杏吧原创

An international conference entitled 鈥淎voiding dangerous climate change鈥� began on Tuesday with a warning that coming up with a global definition of dangerous climate change may be 鈥渕ission impossible鈥�.

Around 200 scientists have gathered for the meeting in Exeter, UK, to thrash out the risks that climate change poses to the world and feed this information to the policy makers who must decide what to do about it.

The known risks were made clear by a number of speakers. Chris Rapley from the British Antarctic Survey revealed that ice sheets in Antarctica 鈥� which in total contain enough water to raise sea levels by nearly 60 metres 鈥� are undergoing dramatic change. The new view of Antarctica is of a 鈥済iant awakening鈥� he said.

Carol Turley from the Plymouth Marine Laboratory in the UK said the oceans are becoming more acidic because they are absorbing carbon dioxide from the atmosphere. Others presented new assessments of the thermohaline circulation, which drives the gulf stream that warms Europe. It may be more likely to collapse than we realised, warned Mike Schlesinger, from the University of Illinois at Urbana-Champana.

But what about defining dangerous? When the British Prime Minister, Tony Blair, announced the meeting in 2004, he promised it would be 鈥渕ore than just another scientific conference鈥�. He said it would address big questions, asking: 鈥淲hat level of greenhouse gases in the atmosphere is self-evidently too much?鈥�

The drive to define what is dangerous also comes from the 1992 United Nations treaty on climate change, through which countries, including the US, committed to 鈥減revent dangerous anthropogenic interference with the climate system鈥�.

Conflicting priorities

But there are obvious problems in defining 鈥渄angerous鈥�. Steve Schneider, a climate scientist from Stanford University, California, US, used melting ice in the Arctic as an example.

The loss of ice will affect Eskimos鈥� way of life. 鈥淚s that dangerous from their perspective? It probably is.鈥� But ships will enjoy faster passage through an ice-free Arctic. So what should be prioritised? 鈥淭hat鈥檚 exactly the kind of conflict you will have,鈥� he said.

In his talk, Schneider explained the complexity of setting specific danger levels. His team has been tasked by the Intergovernmental Panel on Climate Change to study 鈥渒ey vulnerabilities鈥� and assess their resilience to climate change. This involves finding temperature thresholds above which harm is caused to coral, for example, or ocean circulations.

These temperatures have to be linked to concentrations of greenhouse gases through climate models. But that is not simple, because the temperature depends not just on the level of greenhouse gases, but also on the rate at which the concentrations change. If they rise to a peak and then fall, the temperature maximum is different to a scenario in which they stabilise at some elevated level.

That complexity led the session鈥檚 chair 鈥� John Schellnhuber, from the UK鈥檚 Tyndall Centre for Climatic Research, to warn that setting precise global danger levels may be 鈥渕ission impossible鈥�.

However, coming up with such targets could help climate policy to move beyond the Kyoto protocol, which comes into force on 16 February. The protocol will enforce small cuts in greenhouse gas emissions for some industrialised countries. 鈥淜yoto is just the very first step,鈥� acknowledged Margaret Beckett, the UK鈥檚 environment minister, who opened the meeting.