That means we must look towards renewables, especially solar and wind. Both 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 economy fast 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.

What scientists have to say

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 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.

How we can make the switch

Energy production accounts for two-thirds of global greenhouse gas emissions, and thus transformation of this sector is crucial, experts said.

“Decarbonising energy is probably the quickest way to decarbonise the world,” Adnan Amin, director general of the International Renewable Energies Agency, told AFP.

The question, however, is whether solar, wind and other clean energy options can scale up quickly enough.

According the UN climate science body, the Intergovernmental Panel on Climate Change (IPCC), low-carbon energy must account for at least 80 percent of global electricity production by 2050 to have a better-than-even chance of staying under the 2 C threshold .

The good news is that renewables are expanding rapidly and attracting investment.

Nearly half of all new installed power generation capacity in 2014 was in renewables — 37 percent wind, a third solar and a quarter hydro, according to the IEA.

The cost of solar and wind energy too has plummeted .

Can India do it?

In developing 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 Alden Meyer, a veteran climate specialist. India has invested massively in clean energy, and pledged to install 175 gigawatts of renewable capacity by 2022.

But renewables only account for about 20 percent of global electricity generation. Of the total energy consumption — dominated by coal, oil and gas — less than five percent comes from clean technology, excluding nuclear.

Which is why, experts say, the Paris climate summit, which starts at the end of this month, is so crucial.