INTERVIEW-UPDATE 1-CO2 emissions about 2.6 pct down in 09-IEA

* Global carbon emissions to fall about 2.6 pct in 2009

* About twice previous record drop in last 40 years, in 1981

LONDON, Sept 21 - Global emissions of the commonest man-made greenhouse gas carbon dioxide will fall by about 2.6 percent in 2009, the steepest fall in at least 40 years, the International Energy Agency said on Monday.

The world must use the drop to drive a global fight against climate change, rather than allow emissions to rise again as after previous recessions, Fatih Birol, IEA chief economist, told Reuters in an interview.

As well as trimming burning of high-carbon fossil fuels, the financial crisis had stunted investment in energy infrastructure, which could also aid a global transition to low-carbon alternatives, Birol said.

"This fall in emissions and in investment in fossil fuels will only have meaning with agreement in Copenhagen which provides a low-carbon signal to investors," he said.

The world is meant to thrash out in Copenhagen in December a new, tougher climate pact to replace the Kyoto Protocol after 2012, under faltering U.N.-led talks.

U.N. Secretary-General Ban Ki-moon is holding a one-day climate change summit for world leaders at the U.N. headquarters in New York on Tuesday, to generate momentum for the process.

U.S. carbon emissions will fall 6 percent this year, the Energy Information Administration said two weeks ago [ID:nN09339265], and Europe's emissions will fall by 4-5 percent, Deutsche analyst Mark Lewis told Reuters.

Balancing that, industrial output and carbon emissions are rising in developing countries, and especially in the world's biggest carbon emitter China, but global emissions would drop overall, the IEA said.

"The biggest fall was in 1981, at 1.3 percent, after oil price shocks and economic troubles," said the IEA's Birol.

"We estimate this year the fall will be around twice that," he told Reuters.

"We have looked country by country at power, coal, oil and gas consumption for the last eight to eight and a half months and we estimate what could happen in the next three months," he said, referring to IEA analysis of both OECD and non-OECD countries.