increases in atmospheric carbon dioxide.

Not only does the study (published this week in Nature) not claim to discredit what the Independent‘s headline claims it discredits, but it doesn’t even discredit what the scientists behind the study claim it discredits. Moreover, what the scientists claim their work does discredit was, according to prominent Environmentalists, discredited years ago. And finally, what everybody seems to be trying to discredit isn’t even something that sceptics seem to be crediting in the first place.

Yes, sceptics are concerned about the post-war temperature slump, but not because of the sudden steep drop around 1945; it is the downward trend in temperatures between about 1945 and 1975 that they suggest needs explaining (which is actually longer than the upward trend between 1975 and 1998, just so you know), given that greenhouse gas emissions were rising throughout that period.

And as the graph used by the Independent to bolster its case (supplied by CRU, apparently) demonstrates, the Nature study does absolutely nothing to address that concern:

In fact, the most striking thing about the graph is that, once the sampling errors identified by the study have been taken into account, the period of warming in the latter half of the twentieth century was shorter than previously thought, and that the ’45-’75 temperature slump is more pronounced.

According to Phil Jones, a co-author of the paper, the study

lends support to the idea that a period of global cooling occurred later during the mid-twentieth century as a result of sulphate aerosols being released during the 1950s with the rise of industrial output. These sulphates tended to cut sunlight, counteracting global warming caused by rising carbon dioxide.

“This finding supports the sulphates argument, because it was bit hard to explain how they could cause the period of cooling from 1945, when industrial production was still relatively low,” Professor Jones said.

That might be so. But the aerosols issue is supposed to have been done and dusted long ago. One of the central criticisms aimed at the infamous Great Global Warming Swindle, for example, is precisely that it failed to entertain the idea that the post-1940 decline in global temperatures was the result of increases in sulphurous emissions that masked the forcing effect of rising atmospheric CO2. George Monbiot described the omission as ‘straightforward scientific dishonesty‘. After all, he said, that ‘temperatures declined after the Second World War as a result of sulphate pollution from heavy industry, causing global dimming…is well-known to all climate scientists.’ And as we have reported before, this was also one of the main points raised by the Royal Society’s Bob Ward and 36 scientific experts in their open letter to Swindle producer Martin Durkin.

And yet, as we’ve reported elsewhere, other experts in the field just don’t agree. UC San Diego atmospheric physicist Veerabhadran Ramanathan, for example, told us that the empirical evidence for the sulphate masking of warming is ‘pretty flimsy’. We do not doubt that the Nature study is an important contribution to the field. (Although it’s interesting that Steve McIntyre seems to have produced a similar analysis more than a year ago.) What we do doubt is that the headlines, soundbites, and wild interpretations from newspapers and scientists alike bear much relevance to what is a dry, technical, scientific study, which, while increasing our ability to understand and predict climate trends, says little in itself about the truth or otherwise of global warming.

That said, the BBC’s Richard Black has demonstrated uncharacteristic reserve in his coverage of the paper, which includes the following quote from CRU’s Mike Hulme:

Corrections for this measurement switch have not yet been applied to produce a new graph of 20th Century temperatures – that work is ongoing at the UK Met Office – but as the land temperature record shows a flattening of the upwards trend from the 1940s to the 1970s, clearly something did change around the 1940s to ameliorate the warming.

“It perhaps suggests that the role of sulphate aerosols, that cooling effect, was less powerful than we thought,” said Mike Hulme from the University of East Anglia (UEA), who was not involved in the study.

George Monbiot and the Royal Society are just plain wrong – the science is plainly not ‘settled’. And so is Steve Connor, the author of the Independent article. As he wrote last year in response to the Swindle:

The programme failed to point out that scientists had now explained the period of “global cooling” between 1940 and 1970. It was caused by industrial emissions of sulphate pollutants, which tend to reflect sunlight. Subsequent clean-air laws have cleared up some of this pollution, revealing the true scale of global warming – a point that the film failed to mention.

‘Scientists’ have ‘explained’ nothing of the sort. As this case shows, the science is not settled. Indeed science is never settled. It is constantly re-evaluating what it understands about absolutely everything. And that’s especially crucial to bear in mind when the science in question has been bestowed with the kind of political significance that climate science has. To claim otherwise is to do a disservice to both science and politics. It reduces science to a flimsy fig leaf used simply to hide the embarrassing inadequacies of the latest political fad; and it reduces politics to an aimless exercise in number-crunching.

The Black and White Aerosols Show

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:

[The Great Global Warming Swindle] misrepresented the current state of scientific knowledge by failing to mention that the cooling effects of aerosol need to be taken into account when considering the period of slight cooling between 1945 and 1975. 

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.