By Bob Berwyn February 25, 2021 Inside Climate News
The change in the main ocean heat pump could bring more heat waves to Europe, increase sea level rise in North America and force fish to move farther north.
￼￼Since the end of the last ice age, a swirling system of ocean-spanning currents has churned consistently in the Atlantic, distributing heat energy along the ocean surface from the tropics toward the poles, with heavy, cold water slowly flowing back toward the equator along the bottom of the sea.
Collectively known as the Atlantic Meridional Overturning Circulation, the currents played a key role in shaping the climate of eastern North America and Western Europe, and thus the development of civilizations there. But in the 20th century, the circulation has weakened more than at any other time during at least the last 1,000 years, new research shows.
Together with other studies showing that global warming is driving the weakening, the new findings suggest that the circulation will lose even more strength in the decades ahead. That could cause heat and cold extremes in Europe and rapid sea level rise along the East Coast of the United States. As it weakens, pools of warm water form. That can lead to ocean heat waves, with increasing evidence that overheating oceans are linked with droughts and heat waves on nearby land areas.
The overturning circulation loops like a 10,000-mile conveyor belt through the North and South Atlantic, connecting polar regions. It brings cold water up from the deep, sends warmer water across the surface and then drops it back down thousands of miles away as it cools.
The new research zoomed in on the Gulf Stream, which is part of that circulation in the North Atlantic that flows northeast, parallel to the East Coast of the United States, mostly near the sea surface, then veers across the ocean to warm the higher latitudes of Northern Europe. Some of that water also cools and sinks in the subpolar region and starts flowing south.
“If we continue to drive global warming, the Gulf Stream System will weaken further, by 34 to 45 percent by 2100 according to the latest generation of climate models,” said co-author Stefan Rahmstorf, a climate scientist with the Potsdam Institute for Climate Impact Research.
The new study, published today in the journal Nature Geoscience, focuses mainly on measuring the changes in the volume of the Gulf Stream system, which moves 100 times more water than the Amazon River. But the scientists also looked at climate records that measure its strength in other ways.
Chemicals in fossil shells show how water temperature changes, and the size of sediments dropped by parts of the Gulf Stream system show the force of the flow, said co-author David Thornalley, a paleoceanographer at University College, London. When it’s moving slowly, it deposits fine silt and mud, like a lazy river does, but faster flows transport larger sediments, more like the gravels found in swift mountain streams, he said.
Other data sets include measurements that show where different water masses originated. The distribution of subtropical or subpolar water masses says a lot about the vigor of the circulation, Rahmstorf added.
“The strength of this paper is that we have all the different indicators pointing toward the same conclusion,” he said. It’s also important to consider how the changes will affect ocean ecosystems, he said. Some of the currents associated with the overturning circulation are important for dispersing coral larvae, and since changes to the current also affect temperature and salinity, they are likely to shift the production of plankton, the foundation of the ocean food chain.