Climate 'tipping point' warning

Researchers warned projected warming could see ice sheets, forests and weather patterns reach critical thresholds beyond which small changes could cause large-scale negative consequences.

Lead author Tim Lenton of the University of East Anglia said the Greenland ice sheet and the Arctic sea ice were the nearest to their tipping points - which would see widespread melting occur.

Writing in the Proceedings of the National Academy of Science (PNAS), the researchers said the Greenland ice sheet could take more than 300 years to undergo a major change, but could push up global sea levels by two to seven metres.

The loss of the Arctic summer sea ice, which is already in a "striking downward trend" according to Prof Lenton, could take just a decade, altering the ecosystem and amplifying warming as the region loses its heat reflection qualities.

Elements which could have the most obvious and rapid effects on human life include the potential collapse of the Indian summer monsoon, and increases in weather pattern changes caused by El Nino, both of which could spark drought.

Other large-scale systems or "tipping elements" that could be drastically altered after reaching their tipping point within the century are the dying back of the Amazon rainforest and the Boreal forests in the far north, the collapse of the Atlantic thermohaline circulation which could divert the Gulf Stream, and the collapse of the West Antarctic ice sheet.

All of them would directly affect a large number of people, cause the loss of something of great value - such as the biodiversity of the Amazon rainforest - or have a critical impact on the overall climate system.

But the disruption of the West African Monsoon, one of the nine major tipping elements identified, could have a positive effect through the greening of the Sahel/Sahara region.

In some cases the changes will be irreversible, and those that in theory could be reversible may not be if the climate is in an upward trajectory.