Ocean Current Disruption

A study published in Nature Geoscience finds that the Atlantic Meridional Overturning Circulation is at its weakest since 400 CE. Co-author Stefan Rahmstorf:

We risk triggering [a tipping point] in this century, and the circulation would spin down within the next century. It is extremely unlikely that we have already triggered it, but if we do not stop global warming, it is increasingly likely that we will trigger it.

The consequences of this are so massive that even a 10% chance of triggering a breakdown would be an unacceptable risk.

Update (August 6):  A study published in Nature Climate Change suggests there has been a loss of stability in the AMOC. Author Niklas Boers:

We already know from some computer simulations and from data from Earth's past, so-called paleoclimate proxy records, that the AMOC can exhibit—in addition to the currently attained strong mode—an alternative, substantially weaker mode of operation. This bi-stability implies that abrupt transitions between the two circulation modes are in principle possible.

The findings support the assessment that the AMOC decline is not just a fluctuation or a linear response to increasing temperatures, but likely means the approaching of a critical threshold beyond which the circulation system could collapse.

Update (October 3):  A study published in Paleoceanography and Paleoclimatology finds that Pacific Ocean currents are also shifting due to climate change.

[R]esearchers conclusively found, using observational data and modeling studies, that [the Kuroshio Current and Extension] is warming, adjusting its latitudinal position northward, and possibly increasing the amount of warm water that it moves north in the process.

Update (July 25, 2023):  A study published in Nature Communications concludes that the AMOC could collapse sometime between 2025 and 2095 (95% confidence interval).

Hali Kilbourne:

It is very plausible that we've fallen off a cliff already and don't know it.