POTSDAM, Germany — The Atlantic ‘cold blob,’ the patch of ocean south of Greenland and Iceland that has been cooling while the rest of the planet warms, has a cause. A study by Stefan Rahmstorf of the Potsdam Institute for Climate Impact Research and colleagues, published this week in Geophysical Research Letters, finds that the cooling is being driven by a weakening of the Atlantic Meridional Overturning Circulation, the ocean conveyor belt that brings warm water from the tropics to the North Atlantic and keeps Europe’s winters mild. The cold blob is not, the study concludes, a product of surface heat loss to the atmosphere. It is the signature of less heat arriving from below.
The methodological move that produced the finding is the part the climate-science community will be reading. The Rahmstorf team used observation-based ocean reanalysis data from 1955 onward and satellite measurements from 1993, the longest continuous instrumental record available, to test whether the cold blob’s cooling signal extends below the surface. It does. Cooling in the cold blob, the study finds, is present at depth as well as at the surface; atmospheric conditions like winds and clouds, which would be the alternative explanation, have a much weaker influence on temperatures in the deep water column. The AMOC slowdown is the only explanation that fits both signals.
The substantive significance of the finding is the part the policy community will be reading. The AMOC has been one of the tipping points in the IPCC’s climate-tipping inventory since the AR5 cycle in 2014, and the question the science community has been working since then is whether the circulation is weakening fast enough for the tipping to be a near-term risk. The Rahmstorf team’s answer, framed cautiously in the paper, is that the cooling signal is consistent with a weakening trajectory that crosses the tipping point around the middle of the century in a substantial fraction of CMIP6 model simulations. The system, in the authors’ words, is approaching the tipping point closely enough that society and policymakers should treat it as a serious concern.
The downstream consequences of an AMOC weakening are the part that has been in the climate-impact literature for a decade. Sea level on the US East Coast rises faster than the global average if the AMOC slows, because the Coriolis dynamics of the surface current change; European winters, which the AMOC’s heat transport keeps roughly five to ten degrees Celsius warmer than they would otherwise be at the same latitudes, get colder; the West African and South Asian monsoons, which the AMOC’s tropical heat transport helps anchor, shift southward by a measurable margin; and the Amazon, whose moisture budget depends on the same tropical band, becomes drier still. The collapse of the AMOC, which is the bottom-of-the-distribution scenario, would be a global-scale climate reorganisation event.
The British Broadcasting Corporation, covering the AMOC question in 2023, framed the science community’s working position as one of high physical confidence in the direction of change and lower confidence in the timing. The Rahmstorf paper is the latest contribution to the timing question. The reanalysis-based observational record the team used is, by virtue of being instrumental rather than model-based, less subject to the structural uncertainties that have been the principal caveat in the timing literature. The study’s confidence interval on the timing is still wide, but the centre of the distribution has moved earlier.
The scientific reception, even within the AMOC community, has not been uniform. David Thornally, a professor of ocean and climate science at University College London who has been one of the principal voices on the AMOC question, told reporters that the Rahmstorf study bolsters the evidence of a link between the cold blob and the weakening current but cautioned that the sparseness of direct AMOC measurements means the reanalyses are best viewed as good approximations rather than perfect representations. The caveat is the standard one in the field. The Rahmstorf team does not contest it; the study’s contribution is the depth-extension of the cooling signal, not a claim of certainty on the rate.
The timing context of the publication is the part the broader climate week has been making legible. The Indicators of Global Climate Change report published Wednesday by seventy scientists concluded the 1.5-degree carbon budget will be exhausted in three years at current emissions; NOAA declared an El Nino Advisory with a 63 percent probability of a very strong event by next winter; the European Union finalised its ETS2 carbon-market design; and Australia’s COP31 negotiations president told reporters at the Bonn UN climate talks that the world needs to get off fossil fuels. The Rahmstorf paper is the ocean-system slot in the same picture. The system that delivers Europe’s winter heat is approaching a tipping point on a calendar the political community has been arguing over.
The policy translation is the part the next round of UN climate negotiations will read. Stabilising the AMOC requires limiting global warming, because the warming-driven freshening of the North Atlantic from Greenland ice-sheet melt is one of the principal mechanisms slowing the circulation. The Paris Agreement’s 1.5-degree threshold is, on the AMOC literature’s working assumption, also the threshold that gives the circulation the highest probability of staying below the tipping point through the rest of the century. The IGCC report’s finding that the threshold will be crossed around 2030 at current emissions means the AMOC’s risk distribution moves with it. The two papers are reading the same climate-system signal.
The European political-economic implication is the part the European Union has been reasoning about for several years. The European Commission’s climate-resilience strategy explicitly references an AMOC weakening as a tail risk that the bloc’s adaptation planning has to absorb; the energy-system stress-tests run by the European Network of Transmission System Operators for Electricity have included an AMOC-slowdown winter-cold scenario in the modelling since 2024. The Rahmstorf paper is the kind of input that moves an AMOC scenario from the tail-risk band toward the central-scenario band in the next iteration of those models. Whether the political class moves the policy with it is the question the operational community is asking inside.
The cold blob, in the imagery the climate-communication community has used since the anomaly was first identified, is the only part of the planet’s ocean surface that is cooling. The Rahmstorf team’s paper is the strongest published case to date that the cooling is the signature of the broader system that warms the rest of the North Atlantic doing less of that work. The deep-ocean cooling the reanalysis data found is, in the team’s framing, the part of the signal that earlier studies missed because they were looking at the surface. The result is the kind of empirical confirmation that moves a hypothesis from the model-based literature into the observational record. The AMOC slowdown, on this paper’s reading, is no longer a future scenario. It is a current measurement.
