Geophysicist Raymond Pierrehumbert studies global warming on Earth and the climate physics of distant planets.
In the early 19th century, Jean-Baptiste Joseph Fourier discovered what would later become known as the greenhouse effect: that gases in the atmosphere absorb heat from the planet’s surface and radiate it back down to Earth. In an 1827 paper in the Mémoires de l’Académie Royale des Sciences, the French mathematician reasoned that incoming sunlight gives a planet heat, which it loses only by emitting infrared radiation. He also found that having an atmosphere—opaque to infrared, but not to solar, radiation—slows that heat loss. If not for its atmosphere, Fourier calculated, Earth would be considerably colder.
Fourier, says geophysicist Raymond Pierrehumbert, planted the seed for climate science. “Some people seem to think that the whole global-warming concern dates back just to the 1990s, that it was put together by a bunch of environmentalists,” he says. “But in fact, the physics go back almost two centuries.”
This past January Pierrehumbert and his geophysics colleague David Archer edited The Warming Papers (Wiley-Blackwell), a collection of 32 studies that helped to establish the theory of climate change. “These were landmark papers,” says Pierrehumbert, who translated Fourier’s article for the anthology. A few of the studies in the book were published during the past 20 years, but most date back much further: British physicist John Tyndall’s 1861 calculations, for instance, established the heat-trapping effects of carbon dioxide, water vapor, and other trace gases. “Before Tyndall,” Pierrehumbert says, “no one had any reason to connect the Industrial Revolution with climate change.” Thirty-five years later, future Nobel laureate Svante Arrhenius linked coal burning to the buildup of carbon dioxide in the atmosphere.
By 1974 Syukuro Manabe and Richard T. Wetherald had developed a computerized mathematical model of the atmosphere, showing that carbon dioxide not only warms Earth but also causes its atmosphere to hold more water vapor, which warms the planet even further. They published their findings the following year in the Journal of Atmospheric Sciences. “That was critical,” Pierrehumbert says: the paper consolidated the idea that warming would continue for a certain period, even after carbon dioxide concentrations were frozen. “So 1974 was the last time anybody who understood the subject could say there were too many uncertainties to take climate change seriously.”
Climate change has anchored Pierrehumbert’s research since he came to Chicago in 1989; he’s published papers on carbon-dioxide ice clouds, the buildup of subtropical water vapor, the melting of Arctic sea ice. He’s built computer models to predict the pace and consequences of global warming. He’s also studied Earth’s paleoclimate and climate physics on planets in other solar systems.
But Pierrehumbert’s first love was fluid mechanics. “The equations for a fluid are structureless and simple,” he says, but fluids yield “all sorts of structured and intricate behavior: eddies, vortices, tornadoes, instability swirls.” The red spot on Jupiter, Pierrehumbert notes, is an eddy that has held together for hundreds or perhaps thousands of years. “So from an almost structureless, featureless void, the equations of motion call into being a whole universe of structures.”
A physics major at Harvard, he wrote his MIT doctoral thesis on general turbulence theory. While there, he says, “I fell in with some earth-sciences faculty who pointed out that, you know, the atmosphere is a fluid.” After getting his PhD in 1980, Pierrehumbert joined MIT’s meteorology department. He was still “blissfully unaware” of climate change; his work on that subject began when he joined Chicago’s geophysics department.
That work continues. This past February Pierrehumbert and several coauthors assembled a report for the National Research Council, Climate Stabilization Targets: Emissions, Concentrations, and Impacts over Decades to Millennia. Its claims are sobering. “By emitting so much carbon, we’ve really created a new geological era,” called the Anthropocene, he says. “The Holocene is over, and how different the Anthropocene is going to be depends on how much coal and oil we burn.”
He estimates that humans will use all four trillion remaining tons of coal by the year 2100. If that happens, Earth will be seven degrees warmer (Celsius) in 2100 than it is now. And the effect is long lasting. “Because carbon dioxide is removed from the atmosphere so slowly,” he says, “the decisions we make about energy in the next hundred years will be setting the kind of climate we’ll have for more than the next 10,000 years.”
Pierrehumbert’s work takes him from Earth’s future into its distant past, “which is almost like a different planet,” he says. One focus is “snowball Earth,” a phenomenon of the Neoproterozoic era, 1,000 to 542 million years ago, when the globe was almost completely encased in ice. “What does it take to get out of that state?” Pierrehumbert says. “Ice is very reflective; it reflects back a lot of solar radiation; it keeps the planet cold.”
His theory, developed with Chicago postdoc Dorian Abbot and explained in the 2011 Annual Review of Earth and Planetary Science: Over a million years, carbon dioxide accumulated from erupting volcanoes, trapping heat. At the same time, mountains of volcanic and continental dust collected at the equator, absorbing sunlight and eventually melting the ice. “Which gives you open water at the equator,” Pierrehumbert says, “and then the whole thing zips open.”
From studying Earth’s ancient climate, there wasn’t “a big leap” to studying other planets, Pierrehumbert says. As NASA missions detect hundreds of planets beyond the solar system, Pierrehumbert has begun analyzing their characteristics and conditions. “There are whole planetary systems that exist crammed inside an orbit the size of Mercury’s,” he says. “And planets around different kinds of stars and with more elliptical orbits. Suddenly there’s more to think about in terms of how planets might be put together”—including their climates.
For the past five years, Pierrehumbert has been working on a textbook, Principles of Planetary Climate (Cambridge University Press, 2011), that covers fundamentals common to all planets: thermodynamics, infrared radiative transfer, scattering, surface heat transfer. Like fluids, he says, in climate systems simple physical principles yield vast complexities. His goal, says Pierrehumbert, who did every calculation in the 680-page book from scratch, was to “take Fourier’s program of energy balance seriously and generally and formulate it so that it could apply to a whole range of planets, including some that haven’t been observed yet.”