Checking the Planet's Respiration Rate

By Kait Frasier

Imagine lifting the hood of a four-cylinder car you’ve been driving for years, only to discover that it’s been a six-cylinder all along.

This is how Lisa Welp and Ralph Keeling of Scripps Institution of Oceanography, UC San Diego’s CO₂ research group describe their recent work on global photosynthesis rates. Plants, it turns out, have been taking in roughly 40 percent more atmospheric carbon than they’ve been getting credit for.

Many atoms exist in both heavy and light forms, known as isotopes. Scripps researchers have monitored the isotopic ratios of atmospheric carbon dioxide (CO₂) for more than thirty years. The variation of oxygen isotopes contained in that CO₂ was also recorded, because CO₂ with heavy oxygen isotopes skewed the mass ratios measured for carbon. To remove the interference, the oxygen data was used in a calculation known as the “Craig correction,” developed in the 1950s by the late Scripps geochemist Harmon Craig.

No one thought of the oxygen dataset as anything more than a correction factor for many years. However, the global proportion of heavy to light oxygen isotopes in atmospheric CO₂  is largely set by the rate of interaction with water in plant leaves, seawater, and soils. Eventually it became apparent that these data had their own story to tell.

“We're trying to figure out what's under the hood of 'planet-ship' Earth,” Keeling said.

Welp, who led the study, Keeling, and their coauthors took advantage of the unexplored oxygen composition data to recalculate the planet’s photosynthesis rate. Based on their calculations, Earth’s land plant population captures between 150 and 175 petagrams of carbon per year (roughly 20 percent of total carbon in the atmosphere).

Previous estimates of global carbon uptake have been made by scaling up from leaf-level measurements. However, plants uptake atmospheric carbon at different rates (for example, corn is more efficient than rose bushes). They also vary across seasons and are distributed unevenly around the globe. The new estimate is based instead on atmospheric measurements from a worldwide network of study sites.

What do we do with the knowledge that the earth has a bigger engine than we previously thought? Welp and her team are unsure. They have revised one number on the global carbon balance sheet, but carbon is constantly being cycled between rock, water, air, and organic reservoirs. The magnitudes of these exchanges are estimated through a web of interdependent calculations. Increasing estimates of global photosynthesis will have a ripple effect on other estimates.

“Just because we find higher rates of photosynthesis, it doesn’t mean that there’s a bigger land carbon sink than we thought,” Welp is quick to point out. Although plants remove carbon from the atmosphere through photosynthesis, they release most of it back through respiration. Only a small percentage is actually stored for a year or more, and that net storage is well quantified. If anything, plants have been working harder than expected in the face of rising atmospheric CO₂ concentrations.

“It's a good idea to know what you have before you run it too hard,” said Keeling. “Accurately measuring carbon uptake is necessary for understanding the planet’s atmospheric balance and predicting its future.”

According to this study, the way plants are predicted to work and to respond to climate change needs to be adjusted. It also serves as a reminder that our “planet-ship” still has some surprises under the hood.

--Kait Frasier is a 3rd-year Scripps Oceanography graduate student in the laboratory of oceanographer John Hildebrand.