Now trees cause dangerous global cooling

They were supposedly causing global warming just a few weeks back

Europe's "Little Ice Age" may have been triggered by the 14th Century Black Death plague, according to a new study. Pollen and leaf data support the idea that millions of trees sprang up on abandoned farmland, soaking up carbon dioxide from the atmosphere. This would have had the effect of cooling the climate, a team from Utrecht University, Netherlands, says. The Little Ice Age was a period of some 300 years when Europe experienced a dip in average temperatures.

Dr Thomas van Hoof and his colleagues studied pollen grains and leaf remains collected from lake-bed sediments in the southeast Netherlands. Monitoring the ups and downs in abundance of cereal pollen (like buckwheat) and tree pollen (like birch and oak) enabled them to estimate changes in land-use between AD 1000 and 1500. The team found an increase in cereal pollen from 1200 onwards (reflecting agricultural expansion), followed by a sudden dive around 1347, linked to the agricultural crisis caused by the arrival of the Black Death, most probably a bacterial disease spread by rat fleas. This bubonic plague is said to have wiped out over a third of Europe's population.

Counting stomata (pores) on ancient oak leaves provided van Hoof's team with a measure of the fluctuations in atmospheric carbon dioxide for the same period. This is because leaves absorb carbon dioxide through their stomata, and their density varies as carbon dioxide goes up and down. "Between AD 1200 to 1300, we see a decrease in stomata and a sharp rise in atmospheric carbon dioxide, due to deforestation we think," says Dr van Hoof, whose findings are published in the journal Palaeogeography, Palaeoclimatology, Palaeoecology.

But after AD 1350, the team found the pattern reversed, suggesting that atmospheric carbon dioxide fell, perhaps due to reforestation following the plague. The researchers think that this drop in carbon dioxide levels could help to explain a cooling in the climate over the following centuries. From around 1500, Europe appears to have been gripped by a chill lasting some 300 years.

There are many theories as to what caused these bitter years, but popular ideas include a decrease in solar activity, an increase in volcanic activity or a change in ocean circulation. The new data adds weight to the theory that the Black Death could have played a pivotal role.

Not everyone is convinced, however. Dr Tim Lenton, an environmental scientist from the University of East Anglia, UK, said: "It is a nice study and the carbon dioxide changes could certainly be a contributory factor, but I think they are too modest to explain all the climate change seen." And Professor Richard Houghton, a climate expert from Woods Hole Research Center in Massachusetts, US, believes that the oceans would have compensated for the change. "The atmosphere is in equilibrium with the ocean and this tends to dampen or offset small changes in terrestrial carbon uptake," he explained.

Nonetheless, the new findings are likely to cause a stir. "It appears that the human impact on the environment started much earlier than the industrial revolution," said Dr van Hoof.

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MORE GLOBAL COOLING IN BRITAIN

Arctic weather is set to return with a vengeance this week as icy blasts chill most of Britain. Weather forecasters say that temperatures could plunge to as low as minus 6C (22F) in some areas, as snow and severe frost endured in Scotland overnight fan out across England. Last night gritters were out in Lincolnshire, East Anglia and the South West, and the Highways Agency told drivers to look out for stretches of ice on roads that they had not managed to treat. A spokesman said that drivers should watch the forecasts and not go out without a shovel.

Spring is supposed to begin tomorrow. "Mother Nature does not always stick to her calendar," a spokesman for the Met Office said. "The next few days will be a timely reminder that winter is still with us." Overnight temperatures in northern Scotland were expected to drop to minus 15C, with snow settling up to 20cm (8in) deep. Daytime temperatures there are unlikely to rise above freezing.....

From December until mid-February, colder conditions swept west through Europe and across southeast and southern Britain. Average temperatures dropped below the average for this period from 1971 to 2000. In southern England it has been the coldest December and January for nine years...

But the evidence from Britain's gardens is in line with the predictions by the Met Office. Guy Barter, of the Royal Horticultural Society, said that English gardens had suffered colder than average temperatures and little rain, while Scotland's shrubberies benefited from more normal levels of precipitation. At the RHS Garden at Wisley, in Surrey, temperatures fell to minus 6C in November, the lowest recorded there since 1976. "We have seen milder winters in the last four or five years," Mr Barter said. "This one has come as a bit of a shock."

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