Plants might not hold on to carbon as long as we thought

Earth’s plants aren’t holding onto carbon as long as we thought.

A new analysis of pulses of radioactive carbon-14 from 20th-century bomb tests reveals that plants stock more carbon in short-lived tissues such as leaves than previously estimated, scientists report in the June 21 Science. That means that this carbon is probably more vulnerable to re-release to the atmosphere — potentially altering estimates of how much anthropogenic carbon the biosphere can hold, the team says.

In July 1945, the United States detonated the first plutonium bomb. That “Trinity” test kicked off decades of nuclear weapon tests, particularly in the 1950s and early 1960s. Each detonation sent a large spike of radioactive carbon-14, a variant of carbon, into Earth’s atmosphere. The bomb radiocarbon then joined Earth’s carbon cycle, winding its way through Earth’s oceans and biosphere (SN: 4/14/20).

That fact became a scientific silver lining to the bomb tests: The bursts of radiocarbon circulating through Earth’s system, scientists realized, were a lot like pulses of radioactive medical tracers traveling through a human body. They offered a unique opportunity for scientists to follow the carbon, analyzing where and for how long it was being stored and released around the globe.

That intel is now crucial. As the climate heats up due to the accumulation of carbon dioxide and other greenhouse gases in the atmosphere, there is an acute need to understand just how long Earth’s biosphere — including its plants and soil — can sequester some of that carbon, says Heather Graven, an atmospheric scientist at Imperial College London (SN: 3/10/22).

Current computer models of the climate estimate that Earth’s vegetation and soils take up about 30 percent of human-caused carbon dioxide emissions. Graven and her colleagues were curious about that. “We were interested in looking at the models of the biosphere and how well they represented the radiocarbon from the bomb tests,” she says.

In the new study, Graven and her colleagues focused on a brief span of time, from 1963 to 1967, during which there weren’t any bomb tests. That meant no new pulses to confuse the data — only radiocarbon pulses already moving through the system. The team also focused just on the plant-growth part of the carbon storage.

The team started by reassessing how much carbon-14 was estimated to enter the upper atmosphere from the bomb tests, and how much moved into the lower atmosphere and into the oceans during that time. To do this, the researchers updated previous estimates with carbon-14 data collected by aircraft, stratospheric balloons and ocean buoys. From there, they calculated how much carbon-14 must have entered the biosphere. The team then compared satellite-based observations of carbon storage in living vegetation with computer simulations of where the carbon accumulated in the plants.

The results were startling, Graven says. Most current computer simulations of vegetation and climate underestimate how fast plants are growing, they found. Current models suggest that plants are pulling in between 43 trillion and 76 trillion kilograms of carbon each year; the new study increases that to at least 80 trillion — possibly twice as much.

That sounds like good news, when it comes to hopes of storing excess carbon from human activities in the biosphere (SN: 7/9/21). But, the team found, there’s a downside. The bomb radiocarbon tracking also revealed that more carbon is being stored in short-lived biomass such as leaves and thin, fine roots than previously thought. Those tissues are far more vulnerable to degradation that releases carbon back to the atmosphere than longer-lived tissues such as stems and larger roots.

“The carbon going [into plants] now is not going to be there as long as we thought,” Graven says. And that, she says, reemphasizes how important it is to limit fossil fuel emissions. “There is a limit of how much we can store in vegetation.”

What these findings mean for future projections of climate and how best to incorporate the role of vegetation in these models, isn’t yet clear, says Lisa Welp, a biogeochemist at Purdue University in West Lafayette, Indiana, who was not involved in the study. But, she says, they do undermine confidence in how well climate models will be able to simulate that role.