Shutdown of Thermohaline Circulation - Bryden Measurements Reported Late 2005

Bryden Measurements Reported Late 2005

The NewScientist.com news service reported on 30 November 2005 that the National Oceanography Centre in the UK found a 30% reduction in the warm currents that carry water north from the Gulf Stream from the last such measurement in 1992. The authors note that currently the observed changes are "uncomfortably close" to the uncertainties in the measurements. However, the North Atlantic is currently warmer than in the earlier measurements. This suggests that either the circulation is not weakening, or that, even if it is weakening, the weakening is not having the hypothesised cooling effect, or that other factors are able to overwhelm any cooling.

The New Scientist article was based on an article in Nature. In News and Views in the same issue, Detlef Quadfasel reinforces the point that the uncertainty of the estimates of Bryden et al. is high, but says other factors and observations do support their results. Quadfasel continues by pointing out the significance of the possible implications, with palaeoclimate records showing drops of air temperature up to 10 °C within decades, linked to abrupt switches of ocean circulation when a certain threshold is reached. He concludes that further observations and modelling are crucial for providing early warning of a possible devastating breakdown of the circulation.

On 19 January 2006, a News Feature Climate change: A sea change by Quirin Schiermeier appeared in Nature, detailing reactions to the Bryden results. Points made by Schiermeier include the following:

• The results are a surprise to scientists in the field.
• Modelling suggests that increase of fresh water flows large enough to shut down the thermohaline circulation would be an order of magnitude greater than currently estimated to be occurring, and such increases are unlikely to become critical within the next hundred years; this is hard to reconcile with the Bryden measurements.
• The Bryden results could be caused by natural variation, or "noise", that is, coincidence.
• If the results are correct, perhaps thermohaline circulation reductions will not have the drastic effects that have been predicted on European cooling.
• While previous shutdowns (e.g. the Younger Dryas) are thought by some scientists to have caused cooling, the current overall climate is believed to be different; in particular, sea-ice formation is hypothesized to be less because of overall global warming.
• However, a thermohaline circulation shutdown could have other major consequences apart from cooling of Europe, such as an increase in major floods and storms, a collapse of plankton stocks, warming or rainfall changes in the tropics or Alaska and Antarctica (including those from intensified El Niño effect), more frequent and intense El Niño events, 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).

Further measurements support the interpretation of natural variation.