The main Atlantic current that keeps heat in northern Europe could have new variations and tipping points

Northern Europe is relatively warm for its location on the globe. For example, although north of most major Canadian cities, London is warmer than all of them (even Vancouver, British Columbia). But that warmth could disappear by the turn of the century thanks to global warming.

That’s because the main ocean current, the Atlantic Meridional Ocean Current (AMOC), which runs from the Gulf of Mexico to Norway’s Svalbard, could stop working. Today it carries a huge amount of warm water into the North Atlantic, where it cools, descends and changes direction sharply, moving from the east coast of Greenland, then across the mid-Atlantic (and below the northeast boundary of the AMOC) and on to the South Atlantic Ocean. The heat it releases in the process keeps northern European ports ice-free.

With global warming, the salty northeast AMOC mixes with cold fresh water from the melting Arctic and with the increased precipitation characteristic of global warming. This fresh water reduces the current’s density and salinity, so its cooling and sinking in the North Atlantic is limited, and thus its southward flow.

In 1995, climate modelers predicted that the AMOC circulation would stop by 2200. Observations have been available since 2004, and indeed parts of the AMOC seem to be slowing down.

Until now, however, climate models have been unable to observe the AMOC, including its many currents, gyres, and inlets.

Now, using a climate model that looks more closely at the AMOC, scientists have a better view of its future, and have found details that earlier models were missing. In this new, more robust model, the AMOC collapses suddenly in some areas and rises unexpectedly in others. The findings are published in the journal Physical Review Letters.

“Our high-resolution model study reveals a surprising reversal: the Atlantic Meridional Overturning Circulation (AMOC) may intensify in the subarctic Atlantic due to warming,” said Gerrit Lohmann, co-author of the study from the Alfred Wegener Institute in Helmholtz. Center for Polar and Marine Research at the University of Bremen in Germany, “contrary to the widespread belief that this vital current system is uniformly weakening.”

Large global climate models used for climate change projections typically divide land and ocean into 100 by 100 kilometer regions to accommodate time and computing availability. As “low-resolution” models, they may miss smaller physical features such as eddies and eddies in the ocean.

Lohmann and colleagues used a recently developed high-resolution climate model called the Community Earth System Model, which reduced previous grid sizes of 1° latitude and longitude on each side to 0.1°, or about 17 kilometers.

They projected atmospheric carbon dioxide levels to increase at a high rate – the IPCC’s RCP 8.5 scenario, with carbon dioxide increasing rapidly over the century to a level of about 1,250 parts per million (ppm) in 2100.

Both high- and low-resolution models showed an overall slowdown of the AMOC, about 8 million cubic meters of water per second from 2000 to 2100, with a sharp decline near 2020. (For comparison, the total AMOC flow is estimated at 15 to 20 million cubic meters of water per second, it transports about 1.3 million billion joules of energy per second.) But on a smaller, more regional scale, parts of the AMOC collapsed suddenly and even strengthened over time in other parts. .

“Advanced climate models now reveal that under extreme greenhouse gas emissions (RCP 8.5), the AMOC could experience a sharp decline in some regions, while paradoxically increasing in the Arctic,” said Lohmann. “This unexpected regional strengthening occurs despite an overall trend of weakening AMOC activity.”

In addition to regional variations and ocean eddies, the high-resolution model showed tipping points that were not known from lower-resolution studies.

A tipping point is when a system suddenly changes from one kind of state to another—a threshold where another small change causes the system to suddenly go into a new state. For example, you can eat and eat while wearing pants, but at some point the bottom of your pants will suddenly tear and they will be in a different state forever. This is the breaking point for pants.

Subsystems of the climate system have tipping points; for example, studies of the past of the Greenland ice sheet have estimated that a tipping point will occur when the Earth warms about 2.5°C above pre-industrial levels. When a tipping point is reached, melting of the entire ice sheet may be inevitable.

The researchers found that at smaller scales, parts of the AMOC have tipping points that do not appear in previous models of the general AMOC.

“The findings highlight the urgent need to incorporate regional dynamics into AMOC forecasts, as these localized shifts could have profound impacts on climate and marine ecosystems,” said Lohmann.

“As we face an uncertain climate future, these findings highlight the critical importance of advanced climate models for predicting and responding to dramatic changes in our planet’s systems.” What’s more, the feedback between the overall AMOC and the small-scale AMOC “may change in the future,” he said.

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