James Hansen says his new study released today shows that “all coastal cities” could be lost to rising oceans if people fail to reduce greenhouse gas emissions.
The controversial scientist released a draft study a year ago that found sea levels could rise by several meters within decades, triggering catastrophe and superstorms. Many scientists criticized the findings as unrealistic and questioned whether Hansen should have released a draft study that has not undergone peer review.
Now he’s answering them.
Hansen finalized that study, following peer review, and published it in Atmospheric Chemistry and Physics this morning. He sticks to his major conclusions, though he has reorganized the paper so the science is clearer (ClimateWire, July 24). The study has 18 co-authors.
“Consequences [of climate change] include sea level rise of several meters, which we estimate would occur this century or at latest next century, if fossil fuel emissions continue at a high level,” Hansen said. “That would mean loss of all coastal cities, most of the world’s large cities and all their history.”
Melting ice at the poles is already causing a slowdown of an ocean current that transfers heat from the tropics to the North Atlantic, Hansen said. A complete halt of the current, called the Atlantic meridional overturning circulation, or AMOC, would affect the livelihoods of people in Europe and North America.
Scientists who criticized Hansen’s draft study last year were doubtful about the new version, as well.
“Hansen is very good at marshaling data and laying out convincing arguments in support of pretty extreme interpretations, but he’s not so good at determining what actually constitutes a real human emergency,” said Tad Pfeffer, a glaciologist at the University of Colorado, Boulder.
The backlash against one of the most highly respected pioneers of climate science indicates two things: There is still disagreement on the rate of sea-level rise, and many scientists are uncomfortable with Hansen’s mix of science and policy prescriptions.
“I think if Hansen and his co-authors had slowed down just a little bit and presented their work in its scientific context, that is, left out the polemicizing about the urgency to act ‘RIGHT NOW!’, the community would have been more accepting of their work and they’d actually be further forward than they are,” Pfeffer said in an email.
‘No firm justification’
The Intergovernmental Panel on Climate Change says the oceans will rise by between 11 and 38 inches by 2100, potentially submerging low-lying island nations, but most scientists think these estimates are too conservative (ClimateWire, Feb. 18).
During a press call yesterday, Hansen recommended that governments place a fee on carbon in order to make fossil fuels more expensive. He also defended his right to advocate for climate policies.
“If scientists don’t say that, then politicians will tell you what is needed [for policy], and that’ll be based on the politics rather than on the science,” he said. “I don’t see any reason not to try to make the complete story clear rather than just drawing a line and saying I’m not going to step beyond this.”
The biggest disagreement centers on Hansen’s finding that Greenland and Antarctica will lose ice much more rapidly than most glaciologists say. He assumes that the “doubling time,” the amount of time it would take for ice loss from the ice sheets to double, could be as short as 10, 20 or 40 years. At the low end of this range, sea levels would rise by several meters as soon as 2050 (ClimateWire, July 22, 2015).
Hansen used modern-day observations of ice loss from Antarctica and Greenland to derive his 10- or 20-year doubling time. But scientists say these records are too short to assume that the ice loss trends will persist.
“There is no firm justification for short doubling times,” said Ian Joughin, a polar scientist at the University of Washington.
The meltwater from the poles could lead to a shutdown of the AMOC, Hansen finds. Other scientists have independently postulated that the AMOC has already weakened and this weakening may be linked to “superstorms” off the United States’ East Coast, but this remains a theory (ClimateWire, Jan. 29).
“[Hansen is] willing to accept the most extreme interpretation of every piece of evidence and to assume it supports an immediate onset in the very near future,” Pfeffer said.
Hansen unfazed by skepticism
Hansen also peered into Earth’s history, at the Eemian 130,000 years ago when the planet was warmer than today. Using proxy measurements, other scientists have found that sea levels rose by 6 to 10 feet within decades. Hansen points to the Eemian as an analogue.
But Pfeffer stressed that ice sheet conditions in the present-day Holocene Epoch is different than the Eemian. There is not enough ice in the right places on Earth today to produce the rapid and imminent sea-level rise seen back then, he said—a conclusion he and other researchers reached in a 2008 report published in Science.
Michael Mann, a climatologist at Pennsylvania State University, said that the findings from Hansen and his colleagues are compelling, but his results for the shorter term are more speculative.
“Their climate model scenario wherein Greenland and Antarctic meltwater caused by warming poles leads to a near total shutdown of ocean heat transport to higher latitudes, cooling most of the globe (particularly the extratropics), seems rather far-fetched to me,” he said.
Hansen brushed off criticism and said that skepticism is the “lifeblood of science.”
“You can be sure that many scientists, indeed most scientists, will find some aspects in our long paper that they would interpret differently,” he said. “That’s entirely normal. It takes time for conclusions to be agreed upon and details sorted out.”
Everyone agrees that the planet has reached a dangerous tipping point on sea-level rise, he said.
“I think you’ll find that among top experts there’s a strong agreement that we have reached a point that this is really urgent,” he said.
Reprinted from Climatewire with permission from Environment & Energy Publishing, LLC. www.eenews.net, 202-628-6500