Global warming will make some of us a lot colder

Northern Europe is warmer than it should be, all other things being equal, thanks at least in part to ocean currents. The Atlantic Meridional Overturning Circulation (AMOC), of which the better-known Gulf Stream is a part, is an important convection current that moves warm water northwards, in turn warming the climate of northern Europe. The rate of evaporation is higher from warm water than from cold water, and its salinity and hence its density is consequently increased. As water in the AMOC travels north, it becomes increasingly dense and by the time it reaches the Nordic Seas, its density is sufficiently increased that it begins to sink (“overturning”). This overturning contributes to a multitude of processes, for example the movement of dissolved carbon, which the dense, salty water transports to the deeper reaches of the ocean, thus contributing to the role of oceans as carbon sinks.

Figure 1. Trend in sea surface temperature 1993-2023 published by the Copernicus Climate Change Service.

It has been predicted for many years that global warming would lead to a disruption of these critical currents, both because of increased warming of the oceans and also through reductions in the salinity of oceans due to influxes of meltwater. Satellite data have for some years shown a “cold blob” just off the coast of Greenland (see Figure 1), despite the significant warming of the majority of the Atlantic Ocean, but at the moment it is unclear whether this is a precursor to the shutdown of AMOC, or merely a local effect due to the high rate at which meltwater from Greenland is flowing into the sea. Modelling of these complex systems is highly challenging. However, in the sixth report of the Intergovernmental Panel on Climate Change (IPCC), published in 2023, the following rather startling summary was provided:

“The Atlantic Meridional Overturning Circulation (AMOC) will very likely decline over the 21st century for all SSP (Socio-economic Pathways) scenarios. There is medium confidence that the decline will not involve an abrupt collapse before 2100. For the 20th century, there is low confidence in reconstructed and modelled AMOC changes because of their low agreement in quantitative trends. The low confidence also arises from new observations that indicate missing key processes in both models and measurements used for formulating proxies and from new evaluations of modelled AMOC variability. This results in low confidence in quantitative projections of AMOC decline in the 21st century, despite the high confidence in the future decline as a qualitative feature based on process understanding.”

Thus, the consensus view is that a decline in AMOC is highly likely but it is not easy at the moment to predict how soon this will be or how severe the consequences will be. Recent modelling by van Westen et al has suggested that very severe changes in climate could result, including a cooling of winter temperatures by between 10 °C and 30 °C occurring within a century, and dramatic climatic changes in a time period of just a few years. These changes might include a massive decrease in temperatures in the North Sea in winter, with sea ice possibly extending far down the east coast of the UK. Changes of this magnitude would be catastrophic for agriculture in the UK.

There has been debate about the likely time horizon for the AMOC shutdown; however, even where there is scepticism about the likelihood of an imminent shutdown, there is nevertheless agreement that such an outcome is likely and that it will have significant impact on climate systems.

At first sight it seems reassuring to read in the IPCC report that there is medium confidence that there will not be an abrupt collapse before 2100. However, having medium confidence that an absolutely calamitous event will not occur is far from reassuring. Stefan Rahmstorf is a leading researcher in the study of the effects of climate change on AMOC. In an article in the journal Oceanography he says

“A full AMOC collapse would be a massive, planetary-scale disaster. We really want to prevent this from happening. In other words: we are talking about risk analysis and disaster prevention. This is not about being 100% or even just 50% sure that the AMOC will pass its tipping point this century; the issue is that we’d like to be 100% sure that it won’t. That the IPCC only has “medium confidence” that it will not happen this century is anything but reassuring, and the studies discussed here, which came after the 2021 IPCC report, point to a much larger risk than previously thought…For the AMOC and other climate tipping points, the only action we can take to minimize the risk is to phase out fossil fuel use and stop deforestation as fast as possible.”

In the context of my granddaughter’s life (she will be 75 in 2100), this is a terrifying prospect: the IPCC has only medium confidence that the AMOC will not shut down during her lifetime. Of course there are uncertainties in the modelling. New experimental data are needed, and intensive measurement work is under way, as Channel 4 recently reported.

However, the longer that humanity continues to fail to take the corrective action that is needed to halt the harm we are doing to the planetary ecosystem, the more “baked-in” significant warming becomes, and the harder it becomes to avoid such calamitous tipping points.