The major Atlantic current warming northern Europe could see new variations and new tipping points

Current AMOC velocities in (left) a low-resolution climate model and (right) the high-resolution model in this article. Credit: Physics Magazine via APS

Northern Europe is a relatively warm country given its geographical position. For example, although it is located north of most major Canadian cities, London is warmer than all other cities (even Vancouver, British Columbia). But this warmth could disappear by the end of the century due to global warming.

Indeed, a major ocean current, the Atlantic Meridional Current (AMOC), which runs from the Gulf of Mexico to Svalbard, Norway, could 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 AMOC to the northeast) and into the South Atlantic Ocean. The heat it releases in the process helps keep northern European ports ice-free.

Under global warming, salty water from the northeastern AMOC mixes with cold fresh water from melting Arctic ice and increased precipitation characteristic of global warming. This fresh water reduces the density and salinity of the current, reducing its cooling and sinking into the North Atlantic, and thus its southward flow.

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

But until now, climate models have not been able to closely observe the AMOC, including its many currents, gyres and inflows.

Now, thanks to a climate model that looks at the AMOC in more detail, scientists have a better view of its future, finding details that previous models missed. In this new, more resolved model, the AMOC collapses abruptly in some regions and unexpectedly increases in others. The results are published in the journal Physical Exam Letters.

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

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

Lohmann and his collaborators used a recently developed high-resolution climate model called the Community Earth System Model, which reduced the previous grid sizes of 1° of 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’s RCP 8.5 scenario, with carbon dioxide increasing rapidly over the century to a level of about 1,250 parts per million (ppm) by 2100.

Both high- and low-resolution models have shown an overall slowdown of the AMOC, by about 8 million cubic meters of water per second from 2000 to 2100, with a sharp decline around 2020. (By comparison, the total flow of the AMOC is estimated at 15 to 20 million cubic meters of water per second, carrying about 1.3 trillion joules of energy per second.) But on a smaller, more regional scale, some parts of the AMOC have collapsed abruptly, and other parts have even strengthened over time.

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

Besides regional variations and ocean eddies, the high-resolution model showed tipping points that were unknown from low-resolution studies.

A tipping point occurs when a system suddenly changes from one state to another. It is a threshold at which a small additional 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 rip and they will remain in a different state forever. This is a tipping point for pants.

Subsystems of the climate system have tipping points. For example, studies of the Greenland ice sheet’s past have estimated that it will hit a tipping point when the Earth has warmed about 2.5°C above pre-industrial levels. When that tipping point is reached, melting of the entire ice sheet may be inevitable.

Scientists found that on smaller scales, parts of the AMOC exhibit tipping points that do not appear in previous models of the overall AMOC.

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

“As we face an uncertain climate future, these insights underscore the critical importance of advancing climate models to anticipate and respond to dramatic changes in our planet’s systems.” Additionally, the feedback between the global AMOC and the small-scale AMOC “could change in the future,” he said.

More information:

Ruijian Gou et al., Decline of the Atlantic Meridional Overturning Circulation: Small-scale tipping under global warming, Physical Exam Letters (2024). DOI: 10.1103/PhysRevLett.133.034201

Quote:Major Atlantic Current Keeping Northern Europe Warm May See New Shifts and Tipping Points (July 30, 2024) Retrieved July 30, 2024, from https://phys.org/news/2024-07-major-atlantic-current-northern-europe.html

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