24 February, 2015

What Sea Level Rise Models Fail to Take Into Account: Thermohaline Circulation Disruption

Ever wonder why sea level rise is one of the key "doom-mongering" points being used regarding climate science? Won't a gradual rise in sea level give us plenty of time ― hundreds of years ― to get out of harm's way? Actually, no, because it's a known scientific fact that sea level rise isn't linear. Sometimes it is. Other times, however, entire ice sheets collapse, sending sea levels rising abruptly ― I watched a video about this very phenomenon a couple of days ago ― of course, it's a video that dwells on sea level rise with absolutely no regard for any other impacts, but given that it is an alarmist video, that's not surprising. Anyhow, over the past 10,000 years, several so-called "meltwater pulses" triggered by sudden collapses of large masses such as the Laurentide Ice Sheet made sea levels rise at astounding rates ― in one such case, it is believed to have risen as much as 32.5 times faster than today. That being said, there is one key detail that the models regarding MWPs don't take into account: what happens to the ocean currents when abrupt sea level rises occur.

See, there's this global circuit called the thermohaline circulation. It acts like a global heater, transporting warm equatorial water northward. This, in turn, is exactly why some high-latitude places such as Europe and the Pacific Northwest are (usually) as warm as they are during the winter compared to other places, such as Canada and New York, at the same latitude: because of the warm water being transported northward by the thermohaline circulation, which warms the air above through the release of water vapor (a greenhouse gas that, molecule for molecule, is more than 2,000 times as potent as carbon dioxide, but has an atmospheric half-life of only a week compared to hundreds or thousands of years in the case of gases such as methane and CO2). This system, however, has a weakness: the delicate balance of warm, cold, salt, and fresh water that it depends on.

One needs to realize that in order to understand how fragile the thermohaline circulation is, one must first take into account how it works: When warm water moves north from the equator in response to constriction against continents by the normal (not anomalous) east-west flow of the trade winds, it moves into regions of colder, drier air. As a consequence, it evaporates more and more rapidly the further north it gets. That excess evaporation, in turn, results in an increase in salinity, density, and, thus, weight, so it sinks. Then, at the subsurface, the water moves back toward the equator from the Northern and Southern Hemispheres, heating up again. This self-perpetuating cycle normally makes places like the eastern US and Europe relatively temperate as far as climate is concerned. Normally.

Dumping large amounts of fresh water into the ocean from the north (or south) ― yes, even in the form of meltwater pulses ― however, puts this pattern in jeopardy. Remember that water gets denser the more saline it gets. Being less dense, fresh water tends to float on top of the salt water below. This, in turn, forces the warm ocean currents to downwell closer to the equator. Meanwhile, despite being less dense than salt water, fresh water still has a freezing point 4 °F higher (32°F) than salt water (28°F) does. The result? More sea ice. Sea ice which, in turn, increases the albedo of the oceans further north and south. Albedo increases, in turn, reflect more sunlight right back out into space, hampering the Sun's ability to warm the northern and southern regions, resulting in more ice, more snow, more albedo, and more cooling. This runaway process has in fact been responsible for several abrupt climate shifts in the past, including both the Younger Dryas ― likely the result of a meltwater pulse triggered by the late-Pleistocene thermal maximum ― and the Big Chill of 6200 BC, which may in fact have been triggered by none other than the aforementioned Pulse 1A.

So, in regards to sea level rise, therein lies the problem. Although meltwater pulses do indeed raise sea level, they also disrupt the very ocean currents that sustain them. This, in turn, leads to periods of cooling and advancing glaciers, and in some cases, even new Ice Ages. Although, I must admit, those then present a myriad of problems of their own... but that's a topic for another post.