Backs to nature

" There's a perception that global warming is something that will happen smoothly into the future, but some of these ecosystems go into abrupt decline when warming reaches a certain threshold, " said Tim Lenton, an environmental scientist at the University of East Anglia, and lead author of the study. " If we know when the different tipping points are, we can use them to inform targets to limit global warming. It gives us something to aim for, " he added. Last year, the UN's panel of climate scientists warned average temperatures could increase by as much as 6.4°C by the end of the century, with a rise of 4°C most likely. Such a rise poses a threat of food and water shortages to vulnerable parts of the world under many models of subsequent climate change. In the latest study, the scientists calculate Arctic sea ice will go into irreversible decline when temperatures rise between 0.5°C and 2°C above those at the beginning of the century, a threshold that may already have been crossed. There is already a 50 per cent chance that the Greenland ice sheet will soon begin melting unstoppably, although it could take hundreds of years to melt completely. The meltwater would raise global sea levels by seven metres. Warming of 3–5°C could reduce rainfall in the Amazon by 30 per cent, lengthening the dry season. The Boreal forests could also pass their tipping point, with large swathes dying off over the next 50 years. In Africa, more rainfall may lead to a greening of the Sahel — one of the few potentially positive regional outcomes of climate change — but the West African monsoon could collapse, leading to twice as many unusually dry years by the end of the century. The Indian summer monsoon is predicted to become more erratic and, in the worst-case scenario, begin to flip chaotically, unleashing flash floods one year and droughts the next. " Society may be lulled into a false sense of security by smooth projections of global change, " the authors write. " Our synthesis of present knowledge suggests that a variety of tipping elements could reach their critical point within this century under anthropogenic climate change. " The announcement, last month, by President Bush of plans to release three million acres of pristine Alaskan forest for logging, mining and development appeared to generate little public opposition, and a …