Melting water threatens the Gulf Stream | illvet.se

If you turn off the circulation pump in the basement, the items in the living room will soon become cold. The same principle applies to the pump that circulates vast amounts of seawater in the Atlantic and transfers heat through the Gulf Stream from south to north, all the way to the British Isles and Norway.

The Gulf Stream, among other things, ensures that northern Europe and Scandinavia have much milder winters than Canada on the other side of the ocean.

But now large amounts of freshwater melting from the ice sheet threaten to turn off the heat.

according to new mathematical calculations From researchers at the University of Copenhagen, the rapid and powerful supply of fresh water in the sea between Greenland and Iceland could already in two decades bring the pump beyond a fateful point, where it weakens so much that our climate is collapsing.

The Gulf Stream is our hot friend

Earth’s oceans are connected by a huge conveyor belt ocean currents, which orbits all the way around the Earth. The Gulf Stream is a tributary of the conveyor belt that travels from the Gulf of Mexico – hence the name – all the way to the North Atlantic.

The further north the current moves, the more heat it loses. Heat loss makes the water cooler, saltier, and heavier. In the marine area between Greenland and Iceland, heavy surface waters sink to a depth of two to three kilometers.

The descent can be compared to a circulation pump, which pushes a downstream flowing south all the way to Antarctica. There the water returns to the surface and flows north again.

In recent decades, this submersible pump has weakened, as global warming sends increasing amounts of fresh meltwater from the Greenland ice sheet into the North Atlantic. The fresh water is lighter than the cool, salty waters of the Gulf Stream and mixing means that instead of sinking, the water stays on the surface.

The warming could thus bring the global ocean currents closer to a critical point, as the amount of fresh water in the sea outside Greenland becomes so great that the pump behind the currents stops completely.

Melting is out of control

Until the turn of the millennium, the Greenland ice sheet was fairly balanced, with precipitation over the ice roughly making up for the amount of fresh water lost by the ice sheet as the icebergs melted and formed.

At the end of the 1990s, the annual mass loss was 28 billion tons, but in recent years the ice sheet has released 250-300 billion tons of light fresh water into the sea every year. In 2019, mass loss broke all records when it reached 532 billion tons.

Climate researcher Sebastian Mernild of the University of Southern Denmark estimates that the ice sheet in the 1940s may also force a tipping point, where the ice sheet will continue to shrink irreversibly for centuries. Merenild is the lead author on this topic in an upcoming report from the United Nations Climate Committee, IPCC.

If the Greenland ice sheet exceeds this point, it will inevitably lead to a steady increase in the freshwater supply in the North Atlantic, which will weaken the submersible pump and weaken the strength of the ocean current.

If the ocean currents stopped completely, it would have dire consequences on both sides of the Atlantic.

Winter temperatures in northwest Europe will drop by as much as three degrees, and severe winter storms will become more common. The cooling will hit northern Scandinavia hard as the temperature drops by more than ten degrees.

On the other side of the Atlantic, the collapse would cause water levels to rise 15 to 20 centimeters along the eastern coast of the United States, as well as trigger more storms, as the Gulf Stream would not divert large amounts of seawater. from the coasts.

Networks monitor currents

Ocean currents in the Atlantic Ocean move about 17 million cubic meters of water per second and represent a heat transfer of 1.3 billion watts. This is 200 times the energy that all power plants in the world can produce together.

Because of the dangerous consequences of the collapse, climatologists over the past 20 years have established three networks of measuring stations across the Atlantic Ocean, which constantly monitor cold bottom currents and warm surface currents.

The nets consist of threads anchored to the sea floor and extended by buoys near the sea surface. The strings are equipped with instruments placed at different depths, capturing cold bottom currents and warm surface currents, such as the Gulf Stream.

OSNAP plays a central role, as the network measures developments in basin regions around Iceland and in the Labrador Sea, where a circulation pump pushes deep ocean currents south.

Compared to previous measurements of ship shows NotesThe strength of ocean currents has fallen by 15 percent since 1950.

But this is just average. The strength of currents can vary greatly from year to year and decade to decade.

The current, for example, dropped 30 percent in 2009, reducing heat transfer to the North Atlantic by nearly 100 trillion watts — corresponding to the power output of more than 100,000 large power plants. The result was an unusually harsh winter in Europe.

To distinguish these surprising natural differences from the long-term effects of human-caused warming, data going back several centuries is needed.

Now, an international research group led by German climate physicist Lefke Kaiser has found the information in a wide range of nature’s climate archives.

The Gulf Stream is historically weak

The most important source of information on the strength of ocean currents throughout history is the drilling of cores from the sea floor. The cores contain sediments with particles of silt, that is, fine-grained sand, which ocean currents lose along the way.

During periods when currents are strong, they deposit only larger particles, and smaller particles continue to rotate with the current. When the current is weak, even small particles sink to the bottom. Thus the amount of small and large grains in the drill core bearings is indicative of the strength of the old currents.

Drill cores also tell of past temperatures at sea level. There, researchers analyze the content of the cores of tiny microscopic animals with limestone shells that sink to the sea floor when they die. Some species thrive in warm sea water, while others predominate in cold water. In this way, species composition at different levels in the drill core reveals sea temperatures in time.

The research group also collected data from climate archives on land, such as drill cores from the Greenland ice sheet and tree rings that reveal how much water has evaporated from the sea surface, and how warm the air around the Atlantic Ocean is. .

By coordinating the various measurements, the researchers have now obtained a detailed overview of the strength of ocean currents from AD 400. Even today.

Until 1850, the Atlantic Ocean circulation operated as a well-oiled machine. Then a cold period called the “Little Ice Age” ended, and the subsequent warming caused the glaciers of the Northern Hemisphere to melt and from the Greenland ice sheet.

More meltwater seeped into the sinking area around Greenland and Iceland, slightly weakening the ocean current’s flow.

Since 1950 and especially after 2005, increased melting of the ice sheet has seriously weakened the pump, which is now at its weakest level in 1,600 years.

Climate physicists to decode Caesar arrived to That the strength of ocean currents could decline by another 34-45 percent through 2100, if human-made emissions of greenhouse gases continue as they are now.

She estimates that ocean circulation could dangerously approach a critical point in the coming centuries, when the Gulf Stream becomes unstable and threatens to collapse completely.

Fast Track Climate Collapse

Climate physicists Johannes Lohmann and Peter Dettlevsen of the University of Copenhagen have now played this role Model Calculations Who figured out how big the melt would have to be from the Greenland ice sheet in order to stop the circulation pump completely.

In the simulation, the researchers slowly increased the water flow over 300 years, until the critical water supply was exceeded at 230,000 cubic meters of fresh water per second during the summer months. When the flow increased even more, the submersible pump between Greenland and Iceland completely collapsed.

The researchers then simulated a number of processes in which the fresh water supply to the basin area increased faster, but only to levels below the critical point of 230,000 cubic meters per second, posing no danger.

Calculations showed very surprisingly that even small amounts of melt water can stop ocean currents, if the supply of fresh water occurs at a high rate within 10-150 years. The faster the ice sheet melts in the model, the smaller amounts of melt water are required to stop the pump behind the ocean currents.

Hopefully, we’ll never know if the Lohmann and Ditlevsen models are correct. The best way to reduce the risk of a Gulf Stream collapse is to rapidly and drastically reduce global greenhouse gas emissions.

Until then, it is important that measurements from the three transatlantic networks continue in the coming decades, so that researchers can obtain better data on the strength of ocean currents over a longer period.

The measurements increase the possibility of calculating the exact volume and melt speed of the ice sheet required to turn off the circulation pump in the Atlantic basement.