What happens when the unit of measurement that scientists have been relying upon to measure greenhouse gas emissions turns out to be inaccurate?

You wind up with inaccurate policy decisions and public perceptions about agriculture, and in particular, ruminant production, according to Logan Thompson, Kansas State University assistant professor. He spoke at the 2023 K-State Beef Stocker Field Day, Sept. 28. Thompson’s research focuses on measuring greenhouse gas emissions in grazing systems, and how we might be able to use grazing management to improve the ecosystem.

Inaccurate measurements

First off, while institutions and governments are worried about climate change, what they’re really worried about is carbon dioxide emissions, Thompson explained. “We are hyperfocused as a society on carbon dioxide,” he said.

And yet, the current models of measurement don’t show the true picture.

According to the U.S. EPA, greenhouse gases warm the Earth by absorbing energy, slowing the rate at which energy escapes to space. Think of it as a blanket insulating the Earth. GHGs differ in either their “radiative efficiency,” or their ability to absorb energy, or the length of time they linger in the atmosphere, or their “lifetime.”

Now, the Global Warming Potential measurement was developed so scientists could compare the global warming impacts of different gases. It measures how much energy the emissions of 1 ton of gas will absorb over a given period of time, relative to the emissions of 1 ton of carbon dioxide, according to the EPA. Typically, that time is 100 years, so it can also be called the GWP 100.

Thompson explained that scientists have found that calculating emissions based on a carbon dioxide-equivalent basis doesn’t really calculate their warming impact correctly. For one, carbon dioxide has a longer lifetime in the atmosphere — it lingers for centuries, while methane is a much shorter-lived gas. Second, it doesn’t really measure global temperature targets. And finally, the GWP metric tells us that if we adjust methane emissions in the short term, we can buy ourselves time, Thompson said.

“That’s not how this works,” he said. “If we do all this now, we have to still pull carbon dioxide out of the atmosphere and try to sequester it in the soil.”

So, University of Oxford scientists created a more accurate measurement metric, the “GWP*,” or “GWP Star.” CO₂ takes hundreds of years to break down in the atmosphere, giving it a chance to collect and become an even thicker blanket, if you will. Methane, however, breaks down in about a decade, turning into CO₂ and water vapor. GWP* accounts for the removal of the short-lived gases, like methane, in the atmosphere.

Thompson’s team looked at EPA GHG data and the Food and Agriculture Organization data, and compared GWP 100 and GWP* metrics.

“With GWP 100, we’re always going to have a positive emission, or impact on climate, because it says methane never leaves the atmosphere in that calculation,” he said. “But with GWP*, we show a 0.32% decrease in our emissions as a cattle sector, and that keeps us net-neutral in terms of warming.” Anything beyond that means we are providing cooling effects to the atmosphere, he continued.

Ruminant grazing

When you look at emissions using the GWP 100 measurement, 26.9% of the U.S. methane emissions come from enteric sources, or ruminants. That’s the largest source in the U.S. for methane, Thompson said.

But what happens when we use the GWP* measurement? “All U.S. enteric methane is just 0.76% of U.S. greenhouse gas emissions,” Thompson said. And that comes to just 0.17% of all U.S. greenhouse gas emissions.

That shows that cattle grazing is a net sink for GHG emissions, Thompson said. And in the new world of carbon markets and farmers getting paid, potentially, for their environmental contribution, that could be money in cattle producers’ pockets.

It makes sense, since ruminants have long played a role in the Great Plains ecosystems. For centuries, millions of bison roamed the Plains, and were a natural part of the carbon cycle, he said. Ruminants graze; they convert forage into energy and release a bit of methane; the methane goes into the atmosphere; and over nine to 12 years, it’s broken back down into CO₂, which is then taken up by plants.

“A professor at Penn State did a modeling exercise, looking at Great Plains bison herds, and he modeled a low, medium, and high population,” he said. “He [the Penn State professor] estimated for the low population, 73.6 million metric tons of CO₂ emissions per year, up to 166 million metric tons of CO₂ equivalent emissions per year.

“That year, 2007, all ruminants in the U.S. were 134 million metric tons of emissions,” he added. It shows that U.S. cattle production is on the right track, he added.

To learn more about Thompson’s work, watch his full presentation below.