Surely the most conspicuous tipping point in the recent climate record must be the incredibly rapid warming that took place at the Bølling-Allerød event and the end of the Younger Dryas, 14,700 and 11,500 years ago, respectively. In just a few decades air temperature over Greenland increased some 15°C and precipitation nearly doubled. It is generally understood that the north-flowing Atlantic meridional overturning circulation (AMOC) must be playing a major role because it is a nearby and major source of heat and moisture at high latitudes. We also know that prior to these two events the deep North Atlantic was filled with what is called Southern Ocean Water meaning the entire Atlantic Ocean was filled from the bottom up to nearly 2 km depth with dense water from the Antarctic. The water at shallower depths below the main thermocline was formed in the subpolar North Atlantic, which during glacial times was much larger because the Gulf Stream flowed essentially straight east toward southern Europe. So, what happened?
First, it must be recognized is that the rapid transition from cold to warm at these events must imply some form of positive feedback: a self-reinforcing process that causes an initial disturbance to grow rapidly to the point where it modifies the conditions it came from such that no further change is possible; it will have arrived at a new state. This rapid transition from one state to another is what is referred to as a tipping point.
The rapid increase in temperature and precipitation implies a sudden reorganization of the North Atlantic, it suddenly became much warmer. We can envision today how this might have happened. During glacial times the Gulf Stream was a zonal flow that closed the wind-driven circulation much like the Kuroshio and its extension the North Pacific Current does in the Pacific today. It flowed across the ocean and fed the wind-driven gyre to its south, but some water will have turned north in the east, analogous today’s warm Alaska Current. This supply of warm salty water feeds the paleo-subpolar North Atlantic and its production of paleo North Atlantic Intermediate water (NAIW), what was the AMOC during glacial times. There would likely have been some exchange of water with the Nordic Seas, mostly wind-driven. Now let’s suppose that the salinity of the surface water is gradually increasing. The basis for this might be increasing export of water vapor across Central America. Some of this warm salty water will leak into the Nordic Seas as well, this is important. Let us imagine that this increasing salinity will promote the production of dense water that will spill into the North Atlantic over the Greenland-Iceland-Scotland ridge. Most of this spill is mixed up locally and becomes part of the paleo-NAIW. But as the density of the overflow water increases it is plausible that it will remain increasingly intact and slide to depths where it can flow south unhindered by the paleo-Gulf Stream. This export at depth will allow for more warm salty water to be imported into the Nordic Seas. This increased salinity promotes the production of deep dense water further promoting the import of warm salty water – this is the positive feedback resulting in the rapid switch from a paleo-North Atlantic to a modern subpolar gyre circulation, the biggest tipping point of the recent past. This is what I mean by hiding in plain sight: the rapid import and spread of warm water from the Gulf Stream would have had a huge impact on the atmosphere: the warming and concomitant increase in moisture, what we see in the Greenland ice record. This contributed to the huge ice melt that began at the time of the Bølling-Allerød event.
In the paper 2003 Johan Nilsson and I wrote we suppose it is an increase in upper ocean salinity that led to this transition. At that time we didn’t give any thought to the fact that the paleo-SOW, which was several ppt greater than today, was becoming fresher reflecting increasing ice melt as indicated by the rising sea level since the last glacial maximum several thousand years earlier. Regardless, whether it is increasing surface salinity or decreasing deep salinity (or both), it is the salinity difference that matters with respect to the turn-on of the deep AMOC. In the paper we sketch the dynamical links we think might have enabled the North Atlantic to reorganize itself on the short time scales recorded in the Greenland ice record.
Rossby, T. and J. Nilsson, 2003. Current Switching as the cause of rapid warming at the end of the Last Glacial Maximum and Younger Dryas. Geophysical Research Letters, 30 (2).