World leaders are to meet for an unprecedented second summit on climate change this year to try and forge agreement on a tough new treaty by December, and may even get together for a third time before the end of the year.

UN secretary-general Ban Ki-Moon is to call the world's heads of government to New York in September to "galvanise political will" about what he describes as "the defining issue of our time."

And there are plans for another G20 summit to discuss the issue in the autumn.

These will follow a meeting of 17 key world leaders convened at the initiative of US president Barack Obama immediately after the annual G8 summit in July.

Observers cannot remember any similar progression of top level meetings to address any issue over such a short period of time. The moves come as pressure mounts on the leaders to reach agreement at December's vital negotiations in Copenhagen, billed as the world's last chance to get to grips with global warming before it escalates out of control.