Thermohaline Circulation Collapse Mitigation

This is the subject of the movie now in theatres, “The Day After Tomorrow,” and a recent Pentagon commissioned study, that explored the ramifications of this from a geopolitical and economic perspective. We decided to include this topic after reading this report and seeing its proposed mitigation strategy.

The basis for the concern is the oceanic thermohaline circulation (THC) system that brings warm salty water northward from the equator to the Arctic, where its heat is released in the form of moist air. This air moderates the climate of Europe and Eastern North America, making them warmer and wetter than would otherwise be the case.

The North Atlantic portion of this system ceased functioning for 100-1000 years twice in the last 15,000 years, apparently due to an influx of fresh water from melting glaciers and/or increased rainfall and discharge of river water. The result was much colder and in some cases, drier weather for N. America and Europe, bringing a return of the ice sheets southward. Icebergs were found as far south as Portugal.

A similar shutdown could occur in this century if enough melt water and rainwater enters the N. Atlantic. There is some evidence of a drop in the salinity of the N. Atlantic near the upper end of the THC occurring since the 1940’s. The results of such a shutdown in this century, which might occur over a 10-100 year period (and not over a couple of days as portrayed in the movie) would, according to the Pentagon study, cause economic hardship and political instability as resources such as water and food are constrained by unfavorable climatic conditions.

The authors suggested injection of hydrofluorocarbons (HFCs) into the air to counteract the cooling of the Atlantic basin. Gaskill said HFC injection would not necessarily restore the salinity of the N. Atlantic to restart the THC and might instead, prolong the event by increasing melting of Antarctic ice, since the Southern Hemisphere would be expected to get much warmer in this scenario. He said a better approach would involve forcing the event to a more rapid conclusion by initiating a more rapid cooling of the areas contributing the fresh water.

This would be accomplished by injection of aluminum oxide particles into the troposphere or stratosphere over Greenland, the Eastern Arctic Sea and related areas that are the source of the fresh water flowing into the N. Atlantic. By doing so, the air temperatures in this region will be lowered enough due to reflection of sunlight to refreeze the freshwater sources flowing into the N. Atlantic and speed the recovery of the THC.

MacCracken said that tropospheric injection would be ineffective due to short residence times. Gaskill said that the presence of a persistent stagnant air mass over the region would help in maintaining the particle cover. He said other materials such as sulfate aerosol or soil could also be used. He said the delivery system could involve aircraft, but a more efficient method would use naval rocket shells (5-inch) fired from destroyers.

As unlikely as this scenario is, in spite of the movie and the Pentagon study, the potential for it to happen in this century cannot be fully discounted and thus, possible mitigation strategies should be investigated.