by Curt Stager
The Asian summer monsoons, which had been suppressed by glacial cold for thousands of years, reawakened with the return of interglacial warmth, and heavy weathering by seasonal heat and copious rains produced thick layers of soil on the formerly dust-covered plateaus of central China. Pollen grains collected from lake deposits in the rugged mountains of Colombia show that thick, humid forests of oak and glossy-leafed, flowering Weinmannia returned to the high country after temperatures there climbed 1 to 2 degrees higher than those of today.
When the cold began to creep back in about 117,000 years ago, grassy steppes pushed the taiga back from Baikal’s shores, and leafy deciduous forests retreated southward as conifers and tundra reinvaded the increasingly icy landscapes of Europe. Northern Germany was buffeted by dust storms and brush fires as the climate became cooler and drier. The vegetation of central France began to resemble that of northern Scandinavia as gigantic ice masses re-formed at high latitudes and altitudes.
In general, changes in animal life during the Eemian paralleled those among the plants; herbivores followed their food supplies and predators followed their prey as warming reshaped the home ranges of species around the world. A faunal checklist for the Eemian forests of central Europe includes wild boars, wolves, foxes, hares, beavers, martens, and a rich assortment of mice. The descendants of tiny water voles that had scampered and paddled about northwestern Europe during the previous interglacial also reinvaded the territories of their forebears as their more cold-tolerant cousins headed poleward.
It can sometimes be more difficult to reconstruct the ecology of Eemian animals than that of the vegetation because you need to find their bones or cuticles or shells in order to learn much about them. When those kinds of remains are available, though, they can be very informative. We know, for example, that New Zealand was 3 to 5°F (2 to 3°C) warmer than now because diagnostic species of beetles were well preserved in lake sediments from that time period, according to the work of paleoecologist Maureen Marra at the Victoria University of Wellington. Plants, on the other hand, are more commonly studied because their pollen grains are widely dispersed by wind and water; pollen and other plant remains in New Zealand lake sediments confirm the beetle evidence for warming and also show that the weather there was rainier than it is now.
In the case of animals, you also might be fooled into thinking that a certain kind of creature evolved or went extinct simply because it appeared or disappeared at a given location. But we now have enough study sites to show that most Eemian animals simply followed their preferred vegetational settings as climates changed, vanishing in one place and showing up again in another. And some of the migration-related changes were nothing short of spectacular.
At the height of the warming, hippopotami splashed and snorted in the Thames River not far from the future city limits of London. Rhinos stomped the British underbrush, straight-tusked Elephas antiquus munched foliage as far north as Denmark, and water buffalo dipped their heavy crescent horns to drink from the Rhine. Although most of Europe was warmer during the Eemian than it is now, particularly in summer, it wasn’t all that much warmer. Perhaps hippos, rhinos, elephants, and water buffalo could also thrive in modern Europe if we gave them a chance to, especially now that we’re back on a long-term warming trajectory.
In North America, the largest lions of all time—cave-dwelling Panthera leo atrox—shared verdant landscapes with even larger short-faced bears. Enormous beavers, Castoroides ohioensis, gnawed riverside trees in the Midwest as mammoths, wild native horses, and several species of bison looked on. Mammoths and their kind are sometimes called “ice age mammals,” but they actually lived in much of North America and Eurasia during interglacials, too. Such cold-tolerant beasts merely retreated into shrunken remnants of their favored habitats when it warmed, and many of them lived through the Eemian successfully enough to become familiar to us thousands of years later—polar bears, various ice seals, and Arctic foxes among them. To record most of the major changes in species over the last few hundred thousand years at a single fossil site is to watch climatic shifts of the past drive animal and plant communities in and out of the area, with more dramatic effect than the push of genetic evolution.
This raises an important point when we compare the Eemian example to modern times. Although few of us realize it, we live in a species-depleted Anthropocene world, and the full tallies of animals that are now artificially missing could fill many pages in this book. The Americas have lost their sabertooths, cave lions, giant ground sloths, rhinos, and multiple species of elephants, bison, and camels. Australia no longer has its short-faced kangaroos or giant wombats, and New Zealand no longer has moas. No heavy-racked Irish elk live in Ireland today, and no cave bears haunt the painted caves of France.
These absences aren’t simply the legacy of heating at the end of the last ice age; most such species survived multiple warmings in the past, and not just interglacials but also the many abrupt, century-scale hot-cold shifts that disrupted otherwise icy millennia between the Eemian and the Holocene. As all but a small minority of scientists now agree, those worldwide disappearances were more the result of human activity than of climatic change.
Sadly, from the perspective of many of today’s remaining species, it’s only going to get worse from here on out. Even if we take a relatively moderate emissions path into the future and thereby hope to avoid destroying the last polar and alpine refuges, warming on the scale of an Eemian interglacial will still nudge many species toward higher latitudes and elevations. In the past, species could simply move in response to the push and tug of climatic changes, but this time they’ll be trapped within the confines of habitats that are mostly immobilized by our presence. People, more than climate alone, will determine their fates as the Anthropocene continues to unfold.
And what about the early ancestors of those people? What were they doing during that warm respite before the last ice age, back when the plains of North America and the steppes of Asia still supported herds and packs and prides so rich in abundance and diversity as to rival the Serengeti? We have only scattered snapshots of Eemian humanity to go on, for several reasons. For one thing, no humans lived in the Americas or Australia back then because they hadn’t crossed over from Asia yet. For another, Eemian-age deposits are unevenly distributed over the planet; they can be difficult to identify; and they must be conveniently exposed to study, usually by well-placed quarries or roadcuts.
But most importantly, there weren’t many humans anywhere on Earth back then, at least not outside Africa. The Eemian warm period, whether by chance or by causation, marked one of the earliest known dispersals of modern Homo sapiens out of the African homeland.
That’s not to say that early hominids didn’t already occupy much of the Old World then. Some diminutive, hobbitlike beings wandered the moist, rain-forested islands of Indonesia. In what is now France and Germany, spear-toting Neanderthals hunted woolly tuskers and reindeer on “mammoth steppes” under glacial conditions. Later, Eemian Neanderthals switched to elephants, rhinos, brown bears, deer, and steerlike aurochs in dense woodlands as open-ground prey shuffled north with the retreating cold, and on coastlines as far south as Gibraltar and the Middle East they harvested mussels, seals, dolphins, and fish. But these were not precisely human in the strictest modern sense despite their tool use and despite new evidence that some of our ancestors shared genetic material with Neanderthals.
Anatomically modern humans, our most direct ancestors, arose in Africa roughly 200,000 years ago and spread to the rest of the world later on. Many scientists believe that the large mammals of Africa have outlasted their relatives on other continents because they coevolved with humans and were therefore less likely to be taken by surprise when two-legged predators approached them with killing tools in hand.
During the Eemian phase of the Middle Stone Age, one of the first waves of fully human wanderers crossed from the Egyptian mainland eastward into what we now call Israel and Jordan. Geochemical records
from laminated cave formations show that stronger regional rains of the time opened a corridor to the rest of the world for some of our ancestors by making a trek across the Sinai and Negev deserts more of a hunting trip than a waterless death march. Climate change closed that door, too, at the end of the Eemian when the return of cold climates shut the Middle Eastern rains down. The early emigrants vanished from historical view then, perhaps by returning to Africa or by dying out.
Meanwhile, back in the homeland, the bulk of humanity went about its business as usual. On the edge of the Red Sea in today’s Eritrea, teardrop-shaped hand axes litter ancient beach and reef deposits where Eemian humans once feasted on oysters and crabs. On the opposite end of the continent, at South Africa’s Blombos Cave, coastal cave dwellers flaked stone and bone artifacts and munched fish and mollusks. Not far away, at Klasies River, people gathered edible plants and shellfish and also hunted penguins—not because it was unusually cold there but simply because native penguins waddled about on the beaches of the South African Cape region then as they do today. When it cooled off again at the end of the Eemian, sea levels fell so low that the caves at the mouth of the Klasies River overlooked a broad coastal plain rather than a food-rich shoreline. They were only rarely occupied after that until about 3,000 years ago, when rising sea levels once again brought the waves back to within easy walking distance from home.
So there we have it, a taste of what happened the last time a world much like our own warmed above modern temperature ranges. The Eemian example does not show us what a super 5,000-Gton greenhouse could be like; in the next chapter we’ll look much farther back in time in order to find such an example. But for a glimpse of some basic features of a fairly moderate 1,000-Gton future, it does nicely.
Greenhouse gas concentrations rose in response to the Eemian heat, but they didn’t reach nearly as high as they now have with our help. Northern summer warming strengthened tropical monsoons, and sea levels climbed 23 feet (7 m) or so higher than today. Much or all of the Arctic Ocean became open in summer, so we can be fairly certain that continuing our warming trend today will open it again. However, the land-based ice sheets on Greenland and western Antarctica still kept much of their bulk in frozen form, and the eastern Antarctic ice sheet shrank even less.
Eemian warmth sped Arctic warming up to rates considerably faster than those at lower latitudes, almost certainly as a result of amplification processes such as changes in surface reflectivity and the decay of organic matter—changes that are also operating today. However, not all permafrost succumbed to the prolonged thaw, probably because surface materials insulated it; some of the subterranean ice in Alaska is 750,000 years old, according to a study by Canadian geoscientist Duane Froese and colleagues, which means that it has survived multiple interglacials. The Eemian warming did push tundras, steppes, and forests poleward along with their resident creatures, and the subsequent ice age pushed their descendants back again in an early version of the climate whiplash that now lies ahead of us today. In a wide-open world, it’s not much to ask of a mobile organism to shift its geographical range over the course of generations as long as the necessary food and habitat conditions move with it, and many species did this over and over again without obvious difficulty during previous cold-warm-cold oscillations.
But much is different nowadays. Artificially generated greenhouse gases are driving the changes now, and because they operate beyond the limits of any particular latitude, season, or time of day they may destroy more ice in less time than the preceding interglacials did. In addition, the role that our ancestors played in the Eemian world was relatively simple and markedly different from the one that we play in this one. Even though they hunted and gathered widely, those few and technology-poor early peoples generally had less impact on their surroundings than the average beaver colony or mammoth herd—at least until Clovis-age cultures brought heavy spears and other deadly factors into play at the end of the last ice age. But today we spread settlements, farms, factories, and roads far and wide. Our complex technologies allow us to reshape landscapes and relocate or exterminate entire species. Nowadays in this remarkable Age of Humans, we are as much a barrier to biological migrations as high mountains, swift rivers, and broad oceans have been in the past. As Anthropocene warming rises toward its as yet unspecified peak, our long-suffering biotic neighbors face a situation that they have never encountered before in the long, dramatic history of ice ages and interglacials.
They can’t move because we’re standing in their way.
4
Life in a Super-Greenhouse
The farther backward you can look, the farther
forward you are likely to see.
—Winston Churchill
We’ve now glimpsed what a recent warm period was like, using the Eemian interglacial as a rough guide to what a relatively modest 1,000-Gton carbon release could bring us. But none of the events that are recorded in ice closely resemble the super-greenhouse that we could unleash by burning most of our remaining fossil fuel reserves. Just how extreme can Anthropocene climate change become, and what might life in such a world be like?
The details of what it would take to drive climate over a thermal cliff are still unresolved, but history makes one thing clear: the critical tipping point certainly exists because it’s been passed before. Not during the last 130,000 years, though, and not during the lifetime of most species alive today. It happened 55 million years ago, some 10 million years after the demise of the dinosaurs, and it currently represents one of our best historical examples of what radical Anthropocene warming could be like. Unlike the Eemian, it was not caused by orbital cycles, and the mechanisms driving it were global rather than hemispheric in extent. But above all, the case that we’re about to consider here was certainly extreme. By all accounts, it was one of the most abrupt and intense greenhouse warmings ever to occur on this planet.
In order to understand how such a superhothouse could happen long before humans entered the picture, we’ll have to look back into the earliest phase of the Cenozoic era, which began 65 million years ago and continues today through the subset of geologic time that we are now calling the Anthropocene epoch. That great era is sometimes described as the Age of Mammals in acknowledgement of the tremendous diversification of hairy, warm-blooded, milk-producing critters that defines it.
The first 31 million years of the Cenozoic, dubbed the Paleocene and Eocene epochs, were already much warmer than today for reasons that are as yet not fully understood. Some geoscientists attribute it to different patterns of ocean current flow around a slightly different orientation of drifting continents. For example, the Central American land corridor didn’t exist yet, so tropical marine currents and their associated climatic effects were probably somewhat different. Most experts, however, link the warmth to greenhouse gas concentrations that were much higher than they are today—though, again, the causes remain unclear.
For whatever reason, something kept nudging Earth’s temperatures higher and higher during the early Cenozoic. During the Eocene climatic optimum, beginning around 50 million years ago, global average temperatures were 18 to 22°F (10 to 12°C) or more above today’s mean for several million years. From that point on, the prevailing direction of climate change has mainly been toward inexorable cooling. Roughly 34 million years ago, the first permanent ice sheets began to form in Antarctica. The Arctic followed suit about 8 million years ago, and during the last 2 to 3 million years the planet has endured dozens of ice ages. What we face now as we contemplate the onset of an extreme hothouse in the Anthropocene future is, in essence, a sudden reversal of geologic history. If we take the 5,000-Gton emissions path, then we’ll essentially jerk global climate back into the early Eocene.
This outline of events is oversimplified, though. The patterns of long-term change were far from smooth, particularly during the early Cenozoic, when brief surges in the warming trend drove several hot spikes into the rising temperature curve. One of the most notable and
dramatic of these occurred 55 million years ago, and it is considered by many geoscientists to be our best real-world example of a worst-case greenhouse future. For something close to 170,000 years, that Paleocene-Eocene thermal maximum, or PETM, forced the world into an exceptionally heated state that bears a striking resemblance to our extreme-emissions scenario.
As of today, we’ve vaporized about 300 Gtons of fossil carbon. By comparison, most investigators estimate that at least 2,000 Gtons of carbon flooded the atmosphere during the PETM. Some put it as high as 5,000 Gtons, which is close to the magnitude of our worst-case emissions scenario. What could have caused such a thing? For once, we’re off the hook; even our earliest humanoid ancestors wouldn’t show up in the fossil record for another 50 million years.
To address that issue of origins, we can look for clues in the patterns of warming that the gases produced. And to do that, we’ll need information sources other than the ice cores that told us so much about the Eemian interglacial. Our longest ice records reach less than a fiftieth of the way back to the PETM, and the entire early Cenozoic was so hot anyway that it left no ice sheets behind to tell us anything.
Instead of ice, we must turn to sediments for guidance. For example, long cores that scientists pull from ancient marine deposits can contain rich stores of information in the form of tiny protozoans called “foraminifera,” or forams. Forams are saltwater amoebas but, unlike the typical biology-lab blobs, they build beautifully coiled or lumpy shells out of calcium carbonate. Thus armored against small predators, they drift or creep throughout the world’s oceans like microscopic turtles, and when they eventually die and sink to the bottom their hard, empty shells may remain preserved for millions of years.