8 June 2007 Science Magazine
News Focus
Pushing the Scary Side of Global Warming
Richard A. Kerr

Greenhouse warming might be more disastrous than the recent international assessment managed to convey, scientists are realizing. But how can they get the word out without seeming alarmist?

Climate modeler James Hansen knows all about sounding the alarm. In the summer of 1988, drought wracked the country, fire was consuming Yellowstone National Park, and the nation's capital sweltered. Even the Senate hearing room where Hansen was testifying was warm and stuffy--the Democrats had opened the windows the night before. Then Hansen, dubbed NASA's top climate scientist by the media, shouted "Fire!" in the crowded theater: "With a high degree of confidence," he declared, greenhouse warming had arrived. Although many of his colleagues agreed, none chimed in with support; they could not share his high degree of confidence. Still, Hansen's lone authoritative voice was enough to send the media into a years-long brouhaha over global warming.

Figure 1 A goner? Greenland's ice has been accelerating toward the sea lately, and no one is sure why. CREDIT: RICHARD B. ALLEY

   That uproar quieted within a few years, but Hansen, still the director of NASA's Goddard Institute for Space Studies (GISS) in New York City, finds himself at the head of an informal movement to again rouse the public and policymakers. This time he worries that sea level could rise several disastrous meters by the end of the century, as the warming he heralded sends the great ice sheets rumbling toward the sea. If nothing is done to rein in greenhouse gas emissions, he says, "I just can't imagine that you could keep sea-level rise under a meter." Then the sea would flood many kilometers inland along the world's low-lying coasts, from Florida to Bangladesh.
   That was Hansen's warning to Congress in late April, but it's not the message that came out of the U.N.'s Intergovernmental Panel on Climate Change (IPCC) in early February. Many news reports gave the impression that the prestigious international assessment actually downgraded the risk of imminent sea-level rise to a small fraction of a meter.
   So Hansen seems to be out on a limb, again. This time, however, he's got company. No longer reticent, other scientists are going public about how bad things might get by the end of the century. "The IPCC has been overly cautious in not wanting to give any large number to [future] sea-level rise," says climate researcher Stefan Rahmstorf of the Potsdam Institute for Climate Impact Research in Germany.

Figure 3 Asymmetry. The climate debate has focused on problems judged most likely (center) or least damaging (left), but the greatest risk may be at the extreme (right). CREDIT: MODIFIED FROM GRAPH BY RICHARD B. ALLEY

   Scientists are still trying to strike a balance between their habitual caution and growing concern over uncertain but disastrous greenhouse outcomes. "Most scientists don't want to, but I think we need a way to explore" the extreme end of the range of possibilities, says glaciologist Robert Thomas of NASA contractor EG&G at Wallops Flight Facility in Virginia. Thomas says scientists need "a better way" than IPCC's consensus approach, "so we can communicate with the public without becoming scaremongers."

Naturally cautious
Seldom have mainstream climate scientists spoken out about the scary possibilities of global warming. "Most people [in the field] realize this really is an extremely serious problem we're facing" in sea-level rise, says Thomas. But no one understands just why the great ice sheets of Greenland and West Antarctica have accelerated their slide to the sea in recent years (Science, 24 March 2006, p. 1698). Will the acceleration continue? Speed up? Slow down? Stop? In the face of such uncertainty, most climate scientists have traditionally let IPCC speak for them. When they've gone public, it was usually to counter greenhouse contrarians arguing for an inconsequential warming with trivial impacts.
   In the latest report, its fourth since 1990, the IPCC spoke for scientists in a calm, predictably conservative tone (Science, 9 February, p. 754). It is, after all, an exhaustive, many-tiered assessment of the state of climate science based exclusively on the published literature. In IPCC's Working Group I report on the physical science of climate, 600 authors contributed to an 11-chapter report that drew 30,000 comments from reviewers. The report was in turn boiled down to a 21-page "Summary for Policymakers" (SPM). Its central projection of sea-level rise by the century's end--0.34 meter--came within 10% of the 2001 number. And by getting a better handle on some uncertainties, it even brought down the upper limit of its projected range, from 0.89 to 0.59 meter.
   The SPM did add that "larger values [of sea-level rise] cannot be excluded." Whatever has accelerated ice-sheet flow to the sea, the report said, might really take off with further warming--or not. "Understanding of these effects is too limited" to put a number on what might happen at the high end of sea-level rise, it concluded. Lacking such a number, the media tended to go with the comforting 0.34-meter projection or ignore sea level altogether.
   Some scientists believe IPCC did as well as it could in assessing the sea-level threat. "Since 2001, nature has revealed some pretty remarkable behavior in the ice sheets," says glaciologist Waleed Abdalati of NASA's Goddard Space Flight Center (GSFC) in Greenbelt, Maryland, who manages NASA's ice-observation program. That behavior has included the catastrophic collapse of the Larsen B ice shelf--which triggered glacier accelerations--and the galloping glaciers draining the Greenland ice sheet, which have doubled their pace. But "we just don't have the capacity to quantify" that sort of ice-sheet behavior, he notes, so "the best you can do is point to some red flags. The language of the SPM does that, if you're looking for it."
   Quantifying ice-sheet behavior does indeed have its limitations, says climate and sea-level modeler Jonathan Gregory of the University of Reading, U.K., who coordinated the production of the sea-level section in the IPCC projections chapter. A predictive model cited by IPCC would melt more Greenland ice as the air warms, he says, because that is a well-understood and quantifiable process. However, the model would not include the effect of that glacial meltwater lubricating the base of the Greenland ice sheet. Although researchers have seen signs that such lubrication speeds the ice sliding into the sea, they aren't yet able to model it. "If there are no models to give us some numbers," says Gregory, "all you can do is make numbers up. It wouldn't be appropriate to make up numbers."
   Going beyond such physically based models--for example, by extrapolating from past trends--wouldn't be such a good idea either, Gregory says. Lessons taken from how sea level rose as the 20th century warmed, he says, would be useless in predicting sea-level rise in this century if the underlying causes change. "If you don't know what is causing the relationship" between warming and sea-level rise, he says, "is that really a good basis for making projections?"

A bolder assessment
Scientists are well aware of the hazards of straying far from the hard science of climate change, but some are eager to change the IPCC process and even move beyond it. They would begin with wording. "IPCC gets an A+ for scientific assessment," says climate modeler Richard Somerville of the Scripps Institution of Oceanography in San Diego, California, "but a gentleman's C for communication." The communication problem is largely a matter of structure, says geoscientist Michael Oppenheimer of Princeton University. "All the facts are there in the [main-report] chapter," he says, "but the SPM didn't tie those facts together in a coherent statement of risk that would allow a policymaker to make an informed decision."
   Beyond the IPCC's language, a number of climate scientists think the report missed an opportunity to broaden public appreciation of the risk of the most dangerous climate change. "If you don't understand the physics, your uncertainty is larger," says Thomas. That greater uncertainty extends the range of possible ice losses to higher, more dangerous levels. But IPCC didn't capture that increased risk, says climate modeler Michael MacCracken of the Climate Institute in Washington, D.C.
   A big part of IPCC's problem, say MacCracken and others, was its strict adherence to the use of models. By IPCC standards, "if it's not in a model, it's speculation," says Rahmstorf. By ignoring factors that can't yet be modeled, he says, IPCC came up with deceptively reassuring numbers.

More numbers
Although forewarned, some researchers are generating numbers for public consumption by going beyond physics-based models. In a paper published in Science in January, too late for the IPCC to consider it, Rahmstorf took "a semiempirical approach to projecting future sea-level rise." He determined how much sea level rose in the 20th century per year per degree and projected that rate through the 21st century, with its expected warming. That projection produced a sea-level rise in 2100 of 0.5 to 1.4 meters above the 1990 level, well above the IPCC's projection of 0.18 to 0.59 meter.
   Then, Rahmstorf and six co-authors, including Hansen, published a paper in Science on the day the IPCC report was released. They pointed out that warming had been running toward the high side of IPCC projections during the past few decades, while sea levels rose at the upper limit of projections. "These observational data underscore the concerns about global climate change," the authors wrote. IPCC had clearly not exaggerated sea-level rise, they said, and may even have underestimated it. Reinforcing their message, news stories published a few days before the IPCC report's release quoted Rahmstorf and other scientists lamenting the expected shortcomings on sea-level projections.
   That was more media attention than suited some climate researchers. "When we speak to the public, we should not rely on the new result," argues Hans von Storch of the GKSS Institute for Coastal Research in Geesthacht, Germany. "The newest results are not necessarily the best ones. The IPCC should represent a certain filter. That every taxicab driver knows about [the latest result] is a bit premature."
   Some scientists would have IPCC reach even farther back to try to deal with "factors that you don't understand," as MacCracken puts it. He notes that paleoclimatologists and geologists have extracted records of ancient sea level for times when Earth was warmer or colder than today. In the case of the penultimate warm interglacial 120,000 years ago, the globe was only about 1C warmer--a temperature we could reach by 2100--but sea level was 4 to 6 meters higher. Even though that warmth had millennia to shrink the great ice sheets back then, MacCracken says, history still suggests that the world's ice is more vulnerable than IPCC's modeling implies.

Another way
Gregory calls projections drawing on such studies "a scientific hunch." Hansen prefers "insight," but whatever it is called, Hansen says, you won't find much of it in an IPCC report. IPCC "overall does a good job," he says, but "there are limitations on that process. Everybody in [sea-level] research is much more concerned than 6 or 7 years ago" when the previous report came out, he says; yet the latest message from IPCC was seemingly unchanged. "There is a role for something in addition."
   As an example of an alternative to the IPCC report, Hansen cites the U.S. National Academy of Sciences' climate change report of 1979. Chaired by the late meteorologist Jule Charney, then at the Massachusetts Institute of Technology, the small committee delivered, among other things, a best estimate and range for the sensitivity of climate to greenhouse gases, a central figure in climate science. IPCC never ranged far from those numbers, and this year it confirmed them. "There were huge uncertainties back then," says Hansen, "yet the Charney repor t came up with an estimate by coming at it from different angles. That's what you need to do with sea level."
   Hansen practices a multipronged approach himself. With colleagues at GISS, he draws on several lines of evidence--climate modeling, recent observations, paleoclimate records, and the basic physics of the greenhouse--to "gain insight into how the world works." That approach helped him to see greenhouse warming under way in 1988, he says. Now it is revealing positive feedbacks in the ice-climate system that can allow modest warming to accelerate losses from the ice sheets. The world is on a "slippery slope," Hansen has written, that could lead to meters of sea-level rise in the next century or two unless people take immediate actions to cut greenhouse gas emissions.
   As in 1988, Hansen's pursuit of such insights has put him at odds with many in the climate community. "Maybe he's still within the error bars," says glaciologist Robert Bindschadler of GSFC, but "I'm not prepared to put centuries on [the timing] rather than millennia." No matter. Hansen has again taken to the bully pulpit as NASA's top climate scientist, publishing in peer-reviewed journals, testifying to Congress, writing op-ed articles, and appearing in documentaries. Only last week in a GISS press release announcing a new publication, Hansen warned of disastrous effects--including increasingly rapid sea-level rise--if greenhouse gas emissions continue apace for even a couple more decades. And a few days later, he took his boss, NASA Administrator Michael Griffin, to task for publicly questioning the need to tackle global warming. Hansen's take: "remarkably uninformed."
   Besides a streamlined IPCC process and individual scientist activism, Oppenheimer sees another approach: using expert elicitations to broaden the assessment of uncertainty. This survey technique grills selected experts in private on the state of the science. Without the drive to reach a consensus, the experts "give a different view of the probability of various outcomes," says Princeton's Oppenheimer. Ultimately, these and other methods will be needed, he says. "You really need a broad, thorough, and comprehensive assessment of uncertainty and risk," he says. "There is no one answer to the assessing of uncertainty."

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