America's Slumbering Desert-From the book Six Degrees: Our Future on a Hotter Planet by Mark Lynas, © 2008 National Geographic Society

America's Slumbering Desert

It would be easy to walk right pastthem. Not many hikers pass this way, and those who do are unlikely to give asecond thought to a few old stumps rooted in the riverbed. In any case, thislonely spot, where the West Walker River Canyon is at its narrowest as itplunges down the eastern flanks of California's Sierra Nevada, is not a placeto linger?the area is notorious for sudden downpours and flash floods. Theriver runs almost the width of the entire gorge, and there's no place to climbto safety if the heavens open.

But these stumps have a story to tell. Dead trees can talk,in a way. An astute hiker or an observant angler would be puzzled: What arethey doing in a riverbed, a place now treeless because of the constant flowingwater? Investigated by scientists in the early 1990s, the tree stumps werefound to be Jeffrey pines?a common enough species for the area, but one thatcertainly doesn't normally root in rivers. What's more, these trees were old.Very old. Tissue samples revealed that the stumps dated from medieval times,and grew during two specific periods, centered on a.d. 1112and 1350.

The mystery deepened when similar old stumps were revealedin Mono Lake,a large saltwater body about 60 miles south of WalkerRiver, near the state border with Nevada. It's aspectacular location, famous for broad skies and sunsets, with little tointerrupt the gently rolling arid landscape other than a few extinct volcanoes.The Mono Laketree stumps belonged not just to pines but also to other native species likecottonwoods and sagebrush, all rooted far below current-day natural lake levelsand only revealed thanks to water diversion projects that supply faraway Los Angeles. Again,carbon dating revealed the same two time intervals as for the Walker Rivertrees. Clearly, something significant had happened back in medieval times.

More evidence came fromdeeper in the mountains, hidden in two locations today famous for their giantsequoia groves?Yosemite and Sequoia and Kings CanyonNational Parks. Theseenormous trees, which in terms of total wood volume stand as the largest livingorganisms on Earth, are also among the oldest. Some living trees are up to3,000 years old. And because each annual growth cycle leaves a clear ring,these monumental plants are also an excellent record of past climate. Over adecade ago, scientists sampling wood from dead giant sequoias noticed old firescars on the edges of some of their rings. These scars were especially frequentduring this same medieval period?between a.d. 1000 and 1300?that the old trees in West Walker River and MonoLake were growing.Wildfires had raged in both national parks twice as frequently as before, andthere can only be one plausible explanation: The woods were tinder dry.

Raging wildfires, dry rivers and lakes?the pieces of thejigsaw were beginning to make sense. The area we now call California had in medieval times been hit bya megadrought, lasting at different periods for several decades, and alteringboth landscape and ecosystems on a scale that dwarfs today's drought episodes.But just how geographically widespread was this event? Evidence from anotherlake, far away on the Great Plains of North Dakota, provides a partial answer. Moon Lake,like Mono Lakein California,is a closed basin, making it saline. Salinity fluctuates with the climate: Insequences of wet years, more fresh water ends up in the lake and salt levels godown. The converse is also true: In dry years, more water evaporates, leaving amore concentrated salty brine behind. Canadian scientists have nowreconstructed long-term records of MoonLake's saltiness bysampling the remains of tiny algae called diatoms?whose type and numberfluctuate with salinity levels?from old lake sediments. Lo and behold, backbefore a.d. 1200, a series of epic droughts had swept theGreat Plains, the return of which, thescientists agreed, "would be devastating."

An insight into the devastating nature of such a drought wasgained by a team of biologists working in northern YellowstoneNational Park in Wyoming,almost 1,000 miles to the southwest of Moon Lake.They drilled into sediments spilled out by rivers, only to discover a peak inmuddy debris flows?the product of flash floods?about 750 years ago. These flashfloods had poured off mountainsides denuded of forest cover by frequent fires:So, rather oddly, these flood-debris flows are actually a classic sign ofdrought. It appeared that the whole of the western United States had been struck atthe same time.

The effect on NativeAmerican populations in this pre-Columbian era was devastating. Wholecivilizations collapsed, beginning in the ChacoCanyon area of modern-day
New Mexico.One of the most advanced societies on the continent at their peak, the PuebloIndian inhabitants of Chaco Canyon erected the largest stone building on theNorth American continent before the European invasion, a "great house" fourstories high, with over 600 individual rooms?much of it still standing today.Yet when the big drought came in a.d. 1130,they were vulnerable; population growth had already diminished the society'secological base through the overuse of forests and agricultural land. Mostpeople died, while the survivors went on to eke out a living in easily defendedsites on the tops of steep cliffs. Several locations show evidence of violentconflict, including skulls with cut marks from scalping, skeletons with arrowheadsinside the body cavity, and teeth marks from cannibalism.

In fact, the whole worldsaw a changing climate in medieval times. The era is commonly termed theMedieval Warm Period, a time when?so the oft-told story goes?the Vikingscolonized Greenland and vineyards flourished in the north of England.Temperatures in the North American interior may have been one to two degreesCelsius warmer than today, but the idea of a significantly warmer world in theMiddle Ages is actually false. Recent research piecing together "proxy data"evidence from corals, ice cores, and tree rings across the Northern Hemispheredemonstrates a much more complicated picture, with the tropics even slightlycooler than now and different regions warming and then cooling at differenttimes.

However small the globalshift, the evidence is now overwhelming that what the western United Statessuffered during this period was not a short-term rainfall deficit but afull-scale megadrought lasting many decades at least. As recently as 2007, U.S. scientists reported tree-ring studiesreconstructing medieval flows in the Colorado River at Lees Ferry, Arizona, showing thatthe river lost 15 percent of its water during a major drought during themid-1100s. For 60 years at a time, the river saw nothing but low flows?none ofthe floods that normally course down the Coloradoarrived to break the dry spell. Indeed, the remarkable coincidence of dateswith evidence from New Mexicosuggests that this was the very same drought that finished off the Chaco CanyonIndians.

To see the worst that even such a small change in climatecan do, consider that most undramatic of places: Nebraska. This isn't a state that is high upon most tourists' must-see lists. "Hell, I thought I was dead, too. Turns out Iwas just in Nebraska,"deadpans Gene Hackman in the film Unforgiven. A dreary expanseof impossibly flat plains, Nebraskahas as its main claim to fame the fact that it is the only American state tohave a unicameral legislature. Nebraska isalso apparently where the Old West begins; local legend in the state capital Lincoln insists that theWest begins precisely at the intersection of 13th and O Streets, a spot markedby a redbrick star.

But perhaps the mostimportant Nebraskafact is that it sits in the middle of one of the most productive agriculturalsystems on Earth. Beef and corn dominate the economy, and the Sand Hills regionin central Nebraska sports some of the mostsuccessful cattle-ranching areas in the entire United States.

To the casual visitor, the Sand Hills look green and grassy,and in pre-European times they supported tremendous herds of bison, hence theirhigh productivity for modern-day beef. But, as their name suggests, scratchdown a few inches and the shallow soil quickly gives way to something rathermore ominous: sand. These innocuous-looking hills were once a desert, part ofan immense system of sand dunes that spread across thousands of miles of theGreat Plains, from Texas and Oklahoma in the south, right through Kansas,Colorado, Wyoming, North and South Dakota, to as far north as the Canadianprairie states of Saskatchewan and Manitoba. These sand dune systems arecurrently "stabilized": covered by a protective layer of vegetation, so noteven the strongest winds can shift them. But during the Medieval Warm Period,when temperatures in the Great Plains region may have been only slightly warmerthan now, these deserts came alive?and began to march across a fertilelandscape which today is a crucial food basket for humanity. This historicalevidence indeed suggests that only tiny changes in temperature could tip thiswhole region back into a hyper-arid state.

People who remember the1930s Dust Bowl might think they have seen the worst drought nature can offer.In the toughest Dust Bowl years, between 1934 and 1940, millions of acres of Great Plains topsoil blew away in colossal dust storms.One, in May 1934, reached all the way to Chicago,dumping red snow on New England. Hundreds ofthousands of people, including 85 percent of Oklahoma's entire population, left the landand trekked west. All this upheaval took only an average 25 percent reductionin rainfall?enough for plowed farmland to blow away, but the giant dunes stayedput. What awoke the dunes from their long slumber 6,000 years ago was droughton an altogether different scale?with 40 percent less rainfall, sustained overdecades rather than just years.

In a world that is less than a degree warmer overall, thewestern United States could once again be plagued by perennial droughts?devastatingagriculture and driving out human inhabitants on a scale far larger than the1930s calamity. Although heavier irrigation might stave off the worst for awhile, many of the largest aquifers of fossil water are already overexploitedby industrialized agriculture and will not survive for long. As powerful dust-and sandstorms turn day into night across thousands of miles of former prairie,farmsteads, roads, and even entire towns will find themselves engulfed byblowing sand. New dunes will rise up in places where cattle once grazed andfields of corn once grew. For farmers, there may be little choice other than toabandon agriculture completely over millions of square miles of what was oncehighly productive agricultural land. Food prices internationally would rise,particularly if serious droughts hit other areas simultaneously. And althoughmore southerly parts of the UnitedStates are expected to get wetter as theNorth American monsoon intensifies, residents here may not welcome an influx ofseveral million new people.

Farther east, however, agriculture may actually benefit fromwarmer temperatures and higher rainfall. Rather as Californiaoffered sanctuary of a sort to displaced Okies during the Dust Bowl, theMidwest and Great Lakes areas will need toprovide jobs and sustenance to those who can no longer scratch a living fromthe sandy soils far out west, once the rains stop falling and the desert windsbegin to blow.

Already the Day after Tomorrow?

Just as farmers on the High Plainsof North America are watching their fields and grasslands blowing away in therelentless heat, their kinfolk across the Atlanticmay be grappling with another problem: extreme cold. One of the mostcounterintuitive projected impacts of global warming is the possible plungingof temperatures throughout northwest Europe asthe warm Atlantic current popularly known as the Gulf Stream stutters and slowsdown. This is the scenario fictionalized in an exaggerated form by theHollywood disaster movie The Day After Tomorrow,where a collapse in the Atlantic current triggers a new ice age, flash-freezingNew York and London (although the good guy still gets the girl). Real-worldscientists were quick to lambast the film for flouting the laws ofthermodynamics, but they also acknowledged that the reality of a slowdown inthe North Atlantic Ocean current may still be pretty scary, especially forthose who live in a part of the world that is used to a mild maritime climatefar out of keeping with its high northern latitude.

A short technical aside is required here. Only a small partof the great current that delivers warm water into the North Atlantic isactually the real Gulf Stream. It is, as itsname suggests, a stream of warm subtropical water heading northeast out of the Gulf of Mexico, which eventually becomes part of the muchlarger system of currents known to scientists as the Atlantic MeridionalOverturning Circulation. The MOC is partly driven by the cooling and sinking ofwater at high latitudes off the coast of Greenland and Norway, wherefreezing Arctic air lowers its temperature and squeezes fresh water out as seaice, leaving behind a heavy salty brine that quickly sinks to the bottom of theocean. From there it begins a return journey south?eventually surfacing (1,200years later) in the Pacific. Scientists have long feared that a freshening andwarming of the Norwegian and Greenland seas?due to higher rainfall, runoff frommelting land glaciers, and the disappearance of sea ice?could stop this watersinking, and shut down the great ocean conveyor; hence the famous "Shutdown ofthe Gulf Stream" scenarios familiar from newspaper headlines and the Hollywoodmovie.

Far-fetched it may seem,but Atlantic circulation shutdown has always been more than just a theory. Ithas happened before. At the end of the last ice age, 12,000 years ago, just asthe world was warming up, temperatures suddenly plunged for over a thousandyears. Glaciers expanded again, and newly established forests gave way oncemore to chilly tundra. The period is named the Younger Dryas, after anarctic-alpine flowering herb, Dryas octopetala, whose pollen is ubiquitous in peaty sedimentlayers dating from the time. In Norwaytemperatures were seven to nine degrees Celsius lower than today, and evensouthern Europe suffered a reversal tonear-glacial conditions. On the other side of the Atlantic, cooling alsooccurred, and there is evidence of rapid climate change from as far afield asSouth America and NewZealand.

The culprit seems to be the sudden shutting off of theAtlantic circulation owing to the bursting of a natural dam holding back Lake Agassiz,a gigantic meltwater lake that had pooled up behind the retreating NorthAmerican ice sheets. When the dam broke, an enormous surge of water (the lake'svolume was equivalent to seven times today's Great Lakes) is thought to havepoured through Hudson Bay and out into the Atlantic.This freshwater surge diluted the North Atlanticseas and stopped them being salty enough to sink, interrupting the deep-oceancurrent and triggering climatic destabilization across the world.

Obviously today there are no gigantic ice lakes waiting toflood into the North Atlantic, but global warming could still interrupt theformation of deep water by melting sea ice and causing greater freshwaterrunoff from Siberian rivers. Despite the rapidly melting ice cap, however, formany years there was no evidence that changes in the Atlantic MOC were actuallyhappening, and many oceanographers had begun to pooh-pooh the theory. That wasuntil the R.S.S. Discovery, a scientificresearch vessel owned by the British government, began a routine cruise acrossthe Atlantic in 2004. The ship's onboardscientific team set themselves the task of sampling seawater at various depthson a line drawn between the Canary Islands in the east and Florida in the west, aiming to repeatsimilar measurements taken in 1957, 1981, 1992, and 1998. They had not expectedto discover anything terribly exciting; in fact, the team leader, ProfessorHarry Bryden, confided to one journalist: "In 1998 we saw only very smallchanges. I was about to give up on the problem."

But 2004 was different.Bryden and his colleagues found that less warm water was flowing north at thesurface and less cold water was flowing south at depth. Overall, the Atlanticcirculation had dropped by 30 percent, equivalent to the loss of six milliontonnes (6,720,000 tons) of water flow per second. No wonder Professor Brydenadmitted that he was "surprised." Suddenly the slowing down of the great Atlanticcurrent system was no longer just a hypothesis postulated for the distantfuture. It was already happening.

The media reaction wasinstantaneous. "Current that warms Europe weakening," warned CNN, while NPR's All Things Considered show led with "Atlantic Ocean's heatengine chills down." In Europe, the responsewas one of understandable concern. "Alarm over dramatic weakening of GulfStream," reported the UnitedKingdom's Guardian newspaper onDecember 1, 2005. "Global warming will bring cooler climate for the UK" was the Telegraph'stake on the same story. A couple of paragraphs down, the paper reported oneexpert as confirming that "an average temperature drop of a degree or twowithin decades would herald more extreme winters."

Older readers would haveshuddered at the thought of a return to winters as bitter as that of 1962-63,when the United Kingdom was blanketed in snow for more than three months, andtemperatures hit a low of ?16?C (3.2?F) in southern England. In places the seafroze, and ice floes appeared in the River Thames at London'sTower Bridge. That season was about 2.7?Ccolder than average?almost exactly the temperature drop predicted for London in one modelingstudy investigating the possible result of a 50 percent drop in the warmAtlantic current. Was Europe's new ice agejust around the corner?

Apparently not. Almost exactly a year later, and with muchless fanfare, Science magazine reportedthat "a closer look at the Atlantic Ocean'scurrents has confirmed what many oceanographers suspected all along: there's nosign that the ocean's heat-laden 'conveyor' is slowing." Instead of just thesnapshot data generated by a few irregular ship cruises, 19 permanentinstrument-laden sensors had now been stretched across the Atlantic betweenWest Africa and the Bahamas?andthey were able to deliver a much more consistent picture. A year of continuousmonitoring, Harry Bryden now reported to a conference in Birmingham, showed that his original 30percent decline was just a part of random natural variability after all, thesort of thing that happens constantly from one year to the next.

This result was a triumph for the modelers, most of whom hadfor years been pouring cold water on the European ice age theory. They agreedthat huge volumes of fresh water would need to surge into the North Atlantic inorder to shut off the Gulf Stream?far more than was currently being generatedby melt from Greenland or higher precipitation in Siberia.Rather than plunging overnight, the ocean circulation might decline by astately 25 to 30 percent or so, but only after at least a hundred years ofsustained greenhouse gas emissions. Even then, it wouldn't cool Europe?it would simply moderate the otherwise rapid risein temperatures.

As the Intergovernmental Panel on Climate Change (IPCC)concluded in 2007, "It is very unlikely that the MOC will undergo a largeabrupt transition during the course of the 21st century." Although all modelsshowed some weakening by 2100, none of the ones assessed by the IPCC supportedthe collapse scenario. And even with this MOC slowdown, the IPCC reported that"there is still warming of surface temperatures around the North Atlantic Oceanand Europe due to the much larger effects ofthe increase in greenhouse gases." The IPCC's judgment was final: There wouldbe no new ice age for Europe.

Africa's Shining Mountain

Theamateur adventurer Vince Keipper had waited years for this day. Nearing thesummit of Kilimanjaro, the highest point on the African continent, Keipper andhis group were looking forward to panoramic views of the surrounding Kenyan andTanzanian plains. They had climbed through the steep and treacherous WesternBreach and past the towering ice cliffs of the Furtwängler Glacier. The weatherwas perfect, with only a few clouds far beneath. Then, not far from the top ofthe 5,895-meter (19,341 ft) peak, a loud rumbling sound from behind thembrought the group to a sudden halt. "We turned around to see the ice masscollapse with a roar," remembered Keipper. "A section of the glacier crumbledin the middle, and chunks of ice as big as rooms spilled out on the craterfloor."

Keipper and his group knew they had had a lucky escape: Theymight have been buried had the collapse happened only a few hours earlier. Theyalso knew that the event they had just witnessed had a powerful symbolicresonance: Right in front of their eyes, the highest peak in Africawas melting.

Kilimanjaro has become something of a poster child for theinternational climate-change campaign. The Swahili words kilimaand njarotranslate as "shining mountain," testament to the power of this massive volcanoto inspire awe in onlookers through the ages. A recent aerial photo of thecrater, with little more than a few ice fragments encrusting its dark sides,was the centerpiece for a touring global-warming photography exhibitionsponsored by the British Council in 2005. During the 2001 UN climate-changeconference in Marrakech, Morocco, Greenpeace sent a team toKilimanjaro to hold a press conference by video link from beside one of themountain's disappearing glaciers. Kilimanjaro's international celebrity statushas also attracted the attention of climate-change detractors, who suggest thatdeforestation on the mountain's lower slopes is more to blame for glacialretreat than global warming is.

None of the contrarian rhetoric cuts any ice, so to speak,with Lonnie Thompson, a glaciologist at OhioState Universityand a man who is deservedly one of America's most celebrated naturalscientists. Thompson pioneered the drilling of ice cores in inaccessiblemountain regions, bringing back ice tens of thousands of years old fromglaciated peaks as remote and far apart as Peru's Nevado Huascarán and Tibet'sDasuopu, often pushing himself to the edge of human endurance in the process.In 1993 Thompson and his drilling team camped for 53 days at 6,000 meters(19,685 ft) between the two peaks of Huascarán, perhaps setting a world recordfor high-altitude living. (I stayed there for one night in 2002, one of themost freezing, wind-blasted, and wretched nights of my life.) At one point agale blew Thompson's tent, with him inside, toward a precipice?until he jammedhis ice ax through the floor. "I don't understand," he once remarked, "whyanyone would want to climb a mountain for fun."

As Thompson was one ofthe first to recognize, this mountain ice contains a unique record of climatevariations down the ages: preserved in layers of dust, isotopes of oxygen, andtiny bubbles of gas trapped within the frozen layers of water. Once carrieddown in freezer boxes and analyzed in the laboratory, these icy signaturestrace everything from droughts to volcanic eruptions from decades and centuriespast. They also tell a story about past temperature changes: The two isotopesof oxygen, ¹⁶O and ¹⁸O (which have different atomicweights due to the presence of two more neutrons in the latter's nucleus) varyin abundance with water temperature, so their proportions in ice cores are agood "proxy record" of ancient climates.

Thompson and his team also drilled on three ofKiliman-jaro's remaining glaciated areas, and in October 2002 concluded that 80percent of the mountain's ice had already melted during the past century. Thenews made international headlines, along with Thompson's prediction that therest of the ice would be gone by between 2015 and 2020. As he readily admitted,this prediction was not based on complex computer modeling or any otheradvanced techniques. "In 1912 there were 12.1 square kilometers of ice on themountain," he told journalists from CNN. "When we photographed the mountain inFebruary of 2000, we were down to 2.2 square kilometers. If you look at thearea of decrease, it's linear. And you just project that into the future.Sometime around 2015 the ice will disappear off Kilimanjaro."

If there was an urgency in Thompson's voice, this wasbecause he knew that recent melting had already begun to destroy the uniquerecord of past climate preserved in Kilimanjaro's glaciers. In their analysisof dust layers in the ice, the scientific team found evidence of a marked300-year drought 4,000 years ago; a drying so severe that it has been linked tothe collapse of several Old World civilizations across North Africa and theMiddle East. The ice also indicated much wetter conditions even longer ago,when huge lakes washed over what is now Africa's dry Sahel.Close to the surface Thompson's team discovered ice containing a layer of theradionuclide chlorine-36, fallout from the American "Ivy" thermonuclear bombtest on Eniwetok Atoll in 1952. With this precise time control, the scientistscould tell that ice that would have preserved a record of climate fluctuationssince the 1960s had already melted away.

Moreover, the oldest iceat the base of the cores proved to be over 11,000 years old, showing that at notime since the last glacial epoch has the peak of Kilimanjarobeen free of ice. This discovery made Thompson's ice cores even more valuable,for the simple reason that within as little as ten years the sawn-up circularcores in Ohio State University's walk-in freezer will be the only Kilimanjaroice left anywhere in the world. With this in mind, Thompson and his team havealready decided that some of the ice will be kept intact for future generationsof scientists to dissect with new technologies, possibly unlocking climaticsecrets still undreamed of today.

The efforts of climate-change detractors to suggestthat there is something special about the disappearance of Kilimanjaro'sglaciers are undermined by similar changes taking place in mountain rangesright across the world, not least in the RwenzoriMountains of Uganda, nearlya thousand kilometers to the northwest. In this remote region, where Ugandaborders the Democratic Republic of the Congo, the fabled "Mountains of theMoon" generate such heavy rainfall (about 5 meters, or 16.4 feet, per year)that the cloud-shrouded peaks are visible on only a few days out of every year,and form the main headwaters of the River Nile. At the top of the highest peak,the 5,109-meter (16,762 ft) MountStanley (named after theexplorer, who passed by in 1887), ice and snow deny the summit to all but themost determined mountaineers. Yet, as at Kilimanjaro, glacial retreat in theRwenzoris has been profound: The three highest peaks have lost half theirglacial area since 1987, and all the glaciers are expected to be gone withinthe next two decades.

Elsewhere in the world, disappearing mountain glaciers posea major threat to downstream water supplies. But Kilimanjaro's ice cap is sosmall that its final disappearance will make little difference to the two majorrivers?the Pangani and the Galana?that rise on its flanks. Instead, the crucialwater link for Kilimanjaro is not the glaciers but the forests. The montaneforest belt at elevations between 1,600 and 3,100 meters provides 96 percent ofthe water coming from the mountain. This lush tangle of trees, ferns, and shrubsnot only captures Kilimanjaro's torrential rainfall like a giant sponge butalso traps moisture from the clouds that drape themselves almost permanentlyaround the mountain's middle slopes. Much of this water drains undergroundthrough porous volcanic ash and lavas, and emerges in water holes?vital forlocal people as well as for wild animals?far away on the savanna plains.

So is Kilimanjaro's water-generating capacity safe fromglobal warming? Not quite: Rising temperatures and diminishing rainfall increasethe risk of fires, which have already begun to consume the upper reaches ofmontane forest. By the time the glaciers have disappeared, so will the higherforests, depriving downstream rivers of 15 million cubic meters of runoff everyyear, according to one estimate. In contrast, the loss of glacial water inputwill likely add up to less than one million cubic meters annually: significant,but not catastrophic. The diminishing water supply will affect everything fromfish stocks to hydroelectric production downriver in poverty-stricken Tanzania. Muchof the mountain's world-famous biodiversity (Kilimanjaro hosts 24 differentspecies of antelope alone) will also be threatened by the weather changes.

As the snows disappear, so will much of the wildlife and theverdant forests that tourists currently trek through on their arduous journeyto the roof of the African continent.

From the book Six Degrees: Our Future on a Hotter Planet by Mark Lynas, © 2008 National Geographic Society