With Speed and Violence: Why Scientists Fear Tipping Points in Climate Change (14 page)

Larry Smith, of UCLA, estimates that the northern peat bogs of Siberia, Canada, Scandinavia, and Alaska could contain 50o billion tons of carbon altogether, or one third of all the carbon in all the world's soils. If all that carbon were released as carbon dioxide, it would add something like 5°F to average temperatures around the world. But if most of it were released as methane instead, it could provide a much bigger short-term kick. How much bigger would depend on how fast the methane was released, because after a decade or so, methane decomposes to carbon dioxide. If the methane all came out at once, it could raise temperatures worldwide by tens of degrees. That may be an unlikely scenario. Even so, the odds must be that melting along the melting-point isotherm is destined to have a major impact on the twenty-first-century climate. From Stordalen to Pangody, these bogs are primed.

15

THE ACID BATH

What carbon dioxide does to the oceans

The oceans are the ultimate sink for most of the heat from the sun and also for most of the greenhouse gases we are pouring into the atmosphere. The atmosphere may be the place in which we live and breathe, but for longterm planetary systems it is just a holding bay. At any one time, there is fifty times as much carbon dioxide dissolved in ocean waters as there is in the atmosphere. Given time, the oceans can absorb most of what we can throw into the atmosphere. But time is what we do not have, and the oceans' patience with our activities may be limited.

Carbon dioxide moves constantly between the oceans' surface and the atmosphere, as the two environments share out the gas. And, because of ever-rising concentrations in the atmosphere, the oceans currently absorb in excess of 2 billion tons more a year than they release. Much of that surplus eventually finds its way to the ocean floor after being absorbed by growing marine organisms-a process often called the biological pump. Sometimes there are so many skeletons falling to the depths that biologists call it marine snow.

Though they are the ultimate sink for most carbon dioxide, the oceans do not simply absorb any spare carbon dioxide left in the atmosphere. The relationship is much more dynamic-and much less reliable. In the long run, carbon dioxide seems to seesaw between the oceans on the one hand and the atmosphere and land vegetation on the other. Plants on land generally prefer things warm. Certainly the carbon "stock" on land is greater during warm interglacial eras like our own, and less during ice ages. By contrast, ocean surfaces absorb more carbon dioxide when the waters are cold. This seems to be partly because the plankton that form the basis of life in the oceans prefer cold waters, and partly because when the land is cold and dry, dust storms transport large amounts of minerals that fertilize the oceans.

During the last ice age, some 22o billion tons of carbon moved from the land and atmosphere to the oceans. This process didn't cause the ice ages, but it was a very powerful positive feedback driving the cooling. And that is a worry. For if the ice-age pattern holds, future generations can expect the oceans' biological pump to decline as the world warms. The story of the oceans' exchanges of carbon dioxide with the atmosphere may turn out to be rather like that of the carbon sink on land. In the short term, the extra carbon dioxide in the air has fertilized the biological pump and encouraged greater uptake. But in the longer term, warmer oceans are likely to weaken the biological pump and release large amounts of carbon dioxide into the air.

Is something of the sort likely? Very much so, said Paul Falkowski, of Rutgers University, in New Jersey, in a long review of the carbon cycle in Science. "If our current understanding of the ocean carbon cycle is borne out, the sink strength of the ocean will weaken, leaving a larger fraction of anthropogenically produced carbon dioxide in the atmosphere." With tens of millions of tons of carbon moving back and forth between the atmosphere and the oceans each year, it would take only a small change to turn the oceans from a carbon sink into a potentially very large carbon source. This may already be happening. In 2003, the NASA scientist Watson Gregg published satellite measurements suggesting that the biological productivity of the oceans may have fallen by 6 percent since the 198os. It could be part of a natural cycle, he said, but it could also be an early sign that the biological pump is slowing as ocean temperatures rise.

So far, since the beginning of the Industrial Revolution, the oceans have absorbed from the atmosphere something like 13o billion tons of carbon resulting from human activities. While much of it has fallen to the seabed, a considerable amount remains dissolved in ocean waters-with a singular and rather remarkable effect: it is making the oceans more acid.

The carbonic acid produced by dissolving carbon dioxide is corrosive and especially damaging to organisms that need calcium carbonate for their shells or skeletons. These include coral, sea urchins, starfish, many shellfish, and some plankton. Besides eating away at the organisms, the acid reduces the concentration of carbonate in the water, depriving them of the chemicals they need to grow.

Acidity, measured as the amount of hydrogen ions in the water, is already up by 30 percent. To put it another way, the pH has dropped by o.1 points, from 8.2 to about 8.i. If the oceans continue to absorb large amounts of the atmosphere's excess carbon dioxide, acidification will have more than tripled by the second half of this century, badly damaging ocean ecosystems. The most vulnerable oceans are probably the remote waters of the Southern Ocean and the South Pacific. They are distant from land, and so are already short of carbonate-in particular a form known as aragonite, which seems to be the most critical.

"Corals could be rare on the tropical and sub-tropic reefs such as the Great Barrier Reef by 2050," warned a report from Britain's Royal Society. "This will have major ramifications for hundreds of thousands of other species that dwell in the reefs and the people that depend on them." Other species may suffocate or die for want of energy. High-energy marine creatures like squid need lots of oxygen, but the heavy concentrations of carbon dioxide will make it harder for them to extract oxygen from seawater.

"It is early days," says Carol Turley, of the Plymouth Marine Laboratory, a world authority in this suddenly uncovered field of research. "The experiments are really only getting under way." But one set of results is already in. James Orr, of the Laboratoire des Sciences du Climat et de l'En- vironnement, in France, put tiny sea snails called pteropods into an aquarium and exposed them to the kind of ocean chemistry expected later in this century. These creatures turn up all around the world and are vital to many ecosystems. They are the most abundant species in some waters around Antarctica, where a thousand individuals can live in 300 gallons of seawater. As well as being a major source of food for everything from fish to whales, pteropods are the biggest players in the biological pump there.

Orr found that within hours, the acid pitted the pteropods' shells. Within two days, the shells began to peel, exposing the soft flesh beneath. In the real world, predators would break through the weakened shells. "The snails would not survive," he concluded. The demise of the pteropods would cause a "major reduction in the biological pump," the Royal Society agreed. Within a few decades, it could leave the oceans more acid than at any time for 300 million years.

Whatever the outcome, we are seeing the start of an unexpected and frightening side effect of rising atmospheric carbon dioxide levels. Perhaps the nearest parallel to the current situation was 5 5 million years ago-the last time a major slug of carbon was released into the atmosphere over a short period ...

16

THE WINDS OF CHANGE

Tsunamis, megafarts, and mountains of the deep

It was Earth's biggest fart ever. Fifty-five million years ago, more than a trillion tons of methane burst from the ocean, sending temperatures soaring by up to i8°F extinguishing two thirds of the species in the ocean depths, and causing a major evolutionary shock at the surface. The story, while from long ago, is a reminder that methane lurks in prodigious quantities in many parts of the planet-not just in frozen bogs-and that one day it could be liberated in catastrophic quantities.

The first whiff of this prehistoric megafart was unearthed in 1991, from a hole drilled about a mile into a submarine ridge just off Antarctica. Examining the different layers of the ancient sediment removed from the hole, the geologists James Kennett, of the University of California at Santa Barbara, and Lowell Stott, of the University of Southern California in Los Angeles, found evidence of a sudden mass extinction of organisms living on the sea floor 55 million years ago. They had apparently disappeared from the ocean within a few hundred years-perhaps less. Kennett and Stott soon discovered that other researchers had detected evidence of similar extinctions from the same era, in Caribbean and European marine sediments. This was clearly a global event-one of the largest extinctions in the history of the planet.

What happened? Looking at the chemistry of fossils in the drilled sediment, the two geologists found some intriguing clues. There was, for instance, a sudden change in the ratio of two oxygen isotopes, known as oxygen-r8 and oxygen-r6. The ratio in the natural environment is very sensitive to temperature, and this isotopic "signature" in sediments and ice cores is a widely used indicator of past temperatures. Kennett and Stott concluded that after rising gradually for several million years, ocean temperatures had soared much more dramatically about 5 5 million years ago. The change happened at the same time as the extinctions.

The sediments also revealed a second isotopic shift, this time between isotopes of carbon. Earth's organic matter suddenly contained a lot more carbon12. From somewhere, trillions of tons of the stuff had been released into the environment. Clearly a greenhouse gas, either carbon dioxide or methane, had caused both changes. The problem was finding a likely source with sufficient capacity to do the job.

Jerry Dickens, a biochemist at James Cook University, in Townsville, Australia, set himself the task of working out where this carbon12 might have come from. The first suggestion was carbon dioxide in volcanic eruptions, which are a rich source of carbon-I2 in the modern atmosphere. But, says Dickens, that would have required volcanic eruptions at an annual rate a hundred times the average over the past billion years. Fossil fuels like coal, oil, and natural gas were possible sources. But they are mostly buried out of harm's way, sealed in rocks. Given that there were no creatures digging them up and burning them at the time, that, too, seemed implausible. The same was true for methane from swamps and wetlands like those found today in Borneo and Siberia. About three times as many of them existed then, but even so, they could not have delivered the amount of carbon-12 required. Only one last source-big enough and accessible enough to unleash a climatic eruption-was left. That, Dickens suggested, had to be the vast stores of methane that geologists have recently been discovering frozen in sediment beneath the oceans: methane clathrates.

Methane clathrates are an enigma. They have until recently escaped the attention of oil and gas prospectors, because they don't turn up in the kind of deep and confined geological formations where prospectors traditionally look for fossil fuels. Nor are they the product of current ecosystems, such as tropical and Arctic bogs. They are generally close to the surface of the ocean floor but frozen-confined not by physical barriers but by high pressures and low temperatures, in a lattice of ice crystals rather like a honeycomb. Scientists still debate exactly how and when they were formed, but they seem to arise when cold ocean water meets methane created by microbes living beneath the seabed. Seismic surveys have revealed these structures in the top few hundred yards of sediments beneath thousands of square miles of ocean. They exist unseen, usually just beyond the edge of continental shelves. Many of these frozen clathrate structures trap even larger stores of gaseous methane beneath, where heat from Earth's core keeps them from freezing.

Dickens estimates that between i and io trillion tons of methane is tied up today in or beneath clathrates. But its confinement may not be permanent. Release the pressure or raise the temperature, and the lattices will shatter, pouring methane up through the sediment into the ocean and finally into the atmosphere. It seems that some such event must have happened 55 million years ago. Moreover, if this was the source of the great release of carbon-I2, it would also explain why the extinctions appeared to be most serious in the ocean depths, where extensive acidification would have been almost certain. "Right now, most everybody seems to accept that the release of methane clathrates is the only plausible explanation for what happened 55 million years ago," says Dickens.

His chronology goes like this. For several million years, the world was warming, probably because of extraterrestrial influences such as the sun. The warming gradually heated sediments on the seabed until the clathrates started to shatter and release methane. Perhaps it happened in stages, with warming releasing methane that caused further global warming that released more methane. But at any rate, over a few centuries, or at most a few thousand years, trillions of tons of methane were eventually released into the atmosphere-enough to cause the observed global shift in carbon isotopes and a large and long-lasting hike in temperatures.

"The world just went into chaos," as Dickens puts it. Life on Earth was transformed almost as much as by the asteroid hit io million years before that wiped out the dinosaurs. Once the methane releases had ended, the planet's ecosystems gradually absorbed the remainder of the great fart, the climate recovered its equilibrium, and the oceans settled down again. But the evolutionary consequences of that long-ago event have lasted to this day. By the time the climate had recovered, many land and ocean species had become extinct, while others evolved and flourished.