Climate models predict that increasing levels of carbon dioxide and other greenhouse gases will warm the lower part of the Earth's atmosphere, the troposphere, and cool the outer stratosphere.
However, satellite measurements have shown little change in tropospheric temperatures over the last twenty years. The finding has been a puzzle because traditional measuring methods such as weather balloons confirm the expected warming. But sceptics have seized on the discrepancy to argue that global warming is an artefact and that action to reduce carbon emissions is not justified.
A new analysis of data gathered by US weather satellites has eliminated much of the discrepancy between the different data sets.
The satellites use microwave sounding equipment which is not specifically designed for detecting slight trends in atmospheric temperature. The data needs to be corrected to produce temperature trends over the last twenty years. Key sources of error include the satellites' changing orbits, the different times of day they cross the continents and differences in equipment between satellites.
Qiang Fu of the University of Washington in Seattle and colleagues from the US National Oceanic and Atmospheric Administration found that the microwave soundings used for tropospheric temperature measurements were also being influenced by temperatures in the stratosphere. They conclude that warming in the troposphere has been masked by temperature reductions in the stratosphere.
The researchers used a separate microwave channel on the satellites, sensitive to temperature changes in the stratosphere, to correct for stratospheric cooling. This brought the measurements into line with other temperature readings - and climate model predictions.
Scientific reaction to the paper has been largely favourable. Professor Keith Shine from the University of Reading's meteorology department commented that the paper came from "well-respected researchers" and put forward a "disarmingly simple" explanation of the anomaly. "I'm not sure why it hasn't been done before," he said.
Kevin Trenberth, head of climate analysis at the US National Center for Atmospheric Research in Colorado, told Nature that the findings were "a stunningly elegant and accurate method of clarifying global trends."
A second paper, written by NASA scientists, suggests that air travel may be an even greater contributor to climate forcing than current estimates suggest. The study looked at the impact of aircraft condensation trails on the formation of cirrus clouds - high, wispy clouds which are a net contributor to global warming.2Cirrus clouds are only formed at low temperatures and high humidity. Aircraft contrails formed from exhaust gases in the upper troposphere promote cirrus cloud formation by adding extra water vapour and providing particles around which ice crystals condense.
The NASA study examined satellite and observational data on cirrus cloud cover in 1971 to 1995. It found significant increases in the frequency of cirrus clouds across North America, the north Pacific and the north Atlantic - areas of major flightpaths - even though there was no change in humidity in the upper troposphere.
In Western Europe, another of the busiest air travel regions, cirrus cloud formation was constant. However, humidity fell over the period, suggesting that climatic factors may have offset the effect of increased air traffic.
The study shows that cloud cover changes induced by aircraft could have a significant regional effect. Lead author Patrick Minnis said: "Increased cirrus coverage attributable to air traffic could account for nearly all of the warming observed over the United States for nearly 20 years." The predicted increase in surface temperatures from aircraft-induced cloud changes was 0.2-0.37ºC per decade, compared with an observed increase of 0.28ºC per decade between 1975 and 1994.
The study also suggests that aviation's global warming impact may be higher than previously thought. The Intergovernmental Panel on Climate Change has put aircrafts' total impact at between 1 and 5 times that from their CO2 emissions - with a best guess of 2.7. The extra warming is due to the net effect of other emissions such as nitrogen oxides, water vapour and contrails.
Contrails remain the most uncertain factor. The IPPC's figures include the effect of linear contrails but not contrails spreading to form non-linear clouds. Dr Minnis commented that "our new numbers would probably boost the factor [from 2.7] up to 3.0 or so".
Aircraft could reduce contrail formation by avoiding areas of cold, humid air. However, this would require advanced air traffic management - and could also lead to increased fuel use and CO2 emissions. Managing such trade-offs is a challenge for the Government's desire to bring aviation into an emissions trading framework (ENDS Report 347, pp 42-43 ).