The Scare of Rapid Climate Change for our near future…how much is true...

Current global warming has been proven to lie outside the range for the earth’s natural variability (IPCC, 2013). So we know that the state of current warming is likely abnormal and that it might have influences on the Atlantic meridional overturning circulation (AMOC), also known as the thermohaline circulation (THC).

You have seen the melting rates in the Arctic on the bottom of the last post… in coupling with warmer temperatures, waters in the North Atlantic will become fresher and warmer, which reduces their density and slows down deep water formation in the North Atlantic. No more water pushes southward, slowing down the AMOC in return.

Scientists were quite worried about the force of global warming on the AMOC. In 2002,Vellinga & Wood used a HadCM3 model (a coupled ocean-atmosphere model) to investigate the global impact that awaits us, if the AMOC were to shut down. The results are frightening. Within only 20 years, Europe would cool by 1-3°C, and the northwest Atlantic up to 8°C! Even North America and Asia would suffer under cooling of 2°C. The numbers might seem small, but Vellinga & Wood(2002) note that a cooling of >1°C has never been observed since 1659 (the onset of direct air temperature measurements in the UK).

If this gives you an unwell feeling, think about the currently projected warming due to anthropogenic greenhouse gases: 4°C by 2100, if we do not cut our emissions quickly (IPCC, 2013). This implies a warming of 1°C in 20 years, similar to the cooling in the little Ice Age.

In response to the “shut down scare”, many studies investigated the likelihood of current AMOC or THC shut down. However, Stouffer et al. (2006) note that global warming may increase the freshwater input to the North Atlantic, but only by the order of 0.14 Sv (see INFO BOX). To shut down the conveyor, at least 1.0 Sv are needed which is highly unlikely to occur. Similarly, Wood et al. (2003) state a shut down to be highly unlikely under current CO₂ projections.

Model studies are helpful in understanding system behavior. Their problem: models are only as good as the understanding of the system during the time the model was written. Anything we do not know, we cannot imply in a model and cannot reprocess. One such variable are thresholds. As the three modes of the THC show, it is likely that ocean circulation presents threshold behavior. Knutti& Stocker (2002) conclude in their ocean model analysis that today’s models are insufficient for finding the AMOC’s threshold points, mostly due to missing information. Thus, they are unable to surely predict the changes in ocean circulation under climate change scenarios.

Paleoclimatology is unable to help in this case, since CO₂ has not been this high for more than 2 million years…

So the only possibility to get a better insight into the AMOCs behavior is to measure it directly...

…and the RAPID program was born!



http://www.rapid.ac.uk/index.php