- 04 November 2009 by Catherine Brahic and Fred Pearce
- New Scientist issue 2733. Subscribe and get 4 free issues.
TWO-hundred-and-fifty billion tonnes. That's the bottom line. If we are serious about avoiding dangerous climate change, 250,000 megatonnes is the maximum amount of carbon we can put into the atmosphere. Keep going at current rates and we will have used up that ration in 20 years.
The challenge for delegates at the week-long meeting in Denmark's capital is to agree on ways of ensuring we do not exceed it - ever.
Why this year?
Two years ago in Bali, member nations of the UN Framework Convention on Climate Change (UNFCCC), which is convening the Copenhagen summit, agreed that they would accelerate their efforts and draft a long-term plan to avoid dangerous climate change. Their deadline for doing so is the close of this year's summit, on 19 December.
Hasn't the Kyoto protocol shown all this to be pointless?
Not necessarily. The Kyoto protocol was always intended as a first step. There are a number of differences this time around, most notably that the US opted out of the Kyoto protocol but is very much engaged in the Copenhagen process.
Why 250,000 megatonnes?
We have already emitted over 500,000 megatonnes of carbon - equivalent to about 1,800,000 megatonnes of carbon dioxide - mostly by burning fossil fuels and cutting down forests. This year, climate scientists calculated that if we emit no more than 750,000 megatonnes in total, we will have a 75 per cent chance of limiting global warming to 2 °C.
What is the significance of 2 °C?
The objective of the UNFCCC is to prevent "dangerous" climate change. Although any amount of warming may have consequences - including biodiversity loss, changing weather patterns and disappearing coastlines - many climate scientists predict that some of those changes will be irreversible beyond 2 °C and others will pose a serious threat to millions of people. As a consequence, 2 °C has been adopted by politicians as the threshold for dangerous climate change.
Is 2 °C little enough?
That all depends: little enough for what? No amount of warming is risk-free, and modelling studies indicate that at 2 °C an additional 1 billion people will suffer water shortages and most of the world's corals will be bleached. The world's poorest nations, which include a number of island states that are particularly vulnerable to sea-level rise, are campaigning to limit warming to 1.5 °C. Given the effort that is going to be required to reach the 2 °C target, this is unlikely to be achieved. Moreover, lags in climate systems, plus the removal from the atmosphere of the fine aerosol particles now cooling the world, mean past emissions are likely to result in a 1.9 °C warming.
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