Deep Future

by Curt Stager

  When Israeli biologists Maoz Fine and Dan Tchernov exposed Mediterranean corals to high-CO2 conditions, their subjects survived the increased acidity by doing away with their hard parts altogether. After a year of exposure to pH values as low as 7.3 (nearly ten times as acidic as today’s oceanic average, and very close to the acid-base boundary of pH 7), the naked polyps were not only still alive, they were thriving. They resembled bunches of soft, flowery little sea anemones more than typical solid coral formations, but they had three times the body mass of normal polyps, perhaps because they didn’t have to expend energy on depositing new limestone day after day. This hitherto undocumented transformation may explain how some kinds of corals survived extreme natural acidification events in the distant past, such as the PETM super-greenhouse 55 million years ago.

  Unfortunately, even these sparks of hope glow dimly. It’s still not clear how most marine creatures will really respond to future acidification or how many corals can disrobe at will. And even if many corals can go naked indefinitely, a cessation of reef-building could leave other specialized reef dwellers homeless, with unpleasant consequences for many of our descendants, as well. Coral reef fish now represent as much as a quarter of Asia’s annual marine catch and feed close to a billion people.

  Although some corals apparently have adaptive defenses against acidification, they’ll still have to face other problems during the next centuries of the Anthropocene as well. Rising temperatures during the last fifty years have already pushed many tropical reefs close to their tolerance limits. Shallowwater corals are also picky about the depths they grow in, so they will have to keep pace with deepening seas in addition to the physiological drag of acidified water. Even the simple act of respiration could be affected. Warm waters hold less dissolved oxygen than cool waters, and absorbing that life-giving gas from the warmer, increasingly CO2-rich oceans could become more and more like recycling stale breath from a plastic bag.

  Ironically, warming seems to be benefiting some species that live on the outer fringes of their ranges, at least for the time being. New thickets of staghorn coral were recently discovered off Fort Lauderdale, Florida, where they had been absent in previous decades, and elkhorn coral is colonizing new sites in the northern Gulf of Mexico. Some marine biologists suspect that this is due to a temperature-driven expansion of suitable habitats poleward, which makes intuitive sense; as formerly cool waters heat up, they could open new territories for coral settlement just as they did during past interglacial warmings. Unfortunately, ocean acidification runs roughshod over that optimistic scenario. Even if reefs start to grow where newly raised temperatures invite them to follow, the changing chemistry of the water may still work against them.

  The gradual acidification of the oceans will last a long time on the scale of a human lifespan, but like greenhouse gas concentrations and global temperatures the acidity will eventually reach a maximum value and then it will begin to decline. Mineral weathering will wash bicarbonate into the oceans and thereby help to neutralize their acid loads while also removing more CO2 from the atmosphere. Most experts estimate that the worst phase of acid contamination could span several centuries in a moderate-emissions scenario and several thousand years in an extreme scenario.

  On the other hand, any species extinctions that accompany marine acidification will be permanent. Evolution can eventually regenerate the purely numerical tally of biodiversity, but it can’t re-create the unique combinations of features and interactions of species that have been lost. Too much time, too much complexity, and too much chance are behind the origins of today’s organisms for us to win back what we may lose in the near future. Even if the deep future eventually does raise Earth’s species counts close to the already-depleted ranks of today, it will only do so on time scales that dwarf the span of the Anthropocene. The long tail of the CO2 curve is measured in thousands of years, but the ages of many animal and plant taxa are measured in millions.

  Past warm events offer some guidance in this matter. The Eemian interglacial was not accompanied by major increases in acid-producing CO2 concentrations, but the PETM hothouse left a permanent chemical burn on the sedimentary records of the deep sea. About half of all bottom-dwelling foram species died out during that acid bath; this alone demonstrates that the risk of extinctions is real. Coral reefs declined rapidly in the tropics during that event but they persisted in midlatitudes until large and encrusting species such as oysters, bryozoans, and red algae replaced most of them as the dominant reef builders. Some of this reorganization of marine communities may have been due to high temperatures as well as to chemical changes; most of the surviving reef corals apparently clustered in cooler, higher latitudes despite their probable higher acidity. But in any case, sizable coral reefs only returned to the oceans millions of years later, after the CO2-rich Eocene climate optimum had passed and the world switched into long-term cooling mode.

  But the PETM example also offers some hope for the future; that early ecological hit list was selective. Extinctions certainly occurred in the deep sea, but many mollusks, corals, and other calcifiers managed to survive, particularly in shallow waters. Cores from the cool Southern Ocean, where acidification would have struck early, suggest that little net change occurred among calcareous plankton species; some forms vanished but others appeared or persisted throughout the acid pulse. Few of their fossils show signs of altered shell thickness or weight despite clear geological evidence of carbonate undersaturation in their surroundings.

  At this point, we can only speculate about which species will disappear because of future acid buildups, and we can only wonder how their demise will affect others around them. But we can be sure that important ecological changes will occur, and that at least some will be unwelcome. Oceans that lose selected calcifiers from the complex webs of marine life will be like a hockey team that suddenly loses a random assortment of members to flu shortly before the playoffs. The replacement skaters, if they even show up, may be poorly matched to the long-termers, and the patchwork team’s performance could be quite different from what they and their supporters had hoped for.

  Consider starfish and sea urchins. They build their protective spines and shells from calcite, and their larvae secrete a particularly soluble high-magnesium form of the mineral. Starfish eat barnacles and mussels; urchins eat seaweeds and other algae. Together, these hunters and grazers clear away small patches of living space on crowded stony substrates where other bottom-hugging settlers can later attach; cage off a section of seafloor to exclude urchins, for instance, and the enclosures sprout lush algal gardens. Lab experiments show that the larvae of some stars and urchins stop growing and develop dangerously brittle shells when CO2 concentrations are doubled. Loss of these keystone species to acidification could radically alter the community ecology of seaweeds, shellfish, and barnacles.

  In a paper that appeared in Nature in 2008, a team of scientists led by British marine biologist Jason Hall-Spencer described a natural demonstration of what carbonic acidification has already done to such creatures. They studied a volcanically active region of the Mediterranean seafloor near Italy’s east coast where CO2 emerges from cold gas vents and acidifies the local waters by as much as half a pH unit, which is close to what is soon expected to happen at high latitudes. They were careful to select vents that don’t release other toxic substances such as sulfur, so the example may offer a reliable hint as to what such changes could do on a larger scale in the future.

  Corals were common in the general vicinity of the vents but not anywhere close to them, and the encrusting coralline algae that often help to hold reefs together were also absent from the acidified zones. Sea urchins and many snails were either killed or driven out, too. On the other hand, most organisms that lacked soluble hard parts did well. Many-tentacled sea anemones crowded like soft bouquets near the vents, and CO2-loving seaweeds also did well, perhaps because of the dissolved gas, the lack of crowding from the missing bottom-dwellers, and the absence of grazing
urchins. It all sounds rather complicated, as one might expect from a diverse marine environment such as this, but the main point is that local, natural carbonic acidification has clearly changed what lives on that patch of seafloor.

  Much larger habitats and economies may also be at risk where cool, increasingly acidic water wells up from deeper layers of the sea to the surface. Such upwelling zones are among the world’s most productive habitats because they deliver nutrients into sunlit shallows where algae can feed on them and grow like grass on a fertilized lawn. Tiny planktonic animals graze on those algae, and they in turn support great shoals of fish such as sardines and anchovies. Sea lions, squid, dolphins, and other predators feast in such places, and local trawler nets bulge with valuable fish protein. A quarter of the global fish catch comes from near-shore upwellings along the western coasts of South America, California, Spain, southern Africa, and New Zealand.

  Oceanographers are already reporting a drop in pH values in California’s upwelling zones from excess CO2 that entered the deep ocean decades ago, and these sorts of places will bring early signs of acidification to the tropics before the rest of the low-latitude shallows catch up. Predicting what components of these tightly linked food chains might be lost to acidity is merely a guessing game at this point, but sporadic natural events in Peru and South Africa hint at how bad things might become if even one link fails.

  Normally, the upwelling of cold currents along those coasts makes the algae so abundant that it turns the surf foam green, but sometimes that upward flow slows down or even stops. In Peru that change is caused by El Niño climate disturbances and in southwestern Africa it is caused by weather disruptions associated with the coast-hugging Benguela current, but the results are similar in both cases; the algae wane, then so do the fish and the things that eat them. An unusually strong El Niño in 1983 temporarily destroyed the Peruvian anchoveta fishery, and Benguela events can decimate catches, as well. The algae that usually live in these habitats lack carbonate shells so they might not suffer much from a drop in pH, but early acidification there might nonetheless harm shellfish or crabs that live along those coasts. We simply don’t know yet.

  Living ecosystems can be surprisingly complex, of course, and treating one of them like a simple chemical equation in which A minus B produces C can be misleading. Hopefully, we’ll know a lot more about the ecological effects of acidity on oceans sooner than later. Since the Royal Society’s report was published, many other scientific organizations ranging from the U.S. National Science Foundation to the International Geosphere Biosphere Program have launched research initiatives on ocean acidification.

  But time might be running out even faster than we think. Ambitious geoengineering schemes designed to turn sunlight away from Earth with high-altitude reflectors might be able to cool climates down below, but even if they succeed they will still leave the excess CO2 in circulation. Worse still, plans are also afoot to pump massive amounts of atmospheric CO2 into the deep oceans. For those who focus exclusively on ways to avoid temperature changes, that solution seems to be quite sensible. The sea can absorb most of what we emit, so why not help things along with aquatic carbon sequestration?

  To those with a broader environmental perspective, however, this can seem like trying to douse a fire by pouring gasoline on it. As Ken Caldeira and Mike Wickett noted in a recent paper in the Journal of Geophysical Research, injecting CO2 into seawater amounts to trading “greater chemical impact on the deep ocean as the price for having less impact on the surface ocean and climate.”

  Only time will tell us if a general outcry over ocean sequestration stops it in its tracks. But relatively few outside the scientific community are discussing the dark side of this issue yet, much less the deadly effects of carbon emissions on marine ecology. One of the most influential articles about acidification in the popular press was Elizabeth Kolbert’s masterfully written piece that appeared in The New Yorker in 2006, titled “The Darkening Sea.” We need more of that, and soon. But to turn the present situation around in time to prevent widespread damage will require rallying public sympathy for squishy little sea creatures with unpronounceable names that most of us don’t even know exist. Could T-shirts and banners emblazoned with “Save the coccolithophores” or “I love Lophelia” raise much of a response outside of narrow scientific circles?

  Ocean acidification represents one of the most compelling reasons to control our emissions, not only for ourselves but for the sake of countless other species that share this water-dominated planet with us. Temperatures and sea levels will also change in response to our carbon emissions pulse but they will eventually recover in the deep future. When species die out, however, they don’t return. Extinction, far more than global warming, is truly forever.

  7

  The Rising Tide

  And the waters prevailed exceedingly upon the earth;

  and all the high hills that were under the high

  heaven were covered.

  —Genesis 7:19

  And when the seventh day came, the flood subsided

  from its slaughter, like hair drawn slowly back

  from a tormented face.

  —Epic of Gilgamesh

  Invisible but important chemical disturbances in the air and oceans attest to the reality of global carbon pollution, and they also warn of more changes to come. One of the most troubling of those changes, even for people who care little about carbon chemistry or unfamiliar aquatic species, is also invisible but only in the sense that it proceeds too slowly for us to watch it unfold on a daily basis. As the world warms, the physical shape of the sea is changing.

  Global sea-level rise is, in a psychological sense, a mirror image of marine acidification. The latter process is underreported and it rarely generates strong emotional responses among the general public. But sea-level rise is quite another matter. One can easily imagine its effects on coastal settlements, and many of us grew up on tales of epic floods in the days of Noah or Atlantis, so the primal fear of advancing waters runs deep. As a result, ocean encroachment is widely recognized as a problem, but the details of it are often misunderstood. What, exactly, is sea level? How high can it climb, and how fast? How dangerous is it? And what will happen to the oceans far out on the long, cooling tail of the CO2 curve?

  Defining and measuring sea level itself sounds straightforward enough, doesn’t it? “Sea level” is simply the height at which the surface of the ocean rests. Just measure it all over the world and calculate the average. But think more carefully about actually having to go out and do it, and you’ll understand why I have been struggling to nail down various aspects of the concept since tenth grade.

  My first encounters with this surprisingly complex subject began in Mr. Alibrio’s high school physics class in Manchester, Connecticut, during the 1970s. One day he abruptly called my name from the front of the room, brushing with one hand a military-style crew cut that was bleached by age. “Stager! What is sea level? Look it up and tell us next week.”

  A week later, I rose to say that sea level was an average of many measurements of sea-surface heights. “Not good enough!” Alibrio roared. “I want to know exactly how you measure it, too.”

  After another week, I reported that sea level isn’t measured with rulers but with barometers. A former surveyor had told me that the elevations of mountaintops are calculated relative to atmospheric pressure at sea level. “Still not good enough!” came the verdict. “Think about it: air pressure goes up and down from day to day as the weather changes. How can you get a standardized value for sea level with a wavering tool like that?”

  And thus it went. The surface of the sea is roughened by waves, not to mention rising and falling tides, so what part of it are you supposed to measure, and when? And what is it measured relative to? If you measure it relative to a point on land, then you have to account for the long-term shifting of seemingly solid ground as well as of the more obviously fluid oceans. Many regions of Earth’s crust are surprisi
ngly mobile, drifting and bobbing like gigantic rafts afloat on the magma-filled mantle. Most major movements of the Earth are slow, the result of processes such as tectonic plate collisions or local groundwater removal, but they can still be significant. If we were to measure sea level relative to such places, then we might falsely conclude that it is rising or falling faster than it really is.

  Because of this often unsteady relation between land and water surfaces, the elevation of Britain’s official brass sea-level benchmark is represented by measurements at a single location, Newlyn. The data were compiled by observing the position of the local sea surface on a tidal staff every fifteen minutes for six years (yes, that means day and night all year long) between 1915 and 1921, and all other elevations in England, Scotland, and Wales were originally based upon that standard point on the national map. Other countries use different reference points, and even within a single nation the varying needs of different user groups can lead them to use different standards. For instance, nautical maps typically emphasize the hazards of submerged rocks and sandbars by measuring water depths against a chart datum below which tides rarely fall, while warning signs on bridges and low-hanging cables present their heights relative to mean high water.

  I never did give Mr. Alibrio a satisfactory answer to his question, but that may well have been the real point of the lesson. As the years passed and I taught science classes of my own, I began to realize that my old teacher wasn’t so much trying to torment me as to introduce me and my peers to the remarkable depth and detail behind such seemingly simple questions.

  Nowadays, state-of-the-art satellite systems use radar and other energy emissions to monitor and calculate surface elevations worldwide. With such tools we can even see where large sweeps of saltwater bulge or dip by as much as several feet because of tides, currents, and the gravity fields emanating from submarine mountains.