Unearthing Rock-Hard Evidence of Climate Change
Jennifer Wagner, Julia Cole, and Warren Beck in an Arizona cave. Photo by William Peachey.
Julia Cole, a geologist at the University of Arizona, finds evidence of the climate record in some fascinating places. Most recently, Cole's research has led her deep inside a limestone cave 50 miles southeast of Tucson, Arizona. Preserved there, within stalagmites that have formed on the floor of the dark, perpetually humid cave, is what Cole calls "a robust archive" of climate changes over time in the southwestern desert of the United States.
Locked inside the stalagmites, Cole explains, is an almost continuous climate record that dates back tens of thousands of years to the last ice age, when ice sheets covered much of North America. The subtle changes in the composition of a stalagmite as it forms, she says, "establishes a record that ties into large-scale climate patterns." We know about these broad climate changes from many sources, Cole explains, but until her team's efforts, "no one had documented those changes before in such revealing detail over a long period of time in the Southwest of the United States."
The key to Cole's technique stems from her understanding of how stalagmites form. As rainwater percolates down through the soil above a cave, it loses carbon dioxide and drips to the cave floor to leave behind mineral formations of calcium carbonate. These lumps of calcium carbonate, Cole explains, can be seen to represent "preserved records of precipitated rain soaked through a filtering process."
Cole unlocks the stalagmites' secrets by capitalizing on the fact that their chemical composition differs subtly between wet and dry periods. The rare, heavier isotope of oxygen called oxygen-18 is more prevalent in a stalagmite's composition in dry years because this type of oxygen atom in rainwater tends to fall from clouds in higher concentrations at the beginning of a rainstorm and because it is less likely to evaporate in a dry period than the vastly more common lighter form of oxygen.
By shaving the stalagmite's core into 100-micron-thin layers and measuring small changes in the abundance of oxygen-18, and applying a sophisticated dating technique, Cole's team can reconstruct when the region has been wet and when it has been dry.
In a sense, Cole's technique is somewhat like that used by scientists who chart climate variability by studying tree rings—but with some big differences. As she explains, "Tree ring data has limitations. Trees don't live that long. And, even more importantly for this region, there are no trees in the desert, only in the surrounding mountains. Now," she says, "we have a solid set of paleoclimate data below the tree line."
Cole's stalagmite research is already yielding results. In a recent paper in Nature Geoscience (February 2010), Cole's team documents that the wet and dry periods they have chronicled from the stalagmite's composition correlate closely with the record derived from ice-core data in Greenland. "It's a new picture of the climate in the Southwest during the last ice age," Cole says. "What this new evidence shows us is that when it was cold in Greenland, it was wet here. When it was warm in Greenland, it was dry here."
The data has implications for the region's climate in the future as well. As Cole explains, it is well documented that in the southwestern United States, storms tend to move northward toward the pole as the climate warms. The record clearly shows, she says, that "when the climate gets warmer, the U.S. Southwest tends to get drier."
The stalagmite data is just the latest research for Cole, whose remarkable career over the past several decades has also taken her around the world to study what undersea coral reefs (another calcium carbonate formation) can reveal about changes in the ocean and the paleoclimate record. Much of that work has sought to better understand the quasi-periodic climate pattern known as El Niño (or, more officially, the El Niño-Southern Oscillation), which occurs over the tropical Pacific Ocean every three to seven years.
Cole says she has been "hooked on science" ever since she started working as a research assistant in a lab after college. "Global warming was not on the public radar screen in the early 1980s when I was in college," she says, adding that she feels lucky to have wound up studying climate change at a time "when it is so close to the center of the news and public consciousness."
"I have kids and I certainly find motivation in thinking about what kind of world they will inherit and what things will be like for them," Cole says. "But ever since I became a scientist, I have also simply loved the creativity of science—the process of asking interesting questions about the world around us, and building strategies and techniques to try to answer them."