It's being billed as the meeting that will determine the future of humanity. Come early December, we will be inundated with news from the Copenhagen summit. Can it really save us from climate catastrophe? Catherine Brahic and Fred Pearce sift through the mass of science and policy to pick out the key points to watch
Why this year?
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 14 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.
What needs to be done?
There are no two ways about it: to have any chance of avoiding the disastrous consequences of exceeding our carbon budget, we must usher in a new era of low-carbon societies.
How this is done will depend on what deal can be reached between rich and developing nations. Both must agree to cut emissions according to their means and historical responsibility.
Developing nations will also need money and technology to green their industrialisation. Where this will come from will be a key preoccupation for the Copenhagen negotiators
THE HAVES...
must cut emissions by
BY 2020 BY 2050
NEED 25-40 % 80-95 %
AGREED SO FAR 10-24 % 40-80 %
AND HAVE NOTS
must cut emissions by
BY 2020
NEED 15-30 %
AGREED SO FAR 10-15 %
Money
It could cost the poorest nations hundreds of billions of dollars a year to curb their emissions and adapt to inevitable climate change.
Rich nations are responsible for most of the gases that are already heating the planet, and have a duty to help foot this bill. Negotiators in Copenhagen will have to agree on how.
Funds could be raised through taxes on emissions permits, for instance, or on international airline tickets. Or there could be a levy on all carbon emissions above certain national thresholds - as proposed by Switzerland.
The European Union agreed last week to push for a fund worth €100 billion a year by 2020.
Forests
Around 15 per cent of emissions come from deforestation. WWF believes this could be cut by three-quarters by 2020, but that requires giving governments, landowners and forest communities incentives to stop destroying their forests.
Two years ago, climate negotiators promised to sign such a deal - dubbed Reducing Emissions from Deforestation and Forest Degradation (REDD) - in Copenhagen.
The cash could come from rich nations buying carbon offsets to meet their emissions targets.
Brazil and Indonesia - which account for 60 per cent of emissions from deforestation - are keen. But close monitoring is essential to ensure loggers claiming cash for a forest do not continue chopping down individual trees or move their operations elsewhere.
Also, countries such as Costa Rica that have protected their forests say it unfairly rewards those who got rich destroying theirs.
Technology
Two billion people worldwide do not have access to mains electricity.
To bridge that gap and power industry in developing countries, the International Energy Agency says $13 trillion must be invested in the developing world in the next 20 years.
In Copenhagen, negotiators must seal a deal to ensure this goes mostly into low-carbon technologies - but how?
Western engineering firms want an open door to developing markets, perhaps secured by a "green free trade" deal. Countries like India and China want deals with rich nations that would give their own companies free access to western know-how.
Deal breakers
Who might thwart a deal?
The US may not be able to make credible promises if Congress has not passed a climate change bill in time.
If China and India think the US is not serious, they will hold back on pledges to green their own economic development.
Others might wield a veto, too. Some newly industrialised countries - Malaysia and South Korea for instance - now have emissions higher than many European countries. They may protest if asked to sign up to firm targets.
Malaysia's emissions are four times what they were in 1990 and, per head of population, equal to the UK's.
Saudi Arabia's emissions have doubled and, per head, now beat all European countries except Luxembourg.
Qatar's per-capita emissions are four times those of the US.
Gulf states tried to torpedo Kyoto because they felt it threatened oil exports. Copenhagen could threaten their internal industrialisation plans.
