Climate’s Timekeeper

From the Mongolian steppe to a laboratory in New York, one geochemist is uncovering the secret history of climate change — one boulder at a time

Climate scientist Aaron Putnam records his notes while curious locals observe. Putnam led the reconnaissance expedition in August 2014 at the Altai Tavan Bogd National Park in Mongolia. (Photos by Jennifer Draper)

Time travel has always been possible if you know enough about geology. That’s the view of Aaron Putnam, the slender, soft-spoken climate scientist hiking on a brisk August morning through golden plains that lead to the Altai Mountains in western Mongolia. He and five others haul brightly colored packs full of clinking chisels toward larch trees surrounding a stream populated by fat gray rocks. Putnam’s headed toward the hills beyond the rushing water, where the boulders, he insists, hold time’s secrets.

The 33-year-old geochemist belongs to the next generation of climate scientists at Columbia University’s Lamont-Doherty Earth Observatory. Putnam carries on the mission set forth by his colleague, Wally Broecker, the observatory’s researcher credited with coining the phrase “global warming” in 1973. Broecker’s partnership with the founder of Lands’ End, the late Gary Comer, shaped the research foundation that brought Putnam more than 6,000 miles to this Mongolian steppe. Broecker and Putnam suggest that as fossil fuels warm the planet, continents in the Northern Hemisphere will heat up twice as fast as oceans in the Southern Hemisphere, according to their 2013 study in the Proceedings of the National Academy of Sciences. As a landlocked country on Earth’s largest continent, Mongolia may model these changes. Understanding how the climate operates here, Putnam says, could help scientists interpret conditions in the future.

Breakfasts of yak-milk yogurt and Nescafe Gold instant coffee fuel the team for a daylong exercise in drilling, pounding and scraping boulder tops for samples. Trees disappear on the other side of the stream, exposing herds of small but hardy Mongol horses that roam the rugged hills. Those hills, covered in tan grasses and large granite rocks, mean more to Putnam than to the casual observer. He’s been itching to come here for years. Snowcapped mountains separate Altai Tavan Bogd National Park from China to the southwest. A few white dome-shaped gers house Kazakh nomads on the shoreline of nearby Lake Khoton. But it’s those hills in front of us that excite Putnam most. Known as moraines, the hills reveal ancient glacial activity that once pushed boulders, gravel and dirt together. He says each ridge marks a different time period.

Altai Tavan Bogd National Park, August 2014

For Putnam and his team, getting here meant two long flights and a week of driving over bumpy terrain from Ulaanbaatar, Mongolia’s capital. In his right hand he holds an iPad with satellite images of the topography to guide us because there are no maps and no marked roads. Thousands of years ago, the rise and retreat of lakes, ice sheets and mountain glaciers scarred the landscape in ways that Putnam reads now.

Grasshoppers jump around our boots dampened by dew on the grass. Putnam pivots, scanning the panorama. “Glaciers are incredibly sensitive indicators of climate change,” he says. “And they’ve left a spectacular record imprinted on the landscape.” The earth’s ice was thickest about 21,000 years ago, a period known as the Last Glacial Maximum. The last ice age ended abruptly about 3,000 years later. Putnam and his colleagues figure out what naturally causes these massive switches in climate. It helps distinguish the impacts of man-made global warming from the earth’s natural climate dynamics.

Today, Earth’s orbital relationship to the sun is about the same as it was 20,000 years ago. Every year the Northern Hemisphere is tilted closest to the sun, known as the summer solstice, when Earth travels to the farthest point from the sun along the orbital ellipse, during a period known as aphelion. That means the Northern Hemisphere receives roughly the same amount of solar radiation during the summer as it did during the peak of the last ice age, Putnam says. “So the question is why aren’t we in an ice age?” he adds, glancing at his iPad. On the satellite image, the series of moraines look like giant ripples. The outermost ripple marks the oldest, farthest reach of the glacier while each subsequent inner ripple marks a progressive recession of that glacier in more recent times. A set of samples from boulders on each moraine crest establishes a time frame for the ancient glacier’s behavior, Putnam explains.

In the laboratory, Putnam will determine the length of time each rock has been exposed to the sky since it was first deposited by the Mongolian glacier. This information will reveal when and how fast the glaciers melted. “Glaciers are such good thermometers because they have no strategy for survival,” Putnam says. Other scientists measure past climate changes in tree rings or algae. But these living sources can yield complex results since they struggle to adapt to changing environments.

For this geochemist, glaciers hold the key to discovering past climate changes and the effects of global warming today. Solar radiation dominantly drives Mongolia’s temperature, so it’s particularly vulnerable to negative impacts. In recent years, the warming trend has triggered more frequent droughts, killing the majority of the grassland and millions of livestock in Mongolia, according to the Intergovernmental Panel on Climate Change.

Putnam and his colleagues’ analysis of past rapid climate changes in the Asian continent paints a clearer picture for policy makers about what civilizations have faced, and will need to adapt to, as the planet continues to warm.

After an hour of hiking at more than a mile above sea level, the plains end near the swift stream. It separates us from the first glacial moraine, where Putnam’s team will drill three inches into the boulders for samples. “Where we stand marks the onset of the last great global warming that brought an end to the last ice age,” Putnam says, rubbing his unshaven jaw. He reenacts the glacial retreat with his right hand swooping down from the hilltops to the stream in front of us.

“Where we stand marks the onset of the last great global warming that brought an end to the last ice age.”

The water feels cold. Putnam leaps from the bank to a boulder exposed in the stream. His ankles wobble then straighten as his arms fly out from his sides to regain his balance. He pauses, assessing his next move. Humming insects circle his red flannel shirt, grey cargo pants and sturdy brown boots. His left hand clinches the strap of the turquoise pack on his back. Then he bends his knees and his right leg extends forward over the clear, gurgling water. The six-foot-one-inch expedition leader jumps to the next boulder. He crosses the stream like he’s hopping across the backs of baby elephants.

Putnam lands on the opposite bank and turns back to wave at a man who looks remarkably like an older version of himself. It’s his father, David Putnam, an archeologist who’s teamed up with his son on previous climate science expeditions, including to China’s Tarim Basin, the Sierra Nevada and the Rocky Mountains. The pair have also written two scientific papers together about glacial activity in Maine and Wyoming.

“Without the field data, modeling is useless, and vice versa,” says David, a professor at the University of Maine at Presque Isle. “But fewer and fewer people are doing the fieldwork.” He explains that his son’s ability to “ground truth” makes him crucial to the scientific process. Doctors don’t take an X-ray of a tumor and infer that it’s cancerous, he says. “They go in and biopsy it to find out,” he adds, describing the necessity of fieldwork. His burnt-orange button-up shirt and khaki pants remain dry after a hop and a skip across the stream’s boulder tops.

Both men huddle together on the bank, pointing at the moraines ahead, where a Kazakh man with a black newsboy cap herds hundreds of bleating goats. Like his son, David is tall and thin, with ash brown hair and twinkly eyes that scan the terrain like the golden eagle that flies above us. As a kid, Aaron tagged along on his father’s archeological digs. “Then he became my research assistant,” David recalls, adjusting his sage green baseball cap. “That was the point when it wasn’t a father-son relationship, we were partners.”

Aaron and David Putnam

These days, Aaron considers his father an asset to his team, especially during this reconnaissance expedition in Mongolia. “He’s thought about those things in ways that I haven’t,” Aaron says, citing his father’s insights and experience with stratigraphy, or the study of sediment layers. For Aaron, dissecting the climate change problem is a family affair. But it’s not only when gathering clues with his father in the planet’s remotest places. His wife, Kat Allen, unravels relationships between the ocean and past climate changes at the Lamont-Doherty Earth Observatory as well.

Those of us with shorter legs take a few minutes longer to join the Putnams on the other side of the stream. Aaron extends his right hand toward a young woman standing on a boulder three feet from the stream’s edge. After tucking a strand of reddish-brown hair behind her ear, her bright blue eyes calculate the distance in front of her. She leaps forward. Aaron grabs her arm to ensure that the 21-year-old biology student, Hayley Wolcott, lands on solid ground. She’s the granddaughter of distinguished climate scientist George Denton, who taught both generations of Putnams at the University of Maine.

David credits Denton for the most important lesson he learned as a student: never ask a small question. “Always ask a question of global significance, and then figure out little ways to test it,” David says. That’s exactly what the team is doing in this catchment of the Altai Mountains. Figuring out how climate change works here solves a portion of the global climate puzzle, David says, as three other team members reunite with Wolcott and the Putnams on the riverbank. Then, with Aaron leading the way, the team continues its hike on a cattle trail winding toward the closest moraine less than a mile ahead.

White billowy clouds cast large shadows on the team members as they surround the first boulder lodged at the top of the moraine. The perfect boulder, Aaron says, has not rotated or deteriorated since the glacier first dumped it on the land and exposed it to the sky. He bends over the tire-size rock and rubs its surface. His father leans in, too. Their hands feel the glossy texture of the lopsided boulder, assessing it like palm readers. “This one’s perfect,” David declares. An ancient glacier has polished it smooth. It’s also granite, which contains the mineral quartz, needed for testing in the laboratory, Aaron explains. The team deposits its rucksacks on the cheat grass.

Further down the spine of the moraine, a Kazakh woman rides a chestnut horse. She stops to observe the fieldwork. Most of her face hides behind a pink veil and a white headscarf. Her exposed eyes gaze at the team unloading chisels, wedges and hammers from bags. A breeze blows from her direction toward us, bringing with it the odor of the brown and black goats crossing the summit behind her. These pastoral herders become fixtures in the landscape just as much as the boulders.

David attributes the Mongol leader Genghis Khan’s successful expansion across these plains in part to climate change in the thirteenth century. He presented his hypothesis at the 2013 Comer Abrupt Climate Change Conference, suggesting that a period of cooling and glacial growth associated with the Little Ice Age created colder, wetter conditions that benefited the Mongols’ herds. The idea originated in the Taklamakan desert, where he and Aaron found centuries-old signs of water, including mud cracks, mollusk burrows and dried tree stumps. Knowing how climate change influenced the rise and fall of empires would aid the modern world, he says. It would shed light on what today’s societies face dealing with heat waves and drought as the planet warms.

Yet detecting present and future patterns of global warming starts with these boulders’ past. “We’re going to reconstruct the history of this mountain glacier system,” Aaron says, drilling three holes at 20-degree angles into the boulder. He blows the rock dust away, which forms a white nebula around him. The two polite, diligent Mongolian geology students insert metal wedges and shims into each hole. Aaron’s collaborator, Rutgers University biologist Olaf Jensen, hammers one wedge at a time in a slow tempo that sounds like strikes on a xylophone. His cream hat shades his face from the sun directly above us. With an audible crack, the wedges split a fragment from the boulder top.

The group forms an assembly line of drilling, hammering and documenting each boulder that Aaron selects on the moraine ridge. The students record each boulder’s GPS location and measure its dimensions; some of the rocks are bigger than the tents six miles back at basecamp. They shout the numbers to David. He records the data and sketches each boulder in his yellow notebook. His son takes 360-degree photos of each boulder. “Not only are we seeking answers to these great puzzles of earth science from the natural world,” Aaron says, “but we’re also educating young people in the process and forming cross-cultural collaborations that I hope will last well into the future.”

Then Aaron squints through a small, rectangular device that looks like a telescope. The clinometer measures the height of the surrounding topography. It points out any disruptions to the incoming cosmic rays that blast apart silicon and oxygen in the boulders, producing a rare nuclide called beryllium-10, which Aaron will measure in each sample later at the observatory. Because the beryllium-10 builds up in the rock at a specific rate, he can time stamp the sequence of moraines to chart the ancient glacier’s trajectory.

The team collects about a dozen samples during the day. Aaron places the last piece of granite rock, freshly severed from the boulder, into a small canvas bag. The team will stack the bags into large, blue barrels that Aaron purchased at a Mongolian black market. Then the boulder shards will travel across the world to the observatory in Palisades, New York, where Aaron and his colleagues crack climate change codes.


One brisk November morning four months later, Aaron drives his maroon truck on a wooded two-lane road toward the Lamont-Doherty Earth Observatory on the Hudson River, about 15 miles north of New York City. Within the Columbia University complex, scientists like Aaron decipher the earth’s mysteries. The broken window in his truck’s back canopy reminds him of the robbery that occurred the day before he left for Mongolia. The thieves stole his father’s fiddle. Raven feathers hang on his dash mirror, swaying as the road bends between a forest of red and orange foliage. The feathers symbolize his Nordic heritage, Putnam says. It seems fitting for someone who grew up in Barrow, Alaska, where he often explored the northern wilderness.

The radio announcer forecasts frosty weather. “I hope the chill doesn’t kill my tomatoes,” Aaron says. He’s not worried about his carnivorous plants, like Venus flytraps and pitcher plants, that grow in the fenced bog behind his small white house. “They popped up during the period of ice ages, just like humans,” he says, adding that he’s collected the ice age relics since he was a kid. Then he parks near an ultramodern, cubical building with large glass windows. We walk toward the Gary C. Comer Geochemistry Building, where scientists figure out how abrupt climate changes, such as those associated with the end of an ice age, occur across the planet. Putnam stores the boulder samples from Mongolia in the basement, he says.

Inside we climb three flights of steel stairs to the laboratory where Putnam and his colleagues extract minerals from rock samples. After exchanging his boots for black Crocs, Putnam dons his white lab coat that’s missing all but one button, but held together by Velcro strips. A radio sits on the back window’s ledge, playing Queen’s “Bicycle Race” song.

Putnam removes a plastic container the size of a shoebox from an overhead cabinet. The box contains 12 glass beakers filled with what looks like powdered sugar. The contents are quartz grains extracted from boulders, this time from Michigan, Scotland and China. Putnam seeks granitic boulders, loaded with quartz, in the field to use for “surface exposure dating” here in the laboratory. This technique identifies how long cosmic rays from outer space have been bombarding silicon and oxygen atoms that make up quartz in the rock surfaces. This process creates a rare nuclide called beryllium-10 that builds up over time at a certain rate. “It’s crazy that this method works, but it does, and it seems to work really well,” Putnam says. “It’s this cosmic clock based on nuclear physics,” he adds. This clock can answer Putnam’s question of when the climate warmed and triggered the glacier melt that exposed these boulders to the sky.

“It’s this cosmic clock based on nuclear physics.”

The reason why this technique works so well, Putnam says, is because the beryllium-10 nuclide is created by one process that’s similar to a game of pool. “When you have the cue ball and you go in for the break, well this is like the break,” Putnam says. A primary cosmic ray, probably generated by an exploding star somewhere in the galaxy, slams into the atmosphere and releases a whole set of new particles, he explains. It breaks apart atoms that make up air and produces a rain of secondary cosmic-ray particles. A neutron generated from this process comes screaming down from the upper atmosphere and collides with oxygen and silicon atoms in quartz minerals exposed at Earth’s surface. This cosmic-ray neutron contains enough energy to blasts apart oxygen and silicon atoms, creating a series of cosmogenic nuclides. One of these cosmogenic nuclides is beryllium-10.

“So we know that every atom of beryllium-10 was created when one of these cosmic ray particles smashed into the rock,” Putnam says. Scientists tested the technique on geological sites with known ages and identified the rate, or how often the collisions occur to create this particular nuclide. About four atoms of beryllium-10 are created per gram of quartz, per year at sea level. “It’s not many, but enough that we can measure it,” Putnam says. This number changes as the elevation increases because of the greater incidence of cosmic rays making it through the thinner atmosphere, he adds. Yet each chain reaction within these boulders turns the gears in Putnam’s time machine.

Putnam spoons the white mineral into a Teflon container on a small scale in a small, adjoining room. He records the weight of each sample in a red composition notebook. With a small pipette, he squirts a few clear drops of “carrier” into the quartz. This substance contains a known quantity of beryllium-9 atoms, against which the number of beryllium-10 atoms can be measured by an accelerator mass spectrometer at the Lawrence-Livermore National Laboratory in California. Putnam then moves the tray of samples under the fume hood. He adds hydrofluoric acid to begin dissolving the quartz into a solution from which he can eventually extract the beryllium. He returns the slowly dissolving samples to the corner of the fume hood for now.

What’s puzzling Putnam and the head of the laboratory’s Cosmogenic Dating Group, Joerg Schaefer, is how consistent the boulder records are around the world. “Each of these freaking boulders has the same amount of beryllium in them,” Schaefer exclaims in his German accent. This means that the samples Putnam and others collect around the world seem to show that the last ice age abruptly ended in all of these different areas at about the same time. “But you have to prove that,” Schaefer says, running a hand through his blond hair. “We have this perfect key to this treasure box of climate,” he adds, describing the surface exposure dating technique. He describes Putnam as a part of the next generation of scientists using the beryllium breakthrough and applying it in the field. “It’s all related to this big question of when exactly did the last ice age terminate in different areas,” he concludes.

“We have this perfect key to this treasure box of climate.”

The answer to how the last ice age ended is the chicken-or-egg mystery that Putnam and his colleagues are attempting to solve. “One of the big uncertainties in climate science right now is understanding the sensitivity of the atmosphere to carbon dioxide,” Putnam says. If carbon dioxide terminated the ice age, as some have suggested, then the atmosphere is highly sensitive. “It would mean that 100 ppm of carbon dioxide brought the ice age to an end,” he adds, citing that during the last century humans have already increased carbon dioxide by 130 ppm and counting. On the other hand, if instead it was the end of the last ice age that caused carbon dioxide to rise, then this greenhouse gas was a reaction and the atmosphere has a lower sensitivity. If this is the case, Putnam and his colleagues could place limits on how much carbon dioxide it takes to warm atmosphere by a certain amount. Putnam’s quest to find this answer links his ice age research to the information policy makers and disaster planners need to know.

Deciphering what role carbon dioxide played in the last great warming that ended the ice age represents a central problem driving climate research. To crack this climate code, Putnam finds his Rosetta stone in ancient mountain glaciers. Clues in the form of moraines, like those in Mongolia, help him build a timeline of the Earth’s climate history. “It’s similar to how chronology is important to a crime story,” he says. Police need to know exactly when a crime happened and identify where a suspect was when that crime occurred. The suspect in this case is carbon dioxide.

Knowing exactly when and how fast the last ice age ended is a key to solving this riddle. Then Putnam and his colleagues can pinpoint whether or not carbon dioxide was the culprit in the past warming spike and apply it to today’s climatic conditions. Ultimately Putnam hopes to define the circumstances that led to the largest climatic warming in the history of the human species. For now, he’s still collecting clues.

Putnam places the box of quartz samples back in the white cabinet. He takes off his lab coat and hangs it on a wall hook. We leave the laboratory and walk across the hall to his office. He grabs his coat and car keys from his desk, which is surrounded by books, expedition mementos and musty field equipment. He’s headed three miles north to the Sidewalk Bistro in Piermont, New York. It’s the usual Friday evening ritual for a select few scientists that meet to discuss climate change problems over a drink.


The usual group orders martinis and glasses of Malbec wine when Putnam enters the Sidewalk Bistro about 15 minutes later. Wally Broecker, dubbed the father of global warming research, has been sitting at a table here each week to chat with other scientists about climate change. Putnam spots Broecker, wearing a blue sweatshirt and jeans, talking with three others at a square table in the corner of the French restaurant. The waitress sets a breadbasket on the white tablecloth. Putnam grabs a spare wooden chair from a vacant table and squeezes in the space they’ve made for him.

This week the group includes Columbia University professor Sidney Hemming and Rutgers University professor Dennis Kent. Kent, like Broecker, is a member of the National Academy of Sciences. He solves climate change puzzles by using changes in Earth’s magnetic field trapped within rocks to tell time. After he shakes Putnam’s hand, they join the table talk about the late George Kukla, another founding father of climate change science at the Lamont-Doherty Earth Observatory. “He was one of my best friends,” Broecker says, as the group shares fond memories of their colleague.

Putnam sits across from a smiling woman with curly brown hair and a purple sweater. He smiles back at his wife, Kat Allen, an oceanographer splitting her time between the Lamont-Doherty Earth Observatory and Rutgers University. The couple met at the observatory and has been married for two years. Allen helped make Putnam’s fieldwork in Mongolia last summer possible. The year prior, she had gone to an art exhibition that featured photos of Mongolia taken by a Rutgers University student. There, Allen met the student’s mentor, biologist Olaf Jensen. He analyzes the impact of Mongolia’s climate on its lake and river ecosystems. Then Allen made a move that led to Jensen’s future collaboration with her husband. She orchestrated a meeting between the two scientists.

Aaron Putnam (right) and Olaf Jensen (left)

They met in a New York coffee shop and talked for five hours about their research, Putnam says. That’s when he and Jensen started planning his first expedition in Mongolia, he recalls. “Olaf and I share a similar interest in understanding how Earth’s climate operates, and how it will affect humans and ecology,” Putnam says. “We’ve already generated so many ideas about how our two fields of expertise could come together to solve the common problem,” he adds. The fieldwork last summer in the Altai Mountains is only the beginning of his climate change research in Mongolia, according to Putnam. In the next few months, he will process that first set of quartz samples from the Altai Mountains, but he’s already planning a return trip to collect more data.

Mongolia’s sensitivity to climate change illustrates the knowledge needed for people to adapt to its effects there, and elsewhere around the world. During his fieldwork in Mongolia, Putnam says he realized how great of a need there is to protect water resources and food supplies. “The level of these Mongolian lakes is determined by climate change,” Putnam says, because they don’t connect to the ocean and instead lose water through evaporation. “What are the downstream effects on fisheries and human subsistence patterns?” he wonders. As the world heats up, the environment that these traditionally nomadic people depend on is at risk. It’s crucial to understand how past climate change impacted the region and how it relates to the rest of the planet, he says.

As the waitress refills his water glass, Putnam declares that he’s determined to return to Mongolia. He admits that when he first stood at the base of the moraines, he was surprised at the vast scale of the last ice age’s glacier system. It was impossible to grasp it from the satellite images alone, he claims. Those massive hills that we walked across in the Altai Mountains preserved a rich climate history that remains largely untapped, according to the eager geochemist. There are endless numbers of boulders worthy of sampling, he says, scooting his chair closer to the table. “It presents this exceptional opportunity to get a superb record of climate in this part of the world,” Putnam adds. “And we’ve just scratched the surface.”


Jennifer Draper documented Aaron Putnam’s research expedition thanks to a Carnegie Science Reporting grant, the Comer Family Foundation and Northwestern University’s Medill MSJ program. Special thanks to Aaron, his colleagues and family for their exceptional kindness and patience during this once-in-a-lifetime reporting opportunity.

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