Thermohaline circulation, the large-scale movement of surface water through the world’s ocean basins, in latitudes near the poles is sensitive to temperature but even more so to salinity, so the larger concern in the North Atlantic Ocean comes from ice melt from the Greenland ice sheet and other sources of ice in the Arctic. The current melt rate in the Greenland ice sheet is almost 300 gigatons per year and it would take about ten times as much to shut thermohaline circulation down. It is not clear what air temperature increase it would take to reach that threshold, but we know that there have been even larger ice melts in the history of Earth, so it is quite possible that the current trend of increasing temperature may lead to another catastrophic event.
A surprising fact is that the strength of the winds and the temperature and salinity in the Antarctic Circumpolar Current (ACC), which is at the opposite end of the world, contributes to the rate of thermohaline circulation in the North Atlantic Ocean. In fact, the role of the Antarctic Circumpolar Current is as critical as conditions in the North Atlantic because of the meridional overturning circulation, a pole-to-pole cell connecting the surface waters in the high latitudes of the North Atlantic and in the ACC region. This cell is of such importance to oceanographers that there are research programs based in the United Kingdom and the United States devoted to it.
At the moment, despite the warming, ice is not melting in Antarctica, but only near the Arctic. This pattern will tend to slow down or even shut down thermohaline circulation. If ice were to melt in Antarctica as well, then thermohaline circulation may not be affected in the same way. However, we would have an even more serious problem with sea-level rise.
– Paola Cessi, physical oceanographer, Climate, Atmospheric Sciences and Physical Oceanography Division