Just in time for the release of the latest United Nation’s climate change report comes a newly published study revealing the potential shutdown of an ocean system that would dramatically alter life on a scale usually confined to disaster movies.
Too bad it wasn’t overamped fiction, but the recently published study by Niklas Boers, a theoretical physicist at Germany’s Potsdam Institute for Climate Impact Research, posits that 100 years of salinity and sea surface temperature data reveal warning signs that a huge system of currents in the Atlantic Ocean that influences temperatures and the climate may be on the verge of shutting down.
Boers data analysis found increasing instability indicative of what occurs before large global systems shut down or reverse course.
“(The) combined evidence makes it plausible that a critical transition to the weak mode may occur in response to rising temperatures and North Atlantic freshwater inflow,” Boers wrote in the study, “Observation-based early warning signals for a collapse of the Atlantic Meridional Overturning Circulation” published last week in the online journal “Nature Climate Change.”
Known by the acronym AMOC, this system is enormous, complicated and not easily measured.
Cape scientists say that the study should create a sense of urgency, however, Boers did not have enough hard data to support his theory that the system is at the tipping point of a shutdown.
In general, the AMOC is part of a larger global system of currents driven by differences in water temperature, salinity, density, as well as winds and the earth’s rotation. The AMOC is like a conveyor belt flowing from the Southern Atlantic into the Northern Atlantic bringing warm salty surface water north where it cools, becomes denser, and sinks. Driven in part by the mechanics of cold dense water flowing toward less dense warmer water, it returns south along the ocean floor.
The problem some researchers have focused on over nearly two decades is that the water to the north appears to have stopped sinking. Freshwater from melting ice sheets floats on top of the seawater isolating it from the chilling effects of Arctic air and making the seawater less dense so that it doesn’t sink.
The tipping point where the system cannot easily restore itself occurs when warming atmospheric and ocean temperatures along with other factors, such as a shrinking ice sheet, combine to make conditions where the waters coming from the south no longer reach the temperature and density to sink — and pull the Gulf Stream current along with it.
Boers does not predict when that tipping point may be reached, but along with other researchers, he pointed to a cold spot in the sea south of Greenland known as the Cold Blob where the waters have been cooling instead of warming as evidence that a weakening Gulf Stream is no longer traveling into that region. He also cited an area in the South Atlantic where salinity levels have risen as evidence the southern end of the system may also have slowed down considerably.
The Gulf Stream is part of the larger system and it serves to distribute heat from the equator to the north and cool water to the tropics, mitigating the uneven heating of the earth by the sun. Some studies posit that a shutdown could result in dramatic cooling in the northern hemisphere, changes to rainfall patterns, and impacts on ecosystems, wildlife, farming, and sea-level rise.
“I think it’s a really dramatic example of the urgency with which we are now approaching climate change impacts properly described as “tipping points,” said Richard Delaney, the executive director of the Center for Coastal Study in Provincetown and a co-founder of the Cape Cod Climate Change Collaborative. He also is on the board of directors of the Global Ocean Forum.
While Amy Bower, a senior scientist and chair of the physical oceanography department at the Woods Hole Oceanographic Institution, found Boers report interesting she cautioned that there wasn’t enough good data to definitively say that the AMOC was headed for shutdown.
“While Boers’ study is interesting, it is one man’s opinion. We don’t have direct observations (of temperature, salinity and other environmental conditions) that are long enough to corroborate his conclusions,” Bower said.
Boers himself acknowledged this in his paper, expressing concern about only two decades of direct observations and the concern that long-term natural cycles and other causes could be affecting the data. He said that they were insufficient on their own to “infer climatological AMOC weakening and a contribution by anthropogenic (human) climate change.”
But he thinks that using computer modeling and other data using ice and sediment coring revealed the “fingerprints of a weakening AMOC over the last 150 years.”
While Bower appreciated the effort to use modeling and other proxies to reach his conclusions, she thinks the uncertainty was too high to say the AMOC was nearing shutdown.
The AMOC is a complicated system and answers don’t come easy, Bower said.
For instance, sea level rise would have the biggest impact on the Cape and Islands. The strength and speed of the Gulf Stream keeps offshore waters about a meter higher than onshore. As the Gulf Stream slows, that difference diminishes potentially raising sea level along our coastline. But at least one study showed that the Gulf Stream off New England has actually picked up speed, Bower said.
“I think it is valuable to do these global assessments but it does come with a certain level of uncertainty that needs to be recognized,” she said.
Contact Doug Fraser at firstname.lastname@example.org. Follow him on Twitter: @dougfrasercct.
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