“What we get is, ultimately, a record of past atmospheric gas concentration and temperature on the same time scale. You can’t get that from tree rings or ocean sediment samples,” said Severinghaus, a professor at the Scripps Institution of Oceanography in La Jolla.
His work and studies by other researchers, conducted in some of the world’s most forbidding locations, show that “today’s heat-trapping gas concentrations are unprecedented in the entire ice-core record of 800,000 years. Ice cores also show unambiguously that fossil-fuel burning is the cause of the current (carbon dioxide) rise.”
Policymakers and others continue to debate the pace of global warming, why average temperatures have flattened in recent years and how much money countries should invest in adapting to the effects of climate change.
But there’s wide agreement on the value of ice-core science.
Like the carbon-dioxide measurements started by Scripps professor Charles David Keeling, ice-core analysis has been a pillar of climate science since the 1950s, when researchers first began drilling polar ice to scrutinize its chemical timeline.
But in contrast to the Keeling Curve, which tracks atmospheric carbon dioxide from 1958 to the present, ice-core records extend backward in time to measure climate conditions tens of thousands of years ago.
While the Keeling Curve is enshrined at the National Academy of Sciences and received extensive news coverage when carbon dioxide peaked at historic levels last year, studies of ancient ice have proceeded quietly behind the scenes.
Severinghaus discussed his research during a lecture in La Jolla this month, as the Intergovernmental Panel on Climate Change released the third of three sweeping reports about the extent of climatic shifts and how countries should respond to them.
“Taken together with projections of future fossil-fuel burning, ice-core research allows scientists to make predictions of future temperature with high confidence,” he said.
“My goal is to be an honest broker in the legitimate debate over the best response to climate change, by providing impartial scientific advice,” Severinghaus added. “Values and priorities will determine an individual’s tolerance for risk and preferred response to climate change, so the best course of action is not really a scientific problem. It’s a discussion that all citizens of our democracy need to be having.”
ICE AND GEOLOGY
Near the beach at Scripps is Severinghaus’ lab, one of the world’s leading sites for ice-core research.
There, in a subzero freezer, Severinghaus and others store samples mined from polar ice sheets and slice off sections for analysis. Scientists extracted the cylinders using specialized drills.
Outside the storage area is a rack of hats and snowsuits — essential gear for the job, but incongruous among the surfboards stacked in other offices on campus.
Stored in a -7F basement, Dr. Jeff Severinghaus, a professor of geosciences at Scripps Institution of Oceanography, cuts an ice core with trapped bubbles of gases he studies to track changes in ancient climate at Scripps on Friday in San Diego, California. — Eduardo Contreras
In his lab upstairs, Severinghaus and his team use an assemblage of flasks, tubes and burners to melt the samples and decrypt the secrets locked in the ice bubbles.
Born in the Bay Area, Severinghaus studied geology at Oberlin College, earned a master’s degree in geological sciences from UC Santa Barbara and received a doctorate from Columbia University’s Lamont-Doherty Earth Observatory.
“I love working outdoors, so that’s why I got interested in geology,” he said.
He delved into ice core-work as a postgraduate student and found that his geology background was useful in the field, allowing him to scan aerial photos to spot the best research sites. Moreover, the ice itself mirrors features of mineral rock.
“It’s kind of like geology on ice,” he said.
The ice, with its clearly defined annual layers, also serves as a repository for ancient air. By cross-referencing the ice and the air pockets within, scientists can look up information from earlier eras. They liken it to a library of early Earth.
“What’s beautiful about the ice core is we’re actually measuring gas of the prior Earth,” said Shaun Marcott, a postdoctoral researcher at Oregon State University who has worked with Severinghaus in Antarctica. “There’s probably no other archive that can compare to that.”
According to the journal Nature, the field of ice-core research began in 1954 when Danish geochemist Willi Dansgaard discovered the “isotope thermometer.”
Oxygen isotopes — atoms with slightly varied atomic weights — precipitate at different rates depending on temperature. So by measuring their proportions in prehistoric ice, scientists can reconstruct temperatures from the past.
In 1957, U.S. scientists first drilled ice from both poles. Researchers from Europe, Russia, China and Japan quickly launched their own polar-ice studies.
One of their key interests was how carbon-dioxide levels in ice bubbles corresponded with the rise and fall of glacial periods. Carbon dioxide is the main greenhouse gas, so its concentration in the atmosphere plays an important role in how warm the climate is.
In 1997, Severinghaus pioneered a way to calculate ocean temperatures over time by measuring elements called noble gases, which include argon, xenon and krypton. These gases dissolve more readily in cold water than warm water, and evaporate out as water warms.
“So by measuring the concentration of noble gases in the past, we can calculate what the ocean temperature was,” Severinghaus said. “The ocean temperature is the best indicator of global temperature, because it contains about 95 percent of the heat in the climate system.”
That discovery advanced ice-core research far beyond what scientists thought possible, said Kendrick Taylor, a professor at the Desert Research Institute in Nevada and chief scientist for the West Antarctic Ice Sheet Divide Ice Core project, where Severinghaus has conducted part of his work.
“If you said 10 years ago, ‘We’re going to get the temperature of the ocean out of the ice core,’ I would have said, ‘Yeah right, dream on,’ ” Taylor said. “But now we’re doing it.”
Severinghaus also discovered how nitrogen and argon isotopes left a chemical “signature” of rapid temperature change in Greenland’s ice. And he developed a “horizontal ice core” method for collecting large volumes of ice.
“We’ve discovered this outcrop of ice in Antarctica that allows us to get ancient ice at the surface,” he said.
There, researchers collect tons of ice — instead of slim columns — and melt them down in the field to extract 100 liters of air. These massive samples let them trace the sources of methane and carbon dioxide trapped in the ice.
“The (carbon dioxide) that comes from the ocean has a different isotopic signature than CO2 that comes from fossil-fuel burning,” Severinghaus said. “So you can actually discriminate between human-caused and natural CO2.”
Jeff Severinghaus, a professor of geosicences at Scripps Institution Oceanography, cuts 12,000 year-old ice with a chainsaw in Pakitsoq in western Greenland. — Photo by Vasilii Petrenko
The detailed examination of past climate helps explain which chemical triggers flip Earth’s thermostat, and how the cascade of warming or cooling unfolds. That lets scientists test the models used to forecast future climate.
“If you can show that the models are getting it right in the past, there’s more confidence that the models’ projection of the future is going to be accurate,” Severinghaus said.
Low-key and measured, he doesn’t go out of his way to highlight his scientific accomplishments. But his colleagues have taken note of them.
Last year, his innovations in gas measurements earned him the title of Fellow of the American Geophysical Union. It’s kind of an Academy Award for earth scientists.
“In some ways, it wasn’t unexpected that Jeff would get it,” Marcott said. “It was just a question of when he would get it.”
Severinghaus has traveled to Greenland five times and to Antarctica seven times to study ancient air. While Greenland offers opportunities to study abrupt climate change, Antarctica presents the purest and oldest ice.
Scientists fly into McMurdo Station, a logistical hub of 1,000 residents with warehouses, aircraft hangars and construction shops. From there, they branch out to field sites, where they camp in groups of 10 to 60 researchers, and share tasks such as cleaning equipment and repairing snowmobiles.
At Taylor Glacier in Antarctica, scientists gather the gigantic samples of ancient ice. Another location, the West Antarctic Ice Sheet (or WAIS) Divide, has thicker annual layers that allow scientists to measure past climate conditions down to the season.
One technique that Severinghaus helped develop, called replicate coring, lets researchers drill a parallel ice core alongside the main one to mine extra samples from particularly interesting periods of climate change.
“We are able to measure chemical properties we were never able to measure before,” Taylor said. “And we’re able to make the measurements with finer time resolution than we could in the past. So it’s like having a telephoto lens, where you can zoom in on the most interesting parts.”
It’s bitterly cold in Antarctica, even in summer, with temperatures hovering around 10 degrees Fahrenheit. But the toughest part is handling the ice cores. Researchers actually refrigerate their storage sheds so their samples can be preserved at -10 degrees.
“So we step outside (the sheds) in Antarctica to warm up,” Taylor said. “There’s a blizzard outside, and you’re in the middle of Antarctica, and you say, this is nice. It takes a certain physiology and a certain mental attitude to be able to work in that environment.”
For those with the right constitution, the stark landscape offers a quiet thrill.
“The artistic value is very striking; the robins’ egg blue color of the ice,” Severinghaus said.
In the evening, researchers share cooking duties. At bigger sites, they convene in a dining hall where professional cooks ship in delicacies such as frozen veggies and even lobster.
“There’s a nice spirit, where you sit around the dinner table and talk about your day’s adventures,” Severinghaus said.
He is married to a fellow scientist, Scripps oceanographer Lynne Tally, so the couple alternate field time. Since their twin son and daughter were born nine years ago, Severinghaus has limited his expeditions to every three years or so, sending graduate students in his stead during the intervening years.
As the debate over climate change continues, he makes it clear that his realm is science, not politics. But as a father, he looks forward to solutions.
“Speaking as a citizen, I hope that society will figure out a way to get this problem under control so that my grandchildren can have a decent planet to live on,” Severinghaus said. “It’s definitely worth working on it really hard. It’s definitely still possible to turn it around.”