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 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.
Cross that red line, scientists say, and there will, almost literally, be hell to pay.
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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.
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.
India has invested massively in clean energy, and pledged to install 175 gigawatts of renewable capacity by 2022.