Hitting the more generous target would require emissions to be cut to zero within 40 years. Even then, technologies to suck carbon emissions out of the atmosphere — know as negative emissions technologies — may be needed, he said.
But since the last report by the UN’s Intergovernmental Panel on Climate Change, the growth of emissions from rapidly industrializing powers like China and other Asian countries have slowed, while European and US emissions have fallen.
Global emissions have held steady for the past three years even as the world’s economy has grown, the International Energy Agency reported in March. And the Paris pact has a mechanism for countries to stiffen their emissions targets every five years, something the author urged them to do at the first opportunity.
“What our paper shows is keeping warming to 1.5°degrees just went from impossible to very difficult,” added Joeri Roeglj, of Austria’s International Institute for Applied Systems Analysis.
Not so fast, cautions Penn State climatologist Michael Mann, who called himself “rather skeptical” of the study. Recent improvements in emissions may have given the world a running start toward hitting the Paris targets, Mann said — but he said most studies have underestimated how much carbon was building up in the atmosphere before regular record-keeping began in the 1880s.
Mann said he’s “bullish” about hitting the 2-degree mark. “But 1.5°C or less warming may simply be off the table without highly speculative negative emissions technology,” he added.
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Millar and his team also don’t leave much headroom for unexpected events such as an acceleration of warming from unexpected sources, such as a large-scale release of greenhouse gases now trapped in Arctic permafrost — the so-called “methane bomb” scenario. Millar said the study focused on human-induced warming, not natural variability.
While Millar and his team focused on how much carbon the world could still release to hit the Paris targets, researchers at Texas A&M and the Scripps Institution of Oceanography in California have laid out a strategy for hitting those marks.
“It’s clear the current trajectory is not sufficient,” said Texas A&M climate scientist Yangyang Xu, the lead author of a new study published in the journal Proceedings of the National Academy of Sciences. “So we wanted to give a probabilistic estimate of what would happen if we commit more.”
Without rapid reductions in emissions, the world has a 50 percent chance of blowing past the 1.5-degree mark by 2050 and producing catastrophic climate changes in the second half of the century, Xu said. Even if renewable technology that could take care of the world’s energy needs, “it would be likely to take between three and five decades to spread such technology to the whole world,” he and his co-author, Veerabhadran Ramanathan, wrote.
Xu and Ramanathan laid out three “levers” for bringing down emissions rapidly. The first would reduce CO2 emissions through steps like the speedy deployment renewable energy. The second would cut emissions of “super-pollutants” like nitrogen oxides, hydrofluorocarbon coolants, and methane, which are shorter-lived but much more potent heat-trapping effects.
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The third would boost the development of carbon capture and storage technologies, which aim to peel off the carbon byproducts from fossil fuels like coal and pump them underground — a technology that’s shown mixed results so far.
Ideally, all three of those levers would be pulled by 2020, Xu said. And even then, it’s likely the world will overshoot the 1.5-degree mark before the effects of those emissions cuts fully kick in and temperatures fall.
“We need to get serious and think about all the cards we can play and really work hard to reduce warming,” he said.
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