The Amazon River carves a 3,900-mile path through South America, from the cold Peruvian Andes to the tropical Atlantic coast. Its basin covers 2.3 million square miles, draining more fresh water than any other system in the world. That yawning basin is also yielding new information about the role of the tropics in global climate change.
The ancient Amazon's flow appears to explain a mysterious, 10,000-year-old increase in ice-trapped methane, a greenhouse gas 20 times more efficient than carbon dioxide in global warming power, say Mark A. Maslin and Stephen J. Burns of the Environmental Chance Research Center at the University College-London.
Their findings, published in Friday’s issue of the journal Science, match two key pieces of the global climate puzzle: levels of methane imprisoned in polar ice, and the ever-changing “moisture history” of the Amazon Basin, where vast wetlands work overtime as methane biofactories.
The research won’t resolve the debate over how much global warming might result from human activities vs. natural processes. But it should ultimately help scientists paint a more accurate picture of global climate changes over time, taking into account the complex affects of the Amazon River Basin.
How could Amazonian methane reach the far-flung Greenland ice sheet? Like carbon dioxide, methane gas is mixed by air circulation into Earth’s atmosphere, explains Julio Betancourt of the U.S. Geological Survey, author of a Science essay on the study by Maslin and Burns. Methane and other atmospheric gases become trapped in air bubbles that form within the ice in both the Greenland and Antarctic ice sheets, he notes. In Greenland, scientists have drilled to bedrock, 10,000 feet (3,050 meters) below the surface of the ice, to reconstruct past variations in atmospheric chemistry.
The Amazon Basin funnels up to 20 percent of all the fresh water dumped into the world’s seas, along with a billion tons of sediment, Betancourt points out. During periods of heavy rainfall, the Amazon’s wetlands expand, thereby increasing the production of methane, the principal component of natural gas. Also known as “swamp gas,” methane forms in wetlands when water cuts off the oxygen supply to soil, encouraging anaerobic fermentation via bacterial decomposition of plant matter.
At the same time, increased outflow from the Amazon Basin affects water’s movement, as fresh water entering the Atlantic Ocean is swept northward by the North Brazil Coastal Current. The only surface-water current to cross the equator, this strong force grabs hot, salty water and transports it to the North Atlantic, “eventually influencing surface waters that reach the Nordic seas through the Gulf Stream,” Betancourt says. But during glacial periods, strong winds deflect the current, sending it southward and to the east.
To track past Amazonian rainfall events, Maslin and Burns investigated the biochemical composition of fossilized plankton, exhumed from seafloor sediments near the Amazon’s terminal mixing bowl. Since they were mainly interested in the amount of fresh water being carried into the ocean from the Amazon Basin, the researchers focused on a particular species of single-celled marine organisms, Neogloboquadrina dutertrei. These creatures are known to favor cooler, deeper waters, where they avoid changes in salt content. By measuring the oxygen isotope composition in their hard calcite shells, Maslin and Burns documented historical changes in the Amazon’s outflow.
Earlier than 12,000 years ago, they learned, the Amazon Basin was high and dry, with discharge at least 60 percent below current levels. Then, as the ice age drew to a close about 10,000 years ago, Maslin believes, the rain began to fall — lots of it. Throughout the soggy Amazon Basin, swelling wetlands produced more and more methane. In fact, the river’s total discharge jumped by 40 percent, a level that “corresponds to the rapid increase in the atmospheric methane record,” the Science paper concludes.
Today, the Amazon Basin is wetter still, according to Maslin and Burns.
A second Science study, based on pollen analysis, seems to confirm the notion of a wetter-than-ever Amazon. Francis E. Mayle and colleagues at the University of Leicester found that the climate-sensitive Bolivian rain forest has been stretching southward over the past several thousand years. Currently, its border is farther south than at any other time in the last 50,000 years, and some trees at the edge of this newly minted region are younger than 3,000 years old, Mayle reports.
What does all this mean for global climate? Betancourt says the increase in Amazon outflow and trapped methane corresponds with changes in the Amazon’s solar exposure, which dropped during the Younger Dryas period, at the end of the last ice age, then maxed out 3,000 years ago. Changes in solar irradiation may regulate convection over the Amazon, thus triggering shifts in the southward penetration of the Intertropical Convergence Zone. All of these forces alter water cycles and global climate.
Yet, Betancourt says, “Maslin and Burns’ elegant study is probably not the final word.” In particular, he contends, the ice-core methane increased too rapidly — in one or two centuries — to be explained by the gradual development of tropical wetlands.
“A theory now gaining popularity among paleoclimatologists,” he says, “is that these rapid increases in methane are produced by the catastrophic release of gas hydrates stored in marine sediments on the continental slopes.”
For their part, Maslin and Burns say the jury’s still out. Although the Amazon’s moisture level has never been higher, it’s unclear whether methane levels match. “The methane record,” they emphasize,” should still be regarded as a complex signal with varying contributions from other sources.”
The work adds much to our knowledge of complex global cycles, however. Insights into new species emergence may result from the research, too. Because tropical wetlands represent 60 percent of all wetlands worldwide, Burns notes, moisture levels in these regions strongly affect biological diversity. Now, he says: “We can start using realistic models to predict what changes have occurred to the rain forest in the past, and hence, what could have caused such a high degree of diversity.”