Dr. James Hansen, the former NASA scientist who is widely credited with being one of the first to raise concerns about human-caused global warming, is a co-author of a new report predicting that the world will undergo devastating sea level rise within mere decades, not centuries, as previously thought.
The report, published Tuesday in the open-access journal Atmospheric Chemistry and Physics, paints an even bleaker picture of the planet’s future, positing that continued high fossil fuel emissions will “increase powerful storms” and drive sea-level rise of “several meters over a timescale of 50 to 150 years.”
Hansen, who now serves as the director of the Climate Science Awareness and Solutions program at Columbia University Earth Institute, published the findings along with an international team of 18 researchers and academics.
As the abstract states, the predictions “differ fundamentally from existing climate change assessments.” For example, the United Nation’s Intergovernmental Panel on Climate Change (IPCC) in 2013 predicted three feet of sea level rise by 2100 if greenhouse gas emissions continue unabated.
A draft version of Hansen’s paper released last year provoked wide debate among climate scientists. Nonetheless, Michael Mann, a renowned climate scientist with the University of Pennsylvania, who is among those questioning some of the report’s “extraordinary” claims, told the New York Times, “I think we ignore James Hansen at our peril.”
The peer-edited report examines growing ice melt from Antarctica and Greenland and studies how that melting has historically amplified “feedbacks that increase subsurface ocean warming and ice shelf melting.” Taking into consideration “rapid, large, human-made climate forcing,” the study predicts a much more accelerated rate of sea level rise of several meters, beyond that which humanity is capable of adapting to. Or, as Hansen put it, “We’re in danger of handing young people a situation that’s out of their control.”
These staggering claims come as climate scientists continue to reel from the frightening speed at which the Earth is warming. On Monday, the World Meteorological Organization (WMO, issued a report warning that climate change is occurring at an “alarming rate” and that world leaders must act to curb greenhouse gases now, “before we pass the point of no return.”
In a video released alongside the new report, Hansen, who left his position at NASA in 2013 so that he could fully commit himself to fighting climate change, says that the paper explores the consequences of continued greenhouse gas emissions. These include “super-storms stronger than any seen in modern times,” sea level rise that will erase “all coastal cities,” and, finally, “how soon we will pass points of no return.”
Atlantic Ocean overturning found to slow down already
“It is conspicuous that one specific area in the North Atlantic has been cooling in the past hundred years while the rest of the world heats up,” says Stefan Rahmstorf of the Potsdam Institute for Climate Impact Research, lead author of the study to be published in Nature Climate Change.
Previous research had already indicated that a slowdown of the so-called Atlantic meridional overturning circulation might be to blame for this. “Now we have detected strong evidence that the global conveyor has indeed been weakening in the past hundred years, particularly since 1970,” says Rahmstorf.
Because long-term direct ocean current measurements are lacking, the scientists mainly used sea-surface and atmospheric temperature data to derive information about the ocean currents, exploiting the fact that ocean currents are the leading cause of temperature variations in the subpolar north Atlantic. From so-called proxy data – gathered from ice-cores, tree-rings, coral, and ocean and lake sediments – temperatures can be reconstructed for more than a millennium back in time. The recent changes found by the team are unprecedented since the year 900 AD, strongly suggesting they are caused by man-made global warming.
“The melting Greenland ice sheet is likely disturbing the circulation”
The Atlantic overturning is driven by differences in the density of the ocean water. From the south, the warm and hence lighter water flows northwards, where the cold and thus heavier water sinks to deeper ocean layers and flows southwards. “Now freshwater coming off the melting Greenland ice sheet is likely disturbing the circulation,” says Jason Box of the Geological Survey of Denmark and Greenland. The freshwater is diluting the ocean water. Less saline water is less dense and has therefore less tendency to sink into the deep. “So the human-caused mass loss of the Greenland ice sheet appears to be slowing down the Atlantic overturning – and this effect might increase if temperatures are allowed to rise further,” explains Box.
The observed cooling in the North Atlantic, just south of Greenland, is stronger than what most computer simulations of the climate have predicted so far. “Common climate models are underestimating the change we’re facing, either because the Atlantic overturning is too stable in the models or because they don’t properly account for Greenland ice sheet melt, or both,” says Michael Mann of Pennsylvania State University in the US. “That is another example where observations suggest that climate model predictions are in some respects still overly conservative when it comes to the pace at which certain aspects of climate change are proceeding.”
No new ice-age – but major negative effects are possible
The cooling above the Northern Atlantic would only slightly reduce the continued warming of the continents. The scientists certainly do not expect a new ice age, thus the imagery of the ten-year-old Hollywood blockbuster ‘The Day After Tomorrow’ is far from reality. However, it is well established that a large, even gradual change in Atlantic ocean circulation could have major negative effects.
“If the slowdown of the Atlantic overturning continues, the impacts might be substantial,” says Rahmstorf. “Disturbing the circulation will likely have a negative effect on the ocean ecosystem, and thereby fisheries and the associated livelihoods of many people in coastal areas. A slowdown also adds to the regional sea-level rise affecting cities like New York and Boston. Finally, temperature changes in that region can also influence weather systems on both sides of the Atlantic, in North America as well as Europe.”
If the circulation weakens too much it can even break down completely – the Atlantic overturning has for long been considered a possible tipping element in the Earth System. This would mean a relatively rapid and hard-to-reverse change. The latest report by the Intergovernmental Panel on Climate Change (IPCC) estimates there to be an up to one-in-ten chance that this could happen as early as within this century. However, expert surveys indicate that many researchers assess the risk to be higher. The study now published by the international team of researchers around Rahmstorf provides information on which to base a new and better risk assessment.
