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To fix climate change, everything has to change. This much is true and known. But new research out in Nature Climate Change this week shows how tall the task is in particularly stark terms.

The study focuses on what researchers call “residual emissions,” which are basically all the leftover sources of carbon pollution we have to deal with after the comparatively easy task of cleaning up how we get electricity. The findings reveal that the world has a herculean task ahead of it, including developing currently unproven negative emissions technologies. And if we don’t get cracking in the next decade, the task could become Sisyphean.

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Back in the hopeful, heady days of 2015, the world came together to craft the Paris Agreement. It laid out the goal of keeping global warming to within 2 degrees Celsius of pre-industrial temperatures and within 1.5 degrees Celsius if at all possible. There’s a finite pool of greenhouse gases we can put in the atmosphere if we want to meet those goals and avert a climate catastrophe.

The problem is that even with each country’s commitments to the Paris Agreement, we’re still on a path of economic development that depends on fossil fuels for the foreseeable future. The time it takes to shift course will result in residual emissions. A lot of them.

“Most of these residual emissions will be generated in the coming 20-30 years,” Gunnar Luderer, a researcher at the Potsdam Institute for Climate Impact Research who led the nearly two dozen authors on the new study, told Earther. “It simply takes time to replace the current fossil-based system with a climate-friendly one, and even with strong climate action substantial emissions will remain during the transition.”

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Those residual emissions will come from various sources, including electricity production which offers the most pathways to becoming carbon-free by 2050. But the other facets of the global economy that account for three-quarters of the world’s greenhouse gas emissions—including agriculture, forestry, buildings, transportation—are harder to decarbonize.

Take transportation. Electric cars make up just 0.3 percent of all the cars on the road globally. Their numbers will swell dramatically by 2030 in most forecasts, but the combustion engine is still going to be a Thing for the foreseeable future, to say nothing of air and ship travel. The latter two are both areas where there’s no clear path forward to decarbonization, leading to what Luderer said were “decarbonization bottlenecks” on the demand side.

“Even with strong efforts towards better energy efficiency and increasing biofuels use some fossil emissions are likely to remain—and will further add to total residual fossil fuel emissions,” he continued.

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Even with aggressive climate policies, Luderer and his colleagues estimated there’s still about 1,000 gigatons of residual carbon emissions that will accumulate in the atmosphere by 2100. That’s way lower than the 4,000 gigatons humanity would commit to the atmosphere if we continued burning fossil fuels at our current rates, but it’s way over the total carbon budget of keeping warming below the 1.5 degrees Celsius threshold. And that means that any hope of meeting that goal will require removing carbon dioxide from the atmosphere.

There are multiple ideas for doing that, from planting vast swaths of forest to fertilizing the oceans so plankton blooms suck it up. Other researchers are working on machines that can do the same job or even capture emissions at the source and store them as a stable carbonate, inject it into the ground or even back into the oil wells from whence it came.

All are interesting ideas, but they’re also unproven at a scale and cost that’s useful to address climate change. And yet humanity’s fate, especially people living on small, low-lying islands, hinges on finding the solutions.

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There’s one other really important finding in the new research. If we don’t get our asses in gear until 2030, the problem of residual emissions could become an insurmountable one. The issue is twofold. First, there’s the actual emissions between now and 2030, which reduce the carbon we have left over in the bank. Then there’s also the issue of path dependency. If we do very little to address climate change over the next 12 years, the world will continue to rely on carbon-emitting technology for another decade or two longer after 2030. Luderer said that basically means that every ton of excess carbon dioxide emitted before 2030 results in two more tons down the road.

“Their conclusions confirm that meeting 1.5 degree Celsius targets will be nigh on impossible if we delay strong emission reductions much longer as parts of the energy production and transport infrastructure depend on fossil fuels for many years,” Piers Forster, the head of the Priestley International Centre for Climate at the University of Leeds, told Earther. “They also confirm that large amounts of carbon dioxide removal is needed beyond 2030 along with demand reduction.”

This is why Donald Trump proposing to pull the U.S. out of the Paris Agreement is such a big deal (to say nothing of still-committed countries who are, say, still building pipelines to export dirty oil) and why the falling costs of solar and wind are nowhere near enough to stave off catastrophic climate change. I wish I could sugar coat it, but we’ve got our work cut out for us. And the clock is ticking.

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