Thermohaline Circulation Changes
The Thermohaline Circulation (THC) is a global oceanic current that brings heat to northwest Europe which helps raise the average temperatures above locations on a similar latitude, e.g. London, Moscow and Vancouver. The THC transports 1.3 ± 0.1 PetaWatts (1 PW=10,000,000,000,000,000W) of heat across 24ºN in the Atlantic. This is equivalent to the output of about 1 million powers stations, therefore its effect on global climate is enormous.
The THC brings warm water from the tropics to the North Atlantic and releases this heat to the atmosphere. The mass of water gets colder and saltier through evaporation and eventually becomes dense enough to sink to the ocean depths where it returns south. Increased freshwater into sinking zones could potentially decrease water density and cause the conveyor belt to reduce in strength. Freshwater input has already been shown to be increasing from multiple sources. For example, melt rates from Greenland have increased dramatically over the last 25 years with a 31% increase in melt area since 1979.
Already, warning signs are being reported by climate researchers. One team have reported a “widespread, sustained, rapid and surprisingly uniform freshening of the deep North Atlantic.” In 2005 Peter Wadhams produced a report on the measurements of the sinking “chimneys” of cold dense water usually found in the North Atlantic. Normally there are seven to twelve giant columns, but on his most recent survey Wadhams found only two columns, both extremely weak. And finally a 2005 study in Nature by the National Oceanography Centre reported a 30% decrease in the strength of the current since 1992, suggesting a slow down may already be occuring.
If the THC were to decline the effect would heavily impact power production, transport, agriculture and water supply. Temperatures would decrease by up to 5°C in northwest Europe. Such a change would be a shock to the economies in these areas. The years of warming beforehand that we are currently experiencing would also make society more vulnerable to a sudden switch to a colder climate regime.
The above chart shows surface temperature change (ºC) due to a hypothetical collapse in 2049. Anomalies shown are relative to the pre-industrial climate (Wood et al. 2003).