© 1999 Macmillan Magazines Ltd T he effect that global warming might have on the circulation of the Atlantic Ocean has been a topic of much specu- lation and research. On page 572 of this issue, Wood et al. 1 present greenhouse warming scenarios computed with a climate model that, for the first time, gives a realistic simula- tion of the large-scale ocean currents with- out requiring artificial adjustments of the air–sea fluxes. Of more immediate interest to those out- side the modelling business, Wood and col- leagues’ results show a dramatic change in the Atlantic occurring over the next few decades: a complete shutdown of one of the two main ‘pumps’ driving the formation of North Atlantic Deep Water, namely the one in the Labrador Sea (Fig. 1). In 1987, in an article entitled “Unpleasant surprises in the greenhouse?”, Broecker 2 warned that the response of the climate sys- tem to greenhouse warming might involve ‘mode switches’ of the Atlantic circulation. He drew this inference from palaeoclimatic data, indicating that such events had occurred in the past, and from early ocean modelling results. Initially, the idea was sim- ply that a positive feedback meant that the large-scale overturning motion of the Atlantic (sometimes popularly dubbed the ‘conveyor belt’, in which warm surface waters flow northwards and cold deep water returns south throughout the Atlantic, acting like a central-heating system for Europe; see Fig. 1) could exist in two distinct states — switched on (as at present) or switched off. Later work revealed a more complex picture, by showing that individual sites of oceanic convection could also have a tenden- cy towards flip-flop behaviour (switching between quasi-stable states with convection ‘on’ or ‘off’) 3,4 . Given that there are two main sites of convection linked to the formation of North Atlantic Deep Water, in the Greenland Sea and in the Labrador Sea, this led to spec- ulation that global warming could switch off one of these convection sites 5 . Wood et al. 1 provide the first clear model- ling result indicating that this could indeed be the response to increasing concentrations of greenhouse gases. The team works at the Hadley Centre in Britain, and their climate model is remarkable in several respects. Cer- tain improvements, including a higher reso- lution of the ocean and parameterizations of eddy mixing and of the flow of bottom cur- rents over marine sills, mean that the model provides a more realistic representation of the main ocean currents than previous coupled ocean–atmosphere models. In par- ticular, the partitioning of the deep-water formation between the Greenland and Labrador Seas is in good agreement with observations. Furthermore, unlike most previous climate models, this model does not use or require ‘flux adjustments’ at the air–sea interface, which involve adding a pre- scribed heat or freshwater flux to make up for a mismatch between ocean and atmosphere components of the climate model. Flux adjustments had to be used in the past to pre- vent the model climate from drifting slowly to unrealistic conditions, but they may dis- tort the stability of the ocean circulation 6 . The authors subject their model to two greenhouse-gas scenarios: an artificially rapid increase in atmospheric concentra- tions by 2% per year up to a quadrupling of CO 2 , and a more realistic ‘business as usual’ scenario starting in 1860 and extending to the year 2100. In both cases, the overall volume of water transported by the At- lantic conveyor belt (Atlantic overturning) decreases by around 25%, but it does not reach the point of collapse. This is consistent with most simulations by other groups (Fig. 2, overleaf). Previous studies indicate that the threshold for a complete conveyor-belt shutdown could be crossed in the twenty- second century if precipitation and meltwa- ter runoff into the North Atlantic are strong- ly enhanced 7 , diluting the surface waters to the point where the high-latitude sinking motion stops. This, however, is a point on which large uncertainty remains in climate models. What is new in Wood and colleagues’ simulation 1 is the shutdown in Labrador Sea convection and the associated collapse of the Labrador Current, which occurs between the years 2000 and 2030. If such a collapse did occur, it could have serious consequences for marine ecosystems, including seabird popu- lations in the region, as they depend not only on specific temperature conditions but also on nutrients supplied by oceanic mixing and currents. But how likely is this change in ocean cir- culation? From a single model simulation, even one carried out with an advanced cli- mate model, it is difficult to assess the range of uncertainty. Many regional aspects of the ocean circulation, including the energetic synoptic eddies, cannot at present be resolved by this (or any other) climate model. Synoptic eddies are the ocean’s equiv- alent of the high- and low-pressure systems that make up weather in the atmosphere, and they play an important role in mixing ocean waters and in transporting heat in some regions. High-resolution ocean models exist, but their severe computational cost means that they cannot yet be used in climate simulations. Owing to the lack of relevant controlled experiments comparing high- and low-resolution models, it is still an open scientific question to what extent the results of climate models would change if oceanic eddies were resolved explicitly. Another caveat is the uncertainty in regional precipitation changes due to global NATURE | VOL 399 | 10 JUNE 1999 | www.nature.com 523 news and views Shifting seas in the greenhouse? Stefan Rahmstorf Models of the Earth’s possible responses to global warming are continually being improved. The latest simulation of changes in deep flow in the Atlantic operates without several of the fudge factors previously required. Figure 1 Simplified sketch of currents in the North Atlantic, showing the two main convection sites in the Greenland and Labrador Seas. Warm surface currents are shown in red; cold, deep currents in blue. Red-and- blue circles, convection sites. North Atlantic deep water Gulf Str ea m North A tl a n ti c C u r r r e n t Labrador Sea C u rr e n t N o r w e g i a n