Warm water is carried in a stream hundreds of kilometres wide, all the way from the Gulf of Mexico to the Arctic Ocean. The warmth of the south comes to the north to make life more comfortable. But the Gulf Stream is not necessarily a permanent phenomenon. According to two studies published in Nature* this year, the Gulf Stream is now the slowest it has been for as much as 1,600 years.
This comes as no surprise to Professor Petteri Uotila from the Institute for Atmospheric and Earth System Research (INAR).
“Climate modelling suggests that climate change will weaken thermohaline circulation in the Atlantic in the long term.”
Already the IPCC’s 2013 Fifth Assessment Report, which reflects general scientific consensus, considered it to be highly likely that the circulation in the Northern Atlantic will slow down during this century. New observations supporting this prediction have been made during the past few years. The underlying mechanisms are also well known. Melting glaciers are releasing fresh water into the ocean.
“When a large quantity of water from the melted ice is introduced in areas where heavier water would descend towards the bottom, the salinity of the water will decrease, slowing down the descent and, consequently, the circulation,” says Uotila.
Cold water
This is happening in the North Atlantic right now. In satellite images, this area appears as the only northern region that is getting colder while other regions are warming up.
“Known as the subpolar gyre, this is the area where melting sea ice and lower-salinity water from the Arctic Ocean meet the warmer, saltier water from the south. Now that more fresh water is entering the gyre from the glaciers of Greenland, the gyre is cooling and slowing down.”
Climate change can complicate ocean currents in many ways: after all, approximately 90% of the heat that humanity has introduced into the atmosphere by burning coal and cutting down trees is stored in the oceans.
“Most of this heat is stored in surface waters. The oceans have become increasingly stratified, and surface water does not mix with deeper layers as easily. This can slow down circulation,” Uotila explains.
13,000 years ago
The idea of a weakening Gulf Stream is not just based on computer simulations. Climate change has slowed down the Stream before, not even that long ago.
Approximately 13,000 years ago, as the latest ice age receded, the cold fresh water melting from the glaciers and streaming from the glacial Lake Agassiz weakened the circulation in the North Atlantic. This resulted in a significantly colder period in the middle of a generally warming climate and is known as the Younger Dryas.
The cooling of the climate during the Younger Drias was intense and rapid, albeit temporary. Summers became short but warm. It is unlikely that we will see a similar rapid cold period in the foreseeable future.
“The glaciers today are smaller and so are the amounts of meltwater they generate, so the Gulf Stream will weaken more gradually – unless the glaciers melt very rapidly. On the other hand, the climate is currently heating up faster than during the Bølling-Allerød interstadial, a warm period that preceded the Younger Dryas,” Uotila says.
Methane on the ocean floor
In the 2004 disaster film The Day After Tomorrow, glaciers melting due to global warming stop the Gulf Stream, creating tornadoes and bringing about an instant ice age.
However, experts assure us that a new ice age is not around the corner. According to Petteri Uotila, ocean researcher, the climate is becoming warmer at such a rapid pace that even a gradual slowing down of the Gulf Stream would not be sufficient to trigger an ice age.
But could a weaker Gulf Stream mitigate the melting of the ice caps? Uotila says no: the circulation is declining too slowly, while the climate and surface waters are warming up too quickly.
Climate researcher James Hansen has suggested that the rapid warming of the climate more than 50 million years ago could have been the result of methane eruptions. The descent of warmer waters may have shifted to areas where there is methane in the ocean floor.
“The point at which the warmer water in the Gulf Stream sinks in the north also depends on the weather. We have found that as the climate becomes warmer, water from the North Atlantic is spreading to the Arctic Ocean, and there is methane in the ocean floor on the Siberian Shelf,” says Uotila. “However, the threat of methane eruptions is minor in comparison to rising ocean levels or the other likely effects of climate change.”
After Paris
A weaker Gulf Stream will lead to the stratification of the ocean, which may in turn accelerate global warming if the warmer waters are prevented from descending. This will reduce the ability of the ocean to store heat, which may also impact the capacity of the ocean to absorb carbon dioxide.
“In addition to heat, most of the carbon introduced into the atmosphere is stored in the oceans, and as it is transported to deeper waters, it is likely that it will remain in the ocean for a very long time. This mechanism may also become weaker.”
Changes in the circulation have been expected to have local impacts in the form of storms, higher water levels, wind and abnormal weather conditions or floods, such as the recent one in Miami. The weaker the Gulf Stream becomes, the more difficult it will be to predict the future. The same principle applies to the Gulf Stream as does to the impact of climate change in general: the greater the temperature change, the less certain our future
According to Uotila, the greatest uncertainty is not linked to the ice caps, the Gulf Stream or rising ocean levels.
“The future of the Gulf Stream now depends on humanity. We have the Paris Agreement, but it is still unclear what we intend to do about climate change.”
This article has been published in Finnish in Yliopiston magazine Y/05/18
*Nature articles: