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"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.