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Informing Copenhagen

20 April 2010

In the run-up to the Climate Change Conference in Copenhagen, University groups published several articles in prestigious journals

A study in Science (12 November) reported that satellite observations and a state-of-the art regional atmospheric model independently confirmed that the Greenland ice sheet is losing mass at an accelerating rate. This loss is equally distributed between increased iceberg production, driven by acceleration of Greenland’s fast-flowing outlet glaciers, and increased meltwater production on the surface of the ice sheet. Since 2000, the Greenland ice sheet has lost about 1,500 Gt (1 Gt is the mass of 1 cubic kilometre of water), representing a global sea-level rise of five millimeters. Estimates suggest the ice sheet contains enough water to cause a global sea-level rise of seven metres.

Jonathan Bamber, an author on the paper, said: “It is clear from these results that mass loss from Greenland has been accelerating since the late 1990s and the underlying causes suggest this trend is likely to continue in the near future. We have produced agreement between two totally independent estimates, giving us a lot of confidence in the numbers and our inferences about the processes.”

To help preserve the Greenland ice sheet, Pete Irvine and colleagues suggested in Environmental Research Letters (15 December) that light-reflecting geoengineering may be a way of cooling the Earth’s climate. They used a model to predict what might happen if incoming solar radiation were reduced in a future where CO2 is four times higher than pre-industrial levels, either by creating a ‘sunshade’ or by injecting sulphate aerosols into the stratosphere. Their results showed that if only a small fraction, 4.2%, of the sunlight that reaches Earth were deflected, the average temperature could be returned to pre-industrial levels and prevent the Greenland ice sheet from melting. However, although such schemes could cool the Earth’s climate, they would also make it drier and change weather patterns. Reflecting only 2.5% of solar radiation, on the other hand, could reduce these undesired changes and still help prevent collapse of the Greenland ice sheet.

The Greenland ice sheet contains enough water to cause a global sea-level rise of seven metres

Reporting in Nature (19 November), a study re-evaluating records of 340,000-year-old East Antarctic ice cores showed that during brief periods between ice ages, temperatures were up to 6°C warmer than at present. Julia Tindall and others explained that until now, temperatures during warm periods between ice ages – known as interglacials – were thought to be slightly warmer than those of the present day. However, when they analysed Antarctic ice cores they were surprised to find peaks of relatively high temperatures. They concluded that previous temperature estimates from interglacial climates are likely to be too low and that the interglacial temperature of the level suggested by their research ‘indicates there are serious deficiencies in our understanding of warmer than present day climates’. Tindall commented: “It is quite difficult to reconstruct temperatures from long ago. Although it is generally accepted that the climate was warmer during the last warm period, about 125,000 years ago, our results suggests it was much warmer than previously thought.”

Dan Lunt and colleagues publishing in Nature Geoscience (6 December) also used data from the past to help understand the future. He and his colleagues compared temperature reconstructions of the Earth’s environment three million years ago – when global temperatures and carbon dioxide concentrations were relatively high – to results from a global climate model used by bodies such as the Intergovernmental Panel on Climate Change. The temperature reconstructions were derived using data from three-million-year-old sediments from the ocean floor.

Lunt said: “We found that given the concentrations of carbon dioxide prevailing three million years ago, the model predicted a significantly smaller temperature increase than that indicated by the reconstructions. This led us to review what was missing from the model.” The team found that the increased temperatures indicated by the reconstructions could be explained if factors that vary over long timescales, such as changes in land-ice and vegetation, are included in the model. This is primarily because such changes lead to more sunlight being absorbed, which in turn increases warming. Including these long-term processes in the model resulted in an increased temperature response of the Earth to carbon dioxide, indicating that the Earth’s temperature is more sensitive to carbon dioxide than previously recognised.

"Our conflicting results demonstrate what doing cutting-edge science is really like"

The two other papers disagreed on the planet’s ability to absorb carbon dioxide emissions. Wolfgang Knorr, publishing in Geophysical Research Letters (9 November), found that the amount of CO2 absorbed by the oceans and terrestrial ecosystems – the planet’s natural carbon sinks – has stayed approximately constant since 1850, despite emissions of CO2 having risen from 2 billion tons a year in 1850 to 35 billion tons a year now. This suggests that the Earth’s ability to absorb CO2 is keeping pace with the rate at which we emit it into the atmosphere.

The results run contrary to a significant body of recent research which expects that the capacity of terrestrial ecosystems and the oceans to absorb CO2 should start to diminish as CO2 emissions increase, letting greenhouse gas levels skyrocket. Indeed, according to research published in Nature Geoscience (17 November) just a few days later, emissions of carbon dioxide continue to outstrip the ability of the world’s natural sinks to absorb carbon.

The Nature Geoscience team estimated that over the past 50 years, the amount remaining in the atmosphere ‘had likely increased’ from 40 to 45 per cent, suggesting a decrease in the efficiency of the natural sinks. Knorr’s study, on the other hand, also found no increase in the airborne fraction during the past 50 years and that if anything the trend was negative at -0.2 ± 1.7% per decade, which is essentially zero. He therefore concludes that the capacity of terrestrial ecosystems and the oceans to absorb CO2 has not diminished. The strength of his study, argues Knorr, is that it rests solely on measurements and statistical data, including historical records extracted from Antarctic ice cores – although we have already seen these are difficult to interpret – and does not rely on computations with complex climate models.

Both carbon sink studies were based on atmospheric composition data and statistical data on energy use and land use change, but differ in the way they calculate the trend, how they treat uncertainties in atmospheric concentrations and how they account for confounding climatic variability. Knorr explained: “Our apparently conflicting results demonstrate what doing cutting-edge science is really like and just how difficult it is to accurately quantify such data.” Jo House, an author on the Nature Geoscience paper, concurred: “It is difficult to accurately estimate sources and sinks of CO2, particularly in emissions from land use change where data on the area and nature of deforestation is poor, and in modelled estimates of the land sink which are strongly affected by inter-annual climate variability. While the science has advanced rapidly, there are still gaps in our understanding.”

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