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.

Continue reading Russian Rivers and Arctic Salinity: Climate Variation Better Understood →