by Curt Stager
We have fewer geological records of interglacial warm periods from the tropics than we do for the temperate zone, but most do indicate wetter conditions during those hotter times. Sediment records from the eastern Mediterranean Sea show that the Nile River poured huge floods of buoyant freshwater atop the salty waves during the Eemian and again during the early Holocene. During that last warm spell, the Sahara was a game-rich savanna and Lake Chad, now a relatively shrunken puddle of brine, was a huge inland sea of freshwater. Cores from submerged deposits off the coast of Mauritania clearly document the end of that warm Holocene wet phase; about 5,000 years ago, easterly winds began to sweep more and more desert dust into the ocean there, signaling the desiccation of northern Africa.
More recent work, however, has revealed some striking exceptions to the warm-wet, cool-dry rule. My own research (funded by American readers’ tax dollars, thank you, through the National Science Foundation) has shown that although Lake Victoria shrank during major coolings of the distant past, it rose rather than fell during the chilliest phases of the so-called Little Ice Age 600 to 200 years ago. Although most African lakes did not follow that unusual pattern, it has also been recorded in sediment records from Lakes Naivasha and Challa in Kenya and Tanzania, respectively, and it warns us that tropical rainfall conditions can vary in unexpected ways, both geographically and over time.
Does this review of history tell us anything useful about the climatic future of the tropics? Yes, but not as much as we would like. Whatever Anthropocene warming may do to low-latitude climates in the future, its most important effects will probably involve precipitation more than temperature. But recall that the warm-wet, cool-dry pattern doesn’t always hold true, and not all regions respond to environmental changes in the same way. Furthermore, many of those long-term shifts of the past resulted from natural cycles in the orbit and orientation of Earth, which were fundamentally different from the intensification of the greenhouse effect that we’re currently causing. Nonetheless, we can be fairly certain that many future changes will be linked to the primary weather system of the lower latitudes, a meandering belt of clouds and rainstorms that climatologists call the intertropical convergence zone, or ITCZ.
You can clearly see its trademark—or rather, its watermark—in satellite images of Earth. All along the equator, the land turns green wherever the ITCZ makes regular visits during annual rainy seasons. This is where you find the great rain forests and rivers of the Amazon, Congo, and monsoonal Asia and Australia. Farther poleward along either side of those equatorial green belts the colors fade into what seem, by comparison, to be dust-colored dead zones. These are among the world’s largest dry places; the Sahara and Namib and Kalahari deserts, the Arabian Peninsula, and the brick red outback of Australia.
The ITCZ runs on solar energy, and some call the massive atmospheric machinery that drives it the climatic “heat engine” of the planet. The mechanistic metaphor is appropriate because enormous rising and falling loops of air—called Hadley cells—spin like twin sets of interlocking cogwheels as the sun’s most direct rays cook the tropics. Intense solar heating warms and expands the overlying air so it floats upward in the churning central zone between the cogs, the ITCZ proper. There it condenses into clouds and spreads against the base of the stratosphere like smoke against a ceiling. Then the cooled air sinks back down at higher, drier latitudes from whence it pushes ground-hugging trade winds back to the central rising zone. That cyclic motion links convective rains, cloudless deserts, and reliable sailor-friendly winds into a single cohesive system that has dominated tropical climates through the ages.
The two tropical boundary lines rest at 23.5 latitudinal degrees north and south of the equator. Every June the North Pole leans most steeply inward at the sun, and that makes the most direct, high-energy light strike the Tropic of Cancer near the latitude of Calcutta, Aswan, and Havana. Stand anywhere along that line at midday in late June and the sun will beat straight down on your head. In December, when the Southern Hemisphere’s summer begins, a similar leaning of the South Pole makes direct sunlight strike the southerly Tropic of Capricorn near the latitude of Rio, Windhoek, and Alice Springs.
Wherever the zone of vertical rays wanders between those parallel tropic lines, the ITCZ follows. A creamy band of clouds sweeps back and forth with each passing year, and as it passes overhead, tropical residents enjoy their seasonal doses of rain. In this manner, the drumbeats of thunderstorms drive the seasonal rhythms of life in some of the world’s most densely populated regions, from the summer monsoon realms of India and Pakistan to the steep hillside farms and forests of Rwanda and Burundi.
If both common sense and many global climate models are correct, then future warming will drive the heat-sensitive Hadley cogwheels more vigorously and spread the inner zone of ITCZ rains over a wider range of latitudes. At the same time, warming oceans will evaporate more water vapor into the system, so more rain could fall from the rising, condensing columns of air within the ITCZ. This is a recipe for more abundant and more extensive precipitation in the tropical monsoon regions, and it helps to explain why most models predict twenty-first-century wetting in southern Asia and most of equatorial Africa and South America.
Sites that lie at higher latitudes near the outer boundaries of the tropics, though, might face the opposite situation. Where sinking air on the descending flanks of the Hadley cells now inhibits cloud formation, some dry regions could become even more arid. To put it simply; global warming is likely to intensify whatever is currently going on in the tropical climate system. But as tropical circulation intensifies, both wet and dry regions may also expand their ranges poleward, and that could lead to both negative and positive changes in the transitional zones. In the Sahara, for example, the most harm to people and ecosystems might occur along its northernmost margin, where fiercer droughts could reduce rainfall in what are now the somewhat less arid, more densely settled locales of the Mediterranean coast. Along the desert’s midline and southern edges, however, broadening and intensification of the ITCZ rain belt could make life easier. Latitudinal shifts of this sort were important features of the Eemian and PETM-scale warmings of the distant past and are already being reported for some of the prevailing wind tracks in the tropics and the southern temprate zone, so it would be reasonable to expect more of the same in the Anthropocene future, too.
One of the tropical regions that is most likely to face serious water problems in the near future is the long, narrow, hyperarid coastal strip of land between the eastern Pacific and the rugged crest of the Andes. Just a few inches of rainfall on it in a typical year, and yet more than half of the population of Peru manages to live there.
When I first visited the Peruvian desert in 2006, it was in the company of Dan Sandweiss, an archaeologist at the University of Maine’s Climate Change Institute where I hold an adjunct research position. His research had shown that sporadic wet periods strongly influenced local cultures for thousands of years, and on that trip we were traveling north to the Sechura region to study geological evidence of those floods. But even more important to coastal Peruvians than the rain pulses have been the rivers that drain the high peaks farther inland.
As we rode a bus northward along the desolate coast from Lima to Piura, Sandweiss explained what I was seeing in that Mars-like realm of scorched black rocks, rusty gravel, and scalloped, dun-colored dunes. “See the faint greenish tinge on those steep slopes overlooking the Pacific? That’s where fog blows ashore and keeps a few plants alive. Everywhere else, it’s just barren ground.” To me, the scene evoked the earliest days of terrestrial life, when simple vegetation was just beginning to colonize formerly lifeless lands along the edges of the oceans. I wasn’t surprised when he added, “NASA research teams sometimes use the Peruvian desert to represent a terrestrial version of Mars.”
“Now look up ahead,” he continued. The black ribbon before us dipped steeply downward into a narrow, wedge-shaped valley whose wide mouth pressed against the flat bl
ue face of the Pacific. “See how lush it is down there? That’s where the Rio Santa flows in from the mountains. You’ll see this same pattern repeated from valley to valley all the way up the coast. The rivers keep towns and farms alive here, and it’s been this way for centuries. If you want to find archaeological sites, these are the places you want to explore.”
As our bus crossed the valley floor, the crumbling remnants of precolonial mounds shared common ground with bustling twenty-first-century settlements. But most surprising to me were the new dry-country farms. For mile upon verdant mile, sprinklers rained precious moisture onto paper-thin veneers of asparagus, artichokes, and other cash crops. With nothing but desert sands beneath those shallow root systems, the fields were more like hydroponic gardens than dark-earth plots.
“These kinds of farms are appearing all over the place now with the help of new irrigation from the rivers and a recent rise in meltwater flow from the high country. Most of what you see here didn’t exist just a couple of years ago. It’s a huge boost to Peru’s economy, but it all depends on reliable supplies of river water.” And that’s the worrisome part. As the world warms, glacier-fed rivers will be anything but reliable.
Peruvian glaciers, which represent roughly 70 percent of all tropical ice, are like trust funds. They collect rich windfalls of snow in narrow seasonal windows and then dole some of it back out in steadier dividends throughout the year. In some watersheds, up to half of all stream discharge during the annual dry seasons stems from melting ice and snowfields. In that manner they have maintained lifelines of river water to the arid lowlands for centuries, and roughly three-quarters of Peru’s electricity is now generated by hydro dams that depend on such flow. But modern climate change is raiding the principal on those reserves of high-altitude ice.
Many Andean glaciers are now dispensing more water than they take in, and they are visibly shrinking. Total ice cover in Peru has decreased by a quarter since 1930, and some of the smaller glaciers, such as Ecuador’s Cotacachi, have vanished altogether. Increasingly worried Peruvians watch local ice streams shrink farther and farther back into the mountains as if they were white fuses sizzling upward. When the last tendrils of ice disappear among the rugged spires of the Andean skyline, an environmental catastrophe could explode downstream.
Life requires water, and precious little of it falls on western Peru, which lies in the rain shadow of the Andes. Peru has faced great difficulties, from the Incan and Spanish conquests of the last millennium to more recent social unrest, and only now have intensive desert agriculture and hydro power emerged to bring hope of more prosperity by harnessing glacier-fed rivers. It seems a cruel twist of fate that those new industries are already under threat from glacial retreat.
As our bus crossed the razor’s edge between green fields and barren desert and began to climb back out of the valley, I asked Sandweiss what he foresees happening here. “Well,” he began with a rueful look, “there’s talk of building reservoirs in the canyons, or even of tunneling through the mountains to channel water in from the wetter Amazon slopes. But this is earthquake territory, so it’s not yet clear how to maintain a dam or a water tunnel for very long without putting people downstream at risk.”
And what if Peru also becomes even drier as well as warmer? Fortunately, most computer models envision a generally wetter future for the inner tropics as Hadley circulation intensifies. As one might therefore expect, some of the northern mountains are already experiencing a wetting trend, but for some reason, the majority of the Peruvian Andes have been drying in recent years. A regional turnaround to more generous rainfall is predicted by midcentury in most simulations, but the coming decades might still present severe challenges to people and ecosystems in today’s currently drying zones.
In July 2009, I returned to Peru in the company of Kurt Rademaker—a Sandweiss graduate student—and four other promising young geoscientists. This time, the focus was Coropuna, a 21,000-foot (6,425-m) lump of ice-caked volcanic rock that towers over a remote highland wilderness of brown, rubble-strewn plateaus and canyons. Several years earlier and just a short distance from our base camp at 14,000 feet, Rademaker had found a beautifully crafted arrow point flaked from pink coastal chalcedony, proving that native peoples had hiked up there thousands of years ago. But what induced them to go to all that trouble?
One would have been hard-pressed to make much of a living on the flanks of Coropuna in the distant past. For starters, one would need to acclimatize; a lungful there yields about half the oxygen that you’d inhale at sea level, judging from the crumpled state of a plastic water bottle that I emptied and closed at our high camp so I could later watch the maritime air pressure flatten it. There are no trees for shelter and little ground cover; the dominant plant there is yareta (Azorella compacta), which looks like a green lump of boulder coral. There would be few wild animals to hunt apart from the rock-hopping vizcacha, which resemble long-tailed rabbits, but are more closely related to chinchillas, plus a few sleek and elusive vicunas (wild cousins of llamas and alpacas). And the local streams freeze nightly despite their close proximity to the equator. All in all, this is a glorious setting for a scientific expedition, but it can also be a challenging place to call home.
A major key to survival in that high country, as in the lowlands, is water, and I was surprised to see so many sky-tinted streams and pools gleaming on the floors of otherwise desolate valleys and depressions, even though the main December-to-March wet season was long since past. Almost invariably, the blue was cupped in moist carpets of green, the unique upland wetlands known as “bofedales.” Neither mossy bogs nor grassy marshes, Andean bofedales consist of dense mats of Distichia, a strange, low-lying plant whose close-packed, spiky green stems form tough, ruglike versions of the dry-ground yareta. And atop most of those wet bofedales wander the other centerpieces of pastoral life in these highlands—grazing herds of domestic alpacas.
“It’s hard to imagine living up here without these bofedales,” Kurt mused as we drove a dirt track along the margin of the broad Pocuncho wetland. Alpaca herders in brightly colored native garb waved as we passed. “These people have a tough life up here, but they manage to make a living by selling the wool and meat of their animals in the lowlands. Their whole lifestyle depends on these wet areas, and the first folks to move up here after the last ice age probably depended on the bofedales as a place to hunt vicunas.” Whether or not the bofedal wetlands actually did exist in those earlier times was what we aimed to discover by coring and radiocarbon dating the local wetland deposits.
University of Maine graduate student Kurt Rademaker coring a bofedal wetland near Coropuna, Peru. Curt Stager
As I write these words the results of Rademaker’s study are not all in yet, but our preliminary dating of one thick exposure of bofedal peat layers on the flanks of Coropuna shows that the deposits are indeed thousands of years old. In any case, the journey left us with a host of other questions and observations relevant to the story of climate change in the tropics.
Matt Schmitz, an undergraduate student at Pacific Lutheran University who accompanied us to Coropuna, interviewed local herders and found that concerns about climate change have reached even this remote corner of the world. “The people say that they’ve watched the ice retreat on the mountain,” he reported after a long day in the field. “And they’re noticing less rain and snow during the wet season and less water in the bofedales.” Such observations fit those described in the scientific literature as well; Coropuna has lost a quarter of its glaciated area since 1960 as precipitation in southern Peru has fallen off.
But the retreat of mountain ice, in this case, has little to do with freshets of meltwater pouring off the slopes; few of them actually exist. High ice caps are composite beasts whose upper and lower reaches respond differently to weather, and most of Coropuna’s ice and snow lies well above the freeze-thaw elevation where it remains too cold to melt much. Instead, its shrinkage is mainly due to sublimation, the direct escape o
f frozen water into the air.
You may have encountered this effect yourself if you’ve ever wondered why the cubes in your ice tray have shrunken so much after prolonged storage in your freezer. This is the central process behind the de-icing of many of Peru’s mountaintops and of Africa’s Kilimanjaro, too, and it means that drought can be as much of a threat to tropical ice as warming is. Rather than just melting away, much of it is starving for want of replenishing snows. This also means that not all glaciers and snowfields feed rivers significantly. The lower fringes of mountain ice, those that lie low enough to melt in warmer strata of air, do most of the dripping and dribbling that helps to keep rivers running, but they don’t represent the main frozen masses on super-high peaks like Coropuna and Kilimanjaro.
Something else must be feeding those lowland waterways. Bofedal wetlands thrive in isolated gullies that lack any obvious link to ice, but water nonetheless pours through them abundantly on its way to the sea. Such wetlands can trap and dispense the groundwater contributions of seasonal rain and snow in much the same way that glaciers do, and bofedales may therefore represent an important but little recognized source of lowland river water in Peru.
That might be good news in terms of future water supplies, except for two things.
First, the Andes are warming. Most computer models still lack the spatial resolution to simulate the climates of diverse, mountainous terrain accurately, but some experts predict that a moderate-emissions path could leave the region 3 to 5°F (2 to 3°C) warmer by the end of this century, and that an extreme 5,000-Gton scenario could boost the rise by 5 to 9°F (3 to 5°C). Whatever the actual magnitude of that change becomes, additional heating is likely to shrink the white caps of the Andes even further, and it could also increase evaporation from the bofedales, as well.