The Ocean of Life

by Callum Roberts

  On less dynamic coasts there is an alternative to expensive construction or abandonment, and it goes under the name of “soft-engineering”: nature, in other words. Like coral reefs, wetlands are self-repairing breakwaters. Coastal engineers are just beginning to realize their value even as their areas decline. The complex matrix of roots and stems traps and binds sediment and stabilizes the coast. If the health of these habitats is maintained and the input of sediment is sufficient, they can keep pace with sea-level rises by trapping and consolidating a layer of mud and peat whose thickness will increase with the sea level. Vigorous wetlands are like a wall that builds itself higher as the need arises. Replanting mangroves in Vietnam after they were destroyed by herbicides during the war reduced the cost of dike repair from $7 million per year to just over $1 million.31 Likewise, where storm surges are a problem, broad and dense salt marshes require little additional coastal protection, while narrow, scrappy ones need to be backed by dikes.32

  Future wetland growth is expected to get a boost from higher carbon dioxide levels in the atmosphere, since plants use this gas for photosynthesis. Sea grasses and mangroves are among only a handful of marine species that could benefit from higher carbon dioxide concentrations. The new Dutch coastal protection plan calls for much more soft engineering of this kind, even to the point of removing existing dikes to promote the restoration of estuarine habitats and tidal regimes.

  Scientists forecast that the sea-level rises of up to twenty-four inches predicted by the IPCC by this century’s end will lead to the loss of another third of our coastal wetlands. This loss is comparable to or less than the direct destruction that has already been wrought by conversion of wetlands to other uses. Given their growing value in ameliorating impacts of sea-level rises, we need to urgently rethink our attitudes toward wetlands.

  CHAPTER 7

  Corrosive Seas

  Wrapped in the vapors of early morning, the island of Ischia appears to float atop a sea of perfect aquamarine at the entrance to Italy’s Bay of Naples. Its intoxicating beauty and therapeutic thermal springs attract over six million tourists a year. The Victorian war correspondent Sir William Russell captured well the contradiction between the island’s tranquil appearance and its volcanic birthright:

  If one could have been aware of the terrible forces which were at work beneath that smiling surface, how delusive would the whole of that bright pageant—the charming little villas nestling in their gardens, the country houses white as snow, with their green jalousies, and the small spires of chapels piercing the mass of foliage—have appeared.1

  The Castello Aragonese grips the top of a craggy rock at the east end of the island, its ancient ramparts blending into gray cliffs that plunge to the sea. Volcanic gases—mainly carbon dioxide—bubble up from underwater springs under these battlements. Mollusks living close to these springs have paper-thin shells, so weak they can be crushed between thumb and finger. These champagne seas, with their high concentration of carbon dioxide, are a worrying portent of the difficulties that could soon face corals, lobsters, clams, oysters, and other shelled forms of life all across our planet.2

  Ocean chemistry and that of our atmosphere are inextricably linked. Gases dissolved in seawater are in equilibrium with those in the air, which means that as concentrations of carbon dioxide go up in the atmosphere, so does the carbon dioxide dissolved in the sea. When carbon dioxide dissolves in seawater, it produces carbonic acid (which is where fizzy drinks get their tang). This liberates bicarbonate and hydrogen ions and decreases carbonate ions (a union of carbon and oxygen and a key ingredient of chalk).3 The hydrogen ions make the sea more acidic, and the decrease in carbonate ions is bad news for anyone or anything that depend for a living on making chalky shells and skeletons.

  The oceans have absorbed around 30 percent of the carbon dioxide released by human activity since preindustrial times from, primarily, burning fossil fuels, converting forests and swamps into cities and agriculture, and cement production.4 Over that period the pH of seawater, a measure of its acidity, has fallen by 0.1 units. Most of this drop has taken place in the last few decades. Since pH is measured on a logarithmic scale in which one unit equals a tenfold change in acidity/alkalinity, this means the acidity has risen by 30 percent. If carbon dioxide emissions are not curtailed, acidity is expected to rise 150 percent by 2050,5 the fastest rate of increase at any time in at least the last twenty million years, and probably as long as sixty-five million years, which takes us back to the age of dinosaurs.6 As Carol Turley, an expert on ocean acidification from the Plymouth Marine Laboratory put it to me, “The present increase in ocean acidity is not just unprecedented in our lifetimes, it is a rare event in the history of the planet.”

  With a pH that currently stands at 8.1, today’s oceans are actually slightly alkaline. Dissolving carbon dioxide is bringing their pH closer to neutrality (defined as the pH of pure water, which is 7.0). While our great-grandchildren won’t exactly get a chemical peel from a paddle in the sea, animals with shells might. By way of comparison, black coffee is several hundred times more acid than the sea, and a soda drink over a thousand times more acid than the pH of 7.6 that could be reached in some places by 2100.7 But even this small change could completely transform life in the sea.

  Acidification is en route to becoming one of humankind’s most serious impacts on the sea, yet it has inexplicably been overlooked until very recently. One of the first marine biologists to realize just how bad more acidic oceans could be for marine life was Joanie Kleypas, an American expert on coral reefs. At a meeting to discuss climate change in 1998 she experienced one of those rare eureka moments. The sudden realization that coral reefs would, by the end of the twenty-first century, be bathed in water corrosive enough to destroy them was so overwhelming she excused herself and ran to the bathroom to be sick.

  Despite decades of attention to rising levels of carbon dioxide in the atmosphere, the problem of ocean acidification was first highlighted only in 2003 in a study published in the journal Nature by Ken Caldeira and Michael Wickett, American scientists based at Stanford and the Lawrence Livermore Lab. They concluded that if we were reckless enough to burn all of the world’s known fossil-fuel store it would result in the rise of the oceans’ acidity over the next three hundred years to levels not seen in the last three hundred million, save from rare catastrophic events like the one that caused the mass extinction at the end of the Permian period.8 This study was an epiphany for the scientific world. Since then scientists across the world have mobilized to study ocean acidification, and hundreds of papers have been published. They make sobering reading. Increased acidity is the last thing marine life needs given all of the other ways in which we are making oceans a tougher place for them to live.

  Many marine plants and animals take up dissolved carbonate minerals from seawater and secrete it in the form of a calcium carbonate skeleton, often with small amounts of magnesium mixed in. Pteropods (tiny swimming snails), microscopic coccolithophores, and foraminifera do so among the plankton, while corals, snails, crustaceans (crabs, lobsters, shrimp), sea urchins, and coralline algae do so on the seabed, alongside filter-feeding clams, mussels, and oysters. As the pH of seawater falls, it takes extra energy for all of these forms of marine life to secrete carbonate structures. This is because acidification alters their internal chemical makeup, making it harder for them to crystallize carbonate out of solution. So when atmospheric carbon dioxide rises, corals and other calcifying organisms will produce less robust skeletons.

  The industry of countless generations of corals over periods of hundreds of millennia has produced some of the world’s most spectacular and diverse marine habitats. Coral reefs form vast geological structures, some of which, like Australia’s Great Barrier Reef or the Maldives, can be seen from space. I gained a keen sense of the enormous solidity of coral reefs once when I stood in the calm waters of the lagoon of a Pacific coral atoll just inside the reef crest. Great ocean swells expended themselves impote
ntly in angry foam against the coral buttress while calm water gently lapped my ankles. No castle could enjoy greater security from harm than is provided by these self-renewing breakwaters.

  Just as Joanie Kleypas and her colleagues predicted in 1999, recent experimental findings suggest that coral reefs and other habitats built from calcium carbonate could cease to grow within our lifetimes. Scientists are suddenly waking up to the possibility that acidification, in combination with coral bleaching due to global warming, will cause the destruction of reefs worldwide. Indeed, the damage has already begun. The skeletons of corals on Australia’s Great Barrier Reef have weakened measurably in the last twenty-five years and now contain 14 percent less carbonate by volume than they did before.9 Since corals contain annual-growth rings, much like trees, changes in skeletal strength can be measured over time. The results suggest that recent weakening has been unprecedented in the last four hundred years. Experiments with deep sea corals from the Mediterranean indicate that they lay down half as much carbonate today as they did before the onset of the Industrial Revolution.10 Ocean acidification has been dubbed “osteoporosis for reefs” because of this skeletal weakening.

  But corals alone do not make a reef. Without the cement of coralline algae, a group of calcareous seaweeds, to bind corals together into solid structures, reefs would not develop nearly so well. We see this in the deep sea where, far below the reach of light, coralline algae are absent (so are the algae that live harmoniously within coral tissues and boost their growth rates in shallow water). Deep-sea corals grow at glacial rates; at best, they build mounds of loose rubble that over thousands of years often reach only ten feet thick. Healthy shallow-water corals, by contrast, can grow by as much as eight inches per year. Coralline algae are especially susceptible to a fall in pH because they secrete a form of calcium carbonate that is rich in magnesium and is more soluble than the form of calcium carbonate produced by corals. In addition to the “glue” of coralline algae, reefs are also held together by carbonate cement that precipitates chemically from the water that trickles through the interstices of the reef. This form of cementation also weakens as carbonate saturation—the amount dissolved in the water—falls.

  Under normal conditions large regions of the oceans are “supersaturated” with carbonate, which means that it is relatively easy to crystallize it to form skeletal structures. Unprotected mineral carbonate structures cannot form or survive in seawater that is not saturated or supersaturated with carbonate ions. We know this because carbonate saturation changes with temperature and pressure. At low temperatures and high pressure, water dissolves more carbonate. As you go deeper in the sea, carbonate concentrations fall below saturation once you pass depths of around ten thousand to thirteen thousand feet deep in the tropics, shallowing to depths of six hundred or seven hundred feet in polar regions. Solid unprotected carbonate dissolves below this. These effects are enhanced by the increase in dissolved carbon dioxide with depth and low temperature. Cold water holds more carbon dioxide than warm, and the deep sea has more than the shallows because of respiration by the creatures that live there.11

  As carbon dioxide levels in the sea rise, carbonate saturation will fall, and the depths at which carbonate dissolves will become shallower. Recent estimates suggest that this horizon is rising by three feet to six feet per year in some places. So far, most carbon dioxide added by human activity remains near the surface. It has mixed more deeply—to depths of more than three thousand feet—in areas of intense downwelling in the polar North and South Atlantic, where deep bottom waters of the global ocean conveyor current are formed. Elsewhere the sea has been stirred to only a thousand feet deep or less.

  All tropical coral reefs inhabit waters that are less than three hundred feet deep, so they will quickly come under the influence of ocean acidification. If carbon dioxide in the atmosphere doubles from its current level, all of the world’s coral reefs will shift from a state of construction to erosion. They will literally begin to crumble and dissolve, as erosion and dissolution of carbonates outpaces deposition. What is most worrying is that this level of carbon dioxide will be reached by 2100 under a low-emission scenario of the Intergovernmental Panel on Climate Change. The 2009 Copenhagen negotiations sought to limit carbon dioxide emissions so that levels would never exceed 450 parts per million in the atmosphere. That target caused deadlock in negotiations, but even that, according to some prominent scientists, would be too high for coral reefs.12

  Just as Ischia’s carbonated volcanic springs provide a warning of things to come, bubbling carbon dioxide released beneath reefs in Papua New Guinea give us tangible proof of the fate that awaits coral reefs.13 Reef growth has failed completely in places where gas bubbles froth vigorously, reducing pH there to levels expected everywhere by early in the twenty-second century under a business-as-usual scenario. The few corals that survive today have been heavily eroded by the corrosive water. The collapse of coral reefs in the Galápagos following El Niño in the early 1980s was hastened by the fact that eastern Pacific waters are naturally more acid due to their deep-water upwelling than those in other parts of the oceans.14 Corals there were only loosely cemented into reef structures and collapsed quickly.

  Acidification is an even greater problem at the poles, where carbonate dissolves at only six hundred or seven hundred feet deep. Polar seas are renowned for their enormous productivity, which is why many whales trouble themselves to swim thousands of miles every year to gorge themselves in their waters. Pteropods, tiny snails that swim with a foot that expands into a pair of delicate wings (the name means wing foot), are keystone animals in polar food webs. They have been called the potato chips of the sea because of their critical role as a food source, but the name does them no justice. They live within shell castles sculpted from transparent crystal whose cold beauty seems perfectly fitted to the icy seas. They range from the size of a lentil to a fingernail and reach densities of up to ten thousand per cubic meter of water. To visualize such a density, imagine each snail in a space equivalent to about the volume of a kiwifruit. For a predator, hitting a patch like this would be like being caught in the whiteout of a snail snowstorm. If you are a fish lover and have ever eaten salmon, cod, pollock, or any number of other species from frigid waters, you have tasted pteropods secondhand.

  Pteropod chips could be off the menu in less than fifty years, as polar seas become undersaturated with carbonate. Experiments with captive pteropods show that their shells dissolve at acidity levels that may soon be reached. In fact, parts of the sea off northern Canada have already become corrosive to pteropod shells as a consequence of ocean acidification enhanced by sea ice melt.15 The Southern Ocean around Antarctica is predicted to follow by 2030, and the Bering Sea by 2100.

  One of the great unknowns in the future life of acid seas is how fast species will be able to adapt as their environment shifts. The geological record holds clues but does not reassure. Fossils record long stretches of vigorous reef development interrupted by periods with few reefs. Charlie Veron, a veteran Australian coral scientist, believes that periods of elevated carbon dioxide correlate with the periods in the geological record when reef formation ground to a halt. Others are less sure and think that episodes of reef growth are not that closely linked with ocean chemistry. They believe that reefs have developed in the past in seas that were more acidic than the levels we are on target to reach by the end of the century. Even they will concede that periods of high carbon dioxide in the atmosphere and oceans are associated with mass extinctions, followed by bursts of evolutionary diversification. This alone rings alarm bells for me.

  In the opening chapter I charted the emergence of life in the sea. A billion years ago atmospheric carbon dioxide levels were much higher and the oceans more acidic than they are today. The evolution of simple photosynthetic life forms in the pre-Cambrian reduced atmospheric carbon dioxide by withdrawing it from the air and releasing oxygen. Around 570 million years ago, 30 million years before the Cambrian explosio
n of life, carbon dioxide concentrations seem to have come down enough for a signal event in the evolution of marine life. The oceans became less acidic and dissolved carbonate reached saturation levels in shallow seas with sufficient consistency to make it possible for them to construct shells. Fossils reefs in Namibia hundreds of yards across date from this time and are made from giant heaps of tiny calcified tubes produced by Cloudina, possibly an early antecedent of polychaete worms, some of which make shelly reefs today.

  The reduction in acidity of the world’s oceans heralded an evolutionary explosion of animals that produced carbonate skeletons and shells. To begin with, most secreted calcite or calcium phosphate shells. Aragonite, the form of calcium carbonate produced by pteropods and modern reef corals, is more soluble than these compounds. Corals that secreted aragonite began to build reefs about 230 million years ago. They yielded to calcite producers in a period of global warming and high carbon dioxide that prevailed between 145 million and 65 million years ago. Modern corals evolved about 40 million years ago, as carbon dioxide levels fell once more. There is obviously a link between atmospheric carbon dioxide, ocean acidification, and the fortunes of animals with chalky carbonate shells. Some managed to survive, and even thrive, in more acid seas in the deep past. But that does not mean they can adjust fast enough to survive the more rapid acidification of modern times.