Leaders of the G-8 countries, the supposed ruling elite of the world, have declared their ambition to reduce their carbon emissions by a huge 80 per cent by 2050. By that time the developing countries are to reduce their own emissions by 50 per cent.
The leaders also said they recognised that global temperatures should not rise by more than 2 degrees Celsius above pre-industrial levels.
But no roadmap has been drawn, leaving many wonder whether the leaders mean what they have declared.
Difficult talks still lie ahead as negotiators try to firm up the ambitious goals, correspondents say.
Also, the cut in carbon emissions is only a target and will need the co-operation of rapidly industrialising such as China and India.
Wednesday China and India declined to support the objective of halving their greenhouse gas emissions by 2050, and Hu Jintao, the Chinese President, flew home to deal with growing problems in his own country.
While there are signals that India may be prepared to move, the G8 leaders do not expect agreement from the developing countries to halve emissions.
Even so, the G8 deal was being hailed last night by leaders. UK Prime Minister Gordon Brown told reporters that the agreement was historic: "Today in Italy we have laid the foundations for a Copenhagen deal that is ambitious, fair and effective. The change from where we were two, three, four years ago is significant. The world has now agreed that the scientific evidence on climate change is compelling," he said.
The agreement marks a significant step in efforts to limit greenhouse gases, which are blamed for the world's rising temperature. The G8 previously had not been able to agree on that temperature limit as a political goal.
Climate change experts say that the 2C threshold would not eliminate the risk of runaway climate change, but would reduce it. Even a slight increase in average temperatures could wreak havoc on farmers around the globe.
BBC's environment analyst Roger Harrabin says: "This is another example of how the politics of climate change is lagging behind the science. For the first time the G8 has accepted there are scientific limits to the amount of greenhouse gases we can emit - the Bush administration wasn't willing to accept that.
"But scientists insist that the rich nations should cut between 25-50% by 2020 to stabilise the climate - and that's a step too far for the G8. The rich nations will now ask the emerging economies to stem the growth of their own emissions - but India will accuse the West of failing to the action that it knows is necessary."
Meantime the Australian Government has released a report that says the impact of climate change on Australia shows detrimental effects are happening more quickly than expected.
Assistant Climate Change Minister Greg Combet has told ABC 2 News Breakfast that the report shows that there is a risk sea levels will rise by one metre by 2100 and extreme weather events such as storms, floods, bushfires and droughts could have higher impact.
"All of these things have very significant, not just environmental implications but economic implications, for things like food production, how we do our infrastructure, the future of land management," he said.
The Federal Government has committed to cut Australia's carbon emissions by between 5 and 25 per cent of 2000 levels depending on the outcomes of Copenhagen talks.
Its emissions trading scheme legislation will come before the Senate in August, but its chances of passing the Senate are uncertain, with the Opposition and crossbench senators all unhappy with it.
Greens Leader Bob Brown says if the Government supports the two degrees limit it must strengthen its emissions reduction targets.
"The Rudd Government and Climate Change Minister Penny Wong should commit to that two degrees limit and then adjust their weak targets of five per cent, or potentially 25 per cent, up to the levels that are going to make Australia a player in achieving two degrees or less," he said.