North Atlantic Ocean currents are closest to a tipping point in a thousand years

Researchers confirm find that the Atlantic Meridional Overturning Circulation (AMOC), a key system of ocean currents in the North Atlantic that carries heat from the Equatorial region to the mid-latitude and polar regions, seems to be approaching a tipping point from which it may inevitably switch to a much weaker state.

According to lead author Simon Michel, Utrecht University, the Netherlands, the result is based on robust reconstruction of surface AMOC indices and validates earlier findings based on much shorter time series. The work, published in Nature Communications, is part of the Horizon 2020 European TiPES project administered by the University of Copenhagen, Denmark, together with other EU funding. The science was conducted by Simon Michel with colleagues at universities in France, Ireland, and Spain.

”This is an additional warning that we need to act quickly to reduce the risk that such an AMOC transition occur. An irreversible AMOC weakening might have dramatic consequences on our societies and ecosystems. Therefore, we also emphasize the need to further study the effects of an AMOC tipping to also be able to prepare for it,” says Simon Michel, who started the work at the University of Bordeaux, France, before joining the University of Utrecht in the Netherlands.

Known to tip

Paleoclimate data and climate models indicate that the AMOC can shift rapidly and substantially between an active and a much less active state, influencing the climate system globally. A transition to the less active state is expected to impact Northern European temperatures, rainfall in the tropical monsoon systems, and the occurrence of extreme weather events.

In recent years, increasing physical and modelling evidence has raised concern that the AMOC could experience such a transition due to a large temperature increase and strong ice cap melting.

The authors of the paper Early warning signal for a tipping point suggested by a millennial Atlantic Multidecadal Variability reconstruction now add to that concern. They find that a reconstructed sea surface temperature index related to the AMOC is indeed showing signs of a critical slowing down in recovering from its up and down variations, which indicates the approach to an irreversible weakening of the AMOC strength.

Based on the results, the AMOC is estimated to be at its closest to a tipping point for at least a millennium. The findings, however, do not reveal whether the tipping point has been reached or is just approaching, and thus when an eventual shift may occur. That could take decades or even centuries, depending on future human-caused greenhouse gas emissions and the resulting temperature rise and ice melt.

The findings also imply that concerted efforts to reduce greenhouse gas emissions may allow us to avoid crossing this critical threshold.

We have to consider it

”Our reconstruction is based on a robust and reliable way of using statistics, and there is a lot of reason to think that the result we show in terms of an early warning signal is also reliable,” says Simon Michel, and concludes:

”Even if some early warning signals can be falsely detected, we know that for the last millennium, the minimum and maximum in the global temperature ranged between 0.6-0.8 degrees according to existing reconstructions. Today, we stand at 1.1 degrees above the first observations of the global temperature that will keep going up. So, there are reasons that some components of the climate system might shift as a response to such an unprecedented change. And with this kind of early warning signals of tipping points, we really must consider it in the panel of plausible future scenarios.”

The TiPES project is an EU Horizon 2020 interdisciplinary climate science project on tipping points in the Earth system. 18 partner institutions work together in more than 10 countries. TiPES is coordinated and led by The Niels Bohr Institute at the University of Copenhagen, Denmark and the Potsdam Institute for Climate Impact Research, Germany. The TiPES project has received funding from the European Horizon 2020 research and innovation program, grant agreement number 820970.

This research was also funded by Blue Action, EUCP, ROADMAP, and ARCHANGE EU-H2020 research programs.