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

That’s because a major ocean current, the Atlantic Meridional Ocean Current (AMOC), which flows from the Gulf of Mexico to around Svalbard, Norway, may stop flowing. Today it carries huge amounts of warm water into the North Atlantic, where it cools, sinks, and abruptly changes direction, moving off the east coast of Greenland, then across the mid-Atlantic (and under the northeast AMOC) and into the South Atlantic. The heat it releases in the process keeps northern European ports ice-free.

With global warming, the salty northeast AMOC mixes with cool fresh water from the melting Arctic and with increased precipitation characteristic of global warming. This fresh water reduces the density and salinity of the current, so that its cooling and sinking in the North Atlantic is reduced, thereby reducing its southward flow.

In 1995, climate modelers predicted that the AMOC circulation would stop by 2200. Observations have been available since 2004, and it does appear that parts of the AMOC are slowing down.

But until now, climate models have not been able to get a close look at the AMOC, including its many fluxes and gyres and inlets.

Now, using a climate model that looks at the AMOC in more detail, scientists have a better view of its future, discovering details missed by earlier models. In this new, more decisive model, the AMOC collapses sharply in some regions and unexpectedly increases in others. The findings are published in the journal Physical examination letters.

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

Large-scale global climate models used for climate change projections typically divide land and ocean into 100 km by 100 km zones to accommodate the timing and availability of computations. As “low-resolution” models, they can miss smaller physical features such as eddies and gyres in the ocean.

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

They assumed that the level of carbon dioxide in the atmosphere would increase at a high rate—the IPCC 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 the high- and low-resolution models show an overall slowdown of the AMOC of about 8 million cubic meters of water per second from 2000 to 2100, with a sharp decline near 2020. (For comparison, the total AMOC discharge is approximately 15 to 20 million cubic meters of water per second, carrying about 1.3 million billion joules of energy per second.) But on a smaller, more regional scale, parts of the AMOC collapsed suddenly, and in other parts even strengthened over time .

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

Besides 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 the system suddenly transitions from one type of state to another—a threshold at which an additional small change causes the system to suddenly transition to 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 forever be in a different state. This is a turning point for pants.

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

The scientists found that on the smaller scales, parts of the AMOC had turning points that did not appear in previous models of the overall AMOC.

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

“As we face an uncertain climate future, these insights highlight the critical importance of improving climate models to predict and respond to dramatic changes in our planet’s systems.” Moreover, the feedback between the overall AMOC and the small-scale AMOC “can be changes in the future,” he said.

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