Earth's plants – natural scrubbers removing carbon dioxide from the atmosphere – reduced their carbon uptake by some 606 million tons during the past 10 years, according to a new study.
The dip is slight. And it's unclear whether the decline signals the beginning of a trend or merely represents a decade-long lull. But it comes on the heels of two decades of growth in carbon uptake by plants around the world.
Given the important role everything from sage brush to Sequoias plays in removing from the air some of the CO2 from human industrial activities, the decline is "the major punch line to us" from the study, says Steven Running, a forest ecologist at the University of Montana who took part in the research. The results appear in Friday's issue of the journal Science.
Seven years ago, Dr. Running was among a group of scientists who found that between 1982 and 1999, global net primary production – a key measure of plants' carbon uptake – rose 6 percent, or roughly 3 percent per decade.
That trend was driven largely by rising global average temperatures and by the increasing supply of CO2 in the atmosphere. For plants, the combination was like sitting down to an all-you-can-eat banquet in a cozy inn.
But the past 10 years have gone into the record books as the warmest decade since the 1880s, when the instrumental record begins. During the decade, human-triggered increases in atmospheric CO2 concentrations continued at a relentless pace, at least through 2008.
This led Running and colleague Maosheng Zhao, the study's lead author, to ask whether trends in net primary production during the previous 20 years held up during the past 10 years. The short answer was, no: The carbon-uptake trend appeared to shift from positive to negative.
The fall-off is small compared with roughly 88 billion tons of carbon human activities pumped into the air over the past 10 years. But if the decline continues, the duo suggests it could signal a tipping point for one of the two major "sinks" for the long-buried carbon humans have added to the system by burning coal, oil, and gas, as well as through land-use changes.
The other major sink is the ocean, and it appears to be growing less cooperative as well. In their latest report on the state of the global carbon cycle, published in the journal Nature Geoscience last November, scientists associated with the Global Carbon Project noted that the fraction of CO2 emissions from human activity appears to have declined by about 5 percent between 1959 and 2008, essentially as the sinks themselves respond to global warming.
That response appears to be driving the decline the new study records, Running says.
Running and Dr. Zhao used data from NASA's 10-year-old Terra satellite, which monitors changes in vegetation, among other tasks.
During the 2000-2009 period, the data show that net primary production increased in the northern hemisphere
Severe droughts in North America, Europe, and China moderated the increase. Still, those were offset by vegetation at high latitudes taking advantage of the longer growing seasons that global warming has brought.
To the south, however, the story is different. The southern hemisphere lacks expansive, heavily vegetated land masses at high latitudes that might benefit from a longer growing season. Instead, large swaths of the continents stretching south of the equator have deserts and significant arid regions.
In the southern hemisphere, net primary production fell, again largely driven by droughts in Australia as well as long-term drying trends in other regions. The decline in the south hemisphere offset the gains north of the equator.
The results highlight a difference between plant growth in regions where energy – in the form of sunlight and its warmth – has been the key in limiting plant growth and regions where the availability of water is the most influential factor.
On a planet-wide basis, 30 to 40 percent of the land currently is energy-limited, while 50 to 60 percent is water-limited.
The big question moving into the future is "what fraction of the land surface will continue to be energy limited and will benefit from warmer temperatures, and what fraction of the biosphere is going to have continuing droughts" that grow progressively worse?
Current projections hold that arid regions will get drier, while already damp regions will see more precipitation. For this past decade, the effect of droughts beat out the benefits to energy-limited systems, he says.
Even for energy-limited areas, uncertainties remain over how much more precipitation they could receive – and when. A longer growing season based on temperature alone doesn't mean much if most of the precipitation occurs early.
"It's already been noticed that in some of the boreal forests that springtime starts a few weeks earlier, but then they run out of water by August and are in a drought cycle the rest of the summer," Running says.