The World Without Us - Alan Weisman (2007)

Part I

Chapter 4. The World Just Before Us

1. An Interglacial Interlude

F

OR MORE THAN 1 billion years, sheets of ice have been sliding back and forth from the poles, sometimes actually meeting at the equator. The reasons involve continental drift, the Earth’s mildly eccentric orbit, its wobbly axis, and swings in atmospheric carbon dioxide. For the last few million years, with the continents basically where we find them today, ice ages have recurred fairly regularly and lasted upwards of 100,000 years, with intervening thaws averaging 12,000 to 28,000 years.

The last glacier left New York 11,000 years ago. Under normal conditions, the next to flatten Manhattan would be due any day now, though there’s growing doubt that it will arrive on schedule. Many scientists now guess that the current intermission before the next frigid act will last a lot longer, because we’ve managed to postpone the inevitable by stuffing our atmospheric quilt with extra insulation. Comparisons to ancient bubbles in Antarctic ice cores reveal there’s more CO2 floating around today than at any time in the past 650,000 years. If people cease to exist tomorrow and we never send another carbon-bearing molecule skyward, what we’ve already set in motion must still play itself out.

That won’t happen quickly by our standards, although our standards are changing, because we Homo sapiens didn’t bother to wait until fossilization to enter geologic time. By becoming a veritable force of nature, we’ve already done so. Among the human-crafted artifacts that will last the longest after we’re gone is our redesigned atmosphere. Thus, Tyler Volk finds nothing strange about being an architect teaching atmospheric physics and marine chemistry on the New York University biology faculty. He finds he must draw on all those disciplines to describe how humans have turned the atmosphere, biosphere, and the briny deep into something that, until now, only volcanoes and colliding continental plates have been able to achieve.

Volk is a lanky man with wavy dark hair and eyes that scrunch into crescents when he ponders. Leaning back in his chair, he studies a poster that nearly fills his office bulletin board. It portrays atmosphere and oceans as a single fluid with layers of deepening density. Until about 200 years ago, carbon dioxide from the gaseous part above dissolved into the liquid part below at a steady rate that kept the world at equilibrium. Now, with atmospheric CO2 levels so high, the ocean needs to readjust. But because it’s so big, he says, that takes time.

“Say there are no more people burning fuel. At first, the ocean’s surface will absorb CO2 rapidly. As it saturates, that slows. It loses some CO2 to photosynthesizing organisms. Slowly, as the seas mix, it sinks, and ancient, unsaturated water rises from the depths to replace it.”

It takes 1,000 years for the ocean to completely turn over, but that doesn’t bring the Earth back to pre-industrial purity. Ocean and atmosphere are more in balance with each other, but both are still supercharged with CO2 So is the land, where excess carbon will cycle through soil and life-forms that absorb but eventually release it. So where can it go? “Normally,” says Volk, “the biosphere is like an upside-down glass jar: On top, it’s basically closed to any extra matter, except for letting in a few meteors. At the bottom, the lid is slightly open—to volcanoes.”

The problem is, by tapping the Carboniferous Formation and spewing it up into the sky, we’ve become a volcano that hasn’t stopped erupting since the 1700s.

So next, the Earth must do what it always does when volcanoes throw extra carbon into the system. “The rock cycle kicks in. But it’s much longer.” Silicates such as feldspar and quartz, which comprise most of the Earth’s crust, are gradually weathered by carbonic acid formed by rain and carbon dioxide, and turn to carbonates. Carbonic acid dissolves soil and minerals that release calcium to groundwater. Rivers carry this to the sea, where it precipitates out as seashells. It’s a slow process, sped slightly by the intensified weather in the supercharged atmosphere.

“Eventually,” Volk concludes, “the geologic cycle will take CO2 back to prehuman levels. That will take about 100,000 years.”

Or longer: One concern is that even as tiny sea creatures are locking carbon up in their armor, increased CO2 in the oceans’ upper layers may be dissolving their shells. Another is that the more oceans warm, the less CO2 they absorb, as higher temperatures inhibit growth of CO2-breathing plankton. Still, Volk believes, with us gone the oceans’ initial 1,000-year turnover could absorb as much as 90 percent of the excess carbon dioxide, leaving the atmosphere with only about 10 to 20 extra parts per million of CO2 above the 280 ppm preindustrial levels.

The difference between that and today’s 380 ppm, scientists who’ve spent a decade coring the Antarctic ice assure us, means that there will be no encroaching glaciers for at least the next 15,000 years. During the time that the extra carbon is being slowly sopped up, however, palmettos and magnolias may be repopulating New York City faster than oaks and beeches. The moose may have to seek gooseberries and elderberries in Labrador, while Manhattan instead hosts the likes of armadillos and peccaries advancing from the south . . .

. . . unless, respond some equally eminent scientists who’ve been eying the Arctic, fresh meltwater from Greenland’s ice cap chills the Gulf Stream to a halt, closing down the great ocean conveyor belt that circulates warm water around the globe. That would bring an ice age back to Europe and the East Coast of North America after all. Maybe not severe enough to trigger massive sheet glaciers, but treeless tundra and permafrost could become the alternative to temperate forest. The berry bushes would be reduced to stunted, colorful spots of ground cover among the reindeer lichen, attracting caribou southward.

In a third, wishful scenario, the two extremes might blunt each other enough to hold temperatures suspended in between. Whichever it is, hot or cold or betwixt, in a world where humans stayed around and pushed atmospheric carbon to 500 or 600 parts per million—or the projected 900 ppm by AD 2100, if we change nothing from the way we do business today—much of what once lay frozen atop Greenland will be sloshing in a swollen Atlantic. Depending on exactly how much of it and Antarctica go, Manhattan might become no more than a few islets, one where the Great Hill once rose above Central Park, another an outcropping of schist in Washington Heights. For a while, clutches of buildings a few miles to the south would vainly scan the surrounding waters like surfacing periscopes, until buffeting waves brought them down.

2. Ice Eden

Had humans never evolved, how might the planet have fared? Or was it inevitable that we did?

And if we disappeared, would—or could—we, or something equally complicated, happen again?

FAR FROM EITHER pole, East Africa’s Lake Tanganyika lies in a crack that, 15 million years ago, began to split Africa in two. The Great African Rift Valley is the continuation of a tectonic parting of the ways that began even earlier in what is now Lebanon’s Beqaa Valley, then ran south to form the course of the Jordan River and the Dead Sea. Then it widened into the Red Sea, and is now branching down two parallel cleavages through the crust of Africa. Lake Tanganyika fills the Rift’s western fork for 420 miles, making it the longest lake in the world.

Nearly a mile from surface to bottom, around 10 million years old, it is also the world’s second-deepest and second-oldest, after Siberia’s Lake Baikal. That makes it extremely interesting to scientists who have been extracting core samples of the lake bed sediments. Just as annual snowfalls preserve a history of climate in glaciers, pollen grains from surrounding foliage settle in the depths of bodies of freshwater, neatly separated into readable layers by dark bands of rainy-season runoff and light seams of dry-season algal blooms. At ancient Lake Tanganyika, the cores reveal more than the identities of plants. They show how a jungle gradually turned to fire-tolerant, broad-leafed woodland known as miombo, which covers vast swathes of today’s Africa. Miombo is another man-made artifact, which developed as paleolithic humans discovered that by burning trees they could create grassland and open woodlands to attract and nurture antelope.

Mixed with thickening layers of charcoal, the pollens show the even greater deforestation that accompanied the dawning of the Iron Age, as humans learned first to smelt ore, then to fashion hoes for furrows. There they planted crops such as finger millet, whose signature also appears. Later arrivals, like beans and corn, produce either too few pollens or grains too large to drift far, but the spread of agriculture is evidenced by the increase of pollens from ferns that colonize disturbed land.

All this and more can be learned from mud recovered with 10 meters of steel pipe lowered on a cable and, aided by a vibrating motor, driven by the force of its own weight into the lake bed—and into 100,000 years of pollen layers. A next step, says University of Arizona paleolimnologist Andy Cohen, who heads a research project in Kigoma, Tanzania, on Lake Tanganyika’s eastern shore, is a drill rig capable of penetrating a 5-million- or even 10-million-year core.

Such a machine would be very expensive, on the order of a small oil-drilling barge. The lake is so deep that the drill could not be anchored, requiring thrusters linked to a global positioning system to constantly adjust its position above the hole. But it would be worth it, says Cohen, because this is Earth’s longest, richest climate archive.

“It’s long been assumed that climate is driven by advancing and retreating polar ice sheets. But there’s good reason to believe that circulation at the tropics is also involved. We know a lot about climate change at the poles, but not at the heat engine of the planet, where people live.” Coring it, Cohen says, would capture “ten times the climate history found in glaciers, and with far greater precision. There are probably a hundred different things we can analyze.”

Among them is the history of human evolution, because the core’s record would span the years during which primates took their first bipedal steps and proceeded through transcendent stages that led to hominids from Australopithecus to Homos habilis, erectus, and finally sapiens. The pollens would be the same that our ancestors inhaled, even broadcast from the same plants they touched and ate, because they, too, emerged from this Rift.

East of Lake Tanganyika in the African Rift’s parallel branch, another lake, shallower and saline, evaporated and reappeared various times over the past 2 million years. Today, it is grassland, hard-grazed by the cows and goats of Maasai herdsmen, overlying sandstone, clay, tuff, and ash atop a bed of volcanic basalt. A stream draining Tanzania’s volcanic highlands to the east gradually cut a gorge through those layers 100 meters deep. There, during the 20th century, archaeologists Louis and Mary Leakey discovered fossilized hominid skulls left 1.75 million years earlier. The gray rubble of Olduvai Gorge, now a semidesert bristling with sisal, eventually yielded hundreds of stone-flake tools and chopper cores made from the underlying basalt. Some of these have been dated to 2 million years ago.

In 1978, 25 miles southwest of Olduvai Gorge, Mary Leakey’s team found a trail of footprints frozen in wet ash. They were made by an australopithecine trio, likely parents and a child, walking or fleeing through the rainy aftermath of an eruption of the nearby Sadiman volcano. Their discovery pushed bipedal hominid existence back beyond 3.5 million years ago. From here and from related sites in Kenya and Ethiopia, a pattern emerges of the gestation of the human race. It is now known that we walked on two feet for hundreds of thousands of years before it occurred to us to strike one stone against another to create sharp-edged tools. From the remains of hominid teeth and other nearby fossils, we know we were omnivores, equipped with molars to crunch nuts—but also, as we advanced from finding stones shaped like axes to learning how to produce them, possessed of the means to efficiently kill and eat animals.

Olduvai Gorge and the other fossil hominid sites, together comprising a crescent that runs south from Ethiopia and parallels the continent’s eastern shore, have confirmed beyond much doubt that we are all Africans. The dust we breathe here, blown by zephyrs that leave a coating of gray tuff powder on Olduvai’s sisals and acacias, contains calcified specks of the very DNA that we carry. From this place, humans radiated across continents and around a planet. Eventually, coming full circle, we returned, so estranged from our origins that we enslaved blood cousins who stayed behind to maintain our birthright.

Animal bones in these places—some from hippo, rhino, horse, and elephant species that became extinct as we multiplied; many of them honed by our ancestors into pointed tools and weapons—help us know how the world was just before we emerged from the rest of Mammalia. What they don’t show, however, is what might have impelled us to do so. But at Lake Tanganyika, there are some clues. They lead back to the ice.

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The lake is fed by many streams that pour off the mile-high Rift escarpment. At one time, these dropped through gallery rain forest. Then came miombo woodland. Today, most of the escarpment has no trees at all. Its slopes have been cleared to plant cassava, with fields so steep that farmers are known to roll off them.

An exception is at Gombe Stream, on Lake Tanganyika’s eastern Tan-zanian coast, the site where primatologist Jane Goodall, a Leakey assistant at Olduvai Gorge, has studied chimpanzees since 1960. Her field study, the longest anywhere of how a species behaves in the wild, is headquartered in a camp reachable only by boat. The national park that surrounds it is Tanzania’s smallest—only 52 square miles. When Goodall first arrived, the surrounding hills were covered in jungle. Where it opened into woodland and savanna, lions and cape buffalo lived. Today, the park is surrounded on three sides by cassava fields, oil palm plantations, hill settlements, and, up and down the lakeshore, several villages of more than 5,000 inhabitants. The famous chimpanzee population teeters precariously around 90.

Although chimps are the most intensely studied primates at Gombe, its rain forest is also home to many olive baboons and several monkey species: vervet, red colobus, red-tailed, and blue. During 2005, a Ph.D. candidate at New York University’s Center for the Study of Human Origins named Kate Detwiler spent several months investigating an odd phenomenon involving the last two.

Red-tailed monkeys have small black faces, white-spotted noses, white cheeks, and vivid chestnut tails. Blue monkeys have bluish coats and triangular, nearly naked faces, with impressive jutting eyebrows. With different coloring, body size and vocalizations, no one would confuse blue and red-tailed monkeys in the field. Yet in Gombe they now apparently mistake one another, because they have begun to interbreed. So far, Detwiler has confirmed that although the two species have different numbers of chromosomes, at least some of the offspring of these liaisons—whether between blue males and red-tailed females or vice versa—are fertile. From the forest floor, she scrapes their feces, in which fragments of intestinal lining attest to a mix of DNA resulting in a new hybrid.

Only she thinks it’s something more. Genetics indicate that at some point 3 million to 5 million years ago, two populations of a species that was the common ancestor to these two monkeys became separated. Adjusting to distinct environments, they gradually diverged from each other. Through a similar situation involving finch populations that became isolated on various Galápagos islands, Charles Darwin first deduced how evolution works. In that case, 13 different finch species emerged in response to locally available food, their bills variously adapted to cracking seeds, eating insects, extracting cactus pulp, or even sucking the blood of seabirds.

In Gombe, the opposite has apparently occurred. At some point, as new forest filled the barrier that once divided these two species, they found themselves sharing a niche. But then they became marooned together, as the forest surrounding Gombe National Park gave way to cassava croplands. “As the number of available mates of their own species dwindled,” Detwiler figures, “these animals have been driven to desperate—or creative—survival measures.”

Her thesis is that hybridization between two species can be an evolutionary force, just like natural selection is within one. “Maybe at first the mixed offspring isn’t as fit as either parent,” she says. “But for whatever reason—constrained habitat, or low numbers—the experiment keeps getting repeated, until eventually a hybrid as viable as its parent emerges. Or, maybe even with advantage over the parents, because the habitat has changed.”

That would make the future offspring of these monkeys human artifacts: their parents forced together by agricultural Homo sapiens who so fragmented east Africa that populations of monkeys and other species like shrikes or flycatchers had to interbreed, crossbreed, perish—or do something very creative. Such as evolve.

Something similar may have happened here before. Once, when its Rift was only beginning to form, Africa’s tropical forest filled the continent’s midriff from the Indian Ocean to the Atlantic. Great apes had already made their appearance, including one that in many ways resembled chimpanzees. No remnants of it have ever been found, for the same reason that chimp remains are so rare: in tropical forests, heavy rains leach minerals from the ground before anything can fossilize, and bones decompose quickly. Yet scientists know it existed, because genetics show that we and chimps descended directly from the same ancestor. The American physical anthropologist Richard Wrangham has given this undiscovered ape a name: Pan prior.

Prior, that is, to Pan troglodytes, today’s chimpanzee, but also prior to a great dry spell that overtook Africa about 7 million years ago. Wetlands retreated, soils dried, lakes disappeared, and forests shrank into pocket refuges, separated by savannas. What caused this was an ice age advancing from the poles. With much of the world’s moisture locked into glaciers that buried Greenland, Scandinavia, Russia, and much of North America, Africa became parched. No ice sheets reached it, although glacial caps formed on volcanoes like Kilimanjaro and Mount Kenya. But the climatic change that fragmented Africa’s forest, more than twice the size of today’s Amazon, was due to the same distant white juggernaut that was smashing conifers in its path.

That faraway ice sheet stranded populations of African mammals and birds in patches of forest where, over the next few million years, they evolved their separate ways. At least one of them, we know, was driven to try something daring: taking a stroll in a savanna.

If humans vanished, and if something eventually replaced us, would it begin as we did? In southwest Uganda, there’s a place where it’s possible to see our history reenacted in microcosm. Chambura Gorge is a narrow ravine that cuts for 10 miles through a deposit of dark brown volcanic ash on the floor of the Rift Valley. In startling contrast to the surrounding yellow plains, a green band of tropical sobu, ironwood, and leafflower trees fills this canyon along the Chambura River. For chimpanzees, this oasis is both a refuge and a crucible. Lush as it is, the gorge is barely 500 yards across, its available fruit too limited to satisfy all their dietary needs. So from time to time, brave ones risk climbing up the canopy and leaping to the rim, to the chancy realm of the ground.

With no ladder of branches to help them see over the oat and citronella grasses, they must raise themselves on two feet. Perched for a moment on the verge of being bipedal, they scan for lions and hyenas among the scattered fig trees on the savanna. They select a tree they calculate they can reach without becoming food themselves. Then, as we also once did, they run for it.

About 3 million years after distant glaciers pushed some courageous, hungry specimens of Pan prior out of forests no longer big enough to sustain us—and some of them proved imaginative enough to survive—the world warmed again. Ice retreated. Trees regained their former ground and then some, even covering Iceland. Forests reunited across Africa, again from the Atlantic shore to the Indian, but by then Pan prior had segued into something new: the first ape to prefer the grassy woodlands at the forest’s edge. After more than a million years of walking on two feet, its legs had lengthened and its opposable big toes had shortened. It was losing the ability to dwell in trees, but its sharpened survival skills on the ground had taught it to do so much more.

Now we were hominids. Somewhere along the way, as Australopithecus was begetting Homo, we learned not only to follow the fires that opened up savannas that we’d learned to inhabit, but how to make them ourselves. For some 3 million years more, we were too few to create more than local patchworks of grassland and forests whenever distant ice ages weren’t doing it for us. Yet in that time, long before Pan prior smost recent descendant, surnamed sapiens, appeared, we must have become numerous enough to again try being pioneers.

Were the hominids who wandered out of Africa again intrepid risk-takers, their imaginations picturing even more bounty beyond the savanna’s horizon?

Or were they losers, temporarily out-competed by tribes of stronger blood cousins for the right to stay in our cradle?

Or were they simply going forth and multiplying, like any beast presented with rich resources, such as grasslands stretching all the way to Asia? As Darwin came to appreciate, it didn’t matter: when isolated groups from the same species proceed in their separate ways, the most successful among them learn to flourish in new surroundings. Exiles or adventurers, the ones who survived filled Asia Minor and then India. In Europe and Asia, they began to develop a skill long known to temperate creatures like squirrels but new to primates: planning, which required both memory and foresight to store food in seasons of plenty in order to outlast seasons of cold. A land bridge got them through much of Indonesia, but to reach New Guinea and, about 50,000 years ago, Australia, they had to learn to become seafarers. And then, 11,000 years ago, observant Homo sapiens in the Middle East figured out a secret until then known only to select species of insects: how to control food supplies not by destroying plants, but by nurturing them.

Australopithecus africanus.

ILLUSTRATION BY CARL BUELL.

Because we know the Middle Eastern origin of the wheat and barley they grew, which soon spread southward along the Nile, we can guess that—like shrewd Jacob returning with a cornucopia of gifts to win over his powerful brother, Esau—someone bearing seeds and the knowledge of agriculture returned from there to the African homeland. It was an auspicious time to do so, because yet another ice age—the last one—had once again stolen moisture from lands that glaciers didn’t reach, tightening food supplies. So much water was frozen into glaciers that the oceans were 300 feet lower than today.

At that same time, other humans who had kept spreading across Asia arrived at the farthest reach of Siberia. With the Bering Sea partly emptied, a land bridge 1,000 miles across connected to Alaska. For 10,000 years, it had lain under more than half a mile of ice. But now, enough had receded to reveal an ice-free corridor, in places 30 miles wide. Picking their way around meltwater lakes, they crossed it.

Chambura Gorge and Gombe Stream are atolls in an archipelago that is all that remains of the forest that birthed us. This time, the fragmentation of Africa’s ecosystem is due not to glaciers, but to ourselves, in our latest evolutionary leap to the status of Force of Nature, having become as powerful as volcanoes and ice sheets. In these forest islands, surrounded by seas of agriculture and settlement, the last of Pan priorsother offspring still cling to life as it was when we left to become woodland, savanna, and finally city apes. To the north of the Congo River, our siblings are gorillas and chimpanzees; to the south, bonobos. It is the latter two we genetically most resemble; when Louis Leakey sent Jane Goodall to Gombe, it was because the bones and skulls he and his wife had uncovered suggested that our common ancestor would have looked and acted much like chimpanzees.

Whatever inspired our forebears to leave, their decision ignited an evolutionary burst unlike any before, described variously as the most successful and most destructive the world has ever seen. But suppose we had stayed—or suppose that, when we were exposed on the savanna, the ancestors of today’s lions and hyenas had made short work of us. What, if anything, would have evolved in our place?

To stare into the eyes of a chimpanzee in the wild is to glimpse the world had we stayed in the forest. Their thoughts may be obscure, but their intelligence is unmistakable. A chimpanzee in his element, regarding you coolly from a branch of an mbula fruit tree, expresses no sense of inferiority in the presence of a superior primate. Hollywood images mislead, because its trained chimps are all juveniles, as cute as any child. However, they keep growing, sometimes reaching 120 pounds. In a human of similar weight, about 30 of those pounds would be fat. A wild chimpanzee, who lives in a perpetual state of gymnastics, has perhaps three to four pounds of fat. The rest is muscle.

Dr. Michael Wilson, the curly-haired young director of field research at Gombe Stream, vouches for their strength. He has watched them tear apart and devour red colobus monkeys. Superb hunters, about 80 percent of their attempts are successful kills. “For lions, it’s only about one out of 10 or 20. These are pretty bright creatures.”

But he has also seen them steal into the territories of neighboring chimp groups, ambush unwary lone males, and maul them to death. He’s watched chimps over months patiently pick off males of neighboring clans until the territory and the females are theirs. He’s also seen pitched chimpanzee combat, and blood battles within a group to determine who is the alpha male. The unavoidable comparisons to human aggression and power struggles became his research specialty.

“I get tired of thinking about it. It’s kind of depressing.”

One of the unfathomables is why bonobos, smaller and more slender than chimps but equally related to us, don’t seem very aggressive at all. Although they defend territory, no intergroup killing has ever been observed. Their peaceful nature, predilection for playful sex with multiple partners, and apparent matriarchal social organization with all the attendant nurturing have practically become mythologized among those who insistently hope that the meek might yet inherit the Earth.

In a world without humans, however, if they had to fight it out with chimpanzees, they would be outnumbered: only 10,000 or fewer bonobos remain, compared to 150,000 chimpanzees. Since their combined population a century ago was approximately 20 times greater, with each passing year chances weaken for either species to be around to take over.

Michael Wilson, hiking in the rain forest, hears drumbeats that he knows are chimpanzees pounding on buttress roots, signaling each other. He runs after them, up and down Gombe’s 13 stream valleys, hurdling morning glory vines and lianas strung across baboon trails, following chimp hoots until, two hours later, he finally catches them at the top of the Rift. Five of them are in a tree at the edge of the woodland, eating the mangoes they love, a fruit that came along with wheat from Arabia.

A mile below, Lake Tanganyika flashes in the afternoon sunlight, so vast it holds 20 percent of the world’s freshwater and so many endemic fish species that it’s known among aquatic biologists as the Galápagos of lakes. Beyond it, to the west, are the hazy hills of the Congo, where chimpanzees are still taken for bush meat. In the opposite direction, past Gombe’s boundaries, are farmers who also have rifles, and who are tired of chimps who snatch their oil palm nuts.

Other than humans and each other, the chimpanzees have no real predators here. The very presence of these five in a tree surrounded by grass testifies to the fact that they’ve also inherited the gene of adaptability, and are far more able than gorillas, which have highly specialized forest diets, to live on a variety of foods and in a variety of environments. If humans were gone, however, they might not need to. Because, says Wilson, the forest would come back. Fast.

“There’d be miombo moving through the area, recovering the cassava fields. Probably the baboons would take first advantage, radiating out, carrying seeds in their poop, which they’d plant. Soon you’d have trees sprouting wherever there’s suitable habitat. Eventually, the chimps would follow.”

With plenty of game returning, lions would find their way back, then the big animals: cape buffalo and elephant, coming from Tanzania’s and Uganda’s reserves. “Eventually,” Wilson says, sighing, “I can see a continuous stretch of chimpanzee populations, all the way down to Malawi, all the way up to Burundi, and over into Congo.”

All that forest back again, ripe with chimpanzees’ favorite fruits and a prospering population of red colobus to catch. In tiny Gombe—a protected shred of Africa’s past that is also a taste of such a posthuman future—no enticement is readily evident for another primate to leave all that lushness and follow in our futile footsteps.

Until, of course, the ice returns.