The National Academy of Sciences says that global warming could trigger ?large, abrupt and unwelcome? climatic changes that could severely affect ecosystems and human society.

This fulfills the scenario in ?The Coming Global Superstorm,? published last year by Whitley Strieber and Art Bell. The warning comes six months after they informed the White House that global warming is indeed real, largely the result of human activity and likely to cause adverse effects.

Previous discussion of global warming has assumed that change will occur gradually, with average temperatures slowly increasing over the next century. The idea that large changes in climate could occur suddenly and with little warning has remained controversial. Now this idea has reached the scientific mainstream.

?We?re reflecting the thinking,? said Richard Alley, a Penn State University climate expert and the report?s lead author. ?We?re not driving it. We need to deal with this because we are likely to be surprised.?

Sudden climate change involves the Gulf Stream, which is a current of warm water that runs from the Caribbean Sea across the Atlantic Ocean, and keeps the climate of northern Europe temperate. At times in the past, melting of arctic ice has caused a flow of fresh water into the North Atlantic that reversed the Gulf Stream.

Many scientists believe the current could reverse again–over a period of a decade or two, rather than a century–leaving much of Europe far colder than it now is. ?It?s as if climate change were a light switch instead of a dimmer dial,? says Alley.

The possibility of sudden climate means that the amount of time available to adjust is much shorter than government officials once thought. Cutting emissions in greenhouse gasses cannot be put off much longer, and new agricultural areas will have to be researched. Governments may have to plan for shifting population trends if certain parts of the Earth become unlivable. Predicting exactly where changes will occur is hard to do.

Climate change has been slow so far. ?We?re a little spoiled by the last 30 years,? says John M. Wallace, an atmospheric scientist at the University of Washington and the report?s co-author. ?Many years, we?re just barely breaking the previous record.?

Scientists who have studied the past, however, have found repeated instances of sudden and severe change, including the abrupt onset of cooling that drove the Vikings from Greenland in the 14th century and the Dust Bowl drought that devastated the Great Plains during the 1930s. In some areas, temperatures rose 16 degrees within one decade.

New research indicates that climate changes in the past may have decimated forests, speeded the extinction of mastodons and mammoths, promoted the spread of tropical diseases and vastly altered the ocean currents that modify and warm many coastal regions.

Alley and others have used gases within ice cores drilled from the Earth?s remaining ice sheets to produce a detailed record of the Earth?s climate for the last 110,00 years. ?We?ve gotten better and better records, and we?ve been able to say the changes were really big and really fast and affected a lot of the world at the same time,? he says.

Scientists say the possibility of abrupt climate change makes it even more important that the federal government establish a reliable climate monitoring system. Federal agencies have been unwilling to spend the estimated $10 million to $15 million needed for the system, says Bruce A. Wielicki, a climate researcher at NASA?s Langley Research Center in Hampton, Virginia.

Officials of the Global Climate Change Coalition, an industry group, warn against making policy decisions on climate with science so uncertain. ?This really highlights the uncertainties and complexities that remain,? says Frank Maisano, a spokesman for the coalition.

The scientific report suggests nations begin to implement ?no regrets? policies to protect themselves from possible change. These include conserving water in case of drought or planting trees to offset the climate-induced loss of vegetation.

Alley says, ?It?s ?no regrets? because even if nothing goes wrong, you still like the trees.?

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