Thinking like a planet
Paleoclimatologist Maureen Raymo, a research professor at Boston University, is drawn to really big questions, undaunted by million-year time scales, and willing to go just about anywhere to gather evidence. As she explains, “I love being out in nature. I’ve been at sea for months at a time, and I’ve spent weeks in the Himalayas doing research.” Her current fieldwork is a case in point. This summer will be Raymo’s second year leading a multidisciplinary team in the desert of Western Australia digging for evidence to answer this question: exactly how much did the sea level rise three million years ago?
True, the topic might not sit atop most people’s shortlist of burning questions. But the information Raymo’s team unearths will help scientists project how fast and how high sea levels are likely to rise as today’s global warming melts the remaining ice sheets in Greenland and Antarctica.
To understand the connection between Australia’s prehistoric sea level and today’s climate conundrum, you have to engage in something Raymo excels at: thinking on a geological, even planetary, scale. It turns out that long before people began adding heat-trapping gases to the atmosphere, global temperatures fluctuated because of subtle, recurring shifts in Earth’s orbit around the sun. For instance, the tilt of the planet as it orbits varies predictably on a 41,000-year cycle. More tilt of the axis means warmer summers and colder winters at high latitudes; less tilt means cooler summers and milder winters. Scientists believe this 41,000-year variation (known as the Milankovitch cycle), along with other shifts in the shape of Earth’s orbit and the seasonal timing of when Earth is closest to the sun, have influenced the waxing and waning of glaciers and the onset of ice ages as more or less solar radiation reaches Earth at mid-to-high latitudes.
Such observations from the complex field of orbital dynamics have helped scientists like Raymo tease apart the cyclic changes in the climate due to these so-called “orbital forcings”. But Earth’s climate changes on even longer time scales of millions of years as well, changes which can’t be ascribed to the more rapid orbital “wobbles”. Three million years ago, during what is known as the mid-Pliocene climate optimum, Earth was a much warmer place. In fact, this period marked the most recent time during which the climate was consistently warmer than it is today for an extended period, with global temperatures as much as 3 degrees Celsius (5.4 degrees Fahrenheit) above today’s averages. As a graduate student, Raymo proposed this warmth was due to higher levels of atmospheric carbon dioxide caused by tectonic shifts in the Earth, a hypothesis which is still being tested today. Scientists are not sure how high the sea level was during the mid-Pliocene climate optimum. Some past research has suggested that the level was just 15 feet higher than it is today, while other research suggests that it could have been as much as 100 feet higher. Raymo hopes her research will significantly narrow this gap. “If we can determine exactly where the shoreline was three million years ago, we can tell a lot about how much ice remained in Greenland and Antarctica during this warm period,” she explains.
To answer the question, Raymo and her team work their way inland from the Australian coast in search of fossilized coral reefs and other evidence that the land was once covered by ocean. As they move to higher elevations, they will be able to determine the height of the ancient sea level. “Each time you add a data point like this,” Raymo says, “you help to calibrate existing climate models in an important way and build a broader knowledge base for future experiments.”
Raymo is quick to add however, that uncertainty about a specific question such as the maximum sea level in the Pliocene Epoch is entirely different from uncertainty about the fact that global warming is occurring today. Climate-change deniers who jump on uncertainties in scientific understanding to dispute global warming, Raymo says, “completely miss the point: science is always evolving intellectually, processing and incorporating the latest information. Scientists always want more knowledge. They always say ‘I want to try to test out that new idea.’”
Even so,” Raymo says, “everything I’ve learned about the dynamism of the planet’s climate places me, like most all of my colleagues, strongly in the camp that says we need to take preventive action to keep the planet from warming further.” As she explains: “I am extremely concerned that we’ve made such large changes in the composition of our atmosphere with so little understanding of the consequences. The path we are on is completely anomalous to anything that has gone before.”
Given the focus of Raymo’s field on vast stretches of geological time, her assertion is especially powerful. The fact is, paleoclimatologists like Raymo know a great deal about variations in the earth’s climate over the course of the planet’s history. They glean evidence from a variety of sources to track which parts of the planet were once covered by glaciers or oceans. They can see geological evidence of glaciers in shifts of the terrain and in the rocks and other material the glaciers left behind; they can read chemical evidence of oceans in variations in the ratios of isotopes in sedimentary rocks; and the fossil record holds a wealth of clues as well.
The tricky part, of course, is to figure out how to use the hard evidence available today to draw conclusions about past climate. It is a part of her job Raymo particularly enjoys. “I love thinking ‘How does the Earth work?’ I have always felt a natural ability to think about the planet, almost as though I were looking down on it from above,” she says.
Thinking about such big questions seems to run in her family. Her father, Chet Raymo, is a science writer and columnist well known for his musings about large-scale topics in the natural world. But when asked how she got interested in her field, Raymo, without a moment’s hesitation, credits Jacques Cousteau. When she was seven, she says, one of Cousteau’s television programs about the sea completely captured her imagination. “From then on I decided I wanted to be an oceanographer.” Raymo’s passion for oceanography continued all the way to college, where her interests shifted slightly through exposure to the cutting-edge climate research of geologists like John Imbrie and William Ruddiman who became her mentors.
In her research, Raymo tackles many aspects of paleoclimate and orbital forcings. In 2006, for instance, she published a hypothesis in the journal Science to explain why the 41,000 year Milankovitch cycle appears less pronounced over the past 500,000 years. “It was a question that I had literally been thinking about ever since graduate school some two decades earlier,” she says. The verdict is still out on this question, but Raymo’s theory—that ice growth at one pole occurs at the same time as ice decay at the other, thereby canceling out the signal of ice volume change in global climate records—has sparked a great deal of new research and debate. If her hypothesis is correct, it suggests even greater dynamism in the climate than had been previously understood, especially in the Antarctic ice sheet—and even more cause for concern about the long-term effects of a warming planet.
While orbital forcings offer a fascinating window into the historical mechanisms of climate variation, Raymo emphasizes that they are overshadowed today by human-driven effects on the climate. “People sometimes ask me: ‘When will the next ice age be?’ The answer is that I’m pretty sure we have already prevented it. Like it or not,” she says, “we are now the main drivers of the climate, even though so far we’ve been doing it completely by accident.”