Krieger School paleoclimatologists rely on a variety of tools—from isotopes to ocean temperature—to parse the past in order to shed light on Earth’s future.
By Mat Edelson
Photography by Will Kirk
Climate change scientists like Naomi Levin, an assistant professor in the Department of Earth and Planetary Sciences, say you can take this truism to the bank: man—at least the upright and locked Homo sapiens version—is, climatologically speaking, going where no man has gone before. During our roughly 100,000 years on the planet, atmospheric carbon dioxide (CO2) levels have cycled between 180 and 280 parts per million (ppm). Not coincidentally, the low points in the cycle are times of global cooling (known as glacials), the high points are periods of global warming (known as interglacials), but neither is too excessive to take out humans.
Now consider how we’re shredding that cycle. The burning of fossil fuels (coal, gas, oil, etc.) has released a tremendous amount of additional long-lasting CO2 into the atmosphere, beginning in the industrial revolution and accelerating greatly in the last 50 years. Even if CO2 reduction policies go into place, what’s already done is done, and CO2 concentrations of 450 ppm are expected by the year 2100.
That translates into at least a 2-degree centigrade planetary warming, and perhaps, if you believe dire forecasts, as much as five times more. But even at a minimum, a 2-degree hit, according to the Environmental Protection Agency, will melt significant ice and snow, raising sea levels. Ocean acidity will also increase as nature’s sponge, the sea, sucks acidic CO2 emissions out of the sky.
Will the planet survive? Sure—CO2 levels were far, far higher back when dinosaurs roamed and the Earth was one big hothouse. But humans? We could be in for a heckuva ride.
How volatile that roller coaster might be is a question Levin has spent many an hour pondering. But what makes Levin different is that she’s looking to the past for clues about the future. It’s been her way of looking at the world ever since she chanced upon James Michener’s tome The Source. The book centers on a fictional archaeological dig around Jerusalem, and set the teenage Levin’s imagination aflame—imagine being able to hold human history in your very hands!—and on a course that would eventually take her around the globe. Her work leads the way in the field known as paleoclimatology. It’s where basic chemistry preserved in the earth’s equivalent of amber gives insight into mysteries that predate humans—and it may help us survive the climate that appears to be our destiny.
Take the greenhouse gas CO2. How do we know what its levels were when T. rex roamed, especially when scientists didn’t seriously begin to take CO2 atmospheric measurements until 1958 in Hawaii? The answer is literally beneath our feet. Just as fossils tell the history of man and beast, flora and fauna, so too does the chemical composition of those fossils and rocks.
These are the clues Levin and other paleoclimatologists use to parse pre-history. “When we go into the geologic past, we can’t directly observe the process, so we need proxies,” says Levin, who is working with three other “detectives” from the Department of Earth and Planetary Sciences: Professor Darryn Waugh, Associate Professor Anand Gnanadesikan, and Assistant Professor Benjamin Zaitchik.
Levin adds that those ancient CO2 levels came to life “because CO2 gas from the ancient atmosphere is trapped in glacial ice cores, and scientific teams can now measure the concentrations going back 800,000 years. That oscillation, between 180 and 280 ppm, probably extended back to when permanent ice showed up at the North Pole around 2.5 million years ago.” Time also provides Levin with a tool, especially the way its passing, even at a glacial pace, affects certain basic Earth elements. Many of us are familiar with carbon dating; it’s based on the fact that carbon comes in both a stable form, known as carbon-12 (also known as 12C) and an unstable radioactive form, carbon-14 (14C). Since 14C has a known half-life (5,730 years, for those Jeopardy players in the audience), and the ratio of 12C to 14C has been used as a way to establish the age for anything with carbon in it, dating back 50,000 years (the decay rates of other elements, including argon and uranium, are considered more reliable for dating the age of older materials, including Earth itself).
But now imagine being able to tag the movement of a stable, abundant set of isotopes that don’t decay with time. That’s where oxygen comes in, and the implications are staggering. Oxygen isotopes allow the tracking of all sorts of natural processes including photosynthesis, mineral formation, and the origin of rainfall. Did the latter come from the oceans or the rain forest, and what happens if the rain forest is eventually cut to smithereens? All these processes leave different kinds of oxygen footprints. Stable isotopes such as oxygen can be used as proxies for understanding past processes on Earth, and are part of what Levin likes to call the “Rosetta Stone,” for her and her fellow paleoclimatologists.
Take 18O and 16O. No, it’s not Bingo run amuck but rather two isotopes (i.e., variants on a given element; the “18” and “16” refer to the different number of neutrons attached to each respective atom of oxygen). In Levin’s Olin Hall laboratory, a huge black cable shoots electricity into a silver mass spectrometer, ionizing gases derived from multiple types of Earth materials; everything from water samples obtained from tributaries of the Nile in Ethiopia to fossils buried in the ocean floor whose calcium carbonate (i.e., oxygen-containing) shells are still intact, to the fossil teeth of ancient elephants and giraffes that lived among human ancestors in Kenya millions of years ago.
Given that 18O and 16O are stable isotopes—they don’t break down over time—the key difference is that they have slightly different masses, and those weight differences can act as a tracer, perhaps explaining what was going on in the atmosphere at the time a fossilized organism was alive. Bones, teeth, and shells all contain oxygen; measuring the ratio of 18O to 16O in these hard tissue fossils can unlock the Earth’s condition during their lifetime. Also, other researchers are exploring the fact that 18O and 16O come unglued from each other over time and under force—with the heavier 18O tending to sink toward the ocean floor, while the lighter 16O stays in the air—this phenomenon may help explain how the sea rose and fell over eras, along with other mysteries.
“It’s laborious work, but it punches a window into the geologic history, drawing on minerals from the past,” says Levin. “I want isotopes to help me sleuth the past.” How far distant? Levin says she’s working in the past 5 million years, which is a heck of a long time if you’re waiting for Godot, but a mere blink given the Earth’s 4.5-billion-year life span. Still, it’s only in the last generation or so that the use of oxygen isotopes has come into its own. Interestingly, much of the research originated as a byproduct of atomic research in the 1930s and 1940s; Nobel Prize-winning chemist Harold Urey of the University of Chicago helped pioneer the field, discovering in 1947 that oxygen isotopes could be used as “a paleothermometer” to track water temperatures and climate through the ages. “Since then the field has blossomed,” notes Levin. “Ecologists use it to understand the migration of birds, geologists use isotopes to see if there was water on Earth billions of years ago, and police are even using it to track missing people.”
Levin and her colleagues are using their isotopic skills to take on the northeastern region of Africa, which has a notoriously unpredictable climate. Levin says this is borne out by the work of the Intergovernmental Panel on Climate Change. Made up of the world’s top meteorologists and climatologists, the IPCC operates in a competitive environment, where scientists bring their climatology models to the table for scrutiny.
“Every few years [the IPCC] comes together, using a suite of some 20 models,” says Levin. “In some places like North America, these models converge and do relatively well, for example, predicting how much temperature will rise over 20 years. But when you apply those models to East Africa, they totally disagree.”
With some models predicting droughts for the region, and others flooding, one can see the need for more detailed records of regional past climate change. Levin is going there, helping to build permanent and more elaborate research sites in Ethiopia. “I use different isotopic systems to get at the distribution of grasses and trees, to see how arid the area was, where the moisture came from, to look at different points in the landscape because the climate doesn’t just respond at one point.”
Levin’s colleagues say her isotopic work holds great promise for better understanding East Africa’s climatological future. Oceanographer and physicist Anand Gnanadesikan notes that his models must differentiate between linear events—gradual rises in, say, CO2 levels, and slow melting of snow caps—versus nonlinear tipping points that create huge climate changes, such as the temperature at which a giant Antarctic ice shelf might break free and melt into the ocean.
Gnanadesikan says Levin’s work showing that the distribution of lighter 16O levels in ice versus heavier 18O in rainfall and oceans could shed light on past nonlinear events (if any), and hence could yield more accurate models for East Africa. “Naomi and I would like to develop models of what she’s measuring, isotopes, and embed them in our climate models. Maybe her study of the past couple of million years can tell us if East Africa responds in a nonlinear fashion to climate change. And do we need to drop CO2 levels to avoid major shifts to ecosystems? It could affect hundreds of millions of people,” he says.
“One use of isotopic measurements is as an indicator of where the air came from,” says ozone depletion expert and former Earth and planetary sciences department chair Darren Waugh, who hired Levin. “If it evaporated from the ocean, or came from the forest area of the Congo, where water has gone through the trees, it has a different isotopic signature. Naomi has an interest in knowing what meteorological conditions were in the past. Now (with her rain-gathering efforts in Ethiopia), she’ll also do it with current conditions,” the significance being that if patterns have changed, it could factor into models that predict how East Africa will respond to climate change, and other models looking at large-scale air movements such as El Niño.
Or maybe even a nor’easter over Baltimore. As part of her work, Levin is collecting rainwater from the roof of Olin Hall. (Along with colleague Katalin Szlavecz, she’s also teaching students how to collect soil and water samples in a new Environmental Field Methods course.) The idea is to develop methodologies here in Baltimore that could play out globally, while providing hands-on research opportunities for Hopkins students that are closer to home. For while understanding, say, how El Niño works may give clues into future weather on the African continent, there’s also much work to be done to figure out whether jet stream changes over North America may warm the climate, such that today’s South Carolina is tomorrow’s (or maybe the 22nd century’s) Pennsylvania.
Will a few isotopic measurements of Baltimore help yield an answer?
Perhaps, says Levin, especially when added to the growing body of knowledge that’s seen climate-related publications jump tenfold in the last 30 years.
“A lot of Earth science is about how systems relate. If we can understand the [ancient and current] mechanisms, we can better predict the future.”
Even if it turns out to be rather balmy.