Shutdown of thermohaline circulation

A shutdown or slowdown of the thermohaline circulation is a hypothesized effect of global warming on a major ocean circulation.AMOC slowdown that causes cooling ~1°C and perhaps affects weather patterns is very different from an AMOC shutdown that cools the North Atlantic several degrees Celsius; the latter would have dramatic effects on storms and be irreversible on the century time scale. A shutdown or slowdown of the thermohaline circulation is a hypothesized effect of global warming on a major ocean circulation. Data from NASA in 2010 suggested that the Atlantic Meridional Overturning Circulation (AMOC) had not slowed down, but may have actually sped up slightly since 1993. A 2015 study suggested that the AMOC has weakened by 15-20% in 200 years. Don Chambers from the University of South Florida College of Marine Science mentioned, 'The major effect of a slowing AMOC is expected to be cooler winters and summers around the North Atlantic, and small regional increases in sea level on the North American coast.' James Hansen and Makiko Sato stated, .mw-parser-output .templatequote{overflow:hidden;margin:1em 0;padding:0 40px}.mw-parser-output .templatequote .templatequotecite{line-height:1.5em;text-align:left;padding-left:1.6em;margin-top:0} Downturn of the Atlantic meridional overturning circulation, has been tied to extreme regional sea level rise. A 2017 review concluded that there is strong evidence for past changes in the strength and structure of the AMOC during abrupt climate events such as the Younger Dryas and many of the Heinrich events. Lohmann and Dima 2010 found a weakening of the AMOC since the late 1930s. Climate scientists Michael Mann of Penn State and Stefan Rahmstorf from the Potsdam Institute for Climate Impact Research suggested that the observed cold pattern during years of temperature records is a sign that the Atlantic Ocean's Meridional overturning circulation (AMOC) may be weakening. They published their findings in 2015, and concluded that the AMOC circulation showed exceptional slowdown in the last century, and that Greenland melt is a possible contributor, with the slowdown of AMOC since the 1970s being unprecedented over the last millennium. A study published in 2016 found further evidence for a considerable impact from sea level rise for the U.S. East Coast. The study confirms earlier research findings which identified the region as a hotspot for rising seas, with a potential to divert 3–4 times in the rate of rise, compared to the global average. The researchers attribute the possible increase to an ocean circulation mechanism called deep water formation, which is reduced due to AMOC slow down, leading to more warmer water pockets below the surface. Additionally, the study noted, 'Our results suggest that higher carbon emission rates also contribute to increased in this region compared to the global average.' Global warming could, via a shutdown of the thermohaline circulation, trigger cooling in the North Atlantic, Europe, and North America. This would particularly affect areas such as the British Isles, France and the Nordic countries, which are warmed by the North Atlantic drift. Major consequences, apart from regional cooling, could also include an increase in major floods and storms, a collapse of plankton stocks, warming or rainfall changes in the tropics or Alaska and Antarctica, more frequent and intense El Niño events due to associated shutdowns of the Kuroshio, Leeuwin, and East Australian Currents that are connected to the same thermohaline circulation as the Gulf Stream, or an oceanic anoxic event — oxygen (O2) below surface levels of the stagnant oceans becomes completely depleted — a probable cause of past mass extinction events. Hansen et al. 2015 found, that the shutdown or substantial slowdown of the AMOC, besides possibly contributing to extreme end-Eemian events, will cause a more general increase of severe weather. Additional surface cooling from ice melt increases surface and lower tropospheric temperature gradients, and causes in model simulations a large increase of mid-latitude eddy energy throughout the midlatitude troposphere. This in turn leads to an increase of baroclinicity produced by stronger temperature gradients, which provides energy for more severe weather events.