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Cold Climate Shrinks Mountains
18 December 2013 1:00 pm
Every year, billions of tons of rock and soil vanish from Earth’s surface, scoured from mountains and plains and swept away by wind, rain, and other elements. The chief driver of this dramatic resurfacing is climate, according to a new study. And when the global temperature falls, erosion kicks into overdrive.
Scientists have long debated what drives most of the world’s erosion: Is it predominantly triggered by climate, or is it the result of mountain-building, tectonic activity? Most previous studies of erosion have relied on measuring the amounts of sediment that accumulate somewhere after being carried away from their sources and deposited elsewhere. But such analyses focus on the aftereffects of erosion, not the process itself, says Frédéric Herman, a geophysicist at the University of Lausanne in Switzerland. And most research has looked at limited regions of Earth—a particular mountain range, say, and not the planet as a whole.
To more directly estimate rates of erosion, researchers use techniques generally known as thermochronometry, or the measure of how a rock’s temperature has changed through time. Many such techniques rely on assessing how the decay of radioactive elements within a rock has affected its minerals. For their new study, Herman and his colleagues used four such techniques. In two of them, the researchers measured how much decay-produced helium had built up in a rock’s minerals. (Once the rock falls below a certain temperature, the helium stops diffusing out of the minerals efficiently.) In the other two, the team tallied the amount of microscopic damage produced by radioactive decay. (Once the rock falls below a certain temperature, the atoms in a crystal aren’t able to shift and heal the damage.) Using these approaches, the researchers could estimate the dates at which the rocks cooled to temperatures between 250°C and 70°C—and therefore track the speed at which the rocks rose toward ground level as the overlying strata eroded away.
Using data they’d gathered themselves, as well as that gleaned from other studies, the scientists compiled almost 18,000 data points from across the globe. During the past 8 million years, rates of erosion have varied from less than 0.01 millimeter per year (in central and western Australia and in central North America, for example) to as much as 10 mm/yr (at sites in the Himalayas, Taiwan, and New Zealand).
These regional trends may not be surprising: Australia is relatively flat and dry, and the Himalayas and Taiwan host relatively steep terrain that’s often lashed by monsoons. But the big story, Herman says, lies in the global trends seen as those 8 million years unfolded.
About 6 million years ago, as Earth’s climate cooled, erosion rates generally rose at all latitudes but increased most notably in mountainous regions. Then, in the wake of even stronger cooling that helped trigger a series of ice ages and interglacial periods beginning about 2.4 million years ago, erosion rates doubled, the researchers report online today in Nature. Because erosion increased most dramatically in midlatitude mountain ranges—areas most likely to first experience glaciers as climate gradually cooled—Herman and his colleagues blame the acceleration in erosion on glacial scouring.
The new findings, and especially their global scale, “confirms for me that [the increases in erosion rates] are a climate signal,” says David Egholm, a geophysicist at Aarhus University in Denmark. In particular, he notes, the latitude-dependent variations in erosion rates “most probably” can be attributed to glaciers.
If Earth’s climate continues to cool, as it has over the long term in the past few million years, rates of erosion will likely continue to rise. But if that cooling trend stalls, either due to natural climate variations or warming due to human emissions of greenhouse gases, erosion worldwide will gradually decrease. That’s an effect that might not even be noticed in the short term because it would likely unfold over millions of years, plus it would probably be masked by ever-increasing human-driven changes in the landscape.