Much of that warming is already baked in, recent studies say
Climate negotiators inserted a dramatic charge in the 2015 Paris accord, asking world leaders to strive to keep global temperatures at just 1.5 degrees Celsius above pre-industrial levels.
Now new studies have begun to sketch out what the tighter target -- compared to the longtime benchmark goal of 2 degrees (3.6 degrees Fahrenheit) -- actually means. Their overall message to climate envoys meeting in Bonn, Germany this week: Better get cracking.
“We would need an incredibly dramatic reduction in emissions in the very near future,” said Zeke Hausfather, a climate scientist with Berkeley Earth. He called the 1.5 degree target “a little ridiculous and implausible.”
The scientific studies suggest that every year that goes by without global emissions peaking would require larger pollution cuts in the future. As it stands, the world has “room” left in the atmosphere for less than 20 years of emissions at current rates.
An essay Hausfather published on the website CarbonBrief estimates that if emissions peak in 2020, then by 2030, the carbon-emissions rate will have to drop by 9 percent a year. If the peak had come in 1995, required cuts in 2030 would have been just 2 percent -- and off a much lower baseline.
But emissions are still rising. In 2017, they’re expected to go up by 2 percent, according to researchers in the Global Carbon Project. That’s much lower than rates seen in the early 21st century, but still the wrong direction.
Already, even the most optimistic scenarios can’t hit a 2-degree goal without assuming that whiz-bang future technology will emerge to pull carbon dioxide out of the air. The climate models tend to show that it’s unrealistic to reduce pollution by more than 5 percent or 6 percent a year, Hausfather said. To get around that sticking point, the models build in "negative emissions" later in the century -- perhaps the most significant TBD of all time.
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“The idea that we’re going to depend on this largely unknown technology to get us to these targets is a little worrisome,” Hausfather said.
The world has already warmed by about 1 degree Celsius since the end of the 19th century, and there’s momentum in the system. In a thought experiment, authors of a new U. National Climate Assessment found that if atmospheric carbon dioxide stayed at its current level, that would lock in another 0.6 degrees of warming. Another study found that if all emissions magically stopped, the planet would eventually warm between 1.
Veerabhadran Ramanathan of the Scripps Institution of Oceanography in September envisioned an aggressive scenario in which nations yank hard on three main "levers" --zeroing out carbon emissions, slashing other greenhouse gases that don’t hang in the air as long, and deploying machines that suck carbon out of smokestacks and stick it underground. In that scenario, efforts to “bend the warming curve to a cooling trend” should begin by 2020. Negative emissions would come later.
"Since 2020 is just a few years away, this is a highly optimistic option," they write, with an understatement characteristic of the climate scientists.
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A June study in Earth’s Future led by Xuanming Su of Japan’s National Institute for Environmental Studies concluded that temperatures could stabilize below 1.5 degrees, after shooting past it for a brief time, with immediate action including a tripling of carbon prices and doubling of funds for preventing emissions above what would be needed to meet the 2 degree target.
The most forgiving of the studies came in September, when Richard Millar of the University of Oxford and colleagues reported findings that the carbon budget -- a gauge of how much humanity can pump out before it enters the danger zone -- may be bigger than previously believed. While peers criticized the paper for making optimistic assumptions, the paper itself demonstrates that fixing the climate won’t be a cakewalk.
The Paris goal “is not chasing a geophysical impossibility, but is likely to require a significant strengthening” of national commitments, Millar and his co-authors wrote, suggesting “sustained reductions at historically unprecedented rates after 2030.”
The biggest challenge for the scientists may be that the largest source of uncertainty has less to do with the thermal physics of the Earth, or the melt rate of glaciers, than with the actions of the very people who have asked them to study the problem.
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A study led by Swiss scientist Reto Knutti assigns the greatest unpredictability to politicians.
"Current and proposed" policies under the Paris Agreement "are inconsistent with what would be required for the 1.5 degree or 2 degree target, and even these are politically difficult," Knutti and his co-authors write in a study released in September. They brush aside the thought that to make policy more work needs to be done on assessing temperature paths, saying more precision "is not necessary for eliminating those roadblocks."
What to do about climate change is so self-evident at this point that non-scientists needn’t even consider degree targets, said Kate Marvel, a climate researcher affiliated with NASA and Columbia University in New York.
“Things that make us not put as much carbon dioxide in the air are good, and things that make us put more carbon dioxide in the air are bad,” Marvel said. “If you want to think about this in a binary sense, don’t use 2 degrees/not-2 degrees.
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