Andean Ice Gives Scientists Their First Methane Record From the Tropics

A team led by Ohio State University's Byrd Polar and Climate Research Center has published a 2,000-year record of atmospheric methane extracted from ice drilled on a tropical mountain in Peru. Published Aug. 19 in Nature, this is the first methane concentration record from the tropics.

Long methane records have come almost entirely from Greenland and Antarctica, which cannot fully resolve how much of the gas originated in the tropics. The few earlier low-latitude cores did not settle the debate: a Bolivian core showed dust-linked methane spikes, and a Mount Everest core yielded only 15 valid values out of 112 once outliers were removed.

The new ice came from two cores drilled to bedrock in 2019 on the South Peak summit of Nevado Huascaran, in the Cordillera Blanca, at 6,768 meters above sea level. Fifty-one samples were measured for methane at Oregon State University, with a mean standard deviation of 3.7 parts per billion across the set, the authors report.

The Andean record tracks the polar ones in shape but sits above them. Over the pre-industrial period, which the authors define as 0–1800 CE, Huascaran averages 46 ppb above the Greenland GISP2 core and 94 ppb above the West Antarctic Ice Sheet Divide core, per the paper.

Feeding the new data into a four-box model of the atmosphere raised the estimated combined methane source strength of the tropics for 0–1800 CE by 24 percent, from about 163 to 213 teragrams a year. They present that as supporting a long-standing hypothesis that low-latitude emissions dominated methane variability before industrialization.

Five samples spanning roughly 1530–1999 CE were analyzed for the carbon-isotope makeup of the methane, which acts as a chemical signature distinguishing between microbial, burning and fossil sources. The values point to a stronger microbial contribution in the tropics, though the small sample count limits this conclusion since the measurement consumes a lot of ice.