The problem, according to a team led by Dr Wei Liu of the University of California, San Diego, is that previous models ignored freshwater flowing between the Atlantic and Arctic Oceans. Both oceans are naturally salty, but the melting of the polar ice caps introduces cold freshwater and some of this can travel from the Arctic to the Atlantic. Once this is included, the AMOC can run amok, changing dramatically in response to circumstances.
Liu's work explains how the AMOC turned off so often in the past, something previous models struggled with. The impact of this freshwater flow won't be immediate. For the first 50 years after a sudden doubling of carbon dioxide, AMOC strength is similar with and without allowing for the flow, Liu reports, but after that the differences become noticeable.
If Liu's model is right, then global warming will lead to the AMOC weakening relatively steadily for almost 300 years, before stabilizing at a much lower level. Even while the world as a whole warms, Iceland is likely to be a barely habitable 10°C (18°F) colder. Britain could cool approximately 3°C (5°F) in winter. Even the low countries and northern parts of France and Germany may find things rather unpleasant.
Even if there is no flaw in Liu's modeling, this is not inevitable. Radical cuts in greenhouse emissions could minimize the effect. However, if the world refuses to take action, Churchill may have been very premature in diagnosing Britain's “darkest hour”.