Speijer hands me his hammer and gestures to the seam of dirt where the other geologists are working. I climb over and bang off a piece. It is soft and wet and crumbles a little in my hands. I cradle it in my palms, thinking of the power of the archive I am holding, of the mysteries it may reveal. When I show it to Speijer, he expertly breaks it into pieces, looking for the fossil remains of the ancient foraminifera. The sample crumbles through his fingers and falls to the ground, now just dust.
As far back as 2001, the report from the ipcc warned about abrupt climate change. But because it came amid such a wealth of other information and analysis, it was easy to gloss over the consequences. “Greenhouse gas forcing in the 21st century could set in motion large-scale, high-impact, nonlinear, and potentially abrupt changes in physical and biological systems over the coming decades to millennia, with a wide range of associated likelihoods,” according to the summary for policy-makers. “Some of the projected abrupt/non-linear changes in physical systems and in the natural sources and sinks of greenhouse gases could be irreversible, but there is an incomplete understanding of some of the underlying processes.”
That gap in knowledge provoked further investigations. In the United States, the National Research Council set up a committee on abrupt climate change, which reported in 2002 the “surprising new [finding] that abrupt climate change can occur when gradual causes push the earth system across a threshold.” The authors use the analogy of a finger pressing evenly on a light switch. At some point, even though the pressure stays the same, the tipping point is reached and the switch flips. At that stage, the climate sets off on its own path, at its own pace. The committee concluded that “large, abrupt climate changes have repeatedly affected much or all of the earth, locally reaching as much as 10°C change in 10 years. Available evidence suggests that abrupt climate changes are not only possible but likely in the future, potentially with large impacts on ecosystems and societies.”
By 2003, that scenario had prompted the Pentagon to commission a report on the implications of abrupt climate change for national security. Their findings focused on what would happen if the ocean conveyor belt, which transports heat from the equator to the northern Atlantic Ocean, shut off. It predicted shortages of food, water, and energy that “could potentially de-stabilize the geo-political environment, leading to skirmishes, battles, and even war . . . . Unlikely alliances could be formed as defense priorities shift and the goal is resources for survival rather than religion, ideology, or national honor.”
It was, in essence, a request for permission to plan for anarchy. Could this really happen? The report’s answer followed: “Ocean, land, and atmosphere scientists at some of the world’s most prestigious organizations have uncovered new evidence over the past decade suggesting that the plausibility of severe and rapid climate change is higher than most of the scientific community and perhaps all of the political community is prepared for. If it occurs, this phenomenon will disrupt current gradual global warming trends, adding to climate complexity and lack of predictability. And paleoclimatic evidence suggests that such an abrupt climate change could begin in the near future.”
The specific piece of the climate picture that spawned these reports was evidence showing that at various points throughout history, the circulation of ocean waters has changed dramatically at the same time as drastic climate fluctuations were underway. How does this relate to what we’re seeing right now? To take one example, as glaciers melt, the salinity of oceans decreases. Less salty water, being lighter, does not sink as easily into the ocean’s depths. This could produce a discernible change in how the conveyor belt transports heat.
Ice-core samples from Greenland show that the conveyor belt shut off 12,700 years ago, during an event called the Younger Dryas, and was operating only weakly 8,300 years ago during a smaller, regional warming. The reigning theory, originally proposed by the oceanographer Wallace Broecker of the Earth Institute at Columbia University, is that if this conveyor belt should shut off again, the ocean heat that keeps northern Europe warm will stay south, plunging that part of the continent into a prolonged cold spell. (This is the scenario, taken to extremes, in the 2004 Hollywood movie The Day After Tomorrow. The film showed a snap ice age in North America, complete with an icebound Statue of Liberty and valiant Americans freezing to death in mid-stride.)
But while the ocean’s circulation has shut down before and may do so again, many scientists now believe this possibility to be a red herring, drawing attention and research away from graver and more immediate threats. Matthew Huber of Purdue University in Indiana, one of the world’s top climate modellers, says that most computer simulations show that heat circulation is driven by winds, not the conveyor belt. That’s why, for example, Juno, Alaska, is relatively warm despite the lack of a similar conveyor belt in the Pacific Ocean. “The theoretical problem is that when you measure how much heat is transported inland past [a latitude of] 60° N, it’s indistinguishable from zero,” Huber says. In other words, the ocean conveyor belt doesn’t have a major impact on the temperature of inland Europe. The irony is that while the conveyor-belt problem got the attention of at least some policy-makers—to what degree is not clear—and put the concept of abrupt climate change on the fringes of the map, it happened before the discovery of the really worrisome new evidence just now emerging from the microfossils of the petm.
A key question concerns the massive infusion of carbon that took place during the petm: where did it come from? One leading theory is that the carbon exploded in a gaseous burp from the ocean floor when the methane hydrates stored there somehow broke free. Huber describes methane hydrates, when cold, as being like geodesic domes lying stable in the ocean’s depths. But the bonds holding the molecules together are fragile. If they are shaken up or warmed, the bonds break and bubbles of highly flammable methane (a carbon compound) shoot through the water at phenomenal speed. If that happened on a large enough scale, intense global warming would happen almost instantly, Huber says. Another possible trigger could be the warming of the deep ocean by a couple of degrees, and there is some evidence to suggest that during the petm, the warming was caused by the release of the carbon, not vice versa. Or a landslide might have shaken up the methane hydrates.
But Mark Pagani, a professor at Yale University’s department of geology and geophysics and an expert on historical carbon concentrations in the atmosphere, doesn’t buy the methane theory. “It was a very sexy idea and it’s had its day,” he says. According to Pagani, it is unlikely that the volume of methane in the oceans 55 million years ago was enough to ignite the kind of warming that the fossil record shows.
Comments