The idea of a ‘mini ice age’ triggered by a shutdown of the oceans' thermohaline circulation (THC) has been rich fodder for dramatic scenes from Hollywood to the Pentagon. The currents of the THC take cold surface water from high latitudes southwards at depth, driving low-latitude warm surface waters north. This powerful heat conveyor belt is driven by differences in salt-water density. According to models, if sufficient fresh water is added to the ocean, which could happen as a result of global warming, the THC may cease. Indeed, palaeoclimate studies suggest that the THC has shut down a number of times during colder climates in the past 100,000 years.

But most climate researchers have long abandoned the notion of isolated cold regions amidst a globally warming world. It now seems less likely that even a full collapse of the THC — which, although improbable, might still occur towards the end of the century — would significantly cool Britain or Scandinavia (see Climate change: A sea change).

A weakening of the thermohaline circulation may lead to perturbations in global climate systems, with unknown side effects.

The matter is not yet closed, however. A weakening of the THC — and recent observations published in Nature have suggested that the currents have begun to change — may lead to perturbations in global climate systems, with unknown side effects. There are many uncertainties, but it is clear that people in Western Europe and eastern North America are less threatened by a consequent rapid climate change (and are more capable of adapting to it) than many people in poorer societies.

More measurements are clearly needed if we are to fill the enormous gaps in our knowledge of ocean behaviour. Autonomous observation tools, such as drifting floats and moored buoys, are now allowing scientists for the first time to monitor the state of the ocean currents almost in real time. This is an important advance, but observations must be sustained for much longer periods than foreseen in the six-year RAPID programme (see Furthermore, they should be augmented globally if we are to anticipate possible changes in ocean behaviour with any confidence.