Together they produce 78 per cent of the world's greenhouse gases stemming from fossil fuel burning.

Bottom line

More such commitment letters were expected to continue trickling in over the next several days, but Pasztor said that "it is likely, according to a number of analysts, that if we add up all those figures that were being discussed around Copenhagen, if they're all implemented, it will still be quite difficult to reach the two degrees - that is the bottom line".

in depth
The "two degrees" refers to the Copenhagen target of keeping the Earth's average temperature from rising two degrees Celsius above the levels that existed before nations began industrialising in the late 18th century – or no more than 1.3 degrees C above today's average temperatures.

Alden Meyer, the policy director at the Union of Concerned Scientists, called the "political commitment" a breakthrough of sorts.

"This is the first time countries are committed to this goal, that's the good news," he said. "The bad news, of course, is the pledges that have been put on the table to date don't put us on track to meet that goal."

The commitment letters, which largely reaffirm previous pledges, were intended to get an idea of how far the nations most responsible for global warming might be willing to go, towards a legally binding pact at the climate conference planned for Mexico City at the end of the year.

China has pledged to reduce its emissions growth - not make absolute cuts - by up to 45 per cent from 2005 levels by 2020.

India also pledged to reduce emissions growth by up to 25 per cent from 2005 levels by 2020.

The US stuck to its pledge to cut absolute carbon emissions by about 17 per cent below 2005 levels, by 2020.

The EU pledged to cut its carbon emissions 20 per cent below 1990 levels by 2020, and to cut 30 per cent if other nations deepen their reductions.

Mexico talks

Yvo de Boer, the UN climate chief, said the pledges sent in by January 31 should at least help to reinvigorate negotiations towards a legally binding treaty on climate change at this year's conference in Mexico City.

"The commitment to confront climate change at the highest level is beyond doubt," de Boer said, adding that the pledges were "clear signals of willingness to move negotiations towards a successful conclusion".

The Copenhagen accord included collective commitments by developed countries to provide billions of dollars to help poor countries adapt to climate change.

Nations were encouraged to formally "associate" themselves with the accord after the conference, but no deadline for that action was set.

Critics, however, say the accord, which fell far short of a legally binding treaty, was a failure, with world leaders missing a crucial opportunity to commit to greenhouse gas cuts.

Many scientists believe global emissions must be cut in half by mid-century in order to avoid the melting of glaciers and icecaps, the flooding of low-lying coastal cities and islands, and worsening droughts in Africa and elsewhere.