Ocean circulation switched on again abruptly at the end of each shutdown, of which there were at least seven in the last sixty thousand years. We know this because sea surface temperatures suddenly rose more than five degrees as water flowed in from warmer latitudes. This effect extended south at least to Bermuda, where cores through deep sea sediments show sudden rises in surface temperature, from about 60oF to 70oF (today they are about 72oF).23 Nobody is exactly sure what caused these currents to shift. There are no abrupt changes in any of the factors that might drive the shift, like wobbles in the tilt of the Earth’s axis (which change the heat balance of the planet) or fluctuations in atmospheric carbon dioxide that correlate with the timing of North Atlantic temperature flip-flops. However, scientists have come up with several good explanations.
One theory is that the global ocean conveyor current has three different stable states in the North Atlantic: on, off, and partly on, with deep water formation taking place farther south during times when ice sheets were more extensive.24 One of these states predominates most of the time, and the current varies over time as conditions fluctuate. A change between states occurs when a critical threshold has been exceeded, and the system flips into a different regime that itself might remain stable for hundreds or thousands of years before it flips into a different state again. According to this way of thinking a small change in some driver, like carbon dioxide or methane concentration, could cause a large change to the entire climate system because of its sensitivity to jumps between different states.
If this sounds confusing, a simple analogy might help. Imagine a drunkard walks home late at night along a riverside path. He lurches from side to side but remains on the path. Halfway home someone brushes past him from the opposite direction and shifts him slightly nearer the river. The bank now lies within the ambit of his more extreme lurches. A hundred yards along the path the drunkard falls into the river, and so passes from one stable state (on ground) to another (in the water).
Ice cores from Greenland, Antarctica, and glaciers the world over spell out the reality of abrupt climate change. These cores contain an extraordinary wealth of information on snowfall, atmospheric gas concentrations, and dust levels. The chemical isotope concentrations in ice tell us what the temperature was, while trapped dust layers reveal the extent of the deserts and the frequency and size of forest fires and volcanic eruptions. They allow us to stroll back through time, at first season by season, then year by year, and, deep down where the ice is compressed, decade by decade. How far back these records go depends on the depth of the core—from a few thousand years in some glaciers to over seven hundred thousand years in the deepest Antarctic cores. If there is one lesson here it is that the climate of the last few thousand years of human history has been remarkably stable when set against a long-term background of steep fluctuations and sharp flip-flops. Abrupt changes revealed by the ice record were not so sudden that you wouldn’t have had time to buy a polar jacket and upgrade your home boiler. But they were sharp enough that we would really struggle to adjust were one to happen now. If the mighty river that is the global ocean conveyor falters it will have profound impacts on life above and beneath the sea. Radical regional shifts in marine climate alone would precipitate the wholesale reorganization of ecosystems and human societies. There may be some consolation in the fact that the sage Intergovernmental Panel on Climate Change advises us that the North Atlantic arm of the global ocean conveyor is unlikely to fail in the next hundred years. But other shifts are already upon us, like the rapid warming of the Arctic Ocean.
Water flows from the surface to the deep ocean in polar seas at a rate equivalent to twenty thousand Niagara Falls. Since the oceans do not expand by the same volume, this flow must be matched by an equal and opposite upwelling toward the surface somewhere else. Part of that counterbalance is generated by gentle upward mixing in the tropics. In other places upwelling is assisted by wind-driven surface pull. These upwellings are concentrated on the eastern boundaries of the oceans, where winds blow parallel to the coast, like along the western United States.
This upwelling explains why the seas of northern California are at their most bracing in spring and summer, and why San Francisco then wears a foggy shroud: cool deep water upwells under the influence of winds from the north. In a physical quirk of life on a spinning sphere, the average direction of water movement is at a right angle to the direction of the wind.25 In the northern hemisphere the twist is to the right, while in the southern it is to the left. This effect is called the Coriolis force, after Gaspard-Gustave de Coriolis, a nineteenth-century French mathematician who worked out the interplay of forces acting on rotating objects like water wheels. As far as we know, Coriolis never thought to apply his ideas beyond the energy production of rotating machinery, but the Coriolis force has a profound influence on how water and air circulate in both oceans and atmosphere. Indeed, it applies to everything that moves across the surface of the Earth. If they did not correct for the Coriolis force, airline pilots would miss their destinations, and artillery gunners would shoot wide of their targets.
Winds that blow from the south along the coast of South America drive the world’s largest upwelling in the sea off Peru. Upwellings have enormous significance. They generate nearly half of the global fish catch, although they cover just 1 percent of the area of the oceans. Peruvian anchovy alone account for a tenth of world fish landings in good years. This sleek silver fish lives in schools of almost incomprehensible size and feeds mainly on phytoplankton, microscopic plants that live in the open sea. Upwelling fuels spectacular blooms of these phytoplankton (which, as I will come to later, are also caused by nutrient enrichment from human sources).
The ocean has two layers, surface water and the deep sea, over most of its surface area. Surface waters are warmed by sunshine. Since warm water has a lower density than cold water, this layer tends to float on top of the much cooler deep waters. There is little mixing across the boundary between the two, which is called the thermocline because of the abrupt temperature gradient. This creates a problem for marine life. Sufficient light for plant growth only reaches to depths of 30 feet to 300 feet, depending on the water’s clarity. Light will sometimes penetrate as far as 600 feet in the crystal-clear waters of the mid-Pacific, but even that is just a few percent of the average depth of the sea (12,070 feet). Plants grow in this sunlit zone by taking up dissolved nutrients and carbon dioxide. These plants are either eaten by animals or microbes, in which case the nutrients are recycled, or they sink. When they pass through the thermocline, which is typically around 100 feet to 300 feet deep, their nutrients are lost to the deep sea.26 The bright surface layer thus leaks nutrients to the deep, with the result that plant growth is usually limited by a lack of fertilizer. The deep, meanwhile, has plenty of nutrients but no light, so plants cannot grow. Animals living there exist on table scraps that sink down from the surface layer. Life is sparse. Where deep waters are pushed or pulled up to the surface, as they are off Peru, they flush nutrients into the sunlit surface layers to fuel explosive plant growth.
The world’s most intense upwellings occur where winds that blow parallel to the coast are reliable and strong, like West Africa, both north and south of the equator, and the Pacific coast of South America. These winds are a consequence of differential heating between land and sea. The land heats up more than the sea during spring and summer in subtropical latitudes and year round in the tropics.27 Hot air rises above the land, carrying water vapor with it. It cools as it ascends, and the water condenses into those spectacular thunderstorms you can sometimes watch boil upward while sipping piña coladas on a tropical beach. Condensed water falls in showers so intense you feel as if you were underwater.
This condensation of water leaves the air at cloud level warmer and lighter, so it rises even higher, eventually stopping when it is thoroughly chilled. That high-altitude cold air is now pushed sideways by more air welling up from below. North of the equator it blows north, while sou
th of the equator it heads south, carrying heat from the tropics to higher latitudes, where it cools and sinks back down around thirty degrees north or south. If you glance at an atlas you will see that many of the world’s great deserts are concentrated around these latitudes, because the downdrafts are very dry, having had all the moisture squeezed out as the air rose: Mojave, Sonora, and Chihuahua in North America; Sahara and Arabian deserts in Africa; Takla Makan in China; Atacama in Chile; and so on. Since air cannot rise at the equator without being replaced by other air, surface winds blow air back from these drier latitudes to the tropics to complete the loop and drive upwellings as they do. Surface winds blow poleward at temperate latitudes, where tropical circulation loops intersect like cogs with other winds, that then carry heat poleward and draw cool air back to warmer latitudes.
Global air circulation: Simplified view of how air masses circulate on Earth, showing the direction and names of the surface winds prevailing at different latitudes. The loops represent vertical air circulation and show the latitudes at which warm, moist air rises in the atmosphere and cold, dry air sinks.
The tropics will trap more heat as greenhouse gas concentrations rise, which will drive a faster redistribution of warmth and strengthen the winds that power upwelling.28 The productivity boost this will give the oceans could benefit fisheries and help the sea absorb more carbon dioxide (although this is a double-edged sword, as I will come to later). Thumbing his nose at Aristotle, Oscar Wilde once quipped, “Moderation is a fatal thing. Nothing succeeds like excess.” He found out in life, to his dismay, that it was actually the other way round, and so it is with upwelling. Excessive upwelling can make unpleasant things happen in the sea. In southwest Africa we may have a taste of the future of upwelling in a greenhouse world.
The coast of Benguela, offshore from the vast red dunes of the Namib Desert (as scorched and thirsty a place as you are likely to find on Earth), sustains one of the planet’s most intense upwellings. The lack of plant life on shore contrasts vividly with its abundance in the sea. Copiously supplied with nutrients, phytoplankton flourish, turning the sea green. Normally those plant cells would be consumed by zooplankton, which in turn would be eaten by fish and other predators. The trouble is, the wind is so strong that it pushes water offshore before zooplankton, which take longer to develop, have time to complete their life cycles. While phytoplankton turn over at high speeds and continue to bloom right in the midst of upwelling, zooplankton are swept offshore, so they cannot curtail the explosive growth of phytoplankton. The result is devastating, as decaying phytoplankton sink and rob the sea deep down of its oxygen.
Beneath Benguela’s upwelling plumes, the seabed is thick with swirling drifts of dead and decomposing plankton. It can sustain only the ancient microbes that first evolved in the similar toxic broth of the early oceans. This meters-thick sludge is anoxic, or nearly so, and gurgles forth methane and hydrogen sulfide, the waste products of microbial metabolism. Hydrogen sulfide compounds the problem of anoxia, because it reacts with any oxygen present, and strips it from the water as it rises toward the surface. From time to time the sludge effervesces so fast that the rising bubbles rush upward in an eruption of sulfide and methane, turning the sea bright turquoise.29 The color comes from sulfur particles in suspension that are produced when hydrogen sulfide is oxidized.
Local people on the coast have become inured to the stench of rotten eggs that wafts from the sea. The corrosive pall has been a fact of life for as long as people in the Namibian port of Lüderitz can remember. Sulfur eruptions have been reported here since the nineteenth century. They poison the sea, cause mass death of fish, and trigger “walkouts” of lobsters onto beaches as they try to flee one form of death, only to find another. These walkouts provide a short-lived bonanza for locals, who pile them into every container they can find. Similar walkouts in Mobile Bay, Alabama, when all kinds of creatures are forced to the shoreline by plummeting oxygen, are known as jubilees. But they decimate populations, leaving few animals behind to sustain catches later on. In the early 1990s, one particularly energetic eruption off Namibia killed 80 percent of the population of Cape hake, a bottom-dwelling fish, and led to a collapse of catches.
In recent years sulfide eruptions have increased in frequency and intensity, a phenomenon that is most likely linked to overfishing. A few decades ago sardines (also known as pilchards) were to Namibia what anchovies are to Peru. These tiny fish are filter feeders, and they sweep phytoplankton and some zooplankton from the water as they dart around, mouths agape. In good years, sardines shoaled by the billions. Unlike zooplankton, which are poor swimmers and easily swept offshore and away from the heart of phytoplankton blooms, sardines are powerful swimmers and remained in the thick of it. Voracious sardines ate the phytoplankton as fast as it was produced, so dead plankton did not usually accumulate on the seabed and sulfide eruptions were rare. Sardines in turn were eaten by other predators, like tuna, swordfish, and seabirds, and the Benguela upwelling sustained one of the oceans’ most spectacular concentrations of wildlife.
Such productive seas soon drew the attention of the fishing industry, and sardines were targeted heavily beginning in the 1960s. The fishery for sardines collapsed in the 1970s, and it has never recovered. Coastal residents can now blame overfishing for the corrosive sulfide fogs that overwhelm their towns and irritate their eyes and throats. Namibian seas reveal what can happen when we knock holes in the fabric of an ecosystem, whether through fishing or by other means.
Andrew Bakun, from the University of Miami, is the scientist who first made the connection between upwelling intensity and global warming.30 He predicts that warming will intensify upwelling in other regions of the world that will match or exceed the rates seen in Benguela today. If there are insufficient fish to control phytoplankton blooms, they too could experience the poisonous combination of anoxia and sulfide eruptions. One such place is northern California. There is a real possibility that fifty years from now the homely charm of Mendocino will be tarnished by the stench of rotten eggs and its beaches heaped with putrid fish.
There is a further marine twist to the climate change story. The ocean has two layers, as I mentioned: a warm surface layer floats on top of cooler, denser water below. The additional heat absorbed by the surface layer as the world warms increases the temperature contrast, and hence the density difference between these layers. The steeper the density gradient at the thermocline, the band separating the surface water from the deeper sea, the less water mixes across it. The result is that there are fewer nutrients transferred from deep sea to shallow water, and less oxygen mixed from shallow waters to deep, as warm water holds less oxygen than cool water. As the oceans heat up they will thus release some of their oxygen into the atmosphere.
Dissolved oxygen is plentiful at the surface but drops sharply below the thermocline, and it sags further as you descend, eventually rising again toward the bottom. This pattern has a simple explanation. The surface layer is well ventilated and the deep sea gets its oxygen from the water that sinks down at the poles. In between there are regions where the life-giving gas is scarce.
Low oxygen zones form in places where the water column is strongly separated into layers by density differences (caused by differences in temperature and salinity). Dead plankton and animals sink from the surface—marine snow, as oceanographers like to call it—and are then eaten or broken down by microbes, which use up oxygen. Most of the oxygen in the middle layers of the sea has been there since the water was last in contact with air at the surface (in some cases hundreds to a thousand years or more). The longer ago that was, the more time there has been for animals and microbes to use up its oxygen. There is no sunlight in these inky depths, so no oxygen can be produced to replace what is used up. Deep waters in the Pacific and Indian oceans are “older” than those of the Atlantic, meaning that they have been submerged for longer, and so have less oxygen. They also have more carbon dioxide, which has been breathed out by the animals and micro
bes that live there.
The eastern Pacific and Northern Indian oceans have enormous slabs of water in which oxygen concentrations are less than 30 percent of those at surface levels. Fully a quarter of the Pacific is affected by low oxygen at some depth or another.31 These oxygen-starved zones impose physiological stresses that few organisms can tolerate for long. The warm ventilated surface layer is thin in the eastern Pacific, and the low oxygen zone lies just 650 feet to 2,000 feet below the surface. Fast-swimming tunas are confined to well-aerated surface waters, which is why they are easy to catch there. The upper region of low oxygen water is home to many planktonic animals, which migrate toward the surface to feed under cover of darkness, and then retreat by day to a place where few predators can follow. Sharks and tunas can penetrate low oxygen layers for only minutes or hours before they must return to the surface to catch their breath and warm up.
Every animal is different, of course, and some are better able to tolerate low oxygen than others. But to put the conditions into a human context, to enter water with less than 30 percent oxygen saturation would be like a climber venturing into the death zone at the extremes of altitude. At heights of greater than 26,200 feet, oxygen levels are a bit over a third of those at sea level—too low to sustain human life. Without bottled oxygen, we can only make brief sorties into this zone, because we use oxygen faster than our bodies can take it in.
The Ocean of Life Page 8