More than 200 international climate scientists issued a declaration today urging politicians at the United Nations Climate Change Conference in Bali

Scientists issue declaration at Bali

International researchers put their names to a proposal for emissions cuts.

More than 200 international climate scientists issued a declaration today urging politicians at the United Nations Climate Change Conference in Bali to agree on strong targets for tackling climate change.

Global greenhouse-gas emissions need to be reduced by at least 50% below 1990 levels by 2050, the declaration says. For comparison, the Kyoto Protocol aims for a reduction in developed nations of at least 5% below 1990 levels by 2012.

They declare that the goal "must be to limit global warming to no more than 2 ºC above the pre-industrial temperature". Many countries have already taken this limit as a benchmark figure for attempting to avoid dangerous levels of climate change, which would put millions of people around the world at risk from extreme-weather events.

Drawing on data from the 2007 Intergovernmental Panel on Climate Change report, they note that to achieve this, greenhouse-gas concentrations need to be stabilized at a level well below 450 parts per million of CO2 equivalents. This could mean peaking at 475 p.p.m. and then going down to 400 p.p.m. over the next 15 years, says co-signatory Matthew England of the Climate Change Research Centre at the University of New South Wales in Australia.

Even aiming for a stable level of 400 p.p.m. means that "we are only giving ourselves a three-quarter chance" of guaranteeing a less than 2 °C rise, says England.

These numbers have all been discussed before (see for example Dangerous climate change). There is still significant uncertainty when it comes to understanding what level of greenhouse-gas cuts would lead to what level of climate change, and scientists concede that there are no guarantees that the targets in this declaration will be sufficient to avoid dangerous consequences.

Some researchers have recently questioned the wisdom of setting firm targets for greenhouse-gas levels, saying it would be more useful to continually adjust targets as time passes and the consequences of our cuts become apparent (see Climate sensitivity inherently unpredictable).