TO NORTHERN Europeans shivering in the grip of an unusually icy winter, global warming might suddenly seem an attractive proposition. How pleasant to bask in a balmy Mediterranean climate without ever leaving home.
Dream on. Evidence now emerging reveals a risk that global warming could plunge most of Europe into a big chill lasting hundreds of years, bringing with it effects that could be felt right around the world.
What Europeans tend to forget is that the Atlantic Ocean keeps them relatively warm. By rights, northern Europe should experience the same chilly climate as the northern US and Canada, since they are all at more or less the same latitude. But warm surface waters originating in the tropics are drawn northwards by gigantic undersea “pumps”, which pull the balmy water towards the European continent. It is this vital pumping process that could be under threat from global warming.
Advertisement
So how does Europe’s cosy heating system work? The warm surface waters arrive through the Gulf Stream, on the western side of a huge eddy known to oceanographers as a subtropical gyre. Similar gyres are found in all oceans and are driven by the winds and the Earth’s rotation. Normally, this gyre and the Gulf Stream would have little effect on the climate of Europe. But the North Atlantic is home to two mammoth oceanic pumps-one east of Greenland and one in the southern Labrador Sea-which exert an extra tug on the warmed surface water. Just as bathwater is sucked down into the plughole, the pumps pull Gulf Stream water from the surface down into the deep ocean, dragging it far enough north to heat Europe (see Diagram).
The Gulf Stream, and the North Atlantic pumps form part of the global ocean circulation system that has been dubbed the “conveyer belt” by oceanographers. Warm surface waters are drawn north throughout the Atlantic at a flow rate more than a hundred times that of the Amazon River. They then sink to the deeps of the Greenland and Labrador Seas, and return to the Southern Ocean at 2 to 3 kilometres below the surface as the so-called North Atlantic Deep Water. The waters release heat into the cold northern atmosphere at a rate of a trillion kilowatts (1015 W), an amount equivalent to a hundred times the world’s energy consumption. This energy warms the air over Europe by about 5 °C-a free heating service that has operated reliably over the past 10 000 years or so.
But there is increasing evidence of abrupt and dramatic changes in Europe’s climate throughout the last ice age. Even the warm period that preceded the ice age may have had major climatic ups and downs. Average temperatures seem to have swung by 5 °C or more within a few years, leading to icy spells that lasted for centuries. Climatologists have come to view the past 10 000 years following the end of the last ice age as an exception in climate history, and some are saying that human interference with the climate system might trigger a new period of instability.
Much is already known about the basic workings of the conveyor belt. Its flow is driven by differences in water density at different points in the Atlantic. If the North Atlantic Deep Water is to push its way southwards out of the Atlantic basin it needs to be denser than water in the south, near South Africa, where it joins the Antarctic Circumpolar Current, which circles the planet from west to east. Some of this deep water then rises back to the surface near Antarctica, and some travels into the other ocean basins at depth, before finally reaching the North Pacific after a thousand years.
The path by which this water then returns to the Atlantic is still hotly debated. There are two possible routes: a westward “warm water route” passing between the islands of Indonesia and around South Africa, and an eastward “cold water route” around the southern tip of South America via Drake Passage (see Diagram).
The density of the water in the North Atlantic is determined by its salinity and its temperature, so these factors also determine the action of the conveyor belt. When the warm surface water comes into contact with the cold sub-Arctic air, it cools. This increases its density and encourages it to sink. On the other hand, the northward-flowing surface waters are diluted by freshwater coming from rain, rivers and melting snow. This makes the water less dense, and hence more buoyant. Left to itself, the freshwater would win the contest. But the conveyor belt brings a perpetual flow of new salty surface water from regions to the south, which keeps the seawater dense enough to sink. In subtropical regions, the oceans characteristically have more evaporation than freshwater input, so they tend to be more salty.
Detective work
This self-maintaining positive feedback system has at least one glaring hitch: if something interrupts it, and the conveyor belt grinds to a halt, it remains shut down (see “Over the edge”). This effect was seen in one of the first climate modelling experiments to include both ocean and atmosphere. In the late 1980s, Suki Manabe and Ron Stouffer of the Geophysical Fluid Dynamics Laboratory at Princeton in New Jersey found that their climate model had two very different, more or less stable states. One, like our present climate system, had a system of currents resembling the conveyor belt in the Atlantic and a comfortable European climate. In the other, however, the conveyor belt had shut down, and temperatures in northwestern Europe were up to 10 °C colder than today. The existence of these two states has since been confirmed by many experiments using different models.
But other questions remained. Could the climate switch between these different states? Had it done so in the past? And what might trigger such a switch? Luckily, the Earth itself contains several archives of past climatic conditions which, with a bit of detective work, yield many clues. Among the most useful of these are the snow layers that have piled up on the Greenland Ice Sheet and the layers of sediment that have accumulated at the bottom of the Atlantic.
These records show that rapid and severe climatic jumps occurred every thousand years or so during the last ice age, in sharp contrast to the stable conditions of the past 10 000 years. The last of these jumps is the Younger Dryas event which took place as the Earth emerged from the last ice age. Gradual climatic warming was causing the huge continental ice sheets to melt and disintegrate, but then, within a decade, ice-age conditions suddenly returned.
In 1989, a modelling experiment by Ernst Maier-Reimer and Uwe Mikolajewicz at the Max Planck Institute in Hamburg uncovered a neat explanation for the Younger Dryas event. It showed that a massive inflow of meltwater from the Laurentide Ice Sheet could have led to a sudden collapse of the Atlantic conveyor belt, throwing the Atlantic region back into the freezer.
Flickering switch
Now, researchers are asking whether today’s global warming, the result of accumulated carbon dioxide and other greenhouse gases in the atmosphere, could have the same effect as the period of natural warming at the end of the ice age. Global warming is, for instance, expected to warm the surface water in the northern high latitudes. It should also increase the amount of rain and snowfall over the ocean, and speed up the melting of high-latitude ice, which would make the water fresher. Both the warming and the freshening would make the surface water less dense, which could put brakes on the pumping mechanism.
In 1993, Manabe and Stouffer studied the effects of CO2 concentrations on global climate in a model that coupled the ocean, the atmosphere and sea ice. As the atmospheric CO2 concentration slowly increased to four times its preindustrial level, the ocean’s deep circulation came to a complete standstill. However, this change required fairly drastic amounts of CO2 to be released into the atmosphere-the Intergovernmental Panel on Climate Change does not expect such levels to be reached before 2100. Also, the deep circulation in the model ground to a halt slowly-over centuries-unlike the abrupt climate shifts shown by the Greenland ice core record.
Despite these caveats, there is mounting evidence, both from the past climate record and from more recent ocean modelling, that the climate system could be more vulnerable than Manabe and Stouffer’s findings imply. Over the past few years, as researchers have drilled and analysed more ocean sediment cores, the picture has been getting more complex. The new evidence shows that during some cold spells, the conveyor belt may not have switched off but simply shifted south.
Three years ago, marine geologist Michel Sarnthein from the University of Kiel in Germany, with colleagues from France and the Netherlands, published reconstructions of deep water flow in the Atlantic at different times in the past, based on a large number of ocean sediment cores. They found three circulation modes. The first was a warm conveyor belt mode that has operated over the past 10 000 years or so. The second mode was a “glacial” conveyor belt, which was shallower and did not extend north into the Greenland Sea, but ended somewhere south of Iceland. Finally, they found periods where the conveyor belt was very weak because large amounts of meltwater had entered the Atlantic, capping off oceanic convection with a surface “lens” of freshwater.
At that time, I was a researcher at the Institute of Marine Sciences at the University of Kiel running a series of modelling experiments investigating how the conveyor belt would respond if a lot of freshwater suddenly flowed into the Atlantic, and how this would affect surface temperatures. Surprisingly, as well as the then familiar climatic modes of operation with the conveyor belt switched either “on” or “off”, we too found a third possibility of a cold conveyor belt extending not nearly as far north as at present. Although this conveyor belt was almost as vigorous, it hardly warmed the northern Atlantic region, as its waters sank and returned south before releasing much heat to the atmosphere. So a shift in the ocean currents could have thrown the region into a cold spell without the complete collapse of the conveyor belt. Also in 1994, experiments by Andrew Weaver and Tertia Hughes from the University of Victoria in Canada showed that with increased precipitation in the Atlantic the conveyor belt can start to “flicker” between its different modes, leading to strong climatic oscillations over Europe.
The possibility that an influx of freshwater into the Atlantic could have such an effect is worrying. Although the kind of sudden climatic swing shown in these models works through positive feedback, it is primarily associated not with salt transport in the conveyor belt, but with the downward mixing of water in the two pumps. If something-perhaps the effect of global warming-interrupted the downward mixing, or convection, process at one of these sites, the incoming freshwater would start to accumulate at the surface. This would make the surface waters more and more buoyant and it would become harder and harder to restart the pump. The models suggest that in this way the pumping at one of the sites could shut down and the conveyor belt could then change its route within a few years. This breakdown could be triggered by a relatively small change in the amount of freshwater entering the ocean, because the two pump sites are very localised, each being just a few hundred kilometres across. Not surprisingly, these convective pumps have been dubbed the Achilles heel of the conveyer belt.
But we do not know whether this will really happen. Existing models are simply incapable of quantifying how much warming is needed to switch off convection at one of the pump sites. Though some of the fastest computers in the world are used for these simulations, lack of computer power is still forcing models to use a very coarse mesh in their calculations. This means that they cannot represent the kind of regional details that could turn out to be crucial.
The models are also unrealistic in that they overstate the role of salt and heat diffusion. Reducing the amount of diffusion seems to make the ocean currents less stable, according to comparisons that Stouffer and I have made over the past six months. What’s more, most models still work with ad hoc fixes, known as flux adjustments, at the interface between ocean and atmosphere. Without these fixes, the simulated climate and ocean circulation drifts to a less realistic state.
So we can take no comfort from the current global warming scenarios, which tend to show a smooth gradual warming over the next century. We simply don’t know why our present climate is much more stable than the climate of the past, and whether this stability will continue in the face of global warming. Though the models suggest the effect of global warming might be less drastic and rapid than past changes, it could simply be that present models don’t yet capture the physics of abrupt climate change.
Researchers are attacking these gaps in our knowledge on three fronts. First, they are looking to see if past swings in ocean circulation took place only during the last ice age, or whether the ocean was also unstable in the Eemian Interglacial Stage, a warm interlude between 113 000 and 125 000 years ago. If the first of these is true, we could be safe. It implies that swings in the conveyor belt arose only because of meltwater or iceberg flotillas, from the warming of the large amounts of land ice that had formed in the preceding glacial period.
On the other hand, climatic swings in the Eemian suggest that large amounts of land ice are not necessary to cause the changes. That could presage a rough ride for us in the future, when global warming has set in. Two recent Greenland ice cores, the European GRIP core taken in 1992 and the American GISP2 core taken in 1993, provided conflicting views of the Eemian climate. The former showed a record of frequent fluctuation but was probably disturbed by motions of the ice, while the latter revealed a period of stability. Drilling has just started on a new ice core, known as North GRIP, which should help to settle the issue (This Week, 6 July 1996, p 7).
Research is also opening up on the oceanographic front with expeditions by European, American and Canadian teams starting this winter. The oceanographers will be studying the convection processes and their link to climatic conditions in the northern Atlantic. These measurements will in turn provide important data needed to validate and improve model simulations-the third line of attack. More measurements will also help to decide whether convection in the Greenland Sea has already weakened since the early 1980s due to global warming, as some oceanographers have suggested (see This Week, 19 March 1994, p 4), or if wether is a natural climatic fluctuation.
So long as the jury stays out on just how vulnerable the conveyor belt is, there is still a very real possibility that we will unwittingly disrupt it and trigger a calamitous cooling throughout Europe. The consequences for ecosystems, agriculture and society could be severe. And it’s not just a European problem. The effects of past cooling episodes, such as the Younger Dryas, have been seen in the climate record from the US, Chile and even New Zealand. Geochemist Wally Broecker of Columbia University in New York has a blunt way of putting it: “We are playing Russian roulette with climate and no one knows what lies in the chamber of the gun.”
* * *
Over the edge
You might expect that as carbon dioxide levels in the atmosphere gradually rise, the climate will gradually warm. However, the global climate system is very complicated, and does not necessarily respond in such a simple way. Climate may resist change for some time, and then switch very suddenly.
An example is the “conveyor belt”, the global system of ocean currents and pumps. Its strange, nonlinear behaviour was first described by the American oceanographer Henry Stommel in 1961. Stommel’s conceptual model shows how the conveyor belt’s flow depends on the net addition of freshwater (from rain, snow and rivers minus that lost by evaporation) into the North Atlantic (see Figure).
The present climate state is marked by the X on the upper branch of the graph. If the amount of freshwater flowing into the ocean increases-as is expected with global warming-the X shifts towards the right along that branch. Initially this reduces the conveyor belt flow only slightly, but if the freshwater is increased too much, the bifurcation point S is passed, and the conveyor belt circulation collapses and reverses. To turn the conveyor belt back on, you would have to reduce the amount of freshwater coming into the ocean to much lower than present levels, past the second bifurcation point at the origin of the plot.
This behaviour has recently been confirmed in simulations with state-of-the-art ocean circulation models, which also show that the present climate is surprisingly close to Stommel’s bifurcation point. Other bifurcation points have shown up, not captured in Stommel’s simple model. These thresholds determine whether convection continues in certain crucial areas such as the Greenland Sea. Unfortunately, there is as yet no way to be sure how close we are to these critical thresholds, beyond which the ocean circulation, and much of the world’s climate, would change dramatically.