In the North Atlantic there is a crucial pump for global ocean currents and thus an important player in the climate system. Now a study reveals that this circulating current has stuttered several times during the last interglacial periods – it fluctuated between normal strength and a near failure in intervals of one century to a few millennia. According to the researchers, this could indicate that this North Atlantic circulation flow is stable over long periods of time. But especially in times of warm climates, it could more easily than in a period of great fluctuations.
The North Atlantic is a key region for global ocean currents, but also for Europe’s climate. Because here is one of the big circulation pumps in the oceans: Warm, salty water sinks down in front of Greenland and then flows cooled down to the south. On the surface, this subsidence creates a pull that draws warm water from the tropics to the north – including the Gulf Stream and the North Atlantic Stream, which are crucial for Europe’s climate. The North Atlantic Meridional Circulation (AMOC) thus removes heat from the tropics in high latitudes and has a decisive influence on the regional and global climate. It is therefore important that this “pump” also runs reliably. If it stops or weakens significantly, the supply of heat from the southern climes is absent and Europe and North America could experience a kind of regional ice age.
Pump for warm and cold water masses
So far, the North Atlantic circulation seems to have been relatively stable, although there are signs of a slight weakening. However, studies indicate that the inflow of meltwater in particular can disrupt the sinking of the water in the North Atlantic. The expiry of a large meltwater lake at the end of the last ice age some 8000 years ago led to a halt in the circulation, as researchers reported in 2011. And even today, the pump is weakened in summers with high meltwater intakes in the North Atlantic. Climate researchers are therefore concerned that the AMOC could be a tipping element – a component in the climate system that could change state due to the rapidly increasing global warming. Based on previous data, they assume that the circulating flow is difficult to destabilize, but its failure could be irreversible.
However, in addition to the complete “tipping” of the circulating flow, there could be another state of this important flow pump, as Eirik Galaasen from the University of Bergen and his colleagues report. Using drill cores from the North Atlantic, you investigated how the AMOC has behaved in the four interglacial periods of the past around 450,000 years. To do this, they evaluated the carbon isotope values in the limestone shells of foraminifera preserved in the sedimentary rock – shell-bearing unicellular organisms that occurred in almost all seas and at almost all times in the history of the earth. Because these organisms lived at the bottom of the oceans, the isotopes embedded in their shells indicate the temperatures and salinity levels prevailing in the deep water at that time.
Strong but short fluctuations
The analyzes showed that there were major changes in the North Atlantic deep water during the warm interglacial periods. According to Galaasen and his colleagues, the supply of cold deep water fluctuated between normal, modern values and very small amounts, which are more typical of ice ages and large amounts of meltwater, due to the circulation flow. As they determined using a supplementary model, the AMOC apparently experienced a number of exceptionally strong fluctuations during these times. “These circulatory disorders were only short-lived – they may have lasted a century or less,” says co-author Yair Rosenthal of Rutgers University in New Brunswick. However, in two of the interglacial periods there were periods of weakened circulation that lasted up to 3000 years. During this period, the North Atlantic cooled significantly because the supply of warm water from the tropics decreased.
According to the researchers, these results suggest that the North Atlantic circulation flow is quite robust and stable over a long period of time. But that does not exclude phases of short-term fluctuations. “The frequent occurrence of such deep water reductions in the last interglacial periods clearly demonstrates the potential for major changes in the deep Atlantic current,” stated Galaasen and his colleagues. “Our results suggest that rapid and large changes in the circulation flow should not only be seen as a possibility, but even as an intrinsic feature of variability in warm climates.” So far, what triggered these fluctuations can only be partially understood as the researchers explain. For example, there seem to have been major inflows of meltwater in some of these alternating phases, but not in others.
“Our data shows that major disturbances in the circulating flow can occur more easily than previously assumed and that they have occurred in the past under climatic conditions that could also be imminent for us,” said Galaasen and his team. Climate change could therefore not only gradually weaken the AMOC, but also trigger a new phase of abrupt change. Thomas Stocker from the University of Bern sees the same thing in an accompanying comment. He writes: “There is a possibility that the gradual anthropogenic influence on the climate system could move the AMOC into a state in which its variability becomes larger in amplitude and more chaotic.”
Source: Eirik Galaasen (University of Bergen), et al., Science, doi: 10.1126 / science.aay6381