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Sunday, April 14, 2024

The Pandemic Gave Scientists a New Way to Spy on Emissions

Think of the sky as a big bowl of blue soup. Its ingredients include oxygen, nitrogen, and carbon dioxide, which scientists can precisely measure. But ever since the Industrial Revolution, humans have been adding heaps of extra CO2 by burning fossil fuels, warming the planet 1.2 degrees Celsius so far and complicating those calculations. 

While it’s easy enough to know how much total CO2 is in that atmospheric soup, it’s difficult to parse how much humanity is adding at any given time. That’s because Earth’s natural processes also create the gas, and because there are such a multitude of sources for civilization’s own emissions, some of which grow or wane by the hour. It would be like throwing dashes of salt into actual soup and then trying to count precisely how many grains went in after they hit the liquid. 

What atmospheric scientists can do, though, is make an inventory, a “bottom-up” effort to exhaustively count skybound CO2 as it’s produced on Earth. For example, they can add up how much gasoline is being burned and how many fossil fuel power plants are running at a given time, to calculate how much carbon is being exhaled into the atmosphere. While quite accurate, all that inventorying takes time, largely because some data is slow to trickle in. And timeliness matters when taking action on climate change, because we need to identify sources of CO2 and eliminate them as quickly as possible, for instance by replacing coal with renewables, gasoline cars with electric vehicles, and gas furnaces with heat pumps

You might be wondering why researchers can’t take a more “top-down” approach, training satellites on spots on the planet and measuring the CO2 coming off them. It’s been tried on certain parts of the globe, for example when a NASA satellite took readings over the Los Angeles basin. But there are a few issues: Air mixes, and it’s hard to pinpoint exactly where emissions came from. Another is that it can be hard to pick out humanity’s emissions from the CO2 created by Earth’s natural carbon cycle. When plants photosynthesize, they suck in carbon and lock it in their tissues, and in turn expel oxygen. When they die and rot, that carbon is emitted again. 

But now the Covid-19 pandemic has, oddly enough, given scientists a better top-down tool for estimating minute changes in fossil fuel emissions. A team of researchers used the UK’s coastal Weybourne Atmospheric Observatory to test air for carbon dioxide and oxygen separately, then summed the measurements together. Then they used a trick called atmospheric potential oxygen, or APO, which calculates the imbalance between oxygen and CO2 from fossil fuel emissions. 

The key to separating natural and human-caused emissions is the ratio between CO2 and oxygen. Plants process both in a one-to-one ratio: They absorb the same amount of carbon dioxide as the oxygen they expel, so the totals cancel each other out. Burning fossil fuels, on the other hand, consumes more oxygen than it produces CO2.

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When fossil fuel emissions suddenly and dramatically declined during the pandemic, it gave the researchers a unique opportunity to test how well APO can tease out where CO2 comes from. Lockdowns created obvious dips in human-made emissions, while natural emissions stayed constant—and they reckoned their tool should be able to tell them apart.The Weybourne Atmospheric Observatory is a weather station on England's North Norfolk Coast, which tracks meteorological conditions like humidity and temperature, and samples a range of gases beyond CO2 and oxygen, like nitrous oxide. The researchers used a decade’s worth of its atmospheric measurements to train a machine-learning model. This learned under what conditions, like wind speed and direction, fossil fuel emissions had been high or low, plus where those air masses came from and what areas of land they interacted with. “And then we can see what emissions might have interacted with that air mass,” says Penelope Pickers, an atmospheric scientist at the University of East Anglia and lead author of a new paper describing the work in the journal Science Advances. “So when it arrives on site, if we can separate out the fossil fuel and the natural CO2 using APO, then we can say what the recent emissions have been.”

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To validate APO as a tracer for fossil fuel CO2, Pickers and her colleagues then used the algorithm (trained on the observatory’s pre-pandemic measurements) to predict what those emissions levels would have been without two pandemic lockdowns, one between March and July 2020 and another between November 2020 and January 2021. Then they compared these predictions to the actual APO data they collected during the slowdowns. The difference estimated how far emissions dropped during the pandemic, giving a result comparable to ones found with other estimation techniques, like those based on known energy usage. In this way, the researchers demonstrated that APO accurately detected when atmospheric CO2 from fossil fuels dipped during those two time periods.

“The atmosphere sees both the fossil fuel CO2 changes and CO2 changes from vegetation and the terrestrial carbon cycle and ocean uptake,” says Steven Smith, principal investigator of the Community Emissions Data System at the Pacific Northwest National Laboratory, who wasn’t involved in the new research. “And this method is interesting, because it does isolate out that effect.”

APO can’t differentiate the exact sources of anthropogenic CO2 emissions—for example, it can’t discern between those that come from cars versus power plants. It can parse the emissions in a certain geographic area, so this technique could be used at other ground-based observatories to determine how local emissions change in near real time, like after better car emissions standards are introduced. “In my opinion, this is a really, really interesting paper,” says Joshua Laughner, who studies atmospheric carbon at NASA’s Jet Propulsion Laboratory. (He was not involved in the new research, and he was not speaking for NASA or JPL.) “I like what they're doing with this idea of combining CO2 and O2 measurements. This problem of separating out the biosphere signal and the human signal is one that we've tried to solve—or tried to approach—a lot of different ways, and this is a really clever approach.”

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APO is not meant to replace other ways of calculating emissions, but to complement them—each technique has its pros and cons. Satellites are expensive, but they can zoom in on any spot on Earth. Bottom-up inventories can be slow, but they do an excellent job of accounting for emissions, and they can differentiate those derived from different fuels, unlike APO. And while the APO approach gets closer to real-time monitoring than inventorying does, it requires an observatory to collect data, making it more regional than satellite imaging, at least for the time being.

“Ground-based measurements always have this particular challenge when compared to satellites,” says Northern Arizona University climate scientist Kevin Gurney, whose own platform, Vulcan, uses census, traffic, and other data to accurately quantify emissions. (He wasn’t involved in this new research.) “But there's no reason that you couldn't increase the number of ground-based measurements and locate them strategically and densely to carefully isolate countries or regions.”

This kind of work is important, says Gurney, because we have to know where the carbon is coming from before we can get rid of it. “Accuracy just gives you a better sense of prioritization of what you're going to tackle,” says Gurney. Once mitigation is in place—say, a city starts a program to reduce energy waste by insulating buildings—monitoring emissions in real time will help officials determine if it’s working or not, and adapt accordingly. “You want to track it, because if it goes off the rails, you want to know as soon as possible,” says Gurney.

There is no one technique to rule them all—a network of APO observatories could join satellite monitoring and good old inventories to build a better picture of how the carbon soup in the sky is changing. “We already actually have quite a good [observatory] network across some parts of the world,” says Pickers. “Having information quickly, at the relevant scales, for how the change in emissions is occurring is really important if we want to be successful at reducing our emissions.”


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