Tracking global carbon cycle a challenge
Changing ecosystems leading scientists to modify carbon models
December 16, 2009
SAN FRANCISCO –
More frequent natural disasters and other disturbances could make some models that predict the global input and output of carbon off by three to 10 percent, according to estimates by a researcher scheduled to talk at the 2009 fall meeting of the American Geophysical Union.
Ben Bond-Lamberty, a terrestrial ecologist for the Department of Energy's Pacific Northwest National Laboratory, will encourage more carbon modelers to account for such ecosystem changes as an invited speaker at a Dec. 18 session on the impacts of disturbance on the North American terrestrial carbon budget. He will also suggest that researchers reconsider assumptions that were made when carbon models were first developed.
It's increasingly challenging to predict the planet's cycle of carbon, the main component of climate-altering greenhouse gases, especially with natural disasters like forest fires and hurricanes and human-caused disturbances like land-use changes happening more frequently.
"Forests appear stable to us because we only live a handful of decades," said Bond-Lamberty. "But it's when you look at a longer time span or greater geographic area that you have to account for disturbances. Those disturbances make predicting the planet's carbon future difficult."
The carbon cycle starts when carbon is absorbed, or fixed, by plants through photosynthesis. When plants die or are eaten, the carbon is released back into the atmosphere. But if more plants die than usual, such as when trees are knocked down by a hurricane, the cycle can shift. The same happens if humans release large amounts of pollution.
Scientists first developed carbon models in the late 1970s and early 1980s with the understanding that the Earth's ecosystems would remain fairly stable. But that's increasingly not the case today. A Scripps Institute of Oceanography report says the average number of wildfires annually has increased four-fold in the Western United States. About twice as many Atlantic hurricanes form each year on average than did a century ago, according to the National Center for Atmospheric Research. And British Columbia, Canada, has lost at least 33 million acres of lodgepole pine forest because of the unprecedented mountain pine beetle outbreak, the New York Times reported last year.
Such sudden changes aren't reflected in many existing carbon models. As a result, Bond-Lamberty roughly estimates that regional and continental models could be off between three and 10 percent as a result. And larger errors are possible on smaller geographic or time scales, he said.
He will demonstrate this by discussing Canada's boreal forests. The large, woody swaths of land to the north help Canada serve as a carbon sink by taking in about 360 teragrams, or 360 trillion grams, of carbon each year. Woody debris left over from the region's frequent forest fires partly counteracts this. That decomposing debris gives off about 40 to 50 teragrams of carbon, which accounts for about 11 to 14 percent of the overall sink in Canada's forests.
"Carbon models work well, but they aren't perfect," Bond-Lamberty said. "And as world leaders consider their climate and carbon policies, scientists have a responsibility to provide them the best available data. Getting that reliable data isn't easy, but I'm confident ecosystem modelers are up for the challenge."
Many carbon models aren't frequently adjusted for disturbances like natural disasters, logging and land-use changes. But at least some changes are relatively easy to make, Bond-Lamberty said. In other cases, scientists can develop new algorithms to plug into their computational models.
This creates some problems, however. When Bond-Lamberty added such complex mathematical equations, other weaknesses in the models became evident. The models were designed to mimic a simpler world and used assumptions that can create unreliable results when disturbances are added.
For example, models typically assume that trees immediately fall down after a fire ravages a forest. But trees actually remain standing for another 10 to 20 years before they fall and decompose. Models that assume trees will fall right away inaccurately predict a huge carbon flux after a disaster. In reality, carbon from trees will be released over a more gradual time span.
But while that example is fairly easy to fix, other assumptions are not. Updating such assumptions can be more complicated and time-consuming, Bond-Lamberty said. For example, one major disturbance in forest systems is human actions and models do not often take humans into account. Bond-Lamberty wants more models to consider how people moving into forests affect the carbon cycle.
Over the last decade, more scientists have started adding disturbances into their carbon models, particularly regional models. But most global models don't account for such changes yet, Bond-Lamberty said.
Bond-Lamberty works at the Joint Global Change Research Institute, a partnership of PNNL and the University of Maryland, College Park.
REFERENCE: B.P. Bond-Lamberty, "Challenges (and annoyances) in modeling disturbance effects on terrestrial carbon cycling." 11:50 a.m. - 12:20 p.m., Dec. 18. Room 3018, Moscone West, Moscone Convention Center, San Francisco.
The presentation abstract is available online by clicking "plan your itinerary" at http://www.agu.org/meetings/fm09/program/index.php and searching for program "B52C-07."
Information about other PNNL research that will be highlighted at AGU can be found at PNNL's AGU website, http://www.pnl.gov/science/events/default.asp.
Tags: Environment, Fundamental Science, Climate Science, Biology