Summer 1998

<< table of contents  

A    W A R M I N G   A R G U M E N T

Scientists who work with old plants and new
computer models team up to understand
Wyoming's warm winters of 55 million years ago.

By Lila Guterman

IMAGINE CROCODILES creeping under palm trees in Wyoming. Imagine a North Pole with no year-round ice, where forests of tall trees grow above the Arctic circle. Imagine tropical plants thriving outdoors in London and Midwestern winters no colder than comfortably cool.
      These conditions actually existed on Earth. Several million years after the mass extinction that claimed the dinosaurs, another curious worldwide phenomenon took place -- the late Paleocene and early Eocene epochs sustained temperatures as high as 8 degrees warmer than present-day conditions over a period of 5 million years. What's more, winters in continental interiors were surprisingly mild, even for a globally warm period.
      Paleontologists know about the warm winters of 55 million years ago because they have found fossil ancestors of crocodiles and palms -- which cannot tolerate cold weather -- in continental interiors at midlatitudes. Scott Wing, a paleobotanist at the Smithsonian Institution's National Museum of Natural History, has spent two decades unearthing plant fossils from this time period in the badlands of Wyoming. These findings raised a deep question about climate: how did the winters stay temperate so far from the oceans? In today's climate, the interiors of continents at midlatitudes get cold in the winter since they have short days and long nights and lie far from the warming ocean.
      "Lawrence, Kansas doesn't give a darn about what the oceans are doing," UC Santa Cruz paleoclimate modeler Lisa Sloan says. She wondered: did the ocean's stabilizing influence somehow reach farther inland in warmer times? Not likely, she answered in a 1990 paper.
      Sloan and colleague Eric Barron's computer model showed that even during globally warm periods, winters in continental interiors should have been cold, far colder than crocodiles and palm trees could stand. Either something was wrong with their model -- painstakingly constructed to simulate today's climate conditions -- or Wing and other paleontologists had somehow misread their fossils. In short, the notion of Wyoming's warm winters left Sloan cold.
      Her conclusion startled fossil hunters. Many insisted the computer model was simply wrong.

WING AND SLOAN found themselves at opposite ends of the debate, but they agreed on one thing: if fossil gatherers and paleoclimate modelers ignored each others' work, they would miss a great opportunity to understand what the climate was like 55 million years ago, and how it got that way.
      And so, to the surprise of their colleagues, these two scientists started talking to each other at scientific meetings, keeping the lines of communication open even as they disagreed. Wing ultimately decided to come to UC Santa Cruz as a visiting scientist to work with Sloan. "We had this lively discussion about who was right," he says. "I thought it would be much more fun to talk about it in one place."
      Says Sloan, "My hope is that we'll publish a paper together, after nine years of saying, 'She's wrong,' 'He's narrow-minded.'"
      "It's the most productive argument I've ever been involved in," says Wing.
      Fossil gatherers and paleoclimate modelers use very different tools and methods to study a time so distant that any conclusions they reach are based on shaky assumptions.
      Paleontology is the older science: more than 150 years of fossil research point to the warm winters Wing and other paleontologists continue to study. Only since the advent of the supercomputer in 1976 have researchers used models to study the whys and hows of the Earth's climate.
      Paleontology proceeds in small incremental steps as new fossils are unearthed, identified, and analyzed. Each new fossil adds one data point to an already massive body of knowledge. Computer modeling leaps ahead with startling new results, predicting how greenhouse gases or changes in shoreline could have affected the ancient climate.
      But these two disparate approaches feed off each other. Modeling relies on data from the field, such as temperatures, geography, and ages, to make its broad predictions. Fossil hunters and geologists then look for new data to uphold or challenge the model's predictions. The exchange continues as modelers incorporate the new data into their theories. Neither method would explain as much about our world without progress in the other.

THE PROCESS of collecting and analyzing plant fossils is arduous. Because fossilized plant matter is softer than the surrounding rock, wind and weather destroy it. Wing must dig deep into rocks to discover his fossils.
      Furthermore, plants fossilize only under very specific conditions. "Finding plant fossils is part science, part art, and part luck," says Wing. "The part that's science is understanding what kinds of conditions lead to the presence of fossils."
      A fallen leaf fossilizes only if it doesn't get eaten first by earthworms, fungi, ants, or other organisms with a taste for decaying plants. These organisms prefer moist conditions where there is also enough oxygen to breathe. If it's too wet, the leaf-eaters suffocate, leaving the plant to fossilize.
      Quickly settling sediments also favor fossilization. Each layer of sediment protects the leaf from organisms that would destroy it. So paleobotanists like Wing search for plant fossils in sediment that was once very wet and accumulated rapidly. Where floods laid down new earth quickly, layers of sediments are stacked, one on top of the other, making the rock striped. If earthworms had destroyed the plants, they also would have churned up the earth, erasing the boundaries between layers. Wing looks for fossils in dark stripes, because fossil plant material makes rock dark.
      But scientists can look for these visual cues only if the they can see the rock. That's why they end up working in places like Wyoming, where much of the land is desert or semi-desert, and erosion has exposed layers from tens of millions of years ago.
      Once scientists have found dark-colored layers of rock from the geologic time they are studying, they start hunting for fossils. Wing is interested in the climate of the Paleocene and early Eocene, so Wyoming makes the perfect place to hunt -- he estimates that roughly one-third of the exposed rock in the state dates to that time period.
      Using a pick and shovel, Wing collects chunks of rock the size of toaster ovens, and meticulously labels them by location. He splits open a rock by hitting it on its edge. Usually it will break along the surfaces of layers, where the plant fossils hide. If it doesn't, he has to glue the rock together and try again.
      Frequently, Wing says, the Wyoming rocks contain no plant fossils. But where he does find fossils, "there are often gajillions of them." Then the hard work begins. Wing must deduce the fossils' species. And that involves comparing them with other plants, both living and fossilized, to determine where they fit in. Finally, he uses everything he can glean about the plant fossils to draw conclusions about the climate in which they lived.

ONE WAY of reconstructing paleoclimate is to assume the fossil plant required the same climate as its nearest living relative does today. Paleontologists' confidence in that assumption depends on two crucial factors: how old the fossil is and how unified the living relatives are. Younger specimens tend to be more straightforward: the more recent the fossil, the greater its similarity to living relatives. Paleontologists regard many plants from 55 million years ago -- the era Wing studies -- to be at the outer limit of the "nearest living relatives" approach.
      In addition, groups of living plants with similar climate requirements provide more information than diverse groups do about the climates in which their fossilized relatives lived. Oaks, for instance, reveal little about their ancestors' climate, since they thrive in many locales with different temperatures. Palm trees, on the other hand, have much more stringent requirements: none of the thousands of palm species can survive on soil that freezes. Thus, when Wing found palm fronds in Wyoming, he concluded that mild winters were once the rule there. Wing also uses another method to reconstruct climate, one which does not compare fossils to their living relatives but instead draws conclusions from the shape of leaves. Both the size of a leaf and the smoothness of its edges reflect the climate the plant lives in.
      Plants with larger leaves tend to grow in rainy areas. Wing says that's because "most plants are worried about the same things: balancing not losing water and getting plenty of light to make their food." Larger leaves can collect more sunlight, and so are more efficient at photosynthesis, which converts carbon dioxide to food. However, larger leaves also let more water escape.
      So in very dry areas, such as deserts, plants conserve their water by growing only small leaves or sometimes no leaves at all. In wetter locations, such as rainforests, plants can afford to grow gargantuan leaves, Wing says.
      The other shape-based clue to climate rests in the "teeth" along the edges of leaves. Plants with toothy leaves grow in cool climates, where the growing season is short. Having teeth allows increased photosynthesis early in the season. As young leaves grow, the teeth mature fastest and tend to be most efficient at photosynthesis. However, teeth also let more water escape. So teeth can present a problem for a plant living in a warmer environment, where water evaporates more easily. Plants in hot climes tend to have smooth-edged leaves.
      To use these relationships to draw conclusions about past climates, paleobotantists must again assume that the past resembled the present, and that leaf shapes of ancient plants reflect the climate they lived in just as today's leaves do.
      The prevalence of smooth margins in Wing's leaf fossils supports the idea that Wyoming had warm year-round weather from 57 to 52 million years ago, during the late Paleocene and early Eocene. However, leaf shape reveals only the average yearly temperature -- the extremes remain hidden. But since Wing also found fossil ancestors of palms, he concludes that the weather never dropped below freezing. The leaf shape method complements the "nearest living relatives" method, he explains, and they usually agree.

WHEN LISA SLOAN began her modeling studies almost a decade ago, she found Scott Wing's data intriguing. She wanted to know what might have caused the warm winters Wing and others described. At the time, scientists assumed that warm ocean temperatures caused the warm inland winters of 55 million years ago, but no one had tested the assumption. Sloan decided to see if warm sea surface temperatures in a global climate model would cause the ancient conditions paleontologists described.
      The model she used was developed at the National Center for Atmospheric Research and runs on a supercomputer. It's a "terribly complex model developed by teams of dozens of scientists," she says. Climate researchers use the same type of model to predict today's greenhouse warming or El Nino effects. The model is based on the physics of the oceans and atmosphere. Researchers tuned it to simulate modern-day weather conditions and longer-term climate patterns.
      Like the fossil studies, then, paleoclimate modeling relies on the assumption that the present reflects the past -- in other words, that the physical laws linking the land, ocean, and atmosphere today also operated 55 million years ago.
      Sloan enters into the computer such information as the geographic location of land and sea, the elevation of the land, the presence or absence of ice sheets, the types of vegetation and soil, how much energy arrives from the sun, and the composition of the atmosphere. Then the model planet starts spinning and goes into orbit around the sun. To phrase it simply, Sloan says, "I put in all that stuff and let it compute."
      The computer then begins to crunch the numbers. It divides the Earth's surface into 16,200 squares and keeps track of approximately 75 variables for each square. It divides the atmosphere into 18 layers, giving itself almost 22 million points to calculate. It calculates and records these points every 30 minutes on the model Earth. (Time on the model Earth is defined by its rotation just as it is on the real planet -- one spin of the model Earth takes one day. The computer records the variables 48 times in one model day.)
      As the model planet spins, the computer tracks variables such as average annual temperature until the Earth has calmed down from the initial shock of unusual conditions Sloan supplied it with. It typically takes 12 model years for variables such as average annual temperatures to stop fluctuating in the computer model Sloan uses. Then Sloan lets it spin for a few more years. She analyzes the climate variables the computer produces for those years only. The entire process takes about 300 computer hours on a Cray supercomputer.
      When Sloan tested the idea that warm ocean temperatures could cause mild inland winters, the model failed to "predict" the climate conditions the fossils suggested. "I tried all kinds of outrageous temperatures in the ocean and still got cold winters," she says.
      Many paleontologists reacted strongly to her results, insisting the model must be at fault. Despite the backlash, Sloan thinks her results spurred on fossil gatherers who otherwise might have let their finds get dusty in desk drawers. They now felt invigorated to take a second look. Sloan says, "It brought more fossil data out and gave them a framework to put their data in and a windmill to tilt at."

WING AGREES that Sloan's 1990 paper did just that: it gave him a new way to think about the climate of the Eocene. Before the modelers started analyzing the reasons behind warm inland winters, Wing's main concern had been studying ancient plants and how the climate had affected them. Sloan's work made him realize that the implications of the paleoclimate extended beyond its effects on the local flora to understanding the global climate. "The models changed my thinking about why I do what I do," he says.
      He now focuses more on climate, in addition to his continuing work on the plants themselves. He hopes to find more fossil sites in continental interiors to either support or alter the current understanding of the Paleocene and Eocene climate. While at UC Santa Cruz, Wing is also working with Sloan on modeling studies.

IN THE YEARS following her 1990 paper, Sloan continued to seek the cause of the early Eocene's warm winters in continental interiors. Inspired by a controversial suggestion that high levels of greenhouse gases may have existed 55 million years ago, Sloan modeled three scenarios: the early Eocene world with preindustrial levels of carbon dioxide, double preindustrial levels, and six times preindustrial levels. None of these scenarios could predict both Wyoming's warm winters and the ocean temperatures, implying that greenhouse gases alone could not have caused that epoch's climate conditions.
      However, in combination with some other trigger, greenhouse gases may been responsible for the Eocene's warmth. To clarify what role -- if any -- greenhouse gases played in the climate warming and mild winters of 55 million years ago, Sloan needs more data about that epoch's greenhouse gas levels.
      Now she and Wing are changing the continent boundaries in the model, since ocean levels were different and the continental plates have moved in the past 55 million years. The two are also focusing on regional geography. Sloan recently added a large lake in her model of western North America and found warmer winters near the lake. Again she concludes that she needs more data -- in this case, about regional land elevation and where lakes were historically.
      Clouds are another factor that may have been important 55 million years ago. In recent work, Sloan has tried adding high clouds over the polar regions, and she finds that winters in mid- to high latitudes are much warmer when she does this. The model suggests they could be significant, but clouds' importance in the Eocene's climate will be hard to judge until scientists learn more about their location in ancient times. Researchers continually update the models to make them predict today's climate more precisely, in hopes of predicting future climes.
      But an inherent weakness in using models to study ancient or future climates remains: the models are tuned to today's climate. "It's like you've tuned a car to run really well at sea level," Sloan says, "but the minute you take it up to 9,000 feet, it's a dog." The added complication with climate models is that, unlike cars, they don't announce their struggles by sputtering and overheating. Researchers must seek out places where the models fail, and then improve them. "That doesn't give me and other people complete confidence that they're really accurate for other time periods," she says. "I've never considered that the models were completely right."
      A model's best use, then, is testing the potential merit of hypotheses.
      Today, few scientists disagree with Sloan that warm oceans could not have single-handedly warmed the climate interiors. The model results point to gaps in geologic and fossil data. No paleontologist or geologist has yet found evidence that clearly suggests a cause for the early Eocene climate. If Sloan manages to simulate the environment that paleontologists describe, she will still have no proof that those conditions actually existed. "Even if I did find a combination of model factors that produced the climate conditions dictated by the fossil evidence, it would take you right back to the geologic data," she says. Paleontologists' next step would then be to look for more crocodiles and palm trees at midlatitudes on continents not yet examined, such as Asia. Then fossil hunters could throw another challenge to modelers.
      "I tend to keep an open mind about how good the model is," she says. "The data interpretations aren't perfect either. You're looking at two imperfect worlds and trying to get them to match." Sloan doubts these two worlds ever will coincide. But by working together, scientists like Sloan and Wing seem likely to bring them closer together.

IF A MODEL ever succeeds in describing the ancient climate, it will have passed an important test. The model is tuned for the present day, but if the physical laws that governed the planet were the same 55 million years ago, the model should predict the paleoclimate when it's supplied with correct descriptions of such aspects as the land elevation and greenhouse-gas quantities. If those physical laws held true millions of years ago, chances are good that they will in the future as well. "There's a real advantage to testing predictions near the edge of the envelope of understanding," says Wing. A model that can "predict" the conditions of past eras as well as the present may also succeed in predicting the climate of the future.
      In his office at UC Santa Cruz, Wing waits for spring weather. He looks forward to spending weeks in Wyoming collecting new fossils that may add a few tiny pieces to the Paleocene climate puzzle. Suddenly, Sloan rushes in to show him results of her latest model study, involving clouds. By working together to decipher an unfamiliar past, they hope to produce something even more powerful: a model that can describe our future climate.