New study from Hansen suggests 2C limit won’t hold seas back

New study from Hansen suggests 2C limit won’t hold seas back

By Steve Valk

By now, Dr. James Hansen has to feel a bit like the scientist Jeff Goldblum portrayed in “Jurassic Park” who, just before being chased by a T-Rex, says, “Boy, do I hate being right all the time.”

Hansen, the NASA scientist who made headlines back in 1988 with warnings about future disaster related to global warming, has seen his unsettling predictions come to pass in the years since. With carbon dioxide – the principal heat-trapping gas in our atmosphere – now at levels not seen since before humans existed, those predictions are getting more certain and more dire.

Dr. James Hansen (center) appears on an experts panel at CCL's conference last month.

Dr. James Hansen (center) appears on an experts panel at CCL’s conference last month.

Hansen, along with 16 other researchers, released a study this week with particularly bad news for those living in coastal areas: Sea levels, previously thought to be rising at a slow, linear pace, could rise much more rapidly throughout the century because of the “freshening” of oceans in polar regions due to the melting of ice in Greenland and Antarctica.

CCL Legislative and Science Director Danny Richter explains the feedback phenomenon that could quicken sea-level rise:

“The first feedback has to do with density. Salt water is more dense than freshwater, and as it gets colder it stays liquid and sinks, whereas freshwater freezes and floats. If there is more freshwater, there is less mixing between the surface and deeper waters. Since wind and cold air temperatures cool surface waters, less mixing means that deeper waters stay relatively warmer. Warmer waters melt ice sheets with grounding lines underwater faster, increasing the ice melt rate, adding more freshwater to the surface. This is the stratification feedback.”

The bottom line is that the 2C limit for global warming, a number Hansen said was “picked out of a hat in Copenhagen,” may not prevent seas from rising beyond a level that coastal cities will be able to adapt to. The study notes that during the Eemian interglacial period more than 100,000 years ago, sea level rose 15 to 30 feet at a time when temperatures were only 1C warmer than they are today.

The study will appear this week in Atmospheric Chemistry and Physics, an online journal in which a paper is published and the peer-review process occurs in public. In a conference call with journalists, Hansen said they chose the open peer-review journal in order to make the information public before a global climate change accord is finalized later this year in Paris. Waiting on peer review before publishing, Hansen said, might have delayed valuable information becoming available until after the Paris conference.

The study is generating much debate in the scientific community. In a piece from Chris Mooney of the Washington Post, several climate experts weighed in. Here are two:

Michael Oppenheimer, Albert G. Milbank Professor of Geosciences and International Affairs in the Woodrow Wilson School at Princeton University, commented by email that “If we cook the planet long enough at about two degrees warming, there is likely to be a staggering amount of sea level rise.  Key questions are when would greenhouse-gas emissions lock in this sea level rise and how fast would it happen? The latter point is critical to understanding whether and how we would be able to deal with such a threat. The paper takes a stab at answering the ‘how soon?’ question but we remain largely in the dark.  Giving the state of uncertainty and the high risk, humanity better get its collective foot off the accelerator.”

Kevin Trenberth, an influential climate researcher at the National Center for Atmospheric Research in Boulder, Colorado, was critical of the paper, calling it “provocative and intriguing but rife with speculation and ‘what if’ scenarios.” Trenberth objected in particular to the climate modeling scenarios used to study freshwater injection as ice sheets melt. “These experiments introduce a lot of very cold fresh water in various places, and then they see what happens,” he wrote by email. “The question is how relevant these are to the real world and what is happening as global warming progresses?   They do not seem at all realistic to me.” “There are way too many assumptions and extrapolations for anything here to be taken seriously other than to promote further studies,” Trenberth wrote.

Hansen remains optimistic that we can avoid worst-case scenarios, provided we quickly reduce emissions of greenhouse gases and promote practices that pull those gases out of atmosphere, reducing the level of CO2 from the current 400 parts per million to 350 ppm.

Hansen, in short, suggests our current commitments to cut emissions will not be sufficient to ward off catastrophe, that we must pick up the pace on mitigation. In that respect, he’s likely to agree with another observation from Jeff Goldblum in “Jurassic Park” (and “Independence Day”):

Steve Valk is Communications Director for Citizens’ Climate Lobby. 

Steve Valk
Steve Valk is Communications Coordinator for Citizens' Climate Lobby. Steve joined the CCL staff in 2009 after a 30-year career with the Atlanta Journal-Constitution. Follow him on Twitter at @valklimate.