We know the ocean is warming due to climate change. But did you also know there are huge paths that heat and energy takes through the global ocean? Although the ocean may look like an enormous, flat expanse, there is a system of surface and deep-water currents that transport heat around the globe like a large system of conveyor belts. And freshwater from melting glaciers can mess up the system, potentially having climactic impacts.
This heat transport system is mostly driven by the density of different water masses, and is called Thermohaline circulation (THC). Differing density gradients are created by ocean surface heat and freshwater input into the salty ocean from rivers or glacier melt. Evaporation and precipitation also play into this circulation; as water evaporates, the ocean water left behind is saltier, and more dense.
In the Atlantic, the wind-driven surface Gulf Stream travels north from the equator, cooling as it goes, and sinks near Greenland to form new North Atlantic Deep Water (NADW). This process is called Atlantic Meridional Overturning Circulation (AMOC). NADW then flows back south along the seafloor. The water masses transport both heat and matter (solids, dissolved substances, and gases like CO2) around the globe. Thus, the state of the circulation and of AMOC has a large impact on the climate of the Earth.
Global warming tends to weaken the AMOC by warming the upper ocean in the subpolar North Atlantic and by melting more ice for freshwater to flow into the Arctic and North Atlantic. Both processes reduce the density of the upper ocean in the North Atlantic (salty, cold water is denser), which stabilizes the water column and weakens the AMOC. A weakening or shutdown of the AMOC could significantly reduce the poleward transport of heat in the Atlantic, thereby possibly leading to regional cooling in the Atlantic and surrounding continental regions, particularly Western Europe.
The Greenland ice sheet has been experiencing enhanced mass loss since the 1990s, and the increased meltwater flow into the North Atlantic is becoming ever more important in the changing freshwater budget of the subarctic Atlantic. The prolonged and increasing freshwater fluxes from Greenland to the surface ocean could lead to a suppression of deep winter convection in the North Atlantic, with potential threat to the strength of the AMOC. But just how important of a role does this meltwater have?
A meltwater-related freshening trend is difficult to distinguish, because there is strong decadal variation in the sub-polar freshwater content. This also means that the annual formation rates of the NADW vary. From simple ocean observations, it is difficult to say how much freshwater input remains in the subpolar North Atlantic and, in particular, how much of it invades the surface of the Labrador Sea, where it could impact the winter convection. Additionally, ocean current eddies complicate observation of freshwater motion in near-surface waters, making sources and path hard to track.
Böning et al. wanted to cut through all the variation and assess the impact that increasing freshwater may currently have on the AMOC. To do this, the researchers at the GEOMAR Helmholtz Centre for Ocean Research in Germany used a global circulation model that could capture small-scale, eddying transport processes in the subpolar North Atlantic. They used satellite observations to provide detailed reconstructions of the distribution of Greenland ice-mass trends and the corresponding freshwater discharge into the ocean.
The satellite data were used to compare a control simulation of coastal runoffs to a case with a spatially differing but consistently increasing runoff trend over a 30-year period beginning in 1990. Although their simulations show that the meltwater from the West Greenland shelf has initiated a gradual freshening trend at the surface of the Labrador Sea, they found that the freshwater anomaly has not yet had a significant impact on the AMOC. However, in coming decades, the accumulation of meltwater may become large enough to increasingly impede the deep winter convection, which weakens the formation of NADW and the global heat transport through the ocean.
They also conducted an experiment with their model to see what would happen if a constant, gigantic flux of freshwater entered the North Atlantic every year (about 3,000 km3 per year, ). The model showed a rapid dilution of the surface waters, a halt of deep convection of the AMOC after six to eight years, followed by a rapid slowdown of the AMOC. Under this simulation, in the year 2040, the accumulated runoff is more than 20,000 km3 (or 5,300,000,000,000,000 gallons3). That is a lot of freshwater, and more than is currently entering the North Atlantic. Thus, the researchers say that the current accumulation of meltwater is not yet large enough to affect the freshwater budget of the subpolar North Atlantic in a major way.
What does this all mean? Essentially, although the Greenland ice sheets have been melting at an alarming and increasing rate, it is currently not enough to have a major effect on the AMOC. However, that may be changing. The more Greenland ice melts, the slower and shallower the AMOC may become by the latter half of this century, which would affect deep water formation and heat / CO2 transport around the globe. Although this is not a major player in the climate discussion just yet, the freshening of the North Atlantic is something to keep an eye on in coming years.