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Wetter climate to trigger global warming feedback loop in the tropics

Greater rainfall is likely to intensify global warming by increasing microbes' release of CO2 into the atmosphere from soils in tropical drainage basins like that of the Kali Gandaki River, a tributary of the Ganges River in Nepal. Photo by Dr. Valier Galy/Woods Hole Oceanographic Institution
Greater rainfall is likely to intensify global warming by increasing microbes' release of CO2 into the atmosphere from soils in tropical drainage basins like that of the Kali Gandaki River, a tributary of the Ganges River in Nepal. Photo by Dr. Valier Galy/Woods Hole Oceanographic Institution

May 6 (UPI) -- As the tropics get wetter, as many climate models predict, soils are likely to experience greater rates of respiration and decomposition, limiting the carbon storage abilities of tropical soils and intensifying global warming.

Scientists described the unexpected climate change feedback loop in a new paper, published Wednesday in the journal Nature.

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Researchers discovered the pattern after analyzing organic matter in sediment cores collected from the drainage basin of the Ganges and Brahmaputra rivers.

"The Ganges and Brahmaputra rivers 'punch above their weight' in terms of the amount of sediment and organic carbon they discharge to the oceans, as compared with, for example, the area of land they drain," lead study author Christopher Hein, an assistant professor at William and Mary's Virginia Institute of Marine Science, told UPI in an email. "That reflects the fact that these rivers drain the very rugged Himalayan Mountains and Tibetan Plateau, which are ripe for physical weathering of rocks."

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The region, which receives an annual monsoon, has historically experienced heavy rainfall, fueling accelerated erosion and sediment transport rates.

"When you combine the massive amounts of sediments and carbon exported by the rivers, and this rapid burial, it turns out that the delta and fan of the Ganges and Brahmaputra rivers are responsible for the burial of about 20 percent of all of the organic carbon delivered by rivers each year across the world," Hein said. "Thus, this is not just any river-delta-fan system, but a globally important one which, by itself, can affect the earth's climate."

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To better understand the link between precipitation and the carbon cycle in tropical soils, Hein and his colleagues isolated and dated the bits of land-borne organic matter found in the delta sediment cores. The data showed the bigger the monsoon, or the more rainfall, the younger the soils.

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"The best explanation for this is that soils were 'turning over' faster; that is more respiration and decomposition," Hein said.

Rapidly respiring soils can't store as much carbon. In fact, scientists found upticks in precipitation totals more than halved the carbon storage potential of tropical soils. During drier, cooler epochs, scientists observed the presence of older soils capable of storing carbon for longer periods.

While the doubling of soil respiration and carbon turnover was measuring during the 2,600 years that followed the end of the last Ice Age, researchers suggest precipitation can inspire changes across shorter time frames, too.

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"Other studies have shown that warming of soils can increase their carbon turnover rates, and over timescales as short as years; but these increases in soil turnover tend to be short-lived pulses," Hein said. "Our works suggests that increased rainfall too can increase soil turnover, and we see a change by a factor of two (soil carbon age cut in half). This effect is significant and long-lasting."

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This echoes recent research into the future of the Arctic's permafrost. Scientists have found evidence that warming temperatures are likely to diminish the carbon-storing abilities and accelerate the release of CO2. Scientists expect similar trends to play out in the tropics during the centuries ahead.

Moving forward, scientists plan to study the link between precipitation and carbon cycles in river and coastal systems at different latitudes -- systems distinct from the Ganges and Brahmaputra. Hein and his colleagues also plan to study more recent changes in carbon storage cycles.

"Here we go back nearly 20,000 years, but what is the role of humans over the last several thousand years, and especially the last 100 years, on the nature of carbon cycling in this globally important system?" Hein said. "These are the types of questions we are continuing to try to answer using these same tools of organic geochemistry and sedimentology that we've been able to address here."

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