TThe ocean circulation that keeps our relatively northern corner of Europe warm (ish) is often compared to a gigantic conveyor belt bringing warm equatorial water north to the surface, balanced by a cold southerly flow at great depth . The Atlantic Meridional Overturning Circulation, or AMOC for short, brings thermal energy north at the equivalent rate of 10 Hiroshima bombs. each second and keep our weather mild, and just a little too humid, and is essential for the climate at large.
New research has provided important long-term context for scientists’ observations of these Atlantic currents that bring warmth and climate stability to our coasts, with worrying implications for the coming century. Changes in AMOC in the geological past have caused significant local and global impacts, and for several decades now oceanographers have been monitoring its strength.
Computer modeling and theory predict a constant reduction in the strength of AMOC and its heat distribution service in response to human-induced changes in precipitation, river runoff and melting sea ice arctic and Greenland ice cap. In addition, they show that AMOC is one of the global climate “tipping points”. If it is reduced beyond a certain limit, currently uncertain, it can suddenly collapse, with enormous implications for our lives. Our best observational estimates, based on oceanographic data dating back to 1871, show that there has already been an approximately 15% reduction in AMOC force.
What is missing, however, is the longer-term context: is the current decline part of a long natural cycle, or is it due to human influence?
Research published in the journal Nature Geoscience this week provided that background. Authors from Ireland, Germany and the UK have shown that AMOC is the weakest it has ever been for over 1,000 years. They did this using a combination of 11 different “indirect measures” that indirectly infer the AMOC force. These indicators include the composition of marine sediments, tree rings, ice core chemistry, and other exotic metrics that make up the bread and butter of the niche realm in a paleoceanography niche.
On its own, any such recording should be interpreted with caution, but nine of these 11 indicators show a reduction in AMOC strength since the late 1800s, with an even greater decay since the 1960s. Importantly, they show also that before about 1850, the approximate start of human industrial influence, the AMOC force was relatively stable just before AD 400.
This provides critical observational evidence linking human influence to the decline of the AMOC force, confirming what climate models have been showing for decades. These same models also predict that with current emission rates, AMOC could be reduced to less than half of its original strength by 2100. There is still a lot of uncertainty here, but the message is strong. : greenhouse gas emissions drive AMOC lower and lower. Fortunately, the recent Special Report on Oceans and Cryosphere of the Intergovernmental Panel on Climate Change concluded that it is still “very unlikely” to meet the tipping point in the current century, but this does not mean that there are no consequences in the near future. term.
A reduced AMOC is expected to bring colder weather across the UK and northern Europe, with much more intense winters and storms off the Atlantic, as well as reduced summer precipitation and crop productivity and a greater likelihood of extreme weather events such as the 2015 European. summer heatwave. The impacts are not limited to this side of the Atlantic either. Sea level rise is forecast for the US east coast, with associated increased risk of flooding and potentially increased hurricane intensity.
Such significant changes in ocean circulation also threaten the ecosystems and aquaculture on which we depend. Marine deoxygenation and changes in the abundance of key species have been linked to a slowdown in AMOC, as well as an overall reduction in productivity in the North Atlantic Ocean.
The southern end of AMOC around Antarctica is also of concern. The global ocean as a whole has absorbed over 90% of the human-induced warming, absolutely eclipsing the changes in air temperature that concern us all. The huge ocean bordering Antarctica is where most of this extra heat (and carbon dioxide) was injected into the deep ocean, and it is warming and acidifying at an alarming rate. One of the main areas of research for oceanographers like me is whether the ocean will continue to essentially sweep human impacts under the carpet – and what might happen if that stops.
This should not, however, be a cause for despair and inaction. The same models that predict the slowdown in AMOC also show that sharp emission reductions can now lead to a resumption of AMOC towards the turn of the century. Research can reduce uncertainties, but the message is clear: Strong climate action at government and industry levels is needed now, and it’s people’s job to force such action with their portfolios and votes.
Andrew Meijers is a physical oceanographer with the British Antarctic Survey. He is Deputy Scientific Director of the Polar Oceans team and heads the Orchestra’s research program studying ocean circulation and its impact on climate.