That means renewables, especially solar and wind, both of which face fewer constraints to growth than more established clean energy: a river can be dammed only so many times, and nuclear remains expensive and controversial.

But humanity has dithered for so long in the fight against global warming, experts say, that the window of opportunity for decarbonising the global economyfast enough to avoid devastating climate change is barely ajar.

"The cost and difficulty of mitigating greenhouse gases increases every year, time is of the essence," Maria van der Hoeven, executive director of the International Energy Agency, said in a special IEA report on energy and climate change released earlier this year.

The world's nations -- gathering in Paris in a month to ink the first-ever universal climate pact -- have set a target of limiting global warming to two degrees Celsius (3.6 degrees Fahrenheit) above pre-industrial levels.

Cross that red line, scientists say, and there will, almost literally, be hell to pay.

Science also tells us that, if we are to respect the 2 C limit, future greenhouse gas emissions cannot exceed a total "budget" of about 1,000 billion tonnes of carbon dioxide.

Carbon-cutting pledges from nearly 150 nations, unveiled on Friday, put us on track for a 3 C world.

This is a vast improvement on doing nothing. But even this unprecedented effort would use up three quarters of that carbon budget by 2030, leaving very little margin for closing the remaining gap.

That's where the transition from fossil fuels to renewables comes in.

"The economics have been shifting on both sides of the equation," said Alden Meyer, a veteran climate specialist with the Union of Concerned Scientists in Washington. "The least-cost global strategy is to rapidly reduce our reliance on fossil fuels and switch into the clean-energy economy."

In poor countries, this holds out the possibility of skipping past the fossil fuel stage of development, much in the way some regions went from no phones to cell phones.

"I think India" -- where 300 million people are without electricity -- "is realising that it may be easier and more cost effective for them to provide sustainable energy services to hundreds of millions of villagers through a decentralised renewable-based strategy," said Meyer. "They and other countries are poised to leapfrog the fossil fuel age."

India has invested massively in clean energy, and pledged to install 175 gigawatts of renewable capacity by 2022.