With Speed and Violence: Why Scientists Fear Tipping Points in Climate Change (26 page)

Many believe that El Nino and the pattern of ocean temperatures in the Pacific are heavily implicated in the historical megadroughts, perhaps as part of a global reorganization of climate systems linked to Gerard Bond's pulses. And this should set modern alarm bells ringing, says Ed Cook, a leading tree-ring expert at Lamont-Doherty: "If warming over the tropical Pacific promotes drought over the western U.S.... any trend toward warmer temperatures could lead to a serious long-term increase in aridity over western North America." Martin Hoerling, of the National Oceanic and Atmospheric Administration, thinks that such a process is already under way. He blames the increasing droughtiness of the tropics on a persistent ocean warming in the Pacific that, he says, is "unsurpassed during the twentieth century." The pattern of dryness is beginning to look less like a local, short-term aberration and more like a long-term trend, he says, and he predicts that global warming "may be a harbinger of future severe and extensive droughts."

It won't happen everywhere, of course. Climate models predict that a warmer world will, on average, have more moisture in the atmosphere, and that, in general, the wet places will get wetter and the dry places will get drier. They predict that areas of uplift, where rising air will trigger storm clouds and abundant rain, will see the uplift become more intense. But areas of sinking air, which are the traditional desert lands of the world, will see more-intense sinking and drying. In many parts of the world, this "hy- perweather" is likely to set competing forces against each other. Stronger storms will blow off the oceans, and monsoon-type rains may begin again in some places. But the rain-bearing winds will often be confronted by intensifying arid zones of descending air in the continental interiors. It is not obvious which force will win, and where.

Will the Sahara Desert expand and intensify, as drought theorists argue? Or will North Africa be reclaimed by a revived African monsoon? Megadrought or Garden of Eden? Nobody can answer that question yet. Perhaps the greatest likelihood is that in many places, from the Sahara to the American West and Arabia, there will be more and longer droughts, interspersed with brief but devastating outbreaks of intense storms and floods.

27

SEESAW ACROSS THE OCEAN

How the Sahara Desert greens the Amazon

Two of the world's largest and most fragile ecosystems face each other across the Atlantic. On one side is the Amazon rainforest; on the other the Sahara. They seem to be ecological opposites, and unconnected. The Sahara is rainless and largely empty of vegetation. The Amazon is one of the wettest places on Earth, and certainly the most biologically diverse, with perhaps half of the world's species beneath its canopy. But these two opposites are not so far apart. For one thing, the physical gap is surprisingly small. The Atlantic is narrow near the equator, and the two ecosystems are less than half as far apart as London and New York. For another, many believe they have a surprising symbiosis. Their fates may be intertwined in a rather unexpected way-and one that could have important consequences in the coming decades.

The key to the symbiosis lies in the remote heart of the Sahara, a region called Bodele, in northern Chad. Few people go here. It is littered with unexploded bombs and land mines left behind during Libya's invasion of Chad in the i98os. And it is by far the dustiest place on Earth. Satellite images show year-round dust storms raging across Bodele and entering the atmospheric circulation. According to Richard Washington, of Oxford University, two fifths of the dust in the global atmosphere comes from the Sahara, and half of that comes from Bodele.

Some of this dust stays local. But much of it is carried on the prevailing winds, which cross the desert wastes of Niger, Mali, and Mauritania before heading out across the Atlantic. The red dust clouds can grow almost 2 miles high as they approach America. They cause spectacular sunrises over Miami, before falling in the rains of the Caribbean and the Amazon. And there have been a lot of good sunrises in recent decades. The amount of dust crossing the Atlantic grew fivefold between the wet r 96os and the dry r 98os.

The Sahara dust has a series of unexpected effects on the Americas. According to hurricane forecasters in Florida, during dry, dusty years in the Sahara, there are fewer hurricanes on the other side of the Atlantic. It seems that dust in the air interrupts the critical updrafts of warm, moist air that fuel the storms. Equally surprisingly, desert bacteria caught up in the winds are being blamed for bringing new diseases to Caribbean coral reefs, and even for triggering asthma among Caribbean children.

And there is another important link. Saharan dust storms carry huge amounts of minerals and organic matter that enrich soils widely in the Americas. Bodele dust seems especially valuable. Its dunes are the driedout remains of the bed of the vast Lake Megachad, which covered the central Sahara until its abrupt demise. Most of the dunes are made not of sand or broken rock but of the remains of trillions of diatoms, microscopic freshwater creatures that once lived in huge numbers in the lake. These fragments blow freely in the wind. That's why they make such plentiful dust storms. And they also make great fertilizer. If Bodele had any rain, the diatoms would make rich farmland. Instead, Chad's loss is the Americas' gain, says Hans Joachim Schellnhuber, a German physicist turned Earthsystem scientist, who, as director of Britain's Tyndall Climate Centre, in Norwich, has made a study of the unlikely connection. "Bizarre as it may seem, the arid, barren Sahara fertilizes the Amazon rainforest. This process has been going on for thousands of years, and is one reason why the Amazon basin teems with life."

The two unique habitats are on a kind of seesaw, he says. When the Sahara is dry, as it has been for much of the past quarter century, its dust crosses the Atlantic in huge quantities and fertilizes the Amazon, making the rainforest superabundant. When the Sahara is wet, the dust storms subside and the Amazon goes hungry. That the Sahara seems to have only two basic modes, wet and dry, suggests that there may be two distinct modes in the Amazon, too. The last big change in the Sahara came 5,500 years ago, when the region lurched from wet to dry, probably within a few decades. As yet we know little about how the Amazon changed at that time. But if Schellnhuber is right, the Sahara's loss at that time may have been the Amazon's gain. There may have been a major change for the better in the rainforests.

In the twenty-first century, the seesaw could be on the move again. There are hints that the Sahara may become wetter, says Schellnhuber. And if the wetting turns to greening, and the vegetation feedback kicks in, the whole of North Africa could change dramatically. That would be good news for the Sahara, of course. But it might be bad news for the Amazon, which already seems to be close to its own tipping point, as the climate dries and rainforests give up their carbon. Could a wetter Sahara be the final nail in the Amazon's coffin? Schellnhuber believes it could.

28

TROPICAL HIGH

Why an ice man is rewriting climate history

There are two special things about Lonnie Thompson. First, doctors reckon that he has spent more time on mountains above 20,000 feet than any other lowlander on the planet. And second, in his freezer back home in Columbus, Ohio, he has probably the most detailed physical record anywhere of the climate of planet Earth over the past 20,000 years. Not bad for the sixtyyear-old son of a hick from Gassaway, a tiny railroad town in West Virginia.

Make that three things. Because Thompson is, in a mild-mannered but determined way, a revolutionary in the world of glaciology. For four decades now, climate scientists have been drilling ice cores in the polar regions to find the secrets of climates past. They have found a lot, and they have developed some impressive theories about how the world's climate system is driven from these cold wastelands. But thirty years ago, Thompson, then still a graduate student in the geology of coal with a temporary post drilling ice cores in Antarctica, set out to prove them wrong about the origins of climate.

With his early mentor, the legendary British glaciologist John Mercer, Thompson ignored the poles and began drilling ice cores in glaciers high in the Andes, the Himalayas, and other mountain regions of the tropics. This was unheard of at the time. Finding funding was hard, because nobody had a budget for such work. But in the years since, he has uncovered a new, entirely unexpected world of tropical climate change. And now, after fifty expeditions to five continents, and with 20,000 feet of ice cores stored in his freezer, he believes he is on the path to proving that the true triggers and drivers and Achilles heels and thresholds and tipping points for the world's climate lie in the tropics.

For men like Broecker, this is sacrilege. But although Thompson's case is not yet proven, he has found some unexpected fans. Richard Alley, a career member of the "polar school," is an admirer of the senior from Gassaway. He told me with a smile: "Lonnie is a legend, and he may well turn out to be right." Whether he is right or not, Thompson's ice cores and the data he has painstakingly extracted from them are the lifeblood of an emerging debate between the polar and tropical schools-a debate that might not be happening at all without him.

Thompson is a loner. He has always avoided the big organizations and funding bodies that dominate so much climate science. Sometimes that has been out of necessity; now he sees it as a virtue. It has given him the freedom to do and think things his way. With his researcher wife, Ellen Mosley-Thompson, he set up a small team at the Byrd Polar Research Institute, part of Ohio State University. "We started small and we try to be self-contained," he says. "That makes us flexible. We don't have to stand in line for analysis of cores, or for supplies. And we have our own workshops to make everything."

The Thompsons build their own lightweight drills and photovoltaic generators, because these are the only means of getting the right gear by horseback onto the high slopes of the world's tallest mountains. And they have their own four automatic mass spectrometers, working 24 hours a day 365 days a year to analyze the samples brought back from around the world. Thompson doesn't even trust the big science institutions to look after his ice cores when he's gone. With the prize money that has come his way in recent years, he has created a trust fund to keep the freezers going in perpetuity.

Being independent means he can pack his bags and head around the world on a whim if he thinks there is an ice core to be had. Back in 1997, he took advantage of a brief thaw in diplomatic relations between Moscow and Washington to fly to Franz Josef Land, in the Siberian Arctic. There he extracted a thousand feet of ice from near an old Russian nuclear bomber base, and persuaded the bomber pilots to fly it back to Moscow for him. More recently, after years of stonewalling by the Tanzanian authorities, he took his drilling kit on a tourist flight to Dar es Salaam and smooth-talked his way up Kilimanjaro to extract vital evidence of the demise of its ice cap.

Thompson has spent half a lifetime taking his ice pick, crampons, and drilling gear to the Andes and the Himalayas, Tibet and the Russian Arctic, Alaska and East Africa. Back in Columbus, he has interrogated the ice and the bubbles of air trapped inside for signs of dust, metals, salts, and isotopes of oxygen and carbon to discover not just temperatures and rainfall but the comings and goings of El Nino events, forest fires, droughts, and monsoons.

His first love, he says, is Quelccaya, the first ice cap he scaled in Peru with John Mercer. It is the one he keeps going back to. He can see the whole world evolve there, he says, from the revival of El Ninos in the Pacific around 5,500 years ago to the decades of drought that finished off the pre-Columbian Moche empire; from the first record in the tropics of the little ice age to the recent isotopic signature of global warming. Here and elsewhere across the tropics, he has also found a dust "spike" in the ice that shows that dust storms were sweeping across the tropics 4,200 years ago-evidence, it seems, of a sudden near-global megadrought.

Most intriguing for glaciologists, Thompson's collection of worldwide ice cores has revealed a previously unknown pattern in the formation of glaciers across the tropics. The pattern seems to be independent of the great glaciations that waxed and waned in the polar regions. It seems instead to follow latitude, starting in the Southern Hemisphere close to the Tropic of Capricorn, where he has found evidence that glaciers began to form in Bolivia 25,000 years ago. Then, as if by clockwork, other glaciers began to form and grow farther north. One by one, they started through Peru and Ecuador. Then, 12,000 years ago, a continent to the east but following the same northward trajectory, an ice cap began to form at the summit of Kilimanjaro, on the equator. Skipping north again to the Himalayas, around 8,ooo years ago, glaciers started to grow near the Tropic of Cancer. Across three continents, glacier formation was oblivious of longitude or the equator or anything else. Latitude ruled.

Why? Thompson has tied this extraordinary progression to the precession, the wobble in Earth's orbit that gradually alters the line of latitude where the most intense solar heating occurs. This is the same wobble that sustained the African monsoon over the Sahara when the sun was overhead there in the early Holocene, but snuffed out the rains as the sun moved on. In the mountains of the tropics, glaciers generally started where the sun was fiercest. The sun was most intense over the Tropic of Capricorn 25,000 years ago and then moved north, becoming most intense over the Tropic of Cancer. It appears to have triggered the formation of glaciers all the way.

On the face of it, this seems odd. Why would the harshest sun and hottest temperatures create glaciers? Thompson has a simple explanation. The zone of maximum sun in the tropics is also the zone of maximum rainfall, which in the highest mountains means the zone of maximum snow. Up there, he says, it has always been cold enough for glaciers to form. So temperature is not an issue. What the high valleys have often lacked is moisture to feed the growth of glaciers. The sun brought the moisture, and with it the snow and the glaciers.