The Ocean of Life

by Callum Roberts

  Hunting and fishing are the oldest of human influences on the sea. Arguably, fishing also remains the most serious. But climate change from greenhouse gas emissions has been building in the background during the last century. It has burst to prominence in the last twenty years as it has begun to affect our daily lives. In the next four chapters I explore the most important of the many ways in which greenhouse gases are changing the oceans.

  CHAPTER 4

  Winds and Currents

  Benjamin Franklin is celebrated as a Founding Father of the United States, an astute diplomat, the cofounder of the first public lending library, and a prolific inventor whose contraptions ranged from the lightning rod to bifocal spectacles. He is known to have had an exceptional range of interests and abilities, but his pioneering contributions to oceanography are not widely recognized outside scientific circles.

  Before the revolution, Franklin was postmaster general for the British colonial mail, and as such he applied his formidable intelligence to the problem of how to speed up deliveries between the Old and New Worlds. He wondered, more specifically, why it took ships two weeks longer to sail from London to New York than it took them to travel in the opposite direction. His cousin Timothy Folger, a whaling captain, told him that British ships battled a three-knot current while those coming from America sailed with it. Together the pair tapped the knowledge of experienced American whalers to produce, in 1770, the first map of what is now known as the Gulf Stream.

  The Gulf Stream is a fast-flowing surface current that jets into the Atlantic through the Florida Straits and then runs north along the east coast of America. It turns out to sea just south of Cape Hatteras, at which point it crosses the Atlantic to Europe, where it dissipates into a general northerly flow called the North Atlantic Drift. It is part of the “global ocean conveyor,” a system of currents that circles the planet and loops from the surface to the deep sea.

  The idea that the seas might overturn, with water pouring from the surface into the deep and then back again, was first proposed by Benjamin Thompson, a contemporary who, unlike his more famous counterpart, was a loyalist and fled revolutionary Massachusetts for London. In Europe he made a name for himself as a scientist and statesman. He is best remembered today for his discovery of how to increase the draw of fireplace chimneys, helping cure the centuries-old curse of smoke-filled rooms. Thompson, who in 1791 became Count Rumford (after the New Hampshire town where he grew up, today’s Concord), based his proposal that the shallow waters of the oceans overturned and mixed with the deep on his knowledge of the properties of heat. A single measurement of the deep ocean’s temperature from 1751, made thirty-six hundred feet below sea level by a British slave ship in the tropical Atlantic, had set him thinking.1 It was less than fifty-four degrees Fahrenheit, a big contrast to the eighty-four degrees Fahrenheit of the surface.2 This measurement led him to suppose that, in polar latitudes, water

  deprived of a great part of its heat by cold winds, descends to the bottom of the sea, [where it] cannot be warmed where it descends, [and] as its specific gravity [i.e., density] is greater than that of water at the same depth in warmer latitudes, it will immediately begin to spread on the bottom of the sea, and to flow toward the equator; and this must necessarily produce a current at the surface in an opposite direction. There are most indubitable proofs of the existence of both these currents.3

  This down-up current is sometimes called the “thermohaline circulation,” because it is driven by differences between water masses in temperature and salt content. The reasons for downwelling differ between Arctic and Antarctic. Frigid conditions in the Antarctic cause sea ice to form. Freezing separates freshwater from salt and leaves behind a more briny sea. This bitterly cold and saltier water is denser than normal seawater and therefore sinks, in a process known, rather unimaginatively, as “deep bottom water formation.”

  The global ocean conveyor current: The global ocean conveyor current loops around the planet and circulates water between the shallow surface layers and the deep sea. Gray circles show places where water becomes colder and saltier, and therefore denser, and so sinks to create deep bottom water. Water sinking near the poles is counteracted by upwelling from deep to shallow waters at lower latitudes. These upwellings are rich in fish as their nutrients fuel plankton blooms

  In the far north of the Atlantic the Gulf Stream supplies water to polar seas that is more salty than local waters, and it is made even saltier by evaporation from intensely cold, dry winds blowing off Greenland and Europe. At both poles, as cold, salty, and dense water sinks, it pulls in surface water to replace it. This pull is one of the energy sources for the Gulf Stream and North Atlantic Drift (the other being wind). When this water sinks it begins a deep-sea journey that may last fifteen hundred years before it upwells again into the sunlight far away.

  You can create a loop current a little like the global ocean conveyor by blowing on a cup of tea to cool it. Your breath pushes the tea across the cup until it hits the edge, at which point it flows down the inside of the cup, crosses the bottom, and upwells again at the side you are blowing from. (If you are ever short of a teaspoon, this is a handy way to mix milk into your tea.)

  Like the ideas of many gifted thinkers, Rumford’s theory of ocean circulation was not fully accepted until long after he had died. He never returned to America, although his British sympathies were later forgiven and he endowed a professorship at Harvard University. Oddly enough, despite the fact that they were born just twelve miles apart and had such similar interests—Franklin also invented a stove that Thompson later improved—there is no evidence that they ever met or corresponded. Theodore Roosevelt would later declare that Franklin, Rumford, and Thomas Jefferson were the three greatest minds America had ever produced.

  In the 1960s, atmospheric nuclear tests produced radioactive materials that were washed into the Arctic and gave us a way to trace the speed of water flow to the deep sea. Scientists discovered that the global ocean conveyor moved three to four feet per hour.4 This sinking rate means that 550 million cubic feet of water is displaced per second, which is equivalent to the flow of eighty Amazon rivers, or twelve times the flow of all the world’s rivers combined.5 As powerful as this river in the sea is, it would take nearly twenty-eight hundred years to circulate all of the oceans’ waters through what oceanographers call the “North Atlantic pump.”

  As I intimated, there are two other key areas of deep bottom water formation, in the Ross and Weddell seas of Antarctica. They transfer another 740 million cubic feet of water per second from the surface to the deep sea, reducing turnover time for water in the deep oceans to under twelve hundred years. These pumps are critical to the vertical mixing of water in the world oceans. They carry freshly oxygenated water into the deep sea, helping sustain life there. They also transfer carbon dioxide from the atmosphere to the deep ocean, a point I will return to later on.

  One thing that fascinates me is that water bodies in the sea, especially the deep, tend to retain characteristic signatures of temperature and salt content for very long periods. Oceanographers can build up a picture of the three-dimensional structure of the sea from simple vertical profiles of salinity and temperature measured using instruments dangled from boats. It turns out that the oceans are made up of many parcels of water in constant motion, driven by wind and density differences, and their origins and movement around ocean basins can be tracked. At the mouth of the Mediterranean Sea, for instance, water flows in through the Strait of Gibraltar at the surface. Beneath there is a deeper flow back into the Atlantic of dense, salty water concentrated by evaporation as it traveled around the Mediterranean Basin. This pours out across the shallow Gibraltar sill and forms giant turbulent loops that travel west to the Azores and north to Ireland. Similar measurements reveal that the southward flow of deep water from the North Atlantic is not compact, like the surface Gulf Stream, but broad, and it creeps sluggishly along the eastern seaboard of North America.

  The North A
tlantic and Southern Ocean engines of the global ocean conveyor have recently been identified as possible climate “tipping points” by a multinational team of climatologists,6 meaning that a certain threshold of water temperature or density must not be passed if the currents are to remain in a stable state. If that critical point is passed we could see a radical and rapid shift to a different state. Since major ocean currents move at the behest of wind and water density gradients, there is a good chance they would shift when the world’s climates change. The possibility that this might happen is now a cause of great concern to many scientists.

  Unlike past changes, whose causes were geologic or celestial in nature, today’s are down to us. People have multiplied and spread to every corner of the globe. We have shifted from being a species governed by nature to one that can harness nature to its own ends. Unwittingly, our ingenuity has unleashed forces over which we have little control and that threaten the way we live.

  The concept that some form of global warming might be triggered by the burning of fossil fuels was first proposed in 1896 by a Swedish chemist, Svante Arrhenius.7 People at that time were fascinated by the ice ages, whose world-shaping influence had only recently become understood. Arrhenius argued that variation in the amount of atmospheric carbon dioxide could have played a key role in glaciations.8 But he took the idea further. Perhaps his thoughts coalesced while tramping the icy streets of Uppsala one evening as the coal smoke from thousands of hearths wrapped around him. He made the connection that the enormous quantities of fossil fuel being burned could eventually lead to planetary warming. From his rather chilly perspective, he thought warming would be a good thing:

  We often hear lamentations that the coal stored up in the earth is wasted by the present generation without any thought for the future.… We may find a consolation in the consideration that here, as in every other case, there is good mixed with the evil. By the influence of the increasing percentage of carbonic acid [carbon dioxide] in the atmosphere, we may hope to enjoy ages with more equable and better climates, especially as regards the colder regions of the earth, ages when the earth will bring forth much more abundant crops than at present, for the benefit of rapidly propagating mankind.9

  By Arrhenius’s calculation, doubling atmospheric carbon dioxide would increase average temperature by 7oF, a figure since revised by the Intergovernmental Panel on Climate Change to the range of 3.6°F to 8.1oF. Not bad for a nineteenth-century chemist!

  Today we don’t share Arrhenius’s optimism about the benign influence of global warming. A little warming will not simply help shrug off the winter cold or lengthen growing seasons. Now we understand that warming alters patterns of wind, cloud cover, and rainfall, changing conditions in ways that are hard to predict and would be difficult and expensive for us to adapt to. Thousands of studies attest to the reality of global warming, now usually called “climate change” because of the far-reaching influences warming has on climate. A record-breaking snowfall or unexpectedly fierce tornado or monsoon flood is as much a product of global warming as scorching summers and prolonged droughts. Measurements from mountains and lakes, glaciers and ice sheets, deserts and rain forests, coasts and oceans all confirm that temperatures are on the rise.

  As I explained in the opening chapter, Earth’s atmosphere is like a blanket that traps warmth from the sun but also shields us from the ferocity of the sun’s heat and harmful ultraviolet rays. If Earth didn’t have an atmosphere, temperatures would be like those on the Moon, where there are wild swings between extremes.10 In sunshine the Moon’s surface can top 212oF, while on the dark side it can plunge close to -238oF. (The moon averages -9oF, compared to Earth’s clement average of 60oF.) The warmth of the atmospheric blanket depends on the content of its heat-trapping gases. Collectively these greenhouse gases slow the radiation of heat from Earth back into space. The most important are water vapor, carbon dioxide, methane, and ozone, but there are several others in the mix.

  Carbon dioxide concentration in the atmosphere has risen by 38 percent since preindustrial times (before 1750), from 280 parts per million (ppm) to 388 ppm.11 The concentration of methane over the same period has gone up by 150 percent, from 700 parts per billion (ppb) to 1745 ppb. Although there is much less methane than carbon dioxide, it is a far more potent greenhouse gas. You will recall from my description of the early Earth in Chapter 1 that each molecule of methane has twenty-five times the warming potential of a molecule of carbon dioxide.12 The main source of carbon dioxide is the burning of fossil fuel, while methane comes from livestock (cows and other ruminants fart endlessly), landfills, and rice paddies. It is also a by-product of warming, as trapped methane is released by melting tundra near the poles.13 Just as Arrhenius predicted, temperatures have risen as we have burned more fossil fuels. Averaged across the globe, they have increased by 1.3oF since preindustrial times. What is alarming is that the rate of rise has now reached 0.4oF per decade. We are on a slippery slope.

  So far, much of the heat trapped by greenhouse gases has been taken up by the sea. If it hadn’t we would all be sweltering by now. The oceans have sucked heat from the atmosphere because the heat capacity of water is several thousand times greater than air.14 It is for this reason also that temperature increases in the oceans have been less than on land. Averaged from the surface to seabed, temperatures have risen just one fourteenth of one degree Fahrenheit since 1955.15 It doesn’t sound like much, but most of the warming has been near the surface, where average temperatures have increased by 1.1oF in the last century.16 This heating has not been even: Some places have warmed quickly, others are little changed. The tropics have warmed least, while the temperatures at the poles have shot up, which is why polar bears and penguins are icons of the changing climate.

  On the Antarctic Peninsula, where air temperatures have warmed by 11oF in the last fifty years,17 Adélie penguins that once nested on frozen ground now huddle in pathetic groups, ankle-deep in mud. The chicks’ downy feathers are well adapted to snow, but they lose their insulation in sleet and drizzle. Soaked through and frigid, the chicks die. While it seems obvious that they should move to colder rookeries, it is hard for them to abandon traditional sites used for generation after generation. As a result, Adélie penguins on the peninsula have declined by 90 percent in the last thirty years (those farther south are still doing fine). At the other end of the world, polar bears and seals depend on sea ice, the bears to hunt and the seals to breed. Polar bears are frequently seen these days swimming in open water up to sixty miles from the nearest coast or ice. Many now drown far out at sea as exhaustion overtakes them when ice cannot be found.18

  A four-degree rise in global average temperature is expected to produce a seven- to eleven-degree warming at the poles. Summer sea ice cover has declined fast in recent years. In the summer of 2000 Jim McCarthy, a Harvard oceanographer who was then a leading member of the Intergovernmental Panel on Climate Change, was left gasping in amazement when the icebreaker he was on found open water at the North Pole. Summer sea ice has shrunk farther since then, and the region is expected to be regularly ice-free by the summer of 2030.19

  Polar oceans are highly affected by climate change. Less sea ice formation means the surface water at the poles is less salty, and warmer water is also less dense than cold. (The word “warm” is relative—even in a greenhouse world you couldn’t enjoy a swim at these latitudes!) Together these changes will slow the rate of sinking and the replenishment of deep bottom water. There’s another factor related to climate change that’s contributing to slowing the engine of the global ocean conveyor further: More rain and the thawing of frozen tundra have swelled rivers that pour into the Arctic Ocean. This freshwater reduces surface-water density, and so also inhibits deep bottom water formation.

  Ever since Benjamin Franklin’s time, people have generally understood that the Gulf Stream carries the heat of the Caribbean far into northern latitudes. It is held to be responsible for mild, wet winters in Britain and France and for th
e warmth of the seas from Long Island to Boston. Without the Gulf Stream, people have come to believe, a deep freeze will descend on London and New York.20

  The central premise of The Day After Tomorrow, a blockbuster thriller released in 2004, was that the Gulf Stream might suddenly shudder to a halt. In the movie, eastern North America and Europe were plunged into ice age conditions by the sudden failure of the Gulf Stream. While the idea may seem far-fetched, and the near instant freeze in the film was definitely science fiction, there is legitimate cause for concern. Oceanographers have already seen deep currents slow in the North Atlantic. Between 1957 and 2004 there was a 30 percent reduction in deep water flow (although this does not yet seem to have slowed the shallow Gulf Stream and the more diffuse North Atlantic Drift). It is too early to link the slowing conclusively to climate change.

  Over 120,000 years of climate records held in annual layers of snow cored from within the Greenland ice sheet suggest that this part of the global ocean conveyor has stopped many times in the past. It seems that a breakdown can occur rapidly, in as little as a few decades. The trigger for these stoppages in the past was a sudden drop in the salinity of Arctic seas.21 The period covered by the Greenland ice cores lies within the last glaciation, from 110,000 years to 10,000 years ago, when much of North America and Europe were ice-bound. The ice sheets grew in thickness as snowfall accumulated over thousands of years. Eventually it grew top-heavy and became unstable. Ice surged through the Hudson Strait into the North Atlantic, where it melted, freshened the sea, and switched off deep bottom water formation at the northern extremity of the global ocean conveyor current. More ice poured into the eastern Atlantic from the Baltic Ice Lake.22 Each event is marked in bottom sediments far out at sea by the appearance of ice-rafted rubble swept offshore as the ice sheets collapsed.