Impacts of thermohaline circulation shutdown in the twenty-first century

We discuss climate impacts of a hypothetical shutdown of the thermohaline circulation ('THC') in the 2050s, using the climate model HadCM3. Previous studies have generally focussed on the effects on pre-industrial climate. Here we take into account increased greenhouse gas concentrations according to an IS92a emissions scenario. THC shutdown causes cooling of the Northern Hemisphere of -1.7 degrees C, locally stronger. Over western Europe cooling is strong enough for a return to pre-industrial conditions and a significant increase in the occurrence of frost and snow cover. Global warming restricts the increase in sea ice cover after THC shutdown. This lessens the amount of cooling over NW Europe, but increases it over North America, compared to pre-industrial shutdown. This reflects a non-linearity in the local temperature response to THC shutdown. Precipitation change after THC shutdown is generally opposite to that caused by global warming, except in western and southern Europe, where summer drying is enhanced, and in Central America and southeast Asia, where precipitation is also further reduced. Local rise in sea level after THC shutdown can be large along Atlantic coasts (+/- 25 cm), which would add to the rise caused by global warming. Potentially rapid THC shutdown adds to the range of uncertainty of projected future climate change.