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September 1997

Science Briefing

Beth Holland

How does vegetation keep up with carbon dioxide? The earth's plant life appears to be consuming CO2 at an increasing rate as CO2 levels in the atmosphere rise. But the fate of the extra carbon taken up by plants has been unclear. In a number of experiments where plants are grown under enhanced-CO2 conditions, the carbon stocks of the experimental ecosystems haven't reflected the corresponding increase in stored carbon one would expect from the increase in photosynthesis.

Elisabeth (Beth) Holland (ACD) is second author of a paper in the 7 August issue of Nature that posits an explanation for the missing carbon. With lead author Bruce Hungate (University of California, Berkeley) and four others, Beth presents results from a three-year study of California grasslands exposed to doubled CO2 levels. The group found that more carbon than expected was passed to the plants' roots. Previous work had shown little growth in root biomass under such conditions, hinting that the roots didn't serve as a storehouse for extra carbon.

Beth and colleagues found that the additional carbon isn't sequestered (stored) in the roots themselves, but passed back to the atmosphere almost immediately via increased root and microbial respiration, as well as through short-term carbon pools in the surrounding soil that release carbon to the air before it can be fixed in the soil. These so-called labile pools "can drive substantial but difficult-to-measure sequestration of carbon in the short term," write the authors. "The small size and high turnover of the labile pools, however, prevents them from providing quantitatively important long-term carbon storage."

The Nature paper suggests that, because of these labile pools, short-term experiments in CO2-enriched environments "may tend to overestimate the potential for grasslands to sequester carbon in soils in the long term."

Christopher Wikle

Home to the world's warmest oceanic waters, the western half of the equatorial Pacific Ocean features extensive and frequent storminess. The storms are often related to periodic features such as the 30- to 60-day Madden-Julian oscillations. Shorter-term, high-altitude waves also traverse the region in 3- to 4.5-day periods, and a recent paper is the first to find two annual peaks in this wave action. The paper in the 15 July issue of the Journal of the Atmospheric Sciences was authored by Christopher Wikle (NCAR Geophysical Statistics Project). Coauthors are Roland Madden (CGD) and Tsing-Chang Chen (Iowa State).

Although the waves observed by Chris and colleagues were examined at high altitudes (between 150 and 30 millibars, or roughly 14 and 24 kilometers), they are believed to originate closer to the surface, traveling westward and amplifying upward with time. Known as mixed Rossby-gravity waves, they become most evident through spectral analysis of wind data over the region. Chris's team used a new technique, autoregressive cyclic spectral analysis, that allows researchers to retain annual and semiannual cycles that are normally removed from spectral analysis data for clarity. According to the authors, "from a statistical perspective, it makes sense to use the information contained in the [seasonality], rather than to remove it."

With the new analysis, the team uncovered distinct peaks in wave activity that occurred in late winter/spring and late summer/fall for several stations between 5 and 10 degrees north. The dual peaks could be triggered by well-known peaks in thunderstorm activity occurring at those times. Chris and colleagues also found that the horizontal momentum fluxes associated with the waves appear to be in opposite directions for late winter and late summer. The authors suggest that future studies use modeling to explore the horizontal and vertical structure of these waves and their twice-yearly peaks.

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