In 1998, sea temperatures soared across the tropics. In some places, like the Seychelles and Maldives, entire reefs turned white, and almost all the corals were killed. If this had been an isolated event it would have been bad enough, as corals can take decades to recover. But such bleaching events have become common. Scientists are now predicting that corals will be exposed to temperature spikes of the same magnitude every year by the middle of this century.10 If corals cannot adapt, whole reefs will die, taking with them a host of species that call them home. Adaptation is clearly possible. Some of the world’s most northerly reefs are found in the seas off Kuwait, where temperatures swing wildly. When winter winds blow hard from the desert, water temperatures plunge as low as 54oF. In the dead calm of stifling summer they top 95oF. Despite these tough conditions, corals have built reefs large enough to support islands. Fast adaptation will be crucial to reef survival, but the prospects are not good. The rate at which species evolve depends heavily on generation time. Species with high turnover rates evolve rapidly (think of flu viruses), while long-lived species will struggle. Like the ancient trees of the High Sierras in the United States, some corals live for hundreds of years, while a few reach a thousand or more.
The Galápagos Islands offer a harsh warning of what could be ahead. In late 1982 a powerful weather phenomenon called El Niño triggered a sharp rise in water temperatures that lasted five months. By the end, Galápagos reefs were all but dead. Thirty years later they still haven’t recovered, and structures built over thousands of years have now crumbled to rubble and dust. A few years ago I swam through the remains of what had once been a glorious reef there. It was a scene of devastation. The seabed was strewn with chalky fragments, like a field of bones in the aftermath of massacre. Here and there a mound rose above the wreckage where once some mighty coral had stood. It had survived centuries of lesser temperature spikes, only to die in this one. A few young corals struggled to gain a foothold but were quickly overcome by black-spined sea urchins that swarmed over them like ants on a carcass. What that experience and others like it tell us is that today’s bleaching events are unprecedented on millennial timescales.
It is hard to get too animated about a few degrees from our warm-bodied mammalian perspective (although there are ample reasons for us to be anxious as well). But if you live at your thermal limits, as many species do, even this could be intolerable. Warming seas will trigger a global diaspora that will lead to a massive reorganization of life, as long-separated species invade each other’s ranges. There will be a march toward the poles, as we have already seen for fish and plankton in the North Atlantic. These invaders will alter polar ecosystems in profound and unpredictable ways. But where do you go if you already live at the poles and prefer your habitat with ice? The answer, some scientists predict, will be a surge of extinctions among polar residents, as their waters heat and ice melts away. Perhaps they will find refuge in deeper layers of the sea, but for some that is not an option. This is why the polar bear and walrus, icons of the realm of floating ice, are now on the World Wildlife Fund’s top ten list of species in peril.
This global diaspora is not all about loss. Some species will do well, and some countries may prosper from the changes. Others will suffer. In an elaborate thought experiment, William Cheung, from the University of East Anglia, and his colleagues created a global ocean model to try to predict the consequences of life’s reorganization between now and 2055.11 As they cranked up the heat, species shifted poleward. Those that could not move, for want of suitable habitat or because the way was blocked, declined, in some cases to the point of extinction. The movement of all of these species left some places with fewer inhabitants than before, mostly in the tropics, while the poles suddenly became more populous. There will be a knock-on effect on fish catches. Low-latitude nations like Indonesia, Senegal, and Nigeria could see catches dwindle by 10 percent or 20 percent, compounding the loss of agricultural productivity that terrestrial climate models predict. Fish production in the United States will have to shift, as most of the lower continental states could lose 10 percent of their catches, while Alaska’s could expand by a quarter.
Although there are many reasons to worry about the impact of rising temperatures on the sea, the good news is that life can and will move, often quickly. The eggs and larvae of many marine species can travel several tens of miles to more than one hundred and we have erected few barriers to prevent their dispersal. The seas off Greenland experienced a warming spell that lasted between the 1920s and the 1960s. Cod, haddock, and herring expanded their ranges north very rapidly; cod spread more than six hundred miles poleward in less than twenty years and kick-started a fishery that at its peak landed 440,000 tons of fish a year.12 A host of other species shifted alongside these commercial fish, including bottom dwellers and marine mammals.
Cheung’s predictions hold many uncertainties. They assume that the production of food by ocean plants either will not change or might even increase a little. For reasons I will return to in later chapters, how these phytoplankton react as the seas change is largely guesswork right now. There are equally good arguments that productivity will fall. If so, a few countries may break even, but most will see significant reductions in their fisheries.
By now it will be clear where my views lie on climate change. Like thousands of other scientists, I am convinced it is underway. There is abundant evidence of such variety and strength that those who reject it are increasingly isolated. It is important to remain skeptical about every new study and to weigh the solidity of its methods and the rigor of its sampling. Skepticism is the stuff of science. Old theories are overthrown when new data and new ideas provide better explanations for what we see around us. But then there is denial. It is easy to spot those in denial. They use evidence selectively, acknowledging only those findings that support their prejudices and rejecting, or simply ignoring, findings that don’t. As evidence mounts against their ideas, their explanations for the discrepancies become ever more tortuous and far-fetched.
Those who claim there is some elaborate conspiracy among scientists to delude politicians and the public should ask themselves a simple question: Why? What could anyone possibly gain from such a hoax? If the answer to “why” is not sufficient, there is the question of how. Science is a highly competitive pursuit. There are no prizes for coming in second. Scientists constantly put each other’s ideas to the test. A hypothesis is cast aside as soon as it is found wanting or is reworked on the forge of new evidence. No conspiracy could resist such relentless scrutiny for long.
Much of the controversy around climate change stems from the variability of the Earth’s climate over long timescales. We live on a dynamic planet where yearly climatic variations are overlain by multiyear or decadal cycles, known as oscillations. Cores taken through deep sea sediments in California’s Santa Barbara Channel show how marine life tracks these changes. In the aftermath of every frenzied predatory attack on a shoal of sardines or anchovies, a rain of glittering scales makes its way down to the seafloor. Scales trapped in sediments allow us to trace the ups and downs of shoaling fish through thousands of years as they respond to climate events like El Niño and the swings of a cycle known to oceanographers as the Pacific Decadal Oscillation.
Modern measurements have been made on timescales of a few decades to a century, so it can be hard to see the signal for the noise. Much of the change to date may fall within the range of natural variation if you extend the timeline out far enough, but the drivers of our current change—greenhouse gas concentrations—have moved far above historic levels. If you are pushing a broken-down car, you have to apply a lot of force before it starts to move. The force of humanity’s push on the climate system has been building for over a century, and the consequences are beginning to become obvious.
Life is on the move. This is nothing new. Records locked in ice, seabed sediments, and rocks show that change is normal for life on Earth. As conditions shift, new habitats and ecosystems constructed b
y better adapted species will re-form in the place of old. It is not change that we must worry about but its speed. If conditions alter too quickly they could exceed the pace at which ecosystems can respond. It could take centuries for life to adjust, and we can’t afford to wait that long. In a world of growing human need, that is a big concern.
CHAPTER 6
Rising Tides
As I strolled down Ocean Drive in Miami Beach one evening a few years ago, I could almost imagine myself transported back to the 1930s. The party atmosphere from the nightclubs and bars mingled with the smell of seaweed and the distant boom of breaking waves. Today the Art Deco splendor of South Beach is overshadowed by towering hotels and apartments that line the seafront to the north. It is hard to imagine the place any other way, but in the nineteenth century Miami Beach was little more than a windblown barrier island backed by dense mangrove swamps. Its potential was first appreciated by Henry Lum, a veteran of the California gold rush of 1849 who had a keen nose for a bargain. He bought the land for twenty-five cents an acre and planted coconut palms. But Miami Beach was not really born until 1915, when a bridge was laid across Biscayne Bay to connect it with Miami.
Lum fell in love, as many other Americans have after him, with the wild beauty of the barrier islands. These low-lying islands are made of wave-washed sand and run, on and off, along the East Coast, from Long Island to Miami. The twentieth century saw a coastwide building boom that covered many of them with houses and hotels in places like Kitty Hawk, Surf City, and Wrightsville Beach. But after the glory days of speculation and development, many have begun to fear that the ocean is about to reclaim these lands.
We are reaching the end of an era of around five thousand years of unbroken stability in the volume of water in our oceans. This followed a period of rapid increase that came after the last ice age, when melting ice sheets pushed the height of global seas upward by 420 feet. Some coasts have seen levels rise or fall since then, due to coastal uplift or subsidence (sinking), but the overall amount of water in the oceans has been very stable.
Stable sea levels promote the formation of wetlands where rivers meet the sea. Rivers spread sediment with every flood and regularly discharge mud into coastal waters. Deltas, mudflats, marshes, and barrier islands form where the rate of new sediment deposition exceeds the rate of erosion and subsidence. The land inches out to sea when plants bind and stabilize the sediment. People have given wetland formation periodic spurts over the course of history, when they have cleared land to grow crops and build cities.1 Denuded lands shed their topsoils into valleys and rivers far more rapidly than those bound by forests or other vegetation. Soil erosion during the early Middle Ages in parts of Europe choked estuaries and built wetlands that stranded once busy ports far inland, such as Luni in northern Italy and Chester in England. In North America, a pulse of wetland creation followed forest clearing for agricultural purposes between the seventeenth and nineteenth centuries.
Today rivers are again running thick with soil, but less of it reaches the coast than in the past. Dams, which have been constructed along almost all of the world’s large rivers and countless smaller ones, hold back mud that once would have reached the sea. Crop irrigation in arid regions often diverts water and sediment that would have replenished wetlands. Australia’s legendary Murray and Darling rivers, for instance, lose so much water to irrigation that they now reach the coast as mere trickles.
The balance shifts in favor of the sea when sediment supplies are cut off, and wetlands retreat. Deltas naturally subside over time, as sediments settle and the earth sinks below them in a process called isostatic adjustment. In many places today, from Miami Beach and Venice to Tokyo and New Orleans, sea-level rises have combined with coastal subsidence to threaten life and property.
The Earth is ruled by certain immutable physical principles. One such law is that warm substances take up more space than cold ones.2 To us it hardly matters if soil and rock expands as our planet heats up. But it matters a great deal when that heat expands the oceans and the sea begins to reclaim our most valuable coastal land. Ancient Roman fishponds, built at sea level two thousand years ago, have helped define when the rise in sea level began in earnest.3 At the time they were constructed the Mediterranean was five inches lower than today. It didn’t begin to rise until about one hundred years ago, when global temperatures started to respond to greenhouse gas emissions from the Industrial Revolution. The process has sped up considerably since then. Sea levels rose by approximately eight inches between 1870 and 2000.4 We know this from thousands of tide gauges across the world. Since 1993 those recordings have been supplemented by satellite observations that measure sea-level fluctuations with great accuracy. Averaged over 130 years, the rise comes to one fifteenth of an inch per year, but in the last twenty years the rate has accelerated, and it now tops an eighth of an inch per year.5 Three quarters of the rise in sea levels since 1900 has been caused by global warming from carbon dioxide emissions. Water expansion from sea-surface warming contributed a quarter of the rise since 1960, and 30 percent from 1993 to 2009. The rest came from melting glaciers and ice caps, and from groundwater pumping for irrigation.6
Sea levels would have risen a fiftieth of an inch per year faster if we had not embarked on wholesale dam building in the mid–twentieth century, which locked up freshwater on land. This beneficial effect of dams has been rather undermined, though, since they have also blocked the passage of vast quantities of mud, an effect that has hastened erosion around river deltas from the Yangtze to the Mississippi. Parts of the Nile Delta have retreated by as much as five hundred feet per year since the Aswan Dam was built in 1964.7
So far only the upper layers of the sea have begun to warm up. As heat penetrates deeper, the sea will expand, and sea levels will continue to rise. In 2007 the Intergovernmental Panel on Climate Change (IPCC) predicted that sea levels could rise by another seven to twenty-four inches by 2100, depending on how fast we gain control over the drivers of climate change. Their forecasts combine thermal expansion of seawater with ice loss from mountain glaciers and the Greenland and Antarctic ice sheets. But their figures seem increasingly at odds with the recent acceleration: Sea levels have risen faster than the most rapid rate of change they predicted a decade ago.
New research from the ice caps of Greenland and Antarctica suggest that we are close to, and perhaps past, a tipping point for the rapid melt of land-based ice, which will become the major source of sea-level rise in future. If the Greenland ice sheets were to thaw in their entirety, they would add twenty feet to the height of global seas and trigger mass human exodus from low-lying coasts and cities. The thaw of the West Antarctic Ice Sheet would add another ten feet.8 A twenty-foot rise would wipe out most of Florida from just north of Miami.9 It would obliterate the Mississippi delta and drown a third of New York City. It would flood much of London and Hamburg and turn Lagos into a lagoon and Bangladesh into a swamp.
Sea-level records preserved in fossil coral reefs in Central America suggest that we have crossed this tipping point at least once before, 121,000 years ago, when sea levels rose by ten feet within the space of a hundred years (ten times the present rate).10 Some argue that this period is an analog for what a warmed world might be like. Temperatures then averaged 4oF above today’s, and the seas leveled off twenty feet higher than they are now, drowning deltas and plains in many of the low-lying coastal areas that are now densely populated.11 Of course, such extreme levels would not be reached instantaneously. It will take time for sea levels to respond to a warming planet. A range of estimates made since the 2007 IPCC assessment suggest an upper limit to the rise rate of about six and a half feet in a hundred years.12
The melting of Arctic sea ice is perhaps the most striking manifestation of global warming, but even if it were to disappear in its entirety, Arctic ice would not add to sea level, because it floats in the sea. (If you aren’t convinced, add an ice cube to a glass of water and you will see how the level remains the same as
it melts.) It is the ice sheets sitting on land, mainly over Greenland and Antarctica, that we need to worry about. They have proven less durable than the IPCC had hoped. Ice heated from the surface melts and forms pools and streams. Runnels turn into rivulets that coalesce into blue rivers that course across the frozen surface until they find a crevasse. They then plunge in muted waterfalls that would be magnificent if only we could see them. Beneath the ice sheet, this running water lubricates the stuttering creep of ice across rock and accelerates its slide toward the sea.
Ice caps are often held back at their margins by ridges of undersea rock on which the sheet has grounded. Global warming is gnawing away these barriers, too. Tongues of warm water (at least by polar standards) are creeping under the ice and freeing them from their anchors. The West Antarctic ice sheet has already been loosed from one key holdfast.13 Measurements from planes and satellites show the seaward edges of both Greenland and Antarctic ice sheets slipping ever faster into the sea. Ice sheet thaw seems to be accelerating through positive feedbacks. Blue pools and rivers of melted water at the surface mean that less of the sun’s heat is reflected, and so more ice melts. It is hard to predict the future contribution of ice sheets to sea-level rise because climate change has also increased snowfall over their interior regions, helping counteract losses from the margins. How the balance between ice sheet growth and melt develops over the next century will be critical to countless people who live on low-lying coasts.
Another positive proof of global warming is newly emerging in the Arctic Ocean. The seabed has begun to spew forth bubbles of the greenhouse gas methane on a colossal scale as permafrost melts at the bottom of the ocean where ice sheets are in retreat. Russian scientists announced the findings at a San Francisco conference in December 2011 where the leader of the team, Professor Igor Semiletov, described fountains of methane more than half a mile wide.14 He said they saw hundreds of such fountains but estimated there could be thousands more. So methane emissions from the seabed are now adding to those from melting permafrost on land to accelerate global warming.
The Ocean of Life Page 10