Developed and developing nations have agreed that global temperatures should not rise more than 2C above 1900 levels, a G8 summit declaration says.

G8峰会声明，发达国家和发展中国家就全球温度不应该超过1900年的2C水准达成共识。

That is the level above which, the UN says, the Earth's climate system would become dangerously unstable¹.

US President Barack Obama said the countries had made important strides（大步，跨步） in dealing² with climate change.

But the G8 failed to persuade developing countries to accept targets of cutting emissions³ by 50% by 2050.

On Wednesday, the G8 agreed its own members would work towards 80% cuts by the same date.

UN Secretary General Ban Ki-moon said the G8 had not done enough and should also set 2020 targets.

He said that while the G8's Wednesday agreement was welcome, its leaders also needed to establish a strong and ambitious mid-term target for emissions cuts.

The second day of the summit, in the Italian city of L'Aquila, opened its discussions to take in the so-called G5 nations - Brazil, China, India, Mexico and South Africa. Egypt is a special invitee（被邀请者）.

In other developments:

1. The world's biggest economies have agreed to work to reach a global trade deal by 2010
2. Leaders of major developed and developing nations have agreed not to resort to competitive currency devaluations（货币贬值）
3. In a joint⁴ statement, President Obama and UK Prime Minister Gordon Brown said the detention⁵（挽留，延迟） of British embassy staff by the Iranian authorities was unacceptable

Significant step

The latest declaration was issued by the Major Economies Forum⁶, of 16 developed and developing nations - the G8, G5, Australia, South Korea and Indonesia - plus the European Union.

The group accounts for about 80% of the world's total greenhouse gas emissions.

"We recognise the scientific view that the increase in global average temperature above pre-industrial levels ought not to exceed 2C," it said.

It added that the economies would work towards a global goal for substantially reducing emissions by 2050 between now and December, when the UN holds talks in Copenhagen on a successor to the Kyoto treaty.

President Obama, who chaired the meeting, said the countries had had a candid⁷（忠实的，率直的） and open discussion about the growing threat of climate change and what must be done both individually and collectively to address it.

"I believe we've made some important strides forward as we move towards Copenhagen," he said.

"I don't think I have to emphasise⁸ that climate change is one of the defining challenges of our time. The science is clear and conclusive⁹ and the impacts can no longer be ignored."

Early agreement 'unlikely'

RK Pachauri, who chairs the Inter-governmental Panel on Climate Change, praised the declaration's mention of the 2C limit but said more details were needed.

"It certainly doesn't give you a roadmap on how you should get there but at least they've defined the destination," he told the BBC World Service Newshour programme.

Mr Obama added that the United States, as a major polluter, had not met its responsibilities in the past, but those days were over.

But his scientific adviser¹⁰, John Holdren, told the BBC it was unlikely that the US could come to any early agreement on tough reductions in emissions by 2020 proposed by the EU.

"If we had not wasted the last eight years, we could probably achieve that target," he said.

"But we did waste the last eight years and in consequence, it doesn't make a lot of sense for us to officially embrace a target that is not realistically within reach."

BBC environment analyst¹¹ Roger Harrabin says the declaration is a significant step, with all big countries rich and poor agreeing there is a scientific limit on the amount we should warm the climate.

But there is still a huge way to go, he says, as developing nations like India will not sign up to any 2050 targets unless rich nations show more determination and offer more cash.

The G8 summit began in L'Aquila on Wednesday, with the first day largely taken up with discussion of the fragile state of the global economy.

The leaders also issued a statement reaffirming（重申，再肯定） that they were "deeply concerned" by Iran's nuclear programme and condemning¹² North Korea's recent nuclear test and missile launches.

African leaders will join the summit on Friday to push for a new initiative（第一步，首创精神） to fund farming in the developing world and tackle global hunger.