by Curt Stager
The PETM greenhouse wasn’t limited to the oceans, of course, though most of our geological records of it come from marine deposits. To learn what happened on land we turn to terrestrial fossils, especially botanical ones. Many plants back then were remarkably similar to modern forms, more so at least than the animals were, so we can easily identify them from the leaves, wood, and seeds that they left behind, and experts can therefore use them to infer a great deal about past climates.
Fossils exposed along Antarctica’s coastline show that it was green with Nothofagus, the beech trees that now thrive in temperate rain forests of the Southern Hemisphere, and tall deciduous conifers covered much of Arctic Canada. Such finds speak loud and clear; it was obviously warm and moist enough to turn the polar regions lush and leafy. In North America, many plant species shifted their ranges poleward by more than a thousand miles during the PETM, a mass resettlement that would be like relocating an Alabama flower garden to Hudson Bay in northern Canada.
A research team led by Scott Wing, a paleobotanist at the Smithsonian Museum of Natural History, recently sampled PETM-aged flood-plain deposits in Wyoming that were loaded with well-preserved leaf fossils from early relatives of poinsettia, pawpaw, and other southern taxa that had migrated well north of their former ranges. But Wing and his colleagues also sought more subtle details from those ancient remains, as well. What drew their attention most strongly were the shapes of the leaves themselves.
The margins of the leaves fluttering in Wyoming’s forests during the PETM were generally smoother and less toothy than they were just before and after it. In most plants, leaf edges are busy centers of the gasexchange and light-capturing processes that drive photosynthesis. Growing seasons at cool, high latitudes are shorter than those close to the equator, so short-season plants often have to work overtime to get their photosynthetic business finished before winter shuts them down. As a result, the leaves of many high-latitude plants have toothy edges that increase their marginal areas—their “coastlines,” if you will—and thereby boost their productivity. Think of serrated beech, maple, and oak leaves in New England versus smooth-margined, tongue-shaped magnolia blades down in Louisiana. Applying a “leaf-margin index” to their collections of fossil foliage, Wing’s group estimated that Wyoming’s Bighorn Basin warmed by 9°F (5°C), right in line with the marine story: more warming than the tropics felt and less than the poles got.
But how do we know that this seemingly coherent tale wasn’t warped by the tectonic wanderings of continents? Perhaps fossils and cores tell of warming because the plates under them shifted closer to the equator back then.
In fact, continental drift is too slow for that. The Atlantic Ocean was a bit narrower back then, but most of its spreading motions run east-west and therefore don’t affect latitudinal positions much. The Isthmus of Panama was partially open, which allowed some flowthrough of warm currents that dry land now blocks, and India had yet to plow the Himalayas up into their present skyscraping form. But apart from low areas being inundated by the highest sea levels possible on a defrosted planet, most PETM geography more or less resembled today’s layout, at least in a general sense. As a result, we can be sure that the presence of forests in the Arctic means that the far north was indeed remarkably warm, not that it had drifted into the tropics.
It may be difficult to imagine forests growing on what are now stark tundra and polar snowscapes; what were they like? A former student of mine, Chris Williams of Franklin and Marshall College in Pennsylvania, has become a world authority on trees of the early Cenozoic, and his research paints vivid pictures of the past. One of his study sites is the Iceberg Bay Formation on Ellesmere Island in the Canadian Arctic, a storehouse of geologic information whose name and present climatic setting stand in stark contrast to the warm habitats it represents.
“The preservation there is so good that the trees aren’t even fully fossilized,” Chris explained recently. “It’s more accurate to say that they’re mummified instead.” The original cellulose is gone after 55 million years, but enough cell structure still remains that many of the leaves, branches, and trunks look almost fresh. “The Arctic forests were full of deciduous dawn redwoods, some of them a hundred feet tall, and they covered northern Canada and Alaska for tens of millions of years during the early Cenozoic. You could also find lots of ginkgos and cypress there, which makes sense considering that these species like it wet, and ice-free river corridors and swamps were common in the Arctic back then.”
Most of the trees dropped their needles or leaves every year, not necessarily in response to seasonal cold but more likely to the long darkness of high-latitude winter. However, their absence up there today does reflect long-term Cenozoic cooling that drove the trees south into regions where other species eventually outcompeted them; only a few, if any, undomesticated dawn redwoods and ginkgos survive now in the wild, apparently only in China. And during the last 2 to 3 million years, ice ages finished off whatever remained of those high-Arctic forests.
With woodlands thriving in the polar regions, animals spread widely and diversified as they do in today’s tropics, but some of the richest sources of information about those creatures are located in the dry hill country of northern Wyoming and central Utah, parts of France and Belgium, and China’s Hunan Province, where fossil-laden sedimentary rocks are well exposed for sampling. PETM-aged strata paint distinctive red and purple stripes along the exposed flanks of many such formations, luring paleontologists to generous troves of primitive teeth and bones.
Mammal remains in such deposits tend to draw more attention than those of other animals do, partly because there are lots of them to unearth, but also because the PETM marks the noteworthy first appearances of entire groups—“orders,” in the taxonomic sense. This gives it a special significance for us, being mammals and all. A new Eocene order of cloven-hoofed mammals, for example, gave rise to deer and cattle. Another order with solid hooves later spawned modern horses. And a new lineage of big-eyed, large-brained, lemurlike creatures eventually branched out into those of monkeys, apes, and humans.
But although they help to draw new boughs on the mammalian family tree, those long-extinct animals don’t tell us as much about PETM climates as the plants do because we know little about them other than what their hard parts reveal. In fact, most of us wouldn’t know what to make of them if they showed up in our backyards. Back in the early Cenozoic, the ancestors of horses were poodle-sized, hunch-backed omnivorous things, and protowhales walked on four legs. Snaggle-toothed, long-jawed condylarth predators galloped after their prey on toes tipped with tiny hooves; some scientists half-jokingly refer to them as “wolf-sheep.” And then there were the hyaenodontids, Labidolemurs, and Macro-craniums, whose very names evoke the exotic fringes of biological fantasy.
This unfamiliarity factor hinders our ability to use such creatures as temperature indicators. Fossils of tapirlike animals that were recently found on Ellesmere Island appear to make sense in the context of climatic warming because a modern tapir is rather like a floppy-snouted pig that grunts and dashes about in neotropical rain forests. However, tens of millions of years separate the PETM species from those of today, so these animals weren’t necessarily tapirs as we know them now. Putting such a familiar name on them might therefore mislead us in our climatic sleuthing.
A species like woolly mammoths, which appeared much later in the Cenozoic, can better represent past climates because we’ve found their wool as well as their bones. Unfortunately, most PETM fossils are of the strictly bony type. Imagine that we had nothing but bones left to tell us about mammoths, with no clue about the Big Hair that survives in permafrosted specimens and that so clearly links them to cold weather. An unfleshed mammoth skeleton looks much like that of a modern elephant, and a scientist excavating ice age mammoth bones in Europe might falsely conclude that it was warm enough back then for “elephants” to migrate up from Africa, when in fact the great beasts endured frigid conditions every winter. That’s
the situation we face with the Ellesmere “tapirs.” With only skeletons to go on, we couldn’t be sure that the PETM tapirs liked it hot or cold if we didn’t also know that Ellesmere Island and other polar regions were cloaked in forest rather than the ice and tundra that we find there today.
Another possible climatic clue is that most of the PETM mammals were oddly small, about half the size of their counterparts in older and younger deposits. Some paleontologists have attributed this to warmth because smaller bodies resist overheating better than bigger ones do; the inner furnaces of very large animals can sometimes produce more body heat than their skin easily releases into an already-warm atmosphere. But others counter that elephants, rhinos and giraffes do just fine in tropical Africa today, and that huge dinosaurs thrived during the warm Mesozoic era.
A more recent hypothesis suggests that greenhouse gases caused the stunting by altering the nutritional contents of the plants upon which the animals grazed. The sharp dip in body sizes during the PETM roughly parallels the dip in 13C concentrations caused by the greenhouse gas surge, and high concentrations of CO2 are to many plants as candy is to kids—tasty but not very nourishing. They can make vegetation grow rapidly, but the bodies of plants grown under such conditions tend to run low on other nutritional substances such as nitrogen, rather like a building made of cheap, easily assembled cardboard rather than concrete and steel. In the case of animals, one possible response to such an unwholesome menu is to grow less, and that may be what the PETM mammals did. Some experts worry that future plants and animals might also do the same in a super-greenhouse future.
Evidence for just such a reduction in the food value of plants may be revealed in a collection of leaf fossils excavated from the Bighorn Basin of Wyoming. A study led by geologist Ellen Currano, another former student of mine who now holds a faculty post at Miami University, Ohio, found more signs of piercing, tunneling, and chewing in leaves that were deposited during the PETM than in slightly younger or older layers. “We think that insect populations were more diverse at the study site back then because many species migrated in from lower latitudes with the warmer weather,” she explained. “And another factor could be that the leaves became less nutritious because of the high carbon dioxide concentrations, so the insects had to eat more in order to get the same amount of nutrition from their food.” When Currano’s team described their findings in a recent issue of the Proceedings of the National Academy of Sciences, they ended their article with a warning that future greenhouse gas buildups might have similar consequences.
Whatever the reason for their small body sizes, those early Eocene mammals didn’t seem to mind the weather. Though some species went extinct, including the long-tailed, “almost primate” Plesiadapis and the long-jawed, “sort of crocodile” Champsosaurus, many others appeared or persisted all the way through the long thermal excursion. PETM species seem to have lived pretty much everywhere because the global greenhouse more or less leveled the pole-to-equator temperature differences that now isolate penguins from pandas. Animals wandered more widely than usual in that warmer world, and many that invaded the Arctic also crossed the Asia-America land bridge into vast new territories. When an early primate appeared in China, for example, it dispersed into North America so quickly that it almost seemed to have appeared in both places at once.
We aren’t sure exactly how rapidly the PETM’s dramatic environmental and evolutionary changes happened because our methods of dating very old sediments don’t allow for perfect precision. If they were annually layered, all nicely stacked like sequential pages in some ancient ledger, then we could simply count the laminations across the transition zone to tell exactly how long it took to reach full heat or how long the cooling tail of the PETM’s CO2 curve was. But most deposits aren’t like that, so we have to use relatively blunt methods to get at their ages. Carbon-14 doesn’t work on such ancient records because its atomic clock runs down fairly rapidly, and longer-lasting tools, like radioactive potassium or uranium, don’t clearly register the short time increments that characterize truly rapid or brief events.
However, such concerns are minor. There are plenty of lessons in the PETM example that can be applied to our modern world. For starters, it demonstrates conclusively that a super-greenhouse is not just some doomsayer’s morbid dream; it really can happen.
We don’t know exactly how it began, but we do know that its most notable effects lingered for roughly 170,000 years, which is within the ballpark of what most farsighted computer models are predicting for a relatively extreme-emissions scenario.
We don’t know exactly how fast it began, but we do know that it happened abruptly in geologic terms, reaching peak warmth over the course of centuries. In that sense, our own climb toward thermal maximum in modern times is somewhat similar to the carbon-rich path that once led the world into PETM hothouse conditions, but the slope of our climb is apparently even steeper than that of the earlier one.
We don’t know exactly how much the world warmed back then, but we do know that global average heating on the order of 10°F or more (5 to 6°C) shrank the temperature differences between high and low latitudes, turned the Arctic Ocean into a brackish lake, erased the last large frozen habitats from the continents, and lifted sea levels to their highest possible positions. Similar environmental disruptions are therefore also within the range of possibility for a comparably extreme carbon emissions scenario in Anthropocene times.
We don’t know exactly how much carbon-bearing gas was required to trigger the PETM, but we do know that it didn’t necessarily appear all at once. Positive feedback loops almost certainly played a major role, and they could just as well amplify a modern fossil fuel emission into a supergreenhouse like that of 55 million years ago. Unfortunately, we don’t know exactly what critical threshold of temperature or CO2 concentrations would trigger those feedbacks, and we can only hope that a relatively moderate emissions scenario doesn’t exceed it as well.
We don’t know exactly how high greenhouse gas concentrations rose, but we do know that they warmed and acidified the deep sea enough to devastate bottom-dwelling communities and to burn a red layer into the ocean floor. Sediment cores suggest that it took thousands of years for the worst of the acidification to subside. If we take a 5,000-Gton emissions path into the future, then we’re likely to leave a similar scarlet mark of shame on future sediment records.
We’re not sure exactly how the high CO2 concentrations themselves affected biota, but they might have reduced the nutritional value of plants, stunted the growth of mammals, and encouraged herbivorous insects to attack vegetation more vigorously. Any such effects on future crops, herds, or wild communities would be most unwelcome.
The animals that lived through the PETM weren’t exactly the same as those of today, but we can still learn much from the lessons that they offer. Many species thrived in those warm climates, showing that the higher temperatures weren’t necessarily a hindrance to life on land. But we also see that timely migration was an important key to biological success under those conditions.
We can only guess what it might mean to force a PETM-style climate structure onto today’s complex and unpredictable network of nations. Surely there would be winners as well as losers in any such shift. But the international squabbles that have recently erupted over the Arctic’s newly opening sea lanes would pale in comparison to the territorial conflicts that might follow the de-icing of a mineral-rich continent on the South Pole, not to mention the displacements driven by an eventual 230-foot (70-m) sea-level rise or extreme derangements of today’s climatic zones.
On a relatively bright note, we also know that many plants and animals, including our own early primate ancestors, made it through the PETM just fine. If we could time-travel back to the early Eocene, many of us might even find its climates quite pleasant to live in as long as we could also get used to experiencing them above the Arctic Circle in the company of faux tapirs and snarling wolf-sheep. After all, paleontologists label
the more prolonged hot phase that followed the PETM with the positive-sounding “early Eocene climatic optimum,” rather than something more akin to “climatic disaster,” because life in general seems to have thrived in it.
But we also know that the survivors of the PETM passed through those earlier warm times without human activity working against them. If we launch a new superhothouse of our own, the descendants of those Eocene ancestors will find it difficult to follow us into the future. No longer free to colonize new land-and seascapes as their preferred climatic zones shift poleward, the cumulative wealth of modern biodiversity could melt away along with the great ice sheets in this, the carbon-enriched Anthropocene.
5
Future Fossils
I, unfortunately, was born at the wrong end of time,
and I have to live backwards from in front.
—Merlyn, The Once and Future King
One thing about the greenhouse effect that makes it so difficult for some of us to take seriously is that it’s invisible. Carbon dioxide leaves no stain on the air, no odor betrays its presence, and most of its climatic effects are subtle enough to blend into a camouflaging background of natural weather fluctuations. But for many scientists who make their living by studying invisible things, fossil fuel carbon has a decidedly tangible presence. To them, it’s not just an abstract concept that vaguely influences future generations; it’s a demonstrable fact of modern life, something to be remembered constantly and added to the list of routine adjustments made in the course of a day’s work. This little-known aspect of carbon pollution is even more difficult for most of us to detect than a slow rise in global average temperature, but it’s actually even more pervasive because it permeates our bodies as well as the air, water, and sediments around us.