If a ginkgo tree fell in an ancient forest, what could its leaves tell us about climate in dinosaur times? Plenty, scientists hope.

Richard Barclay opens a metal drawer in archives of the Smithsonian Natural History Museum containing fossils nearly 100 million years old.

Despite their age, these rocks are hardly fragile. The geologist and botanist handles them with casual ease, placing one in his palm for closer examination.

Embedded in the ancient rock is a triangular leaf with rounded upper lobes. It fell off a tree around the time that T-rex and triceratops roamed prehistoric forests, but the plant is instantly recognizable.

“You can tell this is ginkgo,” Barclay said of the tree that’s a tough staple of modern streetscapes. “It’s a unique shape. It hasn’t changed much in many millions of years.”

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What’s also special about ginkgo trees is that their fossils often preserve actual plant material, not just an impression. And that thin sheet of organic matter could prove to be key to understanding the ancient climate system — and the possible future of our warming planet.

But first Barclay and his team need to crack the plant’s code to read information contained in the ancient leaf.

“Ginkgo is a pretty unique time capsule,” said Peter Crane, a Yale University paleobotanist.

As he wrote in “Ginkgo,” his book on the plant, “It is hard to imagine that these trees, now towering above cars and commuters, grew up with the dinosaurs and have come down to us almost unchanged for 200 million years.”

“The reason scientists look back in the past is to understand what’s coming in the future,” said Kevin Anchukaitis, a climate researcher at the University of Arizona. “We want to understand how the planet has responded in the past to large-scale changes in climate — how ecosystems changed, how ocean chemistry and sea levels changed, how forests worked.”

Of particular interest are “hothouse” periods when scientists believe carbon levels and temperatures were significantly higher than today, including a period during the late Cretaceous period — 66 million to 100 million years ago — the last era of the dinosaurs before a meteor slammed into Earth, and most species went extinct.

Learning more about hothouse climates also gives scientists valuable data to test the accuracy of climate models for projecting the future.

Climate information about the distant past is limited. That’s where the Smithsonian’s ginkgo leaves come in.

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From a cabinet, Barclay withdraws sheets of paper on which Victorian-era scientists taped and tied ginkgo leaves plucked from botanical gardens of their time. Many specimens have labels written in beautiful cursive, including one dated Aug. 22, 1896.

The leaf shape is virtually identical to the fossil from around 100 million years ago and to a modern leaf Barclay holds. A key difference can be seen with a microscope — how the leaf has responded to changing carbon in the air.

Tiny pores on a leaf’s underside are arranged to take in carbon dioxide and respire water, allowing the plant to transform sunlight into energy. When there’s a lot of carbon in the air, the plant needs fewer pores to absorb the carbon it needs. When carbon levels drop, the leaves create more pores to compensate.

Scientists know the global average level of carbon dioxide in the atmosphere is about 410 parts per million — and Barclay knows what that makes the leaf look like. Thanks to the Victorian botanical sheets, he also knows what ginkgo leaves looked like before humans significantly transformed the planet’s atmosphere.

Now, he wants to know what pores in the fossilized leaves can tell him about the atmosphere 100 million years ago.

But he needs a codebreaker, sort of a Rosetta stone to decipher the handwriting of the ancient atmosphere.

That’s why he’s running an experiment in a forest clearing in Maryland, where he and project assistant Ben Lloyd tend rows of ginkgo trees within open-topped enclosures of plastic sheeting that exposes them to rain, sunlight and changing seasons, “so the plants experience natural cycles,” Barclay said.

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The researchers adjust the carbon dioxide pumped into each chamber, and an electronic monitor outside flashes the levels every five seconds.

Some trees are growing at current carbon dioxide levels. Others are growing at significantly elevated levels, approximating levels in the distant past or perhaps the future.

“We need something to compare with,” Barclay said.

If there’s a match between what the leaves in the experiment look like and what the fossil leaves look like, that will give researchers a rough guide to the ancient atmosphere.

They’re also studying what happens when trees grow in super-charged environments and found that more carbon dioxide makes them grow faster.

But Barclay said, “If plants grow very quickly, they are more likely to make mistakes and be more susceptible to damage.”

Carolyn Kaster / AP