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

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A brief overview of the findings.

A new reconstructed index of sea surface temperatures confirms previous research: The North Atlantic ocean currents appear to be approaching a point of no return, where their strength will inevitably weaken in the coming decades and centuries.

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 a 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.

Difficult to study 

The AMOC cannot be observed in detail because it is a complex ocean current system reaching thousands of meters in depth. Furthermore, measurements of the AMOC strength only date back to 2004 (the international RAPID Climate Change project), which is far too short a period to detect the approach of an AMOC tipping point.

The paper’s conclusion is thus based on a detailed reconstruction of a North Atlantic Ocean sea surface temperature index from 850 to the present in order to retrace AMOC variations in the past. This is possible because the amount of warm water transported northward by the AMOC influences the temperature of the Atlantic sea surface. The strength of the ocean circulation leaves a fingerprint in surface temperatures, which can then be used to retrace AMOC variations. However, varying amounts of CO2 emissions and atmospheric pollution have greatly blurred the AMOC’s sea surface temperature fingerprint in observations.

To disentangle the effect of CO2 emissions and atmospheric pollution, the scientists carefully reconstructed the North Atlantic surface temperature variations driven by ocean circulation from paleo-data from the year 850 to1869. From this, the natural effect of the AMOC on sea surface temperatures was used as an index to estimate the evolution of the AMOC from 850 up till today.

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.”