The sun heats the earth, but unevenly. The excess heat around the equator moves towards the poles, via a number of different mechanisms, the most noticeable for us humans being via air masses. That’s what much of our weather is about. Heat also moves towards the poles, in the ongoing evening-out of energy distribution on the planet’s surface, via ocean currents.
One of the interesting things that happens with ocean currents is this: Warm water tends to move from equator towards polar regions across the surface, then cools down and drops to the deep sea, where it moves back south again, often in a kind of loop that we call a "conveyor." Becuase of some quirky historical stuff, the continents on this planet are mostly in the norther hemisphere, so the loops of ocean water that mariners have long called "currents" are extra strange in the north, and as it happens, there is a big loop of warm water or two that go way farther north (as warm water) than usual, where increased evaporation and cooling cause the water to a) loose it’s heat to the air and b) sink rather dramatically to the bottom of the sea. The sinking helps direct the north-moving surface currents, maintaining the loop. The release of heat keeps England from looking like Canada and Norway from looking like Greenland, as much of this heat leaves the North Atlantic and traverses Europe first. By the time that energy gets around the world all the way back to Greenland, well, it isn’t helping to melt glaciers very much, bit it does in fact have an effect. Without this warming, there would probably be continental glacial masses on Europe and Canada, rather than scattered and small mountain glaciers. In other words, there would be an ice age.
Did I mention the evaporation as a driving force in the conveyor? Yes, of course I did. And the reason this works is that when the warm surface water evaporates, it becomes more saline relative to the rest of the ocean, and sinks, because salty water is denser than fresh water. We believe that there have been times in the past when fresh water being added to the northern seas has mixed with a conveyor, caused the water to be less salty, turned off the flow of warm water to the northerly latitudes, and ushered in a mini-ice age, or perhaps a maxi-ice age. Indeed, there are some theories about paleoclimate that suggest, very strongly, that this is exactly the mechanism that triggers an ice age.
As a yet another irony in the process of anthropogenic global warming, fresh water that is released in large quantities by warming-induced melting of groundwater, glaciers, or other ice in North America or northern Eurasia can cause fresh water to mix with Arctic or other northern sea water, threatening a reduction in saline-density driven conveyor activity. For this reason, scientists are keeping a close eye on fresh water moving into the northerly basins.
A study just released (well, tomorrow) in Nature looks at fresh water from Russia entering the Arctic. At first there was concern that this water would change Arctic Sea salinity, but it turns out that recent changes in currents in the region have redirected the Russian melt-waters in a more benign direction. From the abstract of the study:
Freshening in the Canada basin of the Arctic Ocean began in the 1990s and continued to at least the end of 2008. By then, the Arctic Ocean might have gained [enough] fresh water [to raise] the spectre of slowing global ocean circulation. Freshening has been attributed to increased sea ice melting and contributions from runoff, but a leading explanation has been a strengthening of the Beaufort High–a characteristic peak in sea level atmospheric pressure, which tends to [cause] convergence of fresh surface water. Limited observations have made this explanation difficult to verify, and observations of increasing freshwater content under a weakened Beaufort High suggest that other factors must be affecting freshwater content.
Here we use observations to show that during a time of record reductions in ice extent from 2005 to 2008, the dominant freshwater content changes were an increase in the Canada basin balanced by a decrease in the Eurasian basin. … The freshwater changes were due to a [counterclockwise] shift in the ocean pathway of Eurasian runoff forced by strengthening of the west-to-east Northern Hemisphere atmospheric circulation characterized by an increased Arctic Oscillation index. Our results confirm that runoff is an important influence on the Arctic Ocean and establish that the spatial and temporal manifestations of the runoff pathways are modulated by the Arctic Oscillation, rather than the strength of the wind-driven Beaufort Gyre circulation.
In short, the biggest change in Arctic salinity is in the Beaufort Sea, where it is fresher than ever recorded (over 50 years of study), with most of that water coming from Russian rivers, and a tiny bit coming from the famously melting Arctic sea ice.
What is the significance of this? There are two things you need to know:
1) Middle scale (decades or less) variation in climate are caused by things like the "Arctic Oscillation" or ENSO variation (linked to the Pacific) and so on, and these need to be understood very well in order to interpret long term climate changed data. For the most part, these things are well understood, but as one of the authors of this paper states, "To better understand climate-related changes in sea ice and the Arctic overall, climate models need to more accurately represent the Arctic Oscillation’s low pressure and counterclockwise circulation on the Russian side of the Arctic Ocean." So, we’re moving in that direction and that’s good, and if you are into conveyors and salinity, it’s actually very interesting; and
2) This will be reported by the AGW denialists as somehow disproving global warming. Why? because to them, my Great Aunt Tillie disproves global warming. It will be interesting to see how they manage that. Well, not really, but I’m sure we are going to get it shoved down our unwilling throats soon enough.
Morison, J., Kwok, R., Peralta-Ferriz, C., Alkire, M., Rigor, I., Andersen, R., & Steele, M. (2012). Changing Arctic Ocean freshwater pathways Nature, 481 (7379), 66-70 DOI: 10.1038/nature10705