According to the likes of Bob Ward, George Monbiot, Ben Goldacre and Steve Connor, it is a well established fact that the slump in global temperatures over three decades in the middle of the last century is the result of changes in the composition of atmospheric aerosols following various clean air acts in the western world.
Failure to acknowledge this fact is ‘straightforward scientific dishonesty’, according to Monbiot, and ‘a major misrepresentation of the scientific evidence’, in the words of Ward. Goldacre described the question of the post-war temperature slump as a prime example of a denialist ‘zombie argument’ (it ‘survive[s] to be raised again, for eternity, no matter how many times [it is] shot down’) and wrote that it has ‘been answered already, ages ago’. It’s the aerosols, stupid.
We have stated repeatedly that such certainty is not justified by the state of scientific understanding of atmospheric aerosols (see links above). So it’s good to see Quirin Schiermeier’s piece in today’s issue of Nature – The real holes in climate science – which identifies aerosols as one of four problematic areas of climate change research (the other three being Regional climate prediction, Precipitation, and The tree-ring controversy):
Atmospheric aerosols — airborne liquid or solid particles — are a source of great uncertainty in climate science. Despite decades of intense research, scientists must still resort to using huge error bars when assessing how particles such as sulphates, black carbon, sea salt and dust affect temperature and rainfall.
Overall, it is thought that aerosols cool climate by blocking sunlight, but the estimates of this effect vary by an order of magnitude, with the top end exceeding the warming power of all the carbon dioxide added to the atmosphere by humans.
One of the biggest problems is lack of data…
Schiermeier goes on to describe how little is understood about the relative contributions of white aerosols (eg, sulphates, which have a cooling effect) and black carbon (which has a warming effect); how we don’t know the atmospheric aerosol composition in the present, let alone the past; how little we know about the way in which aerosols interact with clouds and other atmospheric processes, etc.
He doesn’t go into the implications of this lack of knowledge for our understanding of the post-war temperature slump. But it goes without saying that, if ‘Scientists have yet to untangle the interplay between pollution, clouds, precipitation and temperature’, then the claims of Ward, Monbiot, Goldacre, Connor et al are wildly off the mark, if not examples of ‘straightforward scientific dishonesty’.
Thankfully, neither is the post-war temperature slump mentioned in the obligatory list of long-debunked denialist arguments (Enduring climate myths) that accompanies Schiermeier’s article – something of a first for the genre.
The Nature piece is not without its faults. For a start, it is framed in terms of the putative attack on climate science by ‘deniers’. And Hans von Storch is already complaining that Schiermeier has misquoted him, and that the article ignores myths perpetuated by the orthodoxy (of which we could add a few of our own). But perhaps it has dealt the severe head trauma that was needed to finish off for good the real zombie argument – that ‘Temperatures declined after the Second World War as a result of sulphate pollution from heavy industry, causing global dimming. This is well-known to all climate scientists.‘
Schiermeier’s piece also contained this paragraph:
“Climate scientists are worried in particular about tree-ring data from a few northern sites. By examining temperature measurements from nearby, researchers know that tree growth at these locations tracked atmospheric temperatures for much of the twentieth century and then diverged from the actual temperatures during recent decades. It may be that when temperatures exceed a certain threshold, tree growth responds differently.”
i.e. therefore tree rings are useless as a proxy to show that 20th Century temperature rises are unprecedented, and that there was no Medieval Warm Period, since they apparently don’t respond to increases in temperature.
Funny, the phrase ‘hide the decline’ springs to mind…
It is worth noting that the divergence problem is not restricted to a few sites. Briffa describes it as “widespread” in the Northern Hemisphere.
I see that the arch warmist James Hansen is ALSO sceptical about the aerosol *explanation*.
“The different hemispheric records in the mid-twentieth century have never been convincingly explained. The most likely explanation is atmospheric aerosols…However, there are no aerosol measurements to confirm that interpretation. If there were adequate understanding of the relation between fossil fuel burning and aerosol properties it would be possible to infer the aerosol properties in the past century. But such understanding requires global measurements of aerosols with sufficient detail to define their properties and their effect on clouds, a task that remains elusive”
Also discussed at http://wattsupwiththat.com/2009/12/21/hansen-on-the-surface-temperature-record-climategate-solar-and-el-nino/