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C l imatewatch n Warmer world could bring colder winters: The shrinking of Arctic sea ice due to higher global air temperatures may lead to colder Northern Hemisphere winters, according to new research.

Vladimir Petoukhov of the Potsdam Institute for Climate Impact Research in Germany and Vladimir Semenov of the University of Kiel used an elaborate model of air circulation to study the effect of a reduction of sea ice in the Barents-Kara Sea north of Norway and Russia. This region suffered a drastic reduction in ice cover during the cold European winter of 2005–06.

he researchers gradually reduced the values for sea ice cover in the eastern Arctic from 100 per cent to one per cent and analysed the results. ‘Our simulations reveal a rather pronounced non-linear response of air temperatures and winds to the changes in sea-ice cover,’ Petoukhov said. ‘It ranges from warming to cooling to warming again.’

Areas lacking sea ice transfer more heat from the ocean to the air. This can lead to anomalies in the atmospheric air stream, changing the direction in which winds blow and, in this case, bringing colder air down from the north.

‘Our results imply that several recent severe winters do not conflict with the global warming picture but rather supplement it,’ the researchers wrote in a paper that appeared the Journal of Geophysical Research.

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As temperatures rise, tundra will burn

A new study has identified an Arctic-temperature tipping point, above which tundra fires are likely to become significantly more frequent.

In September 2007, 1,000 square kilometres of tundra near the Anuktuvuk River on Alaska’s North Slope was burnt. After the fire, Feng Sheng Hu of the University of Illinois travelled to the region in the hope of determining whether the fire was an anomaly or if large fires such as this were a regular occurrence in the region. ‘If such fires occur every 200 years or every 500 years, it’s a natural event,’ Hu said. ‘But another possibility is that these are truly unprecedented events caused by, say, greenhouse warming.’

Hu collected sediment cores from two nearby lakes and then he and his colleagues analysed the distribution of charcoal particles in the cores. The team found no evidence of a fire of a similar scale and intensity in sediments representing about 5,000 years for that area.

The researchers then created a model that linked the area of tundra burned to mean temperature and precipitation during the warmest period of the year (June–September) using 60 years of data from the Alaskan tundra. The analysis, which is published in the Journal of Geophysical Research, uncovered a striking pattern. ‘There is a dramatic,

non-linear relationship between climatic conditions and tundra fires, and what one may call a tipping point,’ Hu said.

he Anuktuvuk River fire in September 2007 burned an area of about 1,000 square kilometres of Arctic tundra

Once the mean temperature rises above a mean threshold of 10°C, the results suggested, fires become significantly more frequent. For the past 60 years, mean summer temperatures have fluctuated between about 6°C and 9°C, trending upwards since 1995. In 2007, the mean was a record 11°C, while precipitation and soil moisture were at an all-time low.

In brief

GLOBAL A review of four decades of research has demonstrated that the Earth’s lower atmosphere is warming, and that the warming is broadly consistent with both theoretical expectations and climate models. Earlier observations appeared to show that temperatures in the troposphere hadn’t changed, casting doubts on the reliability of climate models, but the new review, which looked at 195 papers, model results and atmospheric data sets, dispelled these doubts.

CARIBBEAN High ocean temperatures in the tropical Atlantic and Caribbean in 2005 caused 80 per cent of surveyed coral in the region to bleach and more than 40 per cent to die, according to a study that combined records from collaborators in 22 countries. The results suggest that the event was the most severe ever recorded in the region.

GLOBAL A new UN study by 30 leading scientists suggests that the emission-reduction pledges made by 80 countries at last year’s climate change conference in Copenhagen fall well short of the level required to keep global temperature rise to 2°C above pre-industrial levels by 2080. The study suggests that there is a gap of at least five billion tonnes between what has been pledged and what is required.

GLOBAL A reanalysis of seasurface temperatures by Met Office scientists has provided evidence that the world warmed more rapidly over the past decade than previously thought. The new analysis adjusted underestimates that arose from the change from predominantly ship-based measurements before 2000 to mostly buoy-based measurements afterwards.

JAnuary 2011 15

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