Global warming: Yes it’s real and yes it’s CO2-driven; a former sceptic speaks his mind

Scientists are supposed to change their minds when the balance of evidence changes. In my experience, this doesn’t always happen, but one very respected scientist who has changed his mind, not once but twice, and very publicly, is Prof Richard Muller of the Berkeley Earth Land Temperature Project, UC Berkeley, and Lawrence Berkeley National Laboratory.

Three years ago, he was among the few remaining respected scientists to reject the IPCC (Intergovernmental Panel on Climate Change) analysis of current climate, and unconvinced that significant climate change was happening at all, let alone that it might be driven by human activity. Not surprising, then, that a consortium of those with an interest in denial funded his BEST (Berkeley Earth Surface Temperature) project, to carry out a completely independent, assumption-free, analysis of the data. They got more than they bargained for.

First, BEST concluded, in findings published last year, that warming is indeed taking place as asserted by the overwhelming majority of the climate science community. Now, even more significantly, BEST has taken the position, in a paper submitted to The Third Santa Fe Conference on Global and Regional Climate Change, that CO2 is the most significant driver, that the IPCC estimate of the effect of CO2 (3^oC warming for each doubling of CO2 concentration) is accurate, and that the amount of warming from the 1950s to the 2000s (0.87 +/- 0.05 ^oC) is if anything slightly more than the IPCC estimate.

Moreover, the BEST publications analyse and dismantle all the standard objections to this work. Yes, there are effects due to volcanoes. I don’t think anyone denies this. The 1991 Mount Pinatubo eruption led to summers bad enough to force up the price of potatoes, but earlier events, such as the eruptions of Laki in 1783, Tambora in 1815, and Cosiguina in 1835, were much more significant. We know this from accounts at the time, and can quantitatively estimate the intensity of the eruptions by the amount of sulphate in ice cores. The late 18th and early 19th century data are good enough to provide a scaling factor (0.15 ^oC cooling per gigatonne emitted sulphate), showing that, by contrast, the overall effect of volcanoes in the 20th century has been insignificant. No, there is no significant effect attributable to the Sun. No, there are no artefacts due to the number and location of climate stations, although this has not stopped WattsUp, a Koch-funded enterprise, from raising this yet again in response to Muller. There is some variability connected with oscillations in the Atlantic, which may be responsible for the 0.17 ^oC variation from the simplest model. This model, which attributes all change to a linear volcanic effect, and a logarithmic CO2 effect, is remarkably successful. In the light of Muller’s work there is no excuse for invoking alleged scientific uncertainty to delay urgent consideration of the effects of further increasing CO2 concentrations, and the appropriate policy responses.

There are some particular words of caution. We don’t understand why the difference between day and night time temperatures decreased from 1900 to 1987, but then started rising again. We can’t be certain that present temperatures exceed those of the Mediaeval Warm Period, so beloved of climate “sceptics”, although it seems clear that if things continue as they are, the issue will be beyond all doubt. Warming has not led to more hurricanes, and the heat wave afflicting the United States this summer is local rather than global.

Muller’s approach includes the effects to date of one important feedback, the positive feedback due to the fact that ocean warming leads to increased concentrations of water vapour, the most significant of all greenhouse gases. However, of necessity, it neglects effects that have not yet kicked in, most of which can only add to our concerns. A tiny minority of climate scientists still maintain that increased cloud cover will moderate the effects of warming, but the evidence (see Science 2009, 325, 376, for a discussion) now shows that the opposite is true. Melting of sea and land ice will speed up warming, by reducing the Earth’s albedo, the fraction of incident sunlight that is reflected straight back into space, since exposed ground, crops, and open ocean absorb more energy and snow and ice. There is the prospect of release of methane from thawing tundra, and increased release of carbon dioxide from soils, as bacterial activity increases with warming. The only negative feedback in prospect is the greater reflectivity of deserts, as compared with cultivated land, but that is the last thing we should be looking forward to.

Thus Muller’s approach offers us the lowest credible estimate of what is in store for us. Despite which, opposition parties in Australia and the US, including one US presidential candidate, and a vocal faction within the United Kingdom’s governing Conservative Party, continue, and may be expected to indefinitely continue, in their denial that any real problem has been shown to exist.

I find this frightening.

This post is also available at http://www.thetwentyfirstfloor.com/?p=4457