We've been hearing variations of the phrase "the world only has 12 years to deal with climate change" a lot lately.

Sen. Bernie Sanders put a version of it front and center of his presidential campaign last week, saying we now have "less than 11 years left to transform our energy system away from fossil fuels to energy efficiency and sustainable energy, if we are going to leave this planet healthy and habitable."

But where does the idea of having 11 or 12 years come from, and what does it actually mean?

Sign up for InsideClimate News Weekly Our stories. Your inbox. Every weekend.

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."

How Do Scientists Know?

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.'

"There's certainly a grain of truth in the phrase, but it's so oversimplified that it leads to comically bad misconceptions about how to get there, conjuring up ridiculous cartoon imagery suggesting we just go on with life normally for the next 11 years and then the world ends," Denning said.

That's not what the IPCC writers envisioned, he said.

The science on the 2030 date is clear, said Michael Mann, a climate scientist at Pennsylvania State University. The controversy stems from people mischaracterizing the carbon reduction timeline as a threshold for climate disaster. He noted that people promoting climate science denial and delay have also latched on to the phrase "to intentionally try to caricature the concern about climate change."

What Would Success Look Like?

It would be helpful if people looked at the 2030 target in terms of what success looks like rather than what failure means, Denning said.

"Solving the problem by 2030, 2040 or 2050 requires a new global energy infrastructure, which is arguably easier and less expensive than past infrastructure shifts like indoor plumbing, rural electrification, the automobile and paved roads, telecommunications, computers, mobile phones or the internet.

"All of these past changes cost tens of trillions of dollars, adjusted for inflation. All of them were hugely disruptive. All of them took a decade or more, completely changed the industrial and economic and social landscape, and created bursts of growth and productivity and jobs. And arguably, all of them made life better for huge numbers of people."

This time, the shift is from heavy reliance on carbon-emitting fossil fuels to carbon-free energy sources, like wind power. And even with a speedy energy transition, the IPCC says keeping temperatures from warming more than 1.5°C will also likely require removing CO2 from the atmosphere on a large scale.

Missing the target doesn't imply the onset of cataclysmic climate change in 2030, Denning said.

"Things just keep getting worse and worse until we stop making them worse, and then they never get better," he said. "But no matter what, the world has to move on from fossil fuels just as we moved on from tallow candles and outhouses and land lines."

What Would Exceeding 1.5°C Warming Mean?

The IPCC report described how increasing greenhouse gas emissions will result in more dangerous and costly disruptions to global societies and ecosystems, including longer, hotter heat waves and more frequent crop-killing droughts.

Mountain glaciers will melt faster as the planet warms, creating new risks for settlements in the valleys below. The meltdown of polar ice sheets is also projected to accelerate, intensifying flooding and speeding up sea level rise to a rate that will be hard to adapt to. More Arctic permafrost will thaw, releasing more greenhouse gases to the atmosphere.

Despite the rising risks, it's important to understand that, "in the physical climate system, there are no scientists claiming that there is a magical threshold that we breach or don't breach that determines whether we have a habitable climate system," said Daniel Swain, a climate scientist at UCLA and the National Center for Atmospheric Research's Center for Climate and Weather Extremes.

The 2030 target is useful because it shows how the "next decade is incredibly consequential for what we do." Swain said. "But I think the emphasis that's being placed on this specific 12-year window as a differentiator between existential crisis or not is problematic.

"First of all, it negates some of the risks that already exist and that will continue to build no matter what. And it also potentially suggests that anything short of complete victory in the next 12 years is pointless, which is exactly the opposite of the truth. At any point along the spectrum, more progress is always going to be better than less progress, less warming is always going to be better than more warming."

Have We Passed Tipping Points Already?

In some ways, the "12 years" narrative may set up a deadline that's too lenient, because some key part of the climate system may already be at or past tipping points, Swain said.

It creates the false illusion that there is some sort of guardrail moving forward, that if we just get in under the deadline we'll be OK, he said. But "twelve years from now, it could be too late for some of these things, like the ice sheets."

Research in the past few years reinforces the idea that some climate tipping points have already been breached. Studies show some parts of the Greenland Ice Sheet are unlikely to recover, and parts of the West Antarctic Ice Sheet may also be at or very near a tipping point to rapid disintegration.

A study published in June suggested that the rate of permafrost thawing is progressing much faster than climate models projected. And scientists studying the link between global warming and European heat waves said those recent extremes are also outside the scope of what they expected at current levels of warming.

The world will still exist if we breach 1.5°C and 2°C, but "the climate impacts and risks will be higher and the temperature will be higher," said Glen Peters, research director at the CICERO climate research center in Oslo. That all seems to be sinking in to public awareness, he said.

"But in terms of deadlines, we have already missed the deadline," he said. "We should have started mitigating decades ago, then we would have the problem solved."

Published Aug. 27, 2019