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A new study says that climate-induced feedback loops could lead to a change in ocean stratification and the more rapid melting of ice sheets.

A new study says that climate-induced feedback loops could lead to a change in ocean stratification and the more rapid melting of ice sheets.

Flickr/mariusz kluzniak (CC BY-NC-ND 2.0)

Climate researcher blasts global warming target as 'highly dangerous'

Climate scientist James Hansen has fired a new salvo in the climate wars. In a new paper, Hansen and colleagues warn that the current international plan to limit global warming isn’t going to be nearly enough to avert disasters like runaway ice-sheet melting and consequent sea-level rise. Hansen told reporters at a press conference yesterday that he hoped the paper—to be published online this week—would influence global climate talks this December in Paris and encourage negotiators to reconsider their goal of keeping warming to less than 2°C above preindustrial levels, a laudable but insufficient target, some scientists say. But how influential this paper will be is unclear, given its flaws.

The new study, which includes nearly 300 references and is 66 pages long, argues that the 2°C target—hard-won as it might be politically—isn’t good enough, and is in fact “highly dangerous.” At that temperature, the study says, enough ice-sheet melting causes a positive feedback loop that leads to more melting and rising seas. Instead, Hansen and his co-authors say, a far better target would be to return to an atmosphere with 350 parts per million CO2. That number currently stands at about 400 parts per million.

The researchers make their case in part by describing paleoclimate data from the Eemian, an interglacial (warm) period that lasted from about 130,000 to 115,000 years ago. During that time, temperatures were less than 1°C warmer than they are today, but sea level stood about 5 to 9 meters higher due to large-scale ice sheet melt. The end of the period experienced powerful storms as well, according to sedimentary evidence the researchers cite.

The paper also describes an atmosphere-ocean modeling study of feedback loops caused by ice sheet melting under 2°C conditions. What they found, Hansen says, is that melting ice sheets in Greenland and Antarctica could inject enough fresh water into the seas to slow the formation of two key water masses: the North Atlantic Deepwater and the Antarctic Bottom Water formations. Both are part of the so-called Great Ocean Conveyor Belt of ocean circulation. The injection of so much cold water, they say, could lead to a stratification of the water column, with warm water buried underneath cold surface water. “Instead of emerging at the surface, much of that heat is melting the ice shelves,” Hansen says, producing more fresh water and amplifying the feedback. That is particularly striking, he added, because it’s what we’re observing right now: an increase in cold surface waters around Antarctica and Greenland, as well as increases in sea ice around some parts of Antarctica.

Hansen’s drive to deliver the message may have been what drove him to publish his latest findings in the open-source Journal of Atmospheric Chemistry and Physics. The paper has not yet been peer reviewed, but it will be online and publicly available during the peer review process. (A linkable draft wasn’t yet available when this story went online.) Hansen told reporters that his goal was to bypass the lengthy peer-review process for fear that the paper wouldn’t be available to its intended audience in time—international negotiators at the Paris talks. Peer review, he said, would instead be a real-time process, occurring in full view of the public. “That’s the merit of a discussion-type journal,” he said.

Other scientists agree that having this discussion is critical. “Too often in debates about climate change risk, the starting point is a presumption that only global warming in excess of 2°C represents a threat to humanity,” says climate scientist Michael Mann of Pennsylvania State University, College Park. “This new article makes a plausible case that even 2°C warming is extremely dangerous, too dangerous to allow.”

But when it comes to the paper’s findings, Mann says, “I am a bit skeptical about some of the specifics.” For one thing, he says, it contains a scenario in which the fresh meltwater from ice sheets increases exponentially over time, “which may not be realistic.” It also uses a low-resolution ocean model that doesn’t include key currents that transfer heat to higher latitudes, such as the Gulf Stream.

Climate scientist Kevin Trenberth  of the National Center for Atmospheric Research in Boulder, CO, agrees that the paper is “provocative and intriguing,” but that “it has many conjectures and huge extrapolations.” Trenberth cites issues from the low-resolution ocean model to the lack of important ocean-climate patterns such as the El Niño-Southern Oscillation. He also calls the freshwater injection experiments “not … at all realistic.”

Whether this paper will become a key point of reference in the ongoing climate talks isn’t clear. In advance of the Paris meetings, negotiators from nearly every country in the world have provisionally agreed to the 2°C target. That there is even such an agreement in the offing seems like a victory, but whether it will be reached is still up in the air. Recognizing this, 24 academic and professional institutions in the United Kingdom yesterday issued a sternly worded joint communiqué that called on the international community to take immediate action on reducing emissions. The statement suggested that to have a chance of reaching that 2°C goal, Earth must become a zero-carbon world by the second half of the century.

Hansen has previously suggested that scientists are often too hesitant to say just how dire the situation is. A 2007 paper he co-authored, titled “Scientific Reticence and Sea Level Rise,” suggested that scientists felt constrained from sounding a full-fledged alarm on how high the waters will get, in part because of the cautious nature of scientific inquiry and the scientific method. But, he says in that paper’s abstract, “there is a danger in excessive caution.” The new paper, he told reporters yesterday, is “significantly more persuasive than anything previously published about just how dangerous 2°C warming would be.”

On that, many scientists do agree. Mann says that although he is skeptical about the details of the study, by putting forth these ideas the authors, “have initiated an absolutely critical discussion” about the 2°C target. “The stakes couldn’t be any higher. If we make the wrong choices, there is no planet B for us to turn to.”

*Updated, 22 July, 10:00 a.m.: This story has been updated to clarify that the Journal of Atmospheric Chemistry and Physics is peer-reviewed. However, peer review of the Hansen paper is ongoing.