The main Atlantic current that keeps northern Europe warm could see new changes and turning points

Northern Europe is relatively warm, given its location on the Earth’s surface. For example, although London is north of most major Canadian cities, it is warmer than all cities (even Vancouver, British Columbia). But this warmth could disappear by the turn of the century due to global warming.

The reason is that a major ocean current, the Atlantic Ocean Current (AMOC), which runs from the Gulf of Mexico to Svalbard in Norway, may be shutting down. It currently carries huge amounts of warm water into the North Atlantic, where it cools, sinks and changes direction sharply, moving off the east coast of Greenland and then across the mid-Atlantic (and under the South Atlantic Ocean Current) into the South Atlantic. The heat it releases in the process keeps northern European ports ice-free.

As global warming occurs, salty water in the Northeast Atlantic mixes with cold fresh water from the melting Arctic, and with the increased rainfall that characterizes global warming. This fresh water reduces the density and salinity of the current, and thus the current cools and sinks in the North Atlantic, and thus its southward flows decrease.

In 1995, climate modelers is expected The AMOC cycle will stop by 2200. Observations have been available since 2004, and in fact, there are parts of the AMOC It seems to be slowing down..

But until now, climate models have not been able to take a close look at the Atlantic ocean circulation, including its currents, eddies and many inputs.

Now, using a climate model that takes a more detailed look at the Atlantic Polar Circulation, scientists have a better view of its future, uncovering details that previous models had missed. In this new, more accurate model, the Atlantic Polar Circulation suddenly collapses in some areas, and unexpectedly increases in others. The results are: Published In the magazine Physical Review Letters.

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

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

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

They assumed that the level of carbon dioxide in the atmosphere would rise at a high rate – which the Intergovernmental Panel on Climate Change has confirmed. RCP 8.5 scenarioWith carbon dioxide levels rising rapidly over the century to a level of about 1,250 parts per million in 2100.

Both high- and low-resolution models showed a general slowdown in the AMOC, of ​​about 8 million cubic meters of water per second from 2000 to 2100, with a sharp decline near 2020. (By comparison, the AMOC’s total flow rate is estimated to be about 15 to 20 million cubic meters of water per second, transporting about 1.3 million billion joules of energy per second.) But on a smaller, more regional scale, parts of the AMOC collapsed abruptly, and in others it strengthened over time.

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

In addition to regional variations and ocean currents, the high-resolution model revealed turning points that were not known from lower-resolution studies.

A tipping point is when a system suddenly changes from one state to another—the threshold at which a small additional change causes the system to suddenly move into a new state. For example, you can eat and drink while wearing a pair of pants, but at some point the bottom of your pants will suddenly rip, and you will be in a different state forever. That’s a tipping point for the pants.

Subsystems of The climate system has tipping points.For example, studies have been conducted on the past of the Greenland ice sheet. estimated The Earth will reach a tipping point when the Earth’s temperature rises by about 2.5 degrees Celsius above pre-industrial levels. When the Earth reaches the tipping point, the melting of the entire ice sheet may become inevitable.

The scientists found that on smaller scales, parts of the Atlantic geophysical circulation have turning points that do not appear in previous models of the general Atlantic geophysical circulation.

“The results highlight the urgent need to incorporate regional dynamics into Atlantic ocean circulation projections, as these local shifts can have profound impacts on climate and marine ecosystems,” Lohmann said.

“As we face an uncertain climate future, these insights underscore the critical importance of developing climate models to anticipate and respond to dramatic changes in our planetary systems,” he said. Moreover, he said, the feedback between the Atlantic macrocycle and the Atlantic small-scale circulation “may change in the future.”

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