When world leaders meet in Paris this December to agree on a new international treaty on climate change, their goal will be to keep atmospheric warming to 2 degrees Celsius, the point after which catastrophic climate change will be nearly inevitable, scientists say.
But a new draft study being published this week by a team of 17 leading international climate scientists warns that even 2 degrees of warming is "highly dangerous" and could cause sea level rise of "at least several meters" this century, leaving most of the world's coastal cities uninhabitable.
"The economic and social cost of losing functionality of all coastal cities is practically incalculable," the authors write. "It is not difficult to imagine that conflicts arising from forced migrations and economic collapse might make the planet ungovernable, threatening the fabric of civilization."
The research was led by climate scientist James Hansen, 74, the former director of NASA's Goddard Institute for Space Studies. Some 35 years ago, Hansen developed one of the world's first climate models and produced prediction after prediction about rising global warming that proved to be correct. He was among the first scientists to show that the burning fossil fuels was to blame, and would lead to dire consequences. His 1988 congressional testimony is often credited for helping launch climate change into the American consciousness.
Sixteen other scientists from the U.S., France, China and Germany contributed to the 66-page paper, which will appear in the journal Atmospheric Chemistry and Physics sometime this week.
The study, which at times reads more like an op-ed than a traditional scientific paper, paints a picture far more drastic than the latest assessment by the Intergovernmental Panel on Climate Change, the world's leading scientific authority on global warming. The IPCC estimates that sea levels could be 3 feet higher in 2100, while Hansen and his colleagues predict a rise of at least twice that amount.
The 17 scientists analyzed how an influx of cold freshwater from the planet's melting ice sheets will disrupt the ocean's circulation—a factor the authors say has not been fully explored in the scientific literature to date. In addition to running climate models, the researchers compared modern warming to similar temperature increases that happened approximately 120,000 years ago in a period known as the Eemian, when global sea level was 5 to 9 meters (between 16 and 30 feet) higher than it is today due to the release of glacial water. They concluded the influx of freshwater from melting ice sheets in modern times would essentially shut down the ocean's circulation, causing cool water to stay in the Earth's polar regions and equatorial water to warm up even faster.
This temperature gradient would generate more intense storms around the tropics and accelerate melting at the poles by so much that the melt could double in just a decade, the study found, making "multi-meter sea level rise practically unavoidable and likely to occur this century."
"Too often in debates about climate change risk, the starting point is a presumption that only global warming in excess of 2 degrees Celsius (3.6 degrees Fahrenheit) represents a threat to humanity," said Michael Mann, a Pennsylvania State University climate scientist who was not involved in the study. "This new article makes a plausible case that even 2 degrees Celsius warming is extremely dangerous, too dangerous to allow."
"The article serves as a sobering wakeup call to those who still dispute the threat posed by our ongoing burning of fossil fuels," he said.
But the study could also be met with skepticism from the scientific community, Mann and other scientists said. Whereas peer review is traditionally done before a study is published, Atmospheric Chemistry and Physics is an open access journal where peer review takes place after publication. Scientists and the public submit their comments and concerns to the study authors, who then address them and update the paper if needed. Only then is the paper considered "accepted."
This type of publishing process is a "bit new and unusual," said Mann, "and it will take time, I imagine, for the scientific community to fully embrace this alternative approach to publication. But it is increasingly widespread and, in my view, totally legitimate."
It is even more rare for such a study to be publicized before the peer review process is complete, said Andrew Shepherd, a polar scientist at the University of Leeds, in England. "I would not recommend reporting on a paper ahead of peer review," he said.
Shepherd said Hansen and his colleagues used a relatively short time period—15 years—to calculate the accelerated loss of ice from Greenland and Antarctica due to climate change, and that he would "exercise caution when interpreting such numbers."
Mann also said he is skeptical that the paper assumed melting of the ice shelves would accelerate constantly with time, and that it used a low-resolution climate model to examine how this meltwater would affect ocean circulation.
Still, Mann said, "the authors have done a real service to the scientific discourse by putting forward some interesting and provocative ideas."
Hansen told reporters during a press call on Monday that he and he colleagues decided to publish the paper in an open access, open peer review journal to ensure the results were available to international leaders ahead of the Paris climate treaty talks in December. Hansen said he will spend the next few months educating treaty delegates on the study, but declined to provide further detail about who he would be meeting with, or when.
"Politicians need to understand that the issue is more urgent than previously realized," Hansen said. If warming reaches 2 degrees Celsius, "we will hand young people a climate system in which it is impossible to avoid large sea level rise."