The ability to make accurate predictions about climate change has always posed a formidable challenge. Projections are based on models, which in turn require the input of vast amounts of accurate data. And that data includes numerous variables gathered from a wide array of sources. Now scientists have discovered that a fundamental assumption about one of those variables - carbon cycling through plants - may have been underestimated by as much as 25 percent.
In a paper published this week in the science journal Nature, a team of Dutch, Australian and American scientists estimate that the rate of global photosynthesis - the process underlying the catch and release of carbon atoms by the plants of the earth and its oceans - happens faster than scientists previously thought. Their estimate is based on a review of several decades worth of data, primarily air samples collected by various monitoring agencies around the world.
While it's relatively easy to estimate the rate of photosynthesis of a single leaf, the problem becomes almost infinitely more difficult when you consider an entire forest, let alone the entire planet. To make their estimate, scientists examined isotopes of oxygen in the atmosphere. "What we have done is to use a naturally occurring marker, an oxygen isotope, in atmospheric CO2 that allows us to track how often it ended up inside a plant leaf, and from oxygen isotopic CO2 data collected around the world we can estimate the mean global rate of photosynthesis over the last few decades," said Dr Lisa Welp from the Scripps Institution of Oceanography, California.
The work allowed scientists to trace the path of oxygen atoms in carbon dioxide, providing a new, revised estimate of the global rate of photosynthesis. Other studies, using ice cores and ocean sediments, have provided a bigger picture of carbon cycling over millions of years. By better understanding the rate of exchange of water vapour and CO2 among plants, scientists can now fine-tune their models of climate change.
"Quantifying this global production, centered on the exchange of growth-promoting CO2 and water vapour, has been historically difficult because there are no direct measurements at scales greater than leaf levels," said Dr Colin Allison, an atmospheric chemist at Commonwealth Scientific and Industrial Research Organisation (CSIRO), Australia's national science agency. "...As a result of this new research we have redefined the rate of biospheric carbon exchange between atmosphere, land and ocean...These results can be used to validate the biospheric components included in carbon cycle models and, although still tentative, may be useful in predicting future climate change."