What Does ‘12 Years to Act on Climate Change’ (Now 11 Years) Really Mean?

2019-08-27

Excerpt from this InsideClimate News story:

The number began drawing attention in 2018, when the United Nations’ Intergovernmental Panel on Climate Change released a report describing what it would take to keep global temperatures from rising more than 1.5 degrees Celsius, a goal of the Paris climate agreement. The report explained that countries would have to cut their anthropogenic carbon dioxide emissions, such as from power plants and vehicles, to net zero by around 2050. To reach that goal, it said, CO2 emissions would have to start dropping “well before 2030” and be on a path to fall by about 45 percent by around 2030 (12 years away at that time).

Mid-century is actually the more significant target date in the report, but acting now is crucial to being able to meet that goal, said Duke University climate researcher Drew Shindell, a lead author on the mitigation chapter of the IPCC report.

“We need to get the world on a path to net zero CO2 emissions by mid-century,” Shindell said. “That’s a huge transformation, so that if we don’t make a good start on it during the 2020s, we won’t be able to get there at a reasonable cost.”

Basics physics and climate science allow scientists to calculate how much CO2 it takes to raise the global temperature—and how much CO2 can still be emitted before global warming exceeds 1.5°C (2.7°F) compared to pre-industrial times.

Scientists worked backward from that basic knowledge to come up with timelines for what would have to happen to stay under 1.5°C warming, said Scott Denning, who studies the warming atmosphere at Colorado State University.

“They figured out how much extra heat we can stand. They calculated how much CO2 would produce that much heat, then how much total fuel would produce that much CO2. Then they considered ‘glide paths’ for getting emissions to zero before we burn too much carbon to avoid catastrophe,” he said.

“All this work gets summarized as ‘in order to avoid really bad outcomes, we have to be on a realistic glide path toward a carbon-free global economy by 2030.’ And that gets translated to something like 'emissions have to fall by half in a decade,’ and that gets oversimplified to '12 years left.’