A new report by top climate change scientists looks back more than 120,000 years to a previous period when the Earth warmed up, and raises the possibility of a near future where sea levels rise far faster and higher than previous projections if carbon emissions are not curtailed.
"It is unlikely that coastal cities or low-lying areas such as Bangladesh, European lowlands and large portions of the United States eastern coast and northeast China plains could be protected against such large sea level rise," the report states.
Authored by former chief NASA climate scientist James Hansen and 16 other researchers, the report takes aim at the 2 degree Celsius "guardrail" for warming that has become a key element of international climate change talks, saying that a global average temperature rise of even that amount could lead to "severely disruptive consequences for human society and ecosystems."
The 66-page report addressed to policymakers has not undergone a formal peer-review process, and was released online Thursday in the European Geosciences Union journal Atmospheric Chemistry and Physics Discussion, where other scientists can comment. The report had already generated discussion before its publication, however, and may continue to do so in the months leading up to international climate talks due to take place in Paris in November and December.
The Intergovernmental Panel on Climate Change has previously projected that sea levels will rise about 3 feet by the end of this century, with those waters continuing to climb higher over several centuries. Even that more conservative estimate would have serious consequences for low-lying regions and coastal cities.
Hansen and his co-authors -- who include researchers from NASA's Goddard Institute for Space Studies; the University of California, Irvine; and the University of North Carolina at Wilmington -- look back to the Eemian period about 125,000 years ago, when global average temperatures were about 1 degree Celsius warmer than they are today, as a rough though inexact parallel to what effect warming might have on the oceans in years to come.
The prospect of increased global temperatures up to the 2 degree Celsius mark above pre-industrial levels "would spur more ice shelf melt," the researchers conclude, and "is highly dangerous." To avert that threat, Hansen has advocated a return to an atmospheric carbon dioxide concentration of 350 parts per million -- well below the 400 ppm reported for the first time in May of this year. He's said in the past that actions including charging fossil fuel companies a carbon fee could be used to help hit that goal.
While some scientists not involved in the new report questioned some of its conclusions, many said they thought it added to the ongoing discussion about the potential widespread impact of a changing climate.
"Too often in debates about climate change risk, the starting point is a presumption that only global warming in excess of 2C (3.6F) represents a threat to humanity," Michael Mann, a climate scientist at Penn State University who was not involved in the study, wrote in an email. "This new article makes a plausible case that even 2C warming is extremely dangerous, too dangerous to allow."
"Let me stress, though, that while I am skeptical of some of the details of the study, I think the authors have done a real service to the scientific discourse by putting forward some interesting and provocative ideas, and by having done so in an open-format journal which encourages discussion and interaction between experts in the field," Mann wrote.
Other experts who reviewed the study at the request of NBC News said they had questions about the report, including the rate at which the researchers say the Greenland and Antarctica ice sheets could melt. The projected sea-level rise put forward by Hansen and his co-authors is "quite an outlier" among other scientific estimates, said Richard Alley, a geologist and polar ice expert at Penn State University.
But Andrea Dutton, an assistant professor of geology at the University of Florida who also was not involved in the study, said such debate will be beneficial as scientists consider the consequences of continued warming.
"As the paper points out, there is no question that there is potential for multi-meter sea level rise over short timescales in our future, even if we stay within the 2 degree target," she said in an email. "The burning question is, how quickly will this sea-level rise take place?
"There is sure to be debate about the rate of sea-level rise that they propose, but given what we know about ice sheet dynamics, we can't rule out such a rapid sea-level rise at this state," Dutton wrote. "Beyond the valuable scientific discussion that this paper will stimulate, given the high risk that this magnitude of sea-level rise poses, the message to society is clear: Prepare now, or be prepared to face the consequences."