http://news.bbc.co.uk/2/hi/science/nature/8414798.stm

Three days of action from ministers are needed to "seal a deal" at the climate talks in Copenhagen, according to UN Secretary-General Ban Ki-moon.

Speaking at the opening of the high-level segment, he told delegates they had the chance to change history.
But governments remain deadlocked on many key issues, including the size of emission targets, finance, and verification of emission curbs.
Demonstrations are expected on Wednesday as ministers convene.
Campaign groups are talking of mounting actions inside and outside the conference centre.
There is anger about the glacial progress of some segments of the talks and about logistical problems that have seen most people from NGOs unable to enter the venue.
UK Climate Secretary Ed Miliband acknowledged that talks "could still fail".
'Decisive moment'
Mr Ban told delegates there was still an "enormous amount of work to be done" if a deal was to be achieved.
"For three years, I have sought to bring world leaders to the table," he said.
"No-one will get everything they want; but if we work together, then everyone will get what they need."
Danish Prime Minister Lars Lokke Rasmussen told ministers they must make this conference "a decisive moment of change".
"Climate change is higher on the agenda than ever before," he said.
"And so it should be; the grim projections from science grow more alarming each day and already many face the dire consequences of global warming."
Different visions of what the science implies are, though, the cause of one of the most fundamental fault lines running between delegations here - what figure should be adopted as the target for limiting the global average temperature rise since pre-industrial times.
Mr Ban came under fire from developing countries after asserting in a BBC interview that a deal here must "put us on the path of limiting global temperature rise within 2C".
Small island states, and other countries that consider themselves vulnerable to climate impacts, have been demanding a lower target of 1.5C and the issue is still under negotiation.
"It is simply a true fact - if temperatures get to 2C, that spells disaster and almost doom to our countries," said Bruno Sekoli of the Lesotho delegation, which chairs the bloc of Least Developed Countries (LDCs).
Bolivia's ambassador, Pablo Solon, added: "This is the Secretary General; he can't take sides on an issue like this."
Britain's Prince Charles, who also addressed the opening of the high-level segment of the conference, said the planet had "reached a crisis" which society had just seven years to solve.
"With issues of such magnitude," he said, "it is easy to focus solely on the challenges, the worst-case scenarios, the 'what-ifs' of failure.
"But take a moment to consider the opportunities if we succeed... a healthier, safer and more sustainable, economically robust world."
Stumbling blocks
Elsewhere in the conference, discussions continued - mainly behind closed doors - on some of the issues that continue to divide governments two years after the process towards a new global climate deal started and just three days before it is due to conclude.
Mr Miliband, who co-chaired talks on finance, said there had been acknowledgement of the need for "significantly scaled-up public funding".
Developing countries have demanded that a majority, if not all, of the money they will receive in future for clean development and climate adaptation should come from public funds; but developed nations have been equally keen that a substantial share - perhaps the majority - should come from levies on the prospective global carbon market.
Another of the discussion groups, on emission pledges by developing countries, made little progress.
A senior source told the BBC that the main stumbling block was US insistence that commitments should be legally binding.
China in particular is adamant that such plans must be voluntary and that emission curbs should not be subject to international verification - something that many members of the US Senate are said to regard as key if they are to back any agreement here.
US chief negotiator Todd Stern said he did not expect the US to increase its current offer of cutting emissions by about 3% from 1990 levels by 2020.
"I am not anticipating any change in the mitigation commitment," he told reporters.
The US announced that Secretary of State Hillary Clinton is to attend the final two days of the summit, in addition to President Barack Obama.
Walk-outs
Around the conference centre, campaign groups gave advance notice of actions expected inside and outside the venue on Wednesday.
Having allowed three times more people to register than the centre can hold, the UN climate convention and the Danish hosts have limited numbers of NGOs to 7,000 on Wednesday, falling to 1,000 on Thursday and just 90 on Friday when heads of state and government are scheduled to attend.
Activists are planning a series of morning marches outside the centre, with some groups attempting unauthorised entry.
A walkout - possibly involving government delegates - is anticipated, and there are also plans for a "sleep-in" on Wednesday night.
Organisers say the actions are intended to be non-violent.