Former NASA climate director James Hansen and a team of scientists claim a mechanism in the climate could rapidly raise sea levels by metres
Dr James Hansen, pictured in 2012
Photograph: Murdo MacLeod for the Guardian
James Hansen’s name looms large over any history that will likely be written about climate change.
Whether you look at the hard science, the perils of political interference or modern day activism, Dr Hansen is there as a central character.
In a 1988 US Senate hearing, Hansen famously declared that the “greenhouse effect has been detected and is changing our climate now”.
Towards the end of his time as the director of NASA’s Goddard Institute for Space Studies, Hansen described how government officials had on other occasions changed his testimony, filtered scientific findings and controlled what scientists could and couldn’t say to the media – all to underplay the impact of fossil fuel emissions on the climate.
In recent years, the so-called “grandfather of climate science” has added to his CV the roles of author and twice-arrested climate activist and anti-coal campaigner. He still holds a position at Columbia University.
So when Hansen’s latest piece of blockbuster climate research was finalized and released earlier this week, there was understandable global interest, not least because it mapped a potential path to the “loss of all coastal cities” from rising sea levels and the onset of “super storms” previously unseen in the modern era.
So what is Hansen claiming?
Well, the first thing to understand is that Hansen’s paper, written with 18 other co-authors, many of them highly-reputable names in climate science in their own right, is far from conventional.
Most scientific papers only take up four or five pages in a journal. Hansen’s paper - in the journal Atmospheric Chemistry and Physics - grabs 52 pages (although it’s hard to quibble over space when you’re laying out a possible path to widespread global disruption and the complete reshaping of coastlines).
Nor was the paper published in a conventional way. If you’re getting a faint sense of déjà vu about Hansen’s findings, then that could be down to how a draft version of the study was published and widely covered in July last year.
The journal runs an unconventional interactive system of peer review where comments and criticisms from other scientists are published for everyone to see, as are the responses from Hansen and his colleagues.
This is arguably a more transparent way of conducting the scientific process of peer review - something usually carried out privately and anonymously.
None of this should really detract from Hansen and his co-author’s central claims.
Firstly, Hansen says they may have uncovered a mechanism in the Earth’s climate system not previously understood that could point to a much more rapid rise in sea levels.
When the Earth’s ice sheets melt, they place a freshwater lens over neighboring oceans. This lens, argues Hansen, causes the ocean to retain extra heat, which then goes to melting the underside of large ice sheets that fringe the ocean, causing them to add more freshwater to the lens (this is what’s known as a “positive feedback” and is not to be confused with the sort of positive feedback you may have got at school for that cracking fifth grade science assignment).
Secondly, according to the paper, all this added water could first slow and then shut down two key ocean currents – and Hansen points to two unusually cold blobs of ocean water off Greenland and off Antarctica as evidence that this process may already be starting.
If these ocean conveyors were to be impacted, this could create much greater temperature differences between the tropics and the north Atlantic, driving “super storms stronger than any in modern times”, he argues.
“All hell will break loose in the North Atlantic and neighbouring lands,” he says in a video summary.
Hansen points to evidence from ancient climates (known as paleoclimate data) to suggest this has previously happened on Earth – an interpretation that was challenged during the interactive peer review process.
Now, according to the IPCC, global sea levels will only rise a maximum of not quite a metre by the end of the century.
But Hansen says this is too conservative. His paper claims that climate models, including those used for his paper, don’t properly capture the consequences from adding freshwater from melting ice sheets. Sea levels could rise several metres beyond the IPCC’s estimates, and the rise would be much faster.
So what’s the upshot? According to Hansen:
The idea that ice sheets are becoming unstable and could raise sea levels several metres is not as controversial as you might think. There is good evidence that sea levels have been many metres higher in earth’s history, well before humans came along.
A May 2015 study covered here on Planet Oz found Antarctica’s ice sheet alone could have pushed sea levels as much as 17 metres higher, but it would have taken thousands of years.
Commenting on Hansen’s study, Dr John Church, a sea level rise expert at Australia’s CSIRO, says multimeter sea level rises are consistent with current research, but where Hansen departs is on claiming it could all happen before the end of this century.
But Church says that as fossil fuel emissions grow, the world is committing future generations to much higher sea levels.
“Even with the lower emission scenarios we have committed the world to ongoing sea level rise for centuries,” he says.
Other scientists have already challenged many of Hansen’s conclusions, as Hansen himself predicted would happen.
“You can be sure that many scientists – indeed most scientists – will find some aspects in our long paper that they would interpret differently,” Hansen says.
But he also commented that these disagreements shouldn’t be misinterpreted.
And so it’s in this spirit that I think Hansen’s paper should be taken.
Hansen and his co-authors have laid out their science and are proposing an idea that will live, die or evolve through the scientific method.
Whether or not March 2016 becomes another seminal moment in Hansen’s storied climate career only time, and a failure to reign in fossil fuel emissions, will likely tell.
One final point.
Hansen has long called for a straightforward rising price on greenhouse gas emissions, which he argues would “quicken the transition to cleaner energy.”
“This is a tragic situation, because it is unnecessary,” he says.