The Scientific Alliance Newsletter - Changing and informed scientific debate

9th December 2011


Climate change uncertainties
As the delegates at the COP17 climate change talks in summit edge towards an agreement of sorts, there has been a flurry of activity among groups wanting to put forward evidence relating to the main drivers of change.

One factor which comes into the headlines from time to time is black carbon, more commonly known as soot. It’s what gets put into the air (or on the inside of chimneys) when fuels are incompletely burnt, and surprising quantities are produced every year.

A recent article in the Guardian (Wood fires fuel climate change – UN) covers a report from the UN Environment Programme (Near-term climate protection and clean air benefits: Actions for controlling short-lived climate factors). Joseph Alcamo, their chief scientist, is quoted as saying "It's nice to sit in front of a wood fire in the winter, but we should all be feeling pretty guilty." This is because all but the newest (and most expensive) wood-burning stoves produce significant quantities of soot, as do most diesel engines more than two years old.  

The problem is that the black carbon absorbs heat, and, when it settles on snow and ice, also accelerates melting. Reducing emissions of soot and other ‘short-lived climate forcers’ could, according to the report, reduce expected average temperature rises by about half a degree, although the prevailing view is still that, in the longer term, it is the enhanced greenhouse effect which will be dominant.

Which makes a report on the Environmental Research Letters website – Is climate sensitivity lower than IPCC finding? – particularly pertinent. A team from a number of American universities and the Universitat Autonòma de Barcelona has come up with a median climate sensitivity (expected temperature rise due to a doubling of CO2 level) of 2.3°C, with a 66% probability of it lying between 1.7 and 2.6°. The most recent (fourth) IPCC Assessment Report gave a figure of 3°, with a 66% chance of between 2 and 4.5°.

This is important, because the current policy response (attempts to make drastic reductions in emissions) is based on keeping temperature rises to a ‘safe’ level, which is directly related to the real climate sensitivity. If this really is 2.3° rather than 3°, this makes a big difference. Of course, if the IPCC is wrong and the greenhouse effect is not enhanced by positive feedback mechanisms, we can expect the climate sensitivity of CO2 doubling to be only about 1°, which would make the cost of forcing rapid emissions reductions much more difficult to justify.

On the other hand, a new article in Nature (Three-quarters of climate change is man-made) reports a study by two Swiss climate modellers, which concluded that at least 74% of observed warming is anthropogenic. This was based on a study of the Earth’s energy balance, and is claimed to offer increased confidence in the IPCC view.

To quote from Nature: “Their findings, which are strikingly similar to results produced by other attribution methods, provide an alternative line of evidence that greenhouse gases, and in particular carbon dioxide, are by far the main culprit of recent global warming. The massive increase of atmospheric CO2 concentrations since pre-industrial times would, in fact, have caused substantially more surface warming were it not for the cooling effects of atmospheric aerosols such as black carbon, they report.” So, we have an another view of the impact of soot on the climate and a claim – based on modelling and without additional evidence – that CO2 emissions are the primary driver of climate change.

These are just a few facets of a complex and fascinating puzzle, and it seems clear that we are still some way from establishing a clear understanding of the influence of different drivers. It could be that changes in the magnetic field of the Sun are finally shown to have a significant impact. Interesting results have come from the CLOUD experiment at CERN, but the effect looks too small at present. It could be that more evidence comes out about the effect of feedback mechanisms, which could, of course, be either positive or negative.

It is also possible that the enhanced greenhouse effect turns out to be the best explanation, although the recent pattern of temperatures suggests that there have been some quite major impacts from other drivers, as yet undefined. Whatever the case, the fact that we are no more certain about how the climate operates now than when the IPCC first started surely means that lack of a strong agreement in Durban is no immediate cause for concern. 


Zion was not the first US National Park. That was in fact Yellowstone, established much earlier, in 1872.

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