Study recalibrates trees’ carbon uptake

Photosynthesis appears to be somewhat speedier than conventional wisdom had suggested, a new study finds.  If true, this could mean that computer projections are at risk of overestimating the potential for forests to sop up carbon dioxide, a major greenhouse gas.

The new study did not measure photosynthesis directly. It instead deduced photosynthetic rates from subtle variations in the molecular weight of carbon dioxide molecules in air. Samples had been collected from across the globe during a 30 year period.

Some CO₂ molecules tip the scales more than usual because one of their oxygen atoms has a molecular weight of 18, not 16. (The heavies pack an extra pair of neutrons.) The likelihood that an oxygen atom in CO₂ will be an O¹⁸ will depend on the proportion of heavy oxygen in a region’s water, explains biogeochemist Lisa Welp of the Scripps Institution of Oceanography in La Jolla, Calif. That ratio can vary by soil moisture and weather conditions, she explains.

As plants breathe CO₂ into their leaves, that CO₂ will exchange its oxygen atoms with those in water, Welp notes. A substantial amount of that CO₂ will eventually be released back into the air, now bearing an O¹⁸-to-O¹⁶ ratio reflective of the plant’s water.

In the September 29 Nature, Welp — and colleagues on three continents — report finding a subtle change in the proportion of CO₂ molecules hosting heavy oxygen. This anomaly appeared to start in the tropics and then quickly spread across the planet. The pattern then repeats, almost in waves.

Each wave lasted about 18 months. And the start of a new wave every four years or so generally coincided with the emergence of a prolonged spell of unusually warm ocean temperatures in the Equatorial Pacific — a climatic event known as an El Ni±o. Explains Welp, “We find that water’s oxygen isotopes get heavier during El Ni±os in the tropics.”

Using the average ratio of heavy-to-normal oxygen in CO₂ (linked to an El Nino event that could be timed), Welp’s group now had a means to evaluate how long it takes plants to transfer an anomalous oxygen signature into atmospheric CO₂ — and then wash it away again once an El Ni±o ended. With this information, the researchers attempted to validate a fairly well accepted estimate of the global rate of carbon taken up by photosynthesis in plants each year —120 petagrams (peta being 10¹⁵).

Their assessment found the 120-petagram figure looked short — by about 25 to 45 percent. As to how such a revision in the rate of carbon cycling through plants might alter estimates of future long-term carbon sequestration by forests, Welp emphasizes: “We don’t know yet. It’s way too early to tell.”

But Matthias Cuntz, a biogeochemist with the UFZ-Helmholtz Centre for Environmental Research in Leipzig, Germany, argues that in fact, carbon-storage implications of the new numbers are not that hard to fathom.

The global value for carbon sequestered long term in plant tissue each year is fairly well established at about 1.6 billion tons, he says. That’s almost 2 percent of the 120 petagram estimate. So if the carbon throughput is revised upward by 25 to 45 percent, then the amount of carbon being sequestered long term must be substantially less than 2 percent, Cuntz says – perhaps “only about 1 percent.”

In a commentary accompanying the new Nature paper, Cuntz likens the old 120-petagram figure to a “gold standard” for the annual rate at which land plants take in carbon for photosynthesis. If Welp’s team is right, he now argues, it has just put “a dent” in that gold standard.

The new estimates in the Nature paper do, however, rest on a lot of assumptions – albeit smart ones, Cuntz explains in his commentary. So the rather unexpected result that Welp and her colleagues report will certainly need confirmation.

But if the new numbers hold up, he told me, it could mean that current computer climate models rely on overly optimistic estimates about how efficiently trees can sequester carbon. “If you change photosynthesis a little, like this,” he says, “it could lead to huge differences [in projections of how climate might change in the future].”

Indeed, it would argue that reining in global warming will prove much harder than biologists had led us to expect.