When Scott Wing is hunting for plant fossils in the badlands of Wyoming’s Bighorn Basin--a vast, lightly peopled zone of sagebrush and weathered hills that the Smithsonian National Museum of Natural History paleobotanist has visited off and on for more than 30 years -- sometimes his attention strays.

"You can hardly help it," he explains. "If you’re collecting fossils, you’re sitting in a hole in the ground. In between using the pick and the shovel to make the hole bigger, and pulling out the biggest pieces of rock and splitting them open with a hammer, it’s not uncommon to think about what the landscape would have been like" back when the Bighorn Basin was still being formed and the fossil leaves Wing seeks were alive and green.

If Wing’s hole in the ground could transport him to that time--55 million years ago, when the late Paleocene era was giving way to the early Eocene era--the scientist would find himself surrounded not by desert sagebrush and rattlesnakes but by palm trees and ferns, alligators and cat-sized horses. "The climate was very different," he says, and the landscape "would have looked something like a forest in Florida."

The Rocky Mountains that surround the Bighorn Basin would still be rising, rivers crossing the basin would be depositing massive quantities of mud and sand shrugged off by the mountains, and those sediments would be capturing the plant parts that Wing digs for every summer.

What Wing is learning about this long-ago world is helping scientists understand how plants will respond to the global warming being caused today by the burning of massive amounts of fossil fuel and the resulting build-up of greenhouse gases, such as carbon dioxide, in the atmosphere.

Thermal maximum
Wing is studying fossils from a sliver of time known as the Paleocene-Eocene Thermal Maximum, or PETM, a geologic eyeblink when that warm, moist world of 55 million years ago grew even warmer. In just about 10,000 years, the basin’s climate went from Floridian to something more like southern Mexico.

"It was a global warming on top of an already globally warm situation," Wing says. Earth’s average surface temperature rose 4 to 8 degrees Celsius and stayed that way for the next 80,000 to 100,000 years.

The Bighorn Basin’s eroded hills expose rock from the Paleocene-Eocene boundary. That rock contains evidence of changes brought on by the PETM: fossils of primates and other mammals that migrated to North America when the warming exposed Arctic land bridges to Asia and Europe. But only recently have paleobotanists like Wing had proof of how the PETM affected plant life.

"Things started to click in the summer of 2003," says Wing, who that season found the first fossil leaves and pollen known to be from the PETM. Since then, six PETM plant sites have been identified, all in the Bighorn Basin, and in November 2005, Wing and his team reported their findings in the journal Science.

Pushing north
What they have learned is that during the PETM, global warming pushed several kinds of plants northward into the basin. These include ancestors of warmth-loving plants like the paw-paw and poinsettia. Fossils reveal these same plants had long thrived in Mississippi, Louisiana and Texas. But during the 10,000 years that worldwide temperatures climbed to PETM levels, several species of Southern plants surged north some 1,000 miles. Then, at the end of the period of global warming, those plants disappeared from the Bighorn Basin.

The last time plants anywhere on Earth changed their ranges so widely and rapidly was when glaciers retreated from the Northern Hemisphere some 20,000 years ago. "So it is known that plant populations have the capacity to adjust their ranges relatively quickly," Wing says. But when the glaciers withdrew, they left behind a landscape scraped clean of vegetation, exposing territory ripe for colonization by plants.

That wasn't the case during the PETM. Even before that global warming event, North America was carpeted with forests to the shores of the Arctic Ocean. "There were already plants living all over the place," Wing says. Thus the climate change caused by the PETM favored some plants--invading Southern species that replaced earlier basin plants--over others. That makes the fossil plants from the Bighorn Basin the first "real examples" of how rapidly plants can respond to warming in an already warm world, Wing says.

Hot topic
Knowing how plants have responded to global warming in the past may help in predicting the effects of future climate change. In fact, the PETM is a hot topic among scientists, and not just those with their eyes on the past. "We had a conference last summer, and there were close to 200 people there who were all working on various aspects of this event. It’s a very lively topic right now," says Wing, adding that there is a good reason for all the interest in the PETM. "It’s seen as being relevant to the future."

Not only was the PETM a rapid change in climate, like the accelerated warming we are witnessing today, but the massive quantity of carbon (about 5,000 gigatons) that was released into the atmosphere during the past event "is roughly the same amount of carbon that we estimate humans are going to produce during the next 500 years by burning fossil-fuel reserves," Wing says.

That makes the PETM the best geologic analog to climate change now being caused by human production of carbon dioxide and other greenhouse gases.

What caused the PETM? No one knows for sure, but one theory suggests that rising ocean temperatures, or perhaps an undersea earthquake, led to the melting of ice containing methane, which was trapped in sediments on the ocean floor. Methane is a powerful greenhouse gas. Released into the ocean and atmosphere, it would have reacted with oxygen, producing still more greenhouse gases in the form of carbon dioxide and water vapor. Those gases may have set in motion the abrupt, worldwide warming, the effects of which Wing reads in the fossils he excavates in the Bighorn Basin.

Field guide
Good finds from his summers of fieldwork are carefully wrapped and brought back to the National Museum of Natural History in Washington, D.C. There, Wing and museum volunteers "prepare out" the specimens. "Almost always, part of the thing that you’re interested in is still covered up," Wing explains, so he and his helpers must painstakingly chip away the fine-grained, pale mudstone that contains each fossil leaf.

Photos and descriptions of the leaves end up in fat binders that Wing calls a "field guide to the flora" of the Bighorn Basin of 55 million years ago. The shapes and sizes of leaves, whether of plant species still living today or those from the distant past, are highly correlated with climate. By analyzing each fossil leaf--its overall size, as well as the features of the leaf margin--Wing can draw conclusions about changing levels of precipitation and temperature during an event like the PETM.

'Postdictions'
For several years, Wing has shared his findings with climatologists who create  the general-circulation computer models used to predict the rate of future climate change, including global warming. Wing calls this work "one of the most important things that we do with this fossil record."

"The projections of how much warming there will be as the result of anthropogenic carbon dioxide emissions are based on computer models," Wing adds, "and the truth is that we don’t really know how well those models work at doing their job. They all show warming, but they show different amounts of warming."

Wing and other paleontologists are helping climate scientists test the accuracy of the models’ 'postdictions' of temperature and rainfall patterns in the past against what is actually known about those past conditions from the fossil record. That work can help reveal the strengths, as well as the weaknesses of models that government policy makers are relying on to predict the severity of future warming.

"It’s the time-machine aspect of things that I enjoy," Wing says of his work in the Bighorn Basin, where pick-and-shovel labor among the sagebrush can set his mind roaming backward across the ages. But Wing’s time machine clearly travels forward, too, with the fossil leaves he unearths and studies revealing as much about our world’s warm future as about its past.