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Ice age ‘weather balloon’ probes ancient temperatures high in the atmosphere

Reconstructing ancient climates is murky business. Cores of sediment or ice contain records of temperatures that date back hundreds of thousands or even millions of years, but typically only near Earth’s surface or in the ocean. Now, by measuring rare oxygen isotopes, a team of geochemists has developed a way to deduce long-ago global temperatures 10 kilometers up in the atmosphere, creating, in effect, a paleo–weather balloon. The technique will help scientists study past climates, not only to better understand what conditions were like as life evolved, but also to ground-truth computer models’ predictions about global warming.

“There are not a lot of geological archives that store information about past temperature at high elevations,” says James Russell, a paleoclimatologist at Brown University who wasn’t involved in the work. “This new method gives us a way to see what the Earth’s upper atmosphere was doing.”

To create the new probe, Laurence Yeung, a geochemist at Rice University, and his colleagues relied on so-called “clumped isotopes,” where two or more atoms in a molecule are replaced by isotopic cousins of different weights. They targeted oxygen molecules (O2) in which rare oxygen-18 isotopes replaced both of the more common atoms of oxygen-16. In today’s atmosphere, the molecule exists at levels of just 4 parts per million, forming naturally at an altitude of about 10 or 11 kilometers. The colder it gets, the more these rare molecules are created, which means tiny variations in their abundance can be used as a thermometer. Once formed, winds mix the molecules throughout the atmosphere, and some get trapped in air bubbles within compacted ice. Yeung was keen to apply the method to the Last Glacial Maximum (LGM), a cold period 20,000 years ago, which plays a critical role in many climate models.

The challenge was measuring small variations of an exceedingly rare gas, which Yeung says was akin to weighing a Boeing 737 jet to within the weight of a grain of sand. To achieve such precision, Rice graduate student and co-author Asmita Banerjee worked in a freezer room, cutting 100-gram chunks of ice taken from ice cores from Greenland and Antarctica that dated to the LGM. She carefully melted the ice and collected the gasses in the air bubbles. After filtering out nitrogen, argon, and other trace gases, she ran the remaining oxygen through a souped-up mass spectrometer customized to tally all the oxygen atoms present by weight. “We kind of hot-rodded it,” Yeung says.

Banerjee put in a “heroic effort,” he says. She spent weeks performing the analysis for 102 samples, with each laboratory run taking about 9 hours. “I do try to make sure I’m not sleeping here,” Banerjee says. “That’s a personal boundary for me.”

Based on the proportional abundance of O2 made of oxygen-18 in the samples, the team concluded that the upper troposphere—a region of the atmosphere corresponding to an altitude of 10 to 11 kilometers—was between 6℃ and 9℃ colder in the LGM than today, they report in the 17 August issue of AGU Advances. Those temperatures are not as cold as previous high-altitude estimates that were based on records from mountain lake sediments. “This illustrates the power of having this tracer that integrates the global signal rather than looking at a specific location,” Yeung says.

The number also allowed the team to calculate the “lapse rate,” or the rate at which air temperatures fall with height, an important damper on climate change. When Earth’s surface warms, it puts more water vapor and heat into the upper troposphere. But in the upper troposphere—above the planet’s clouds and heat-trapping gases—heat can efficiently radiate into space, offsetting some of the heating below. The reverse is also true, the new study suggests. During the frigid LGM, the upper troposphere dried and cooled, reducing some of the leakage to space and keeping the planet a bit warmer. The lapse rate is “always buffering against changes in the surface temperatures,” Russell says.

The study’s estimate of the strength of this buffering effect fits with climate model predictions better than those based on the mountain lake records, says Brian Soden, a climate scientist at the University of Miami. “They came up with results that are more consistent with how we think the climate works,” he says. “That’s reassuring, from my perspective.”

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