A study, just out Monday in the journal Proceedings of the National Academy of Sciences, supports a long-standing theory that some plants become more efficient at using water under higher carbon dioxide concentrations. This is a boon for the plant, allowing for more efficient photosynthesis, the chemical process by which plants make food for themselves, which requires both carbon dioxide and water.
Better photosynthesis helps plants grow bigger — which in turn allows them to store more carbon away. That means if plants around the world continue to adjust to rising carbon dioxide concentrations, increasing their biomass on a global scale, they could actually help offset some of our human carbon emissions by removing more carbon dioxide from the air.
“There’s more photosynthesis going on than in the past, and there’s more biomass,” said the new study’s lead author, Ralph Keeling, a professor of geochemistry at the Scripps Institution of Oceanography and program director of the Scripps CO2 Program. “And the accumulation of biomass is important, because it’s carbon that otherwise would have been in the air that got taken out and is slowing down the growth rate of atmospheric carbon dioxide. These things all hang together.”
In fact, the researchers suggest not only that changes in global plant behavior may actually be enough to alter the composition of carbon atoms in the atmosphere, but that this process can help solve a puzzling mystery about what’s happening to all the carbon dioxide we put into the atmosphere.
Carbon atoms can come in several different forms, called “isotopes.” The most common is carbon-12, which has six protons and six neutrons and accounts for about 99 percent of all naturally occurring carbon on Earth. Another isotope is carbon-13, a slightly heavier atom with six protons and seven neutrons, which accounts for the other 1 percent.
It turns out that most plants tend to favor carbon-12 over carbon-13 for carrying out the chemical processes involved in photosynthesis. This means plants tend to contain an even smaller ratio of carbon-13 to carbon-12 than naturally occurs in the atmosphere.
Because fossil fuels, such as oil, are naturally formed from dead plant matter, they also contain a smaller ratio of carbon-13 to carbon-12. Burning them releases this plant-derived carbon — rich in carbon-12 and deficient in carbon-13 — into the air all at once, changing the naturally occurring ratio of carbon isotopes found there. Indeed, measurements show that the atmospheric ratio of carbon-13 to carbon-12 has been growing even smaller over time.
But the rate at which it’s been decreasing doesn’t match the level scientists would expect based on the amount of fossil fuels humans have been burning since the Industrial Revolution. That pulse of plant carbon should mean that the ratio would be changing even faster than it actually is.
But Keeling and his colleagues, a group of scientists from the United States, Britain and the Netherlands, now demonstrate that accounting for changes in plant behavior can explain the discrepancy. They suggest that plants are sucking up even more carbon-12 from the air than they used to, on a scale large enough to affect this atmospheric ratio.
“We looked across a range of processes and couldn’t find anything to explain this offset except if there were changes going on in photosynthesis that were making the plants even more selective for carbon-12 than they were in the past,” Keeling said. “They become more choosy.”
This choosiness ties back to the theory that plants use water more efficiently under higher levels of carbon dioxide. Scientists have suggested that when carbon dioxide is more abundant in the air, plants can take in the same amount with fewer or smaller pores in their leaves, called stomates, losing less water in the process and increasing the efficiency of their photosynthesis. And as they increase their efficiency, the changes in their stomates cause them to become even more selective for carbon-12.
Keeling and his colleagues tested this idea using a model relying on long-term atmospheric carbon dioxide data. It produced the standard puzzling off ratio of atmospheric carbon-13 to carbon-12, but assuming plants became more selective over time accounted for the difference.
“The authors present the first evidence of a long-term global increase in the efficiency with which plants use water, confirming previous results that were limited to the leaf scale or a few specific forests with long-term measurements,” said Trevor Keenan, a research scientist and expert on carbon cycling at Lawrence Berkeley National Laboratory, who was not involved with the new study, in an emailed comment.
The study is an example of how analyzing fine details — measuring carbon isotopes in addition to overall levels of carbon dioxide in the atmosphere, in this case — can yield surprising insights into the processes shaping the planet, said Joseph Berry, an expert on global ecology at the Carnegie Institution for Science, who was not involved with the research.
“If we just look at the concentration of CO2, which changes with time in the atmosphere, we have a lot of options open to us to explain what’s going on,” he told The Washington Post. “If we add information on changes in the isotopic composition of that CO2, it reduces the options that we have to explain things with, and provides a better, more precise characterization of how the biosphere’s changed.”
What this all means for the carbon cycle on Earth remains to be seen. The findings indicate that plants are adjusting in proportion to the rate at which carbon dioxide levels are rising, Keenan noted, suggesting they may continue to adapt into the future. And if plants around the world increase and flourish under rising carbon dioxide levels, they could actually help offset some of our future carbon emissions by taking in more carbon dioxide.
But some scientists believe that vegetation will eventually hit a sweet spot in terms of efficiency, and can’t continue adapting indefinitely. And while the new study does suggest that changes in plant photosynthesis are producing global effects, it can’t tell us whether vegetation in certain regions of the world is changing faster than others, or what will happen to plants in the future. In fact, scientists have cautioned that rising carbon dioxide levels aren’t necessarily all good for global vegetation. As carbon dioxide levels climb, so do global temperatures — and in some warm regions of the world, conditions could eventually become too hot or dry for plants to tolerate.
So for now, the prognosis for global vegetation on the whole remains unclear. But the study does suggest that the burning of fossil fuels could already be changing the planet’s ecology on a large scale. And it may shed new light on a tricky climate conundrum — an important breakthrough for climate science, even if we don’t fully understand its implications, according to Berry.
“We need to know more than just the world is changing,” he said. “We need to know how it’s changing.”