Climate Change Could Stall Global Ocean Circulation System

The global system of ocean currents redistributes warm waters from the tropics north into the Atlantic and sends cooler water south past the Equator. It also regulates climate and without it, we could be doomed.

A study published in PNAS found that continued infusion of freshwater ice melt into the ocean could throw a global ocean climate circulation system off balance. (Courtesy of TiPES/HP)

(CN) — A system of ocean currents that circulate warm and cool sea water across the globe and help regulate Earth’s climate could become dormant or even collapse if global warming goes unchecked, according to a study released Monday.

The ocean current system, called the Atlantic Meridional Overturning Circulation (AMOC), redistributes warm and cool water across the equatorial regions and northern hemispheres.

The system directly influences the strength and frequency of tropical monsoons and North Atlantic storms.

Agricultural systems and biodiversity are also impacted by AMOC as it shapes rainfall and weather patterns year-round.

Several central players of the global climate system have already demonstrated their fragility under the weight of climate change, including the Amazon rainforest, the Arctic Sea ice system and the boreal permafrost.

The study led by Johannes Lohmann and Peter D. Ditlevsen from the Niels Bohr Institute at the University of Copenhagen in Denmark examined how close AMOC is to surpassing its “tipping point” or its threshold for system collapse.

Specifically, the study analyzed whether the infusion of ice melt into the sea — a mass injection of cold freshwater water into the salty ocean — could induce collapse of a relatively stable climate system such as AMOC.

Scientists focused on this phenomenon because of already available evidence of existing ice melt in Greenland spilling into the sea and disrupting components of AMOC.

Researchers found that continued ice melt infusion into the ocean could force AMOC to reach its tipping point much earlier and faster than ever anticipated, according to the study published Monday in the journal PNAS.

The finding reveals the fragility of climate systems worldwide — described in the study as a “rate-induced tipping point” — when global warming introduces unpredictable, rapid change into them, the study said.

“We show that due to the chaotic dynamics of complex systems there is no well-defined critical rate of parameter change, which severely limits the predictability of the qualitative long-term behavior,” the study said. “The results show that the safe operating space of elements of the Earth system with respect to future emissions might be smaller than previously thought.”

Researchers said in a news release attached with the study that because of the unpredictability of climate change it remains unclear what conditions must be present for rate-induced tipping to occur.

“These effects manifest themselves as a tipping of the system to a new state even before a theoretical threshold in the external conditions (such as the atmospheric CO2 levels) is reached,” the statement said. “In rate-induced tipping, the rate of change — not the amount of change — is the important factor. This is because tipping occurs more readily when the system’s conditions change rather quickly.”

Lohmann and Ditlevsen analyzed the effects of various conditions on AMOC using simulations played out within the ocean modeling tool Veros.

Veros allowed researchers to identify the AMOC tipping point at varying rates of freshwater injection.

The model showed that over periods of between 10 and 150 years, the input of freshwater from a source like Greenland ice melt could tip AMOC into a dormant state, even before its threshold had been reached.

Due to the chaotic nature of the ocean model, scientists were unable to determine whether ocean circulation would remain dormant or collapse entirely, the study said.

Researchers explain that it’s still unclear what level of atmospheric carbon dioxide would push the AMOC and other climate sub-systems beyond the thresholds for irreversible tipping.

“Our results suggest that the existence of alternative and undesired stable states may be already a risk even if the tipping point is relatively far away,” the study said. “Our findings point to fundamental limitations in climate predictability and corroborate the need to keep the boundary conditions of vulnerable elements of the climate system as stable as possible.”

Lohmann said in a statement released with the study that while the findings solidify the urgency in requiring CO2 emission reductions globally more research is needed to understand how global warming will push climate systems beyond their tipping points.

“It is worrying news,” Lohmann said. “Because if this is true, it reduces our safe operating space.”

Researchers did not immediately respond to a request for further comment on the study.

The study is part of the Tipping Points in the Earth System (TiPES) project and received funding from EU Horizon 2020.

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