Some start to get over environmental denial.

Scientists are admitting that it is too late to prevent carbon dioxide from getting to levels where 2 degrees of warming can be prevented according to climate models. The entire approach of asking for costly economic sacrifices

Countries have delayed action for so long that the necessary emissions cuts will have to be extremely sharp. In April 2014, the UN’s Intergovernmental Panel on Climate Change (IPCC) concluded that if we want to stay below the 2°C limit, global greenhouse-gas emissions would have to decline between 1.3 percent and 3.1 percent each year, on average, between 2010 and 2050.

The fastest that any country has ever managed to decarbonize its economy without suffering a crushing recession was France, when it spent billions to scale up its nuclear program between 1980 and 1985. That was a gargantuan feat — emissions fell 4.8 percent per year — but the country only sustained it for a five-year stretch.

So how do we cut emissions that sharply? First, the IPCC says that the world would have to triple or quadruple the share of clean energy that it uses. Second, we’d have to get dramatically more efficient at using energy in our homes, buildings, and cars.

Right now, about 17 percent of the world’s energy is “low-carbon” — a little bit of wind and solar power, some nuclear power plants, a bunch of hydroelectric dams. Countries would have to ramp those sorts of technologies up dramatically — tripling or quadrupling their share.

That means two things. First, it’s tough to rule out any particular technologies. For instance, some environmentalists are opposed to nuclear power. But the IPCC estimates that the task of cutting emissions becomes 7 percent more expensive if we shuttered all our nuclear plants. Likewise, the technology to capture carbon emissions from coal plants and bury it underground is still in its infancy. But if that technology proves unworkable, then the task of cutting emissions becomes twice as expensive.

Second, the IPCC notes that investment in fossil fuels — coal, oil, and natural gas — would have to decrease by 20 percent in the next few decades. After all, if solar power ramps up, but conventional coal expands even faster, emissions would rise, not fall.

Is this all doable? The IPCC report suggests that it’s at least technologically feasible. Whether it’s politically realistic is another matter.

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