Human-caused carbon emissions islikely to trigger irreversible damage to the planet, according to a comprehensive international study released Wednesday. Researchers studying one of the most important and vexing topics in climate science — how sensitive the Earth’s climate is to a doubling of the amount of carbon dioxide in the atmosphere — found that warming is extremely unlikely to be on the low end of estimates.

 

These scientists now say it is likely that if human activities — such as burning oil, gas and coal along with deforestation — push carbon dioxide to such levels, the Earth’s global average temperature will most likely increase between 4.1 and 8.1 degrees Fahrenheit (2.3 and 4.5 degrees Celsius). The previous and long-standing estimated range of climate sensitivity, as first laid out in a 1979 report, was 2.7 to 8.1 degrees Fahrenheit (1.5 to 4.5 Celsius).

25 scientists have significantly narrowed the bounds on critical factor, known as climate sensitivity. The study is the payoff of decades of advances in climate science, says James Hansen, the famed retired NASA climate scientist who helped craft the first sensitivity range in 1979. “It is an impressive, comprehensive study, and I am not just saying that because I agree with the result. Whoever shepherded this deserves our gratitude.”

The assessment, conducted under the World Climate Research Program (WCRP) and published this week in Reviews of Geophysics, relies on three strands of evidence: trends indicated by contemporary warming, the latest understanding of the feedback effects that can slow or accelerate climate change, and lessons from ancient climates. They support a likely warming range of between 2.6°C and 3.9°C, says Steven Sherwood, one of the study’s lead authors and a climate scientist at the University of New South Wales. “This is the number that really controls how bad global warming is going to be.”

The study dispels uncertainty introduced by the latest climate models. Models have historically been used to estimate sensitivity, beginning in 1979, with the world’s first comprehensive assessment of CO2-driven climate change. That summer, at a meeting in Woods Hole, Massachusetts, led by Jule Charney, scientists produced a paper, known ever since as the Charney report, that predicted between 1.5°C and 4.5°C warming for a CO2 doubling. Those numbers — based in part on a model Hansen had developed — stuck around far longer than anyone imagined: The latest IPCC report, from 2013, gave the same range.

The scientists involved in the research say they have narrowed the range of probable climate outcomes, which reduces the uncertainty that has long plagued public debate about this field.

Their increased confidence about the sensitivity of the climate should ease the job of policymakers and diminish the scope for skepticism but it is far from reassuring for the future of the planet.

Humanity has already emitted enough COto be halfway to the doubling point of 560 parts per million, and many emissions scenarios have the planet reaching that threshold by 2060. The report underscores the risks of that course: It rules out the milder levels of warming sometimes invoked by those who would avoid emissions cuts. “For folks hoping for something better, those hopes are less grounded in reality,” says David Victor, a climate policy researcher at the University of California, San Diego, who was not part of the study.

The WCRP sensitivity estimate is designed to be used by the UN’s Intergovernmental Panel on Climate Change (IPCC) when it publishes its next major report in 2021 or 2022. The estimate will also inform projections for sea-level rise, economic damage, and much else.

A clearer picture of those consequences could do much to spur local governments to cut emissions and adapt to warming, says Diana Reckien, a climate-planning expert at the University of Twente. “The decreasing uncertainty could potentially motivate more jurisdictions to act.”

Recent models suggest the range might even go higher. They run hot, some predicting warming of more than 5°C for a COdoubling, apparently because of the way they render clouds, especially over the Southern Ocean. Yet these high-end models struggle to accurately recreate the climate of the 20th century, undermining their credibility. Such models play only a supporting role in the new assessment, says Robert Kopp, a climate scientist at Rutgers University, New Brunswick, who was not involved in the effort. “We now have enough independent lines of evidence that we don’t need to use the climate models as their own line.”

The scientists probed individual climate feedbacks. Some of these, like the warming effect of water vapor, are well known. But clouds, which can cool or warm the planet depending on how they reflect sunlight and trap heat, have long been a wild card. In particular, climate scientists want to understand the decks of stratocumulus clouds that form off coastlines. If they grow more extensive in response to warming, as some suspect, they could have a cooling effect.

The scientists looked at records from two past climates — 20,000 years ago, at the peak of the last ice age, and a warm period 3 million years ago, the last time atmospheric COlevels were similar to today’s. Recent work suggests climate sensitivity is not a fixed property of the planet, but changes over time. During warm periods, for instance, the absence of ice sheets probably raised sensitivity. Records of ancient temperatures and CO2 levels enabled the team to pin down sensitivities of 2.5°C and 3.2°C for the cold and warm periods, respectively. “It’s really comprehensive,” says Jessica Tierney, a paleoclimatologist at the University of Arizona, who was not part of the report. Even for the coldest climate state, she says, the possibility of a sensitivity below 2°C seems negligible.

Assembling the three lines of evidence was a huge task. But wiring them together for a unified prediction was even tougher, Marvel says. The team used Bayesian statistics to churn through its assembled data, which allowed the researchers to test how their assumptions influence the results. “The real advantage” of Bayesian statistics, Tierney says, is how it allows uncertainties at each stage to feed into a final result. Co-authors often butted heads, Marvel says. “It was such a long and painful process.” The final range represents a 66% confidence interval, matching IPCC’s traditional “likely” range. The WCRP team also calculated a 90% confidence interval, which ranges from 2.3°C to 4.7°C, leaving a slight chance of a warming above 5°C.

Either way, the report has a simple takeaway, Sherwood says: A doubling of COall but guarantees warming of more than 2°C. “Three major lines of evidence are all very difficult to reconcile with the lower end of climate sensitivity.”

In recent years, another uncertainty in the climate future has also narrowed: Global emissions seem unlikely to reach the worst-case scenarios IPCC helped craft 15 years ago, ruling out some forecasts of extreme warming. “We’re light-years ahead of where we were in 1979,” says Reto Knutti, a co-author and climate scientist at ETH Zurich.

Unfortunately, the years of work needed to attain that certainty came with a cost: 4 decades of additional emissions and global warming, unabated.

“The main message is that unfortunately we can’t expect that luck will save us from climate change,” Reto Knutti, professor of climate physics at ETH Zurich’s Institute for Atmospheric and Climate Science.

“The good thing is that we’ve somewhat narrowed the range of future long-term warming, the bad thing is that we can no longer hope or claim that the problem will just magically go away.”

“It is moderately good news. It reduces the likelihood of some of the catastrophically high estimates. If we were planning for the worst, the worst has become less likely,” said one of the authors, Zeke Hausfather, of the Energy and Resources Group at University of California Berkeley. “But fundamentally, it means we must do more to limit climate change. We are not anywhere near on track to do that.”

This confluence of sources has allowed scientists to estimate with a 90% level of probability that climate sensitivity is between 2.3C and 4.7C. The most likely level of climate sensitivity has nudged slightly above 3C. Hausfather says a figure below 2C is extremely unlikely. Above 5C remains possible, though the study lowers that likelihood to 10%.

Earlier this year, a handful of climate models, including some of the world’s most advanced, suggested climate sensitivity could be above 5C, prompting alarm.

These high figures were not included in this week’s study, but many climate scientists regard the recent higher numbers as outliers that should not be taken out of the broader context.


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