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Climate shift could be most unwelcome

The big wild card in the game of climate change is the possibility of unwelcome surprises that would suddenly occur if some threshold of warming caused the climate system to shift to a new state.

For example, some scientists say, the warming could melt more snow and ice, flooding the North Atlantic with fresh water and upsetting the workings of large ocean currents. If that happened, the world might suddenly find itself with serious disruptions in currents like the Gulf Stream that transport heat around the globe, keep Europe warm and maintain favorable conditions for marine life.Just such an event may have been responsible for plunging the world back into glacial conditions for several centuries, starting about 12,000 years ago, after it had temporarily warmed near the end of the last ice age.

A study reported recently in the journal Nature suggested that the critical ocean currents could be shut down altogether should atmospheric concentrations of heat-trapping carbon dioxide double within 100 years, as mainstream scientists expect to happen if emissions of the gas from power plants, industries and motor vehicles are not reduced.