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Carbon Sequestration
Weathering climate change

Policy-makers looking to curb future increases in atmospheric carbon dioxide, could turn to a simple plan: Plant trees. The Intergovernmental Panel on Climate Change (IPCC) estimates that converting unproductive cropland and grasslands to agroforests could sequester about 400 million tons of carbon dioxide in wood each year by 2010. Together with other methods for increasing and preserving carbon reservoirs in soils and vegetation, planting trees could sequester as much as 100 gigatons of carbon dioxide by 2050, according to the IPCC — offsetting 10 to 20 percent of potential fossil fuel emissions during that period.

Landsat image of the Mississippi delta, where weathering products from the Mississippi and its tributaries reach the Gulf of Mexico. A recent Science study links enhanced weathering rates to increased carbon sequestration. Photo courtesy of NASA/GSFC/METI/ERSDAC/JAROS, and U.S./Japan ASTER Science Team.

But, according to a paper in the July 4 Science, past studies may have overestimated the potential for planting trees by overlooking geologic controls on carbon sequestration. The weathering of carbonate and silicate minerals in soils pulls carbon dioxide out of the atmosphere. The Science study found that mineral weathering occurs five times faster in cropland soils than forest soils. Converting croplands to forests could therefore decrease weathering rates, offsetting carbon gains from tree growth by approximately 25 percent.

“Forests are still probably stronger sinks than agricultural fields. However, the weathering effect I report could reduce the net size of the forest sink, probably by enough to question its usefulness,” says lead author Pete Raymond, a biogeochemist at the Yale School of Forestry and Environmental Studies.

Geologists have long known that mineral weathering regulates atmospheric carbon dioxide concentrations, but have traditionally thought that the process only operates over millions of years, says Venugopalan Ittekkot, a biogeochemist at the Centre for Tropical Marine Ecology in Bremen, Germany. The new NSF-funded study suggests that weathering rates can change over decades, and may respond to changes in land use, he says.

Raymond and co-author Jon Cole, a biogeochemist at the Institute of Ecosystem Studies in Millbrook, N.Y., quantified weathering rates by analyzing the export of alkalinity from the Mississippi River and its tributaries over the past 40 years — a dataset collected by the U.S. Geological Survey. Alkalinity directly measures the capacity of water to buffer against changes in acidity. In the Mississippi and most other rivers, alkalinity serves as proxy for bicarbonate, which forms from the weathering of silicate and carbonate minerals, Cole says.

The authors report that alkalinity export at the mouth of the Mississippi has steadily increased over the past half century, corresponding to an increase in weathering. Raymond says that this increase largely results from a rise in precipitation in the region during that time, without any simultaneous drop in the amount of alkalinity in the river water.

Additionally, streams draining cropland areas exported five times more alkalinity than streams draining forests. From this relationship, the authors calculated that converting cropland to forest could decrease carbon sequestration through weathering by roughly 10 grams of carbon per square meter every year. In contrast, the expected increase in carbon uptake in trees from reforesting is about 40 grams of carbon, Raymond says, yielding a net uptake of 30 grams — the 25 percent drop in sequestration efficiency.

The reasons why croplands enhance weathering are not clear, Raymond says. However, tilling may increase the rate and extent to which water interacts with soil minerals, stimulating their weathering. Additionally, he says, high biological productivity on croplands enhances the rates at which plant roots pump carbon dioxide into the soil solution where it can drive weathering.

“The study is not a silver bullet against re-conversion of cropland to forests, but it is a piece of evidence to consider,” Raymond says. He adds that several other factors make re-conversion unwise. For one, trees are not a safe place to store carbon — they can be cut in the future when land-use priorities change. Furthermore, forests reach their peak biomass quickly, limiting their maximum potential for storing carbon, he adds.

Richard Houghton, a biogeochemist at the Woods Hole Research Institute on Cape Cod, Mass., applauds the study for showing that weathering plays an important role in terrestrial carbon sinks over decades. However, he cautions against overextending the results. “In terms of carbon, you might not be any better off reforesting as you were cultivating. But the world is bigger than carbon. … Forests have an ameliorating effect on local climate and can reduce droughts, floods and erosion.”

Greg Peterson

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