Towers of Knowledge

A radio tower? A cell-phone tower? No, it's a flux tower.

Climate and biogeochemistry models honed to run on ORNL's world-class supercomputer are fed data on air-surface exchanges of carbon dioxide, water vapor and energy. A global network of more than 400 micrometeorological tower sites measures these exchanges of two greenhouse gases and solar radiation between the atmosphere and terrestrial ecosystems. Researchers also collect data on the vegetation, soil, hydrologic and meteorological characteristics of tower sites.

Two towers are located on the Oak Ridge Reservation's Chestnut Ridge and Walker Branch Watershed. Tower site data are downloaded, processed and archived at ORNL for DOE's Carbon Dioxide Information Analysis Center.

Sensitive instruments suspended on these "flux towers," some almost as tall as 200 feet, measure tiny CO₂ fluctuations in air columns over a variety of landscapes. The towers, which form a 10-year-old U.S. network called AmeriFlux, provide carbon cycle modelers with essential data on exchanges involving numerous terrestrial ecosystems worldwide, from forest to grassy field to desert.

FLUXNET was formed to coordinate global analysis of flux tower data. FLUXNET data are stored at ORNL at one of NASA's nine Distributed Active Archive Centers. Using FLUXNET data, researchers discovered that the drought linked to the 2003 killer heat wave in southern Europe triggered considerable respiration—accelerated emission of CO₂ to the atmosphere by plants and soil bacteria. The reason: photosynthesis had shut down in response to less water so respiration picked up.

More and more towers are measuring sunlight entering the forest from all directions at low angles because of a recent discovery by ORNL's Lianhong Gu. He found that diffuse solar radiation scattered by airborne volcanic ash enhances photosynthesis in plants shaded from direct sunlight by leaves in the forest canopy. Moderate cloud cover and sulfate aerosols injected into the atmosphere by coal-fired power plants can increase diffuse radiation. Accurate estimates of the resultant enhanced photosynthesis in the lower forest help scientists predict more precisely the amount of carbon that will be stored in the forest soil.

Advances in instrumentation and data collection over the past 30 years have greatly improved flux measurements. The number and accuracy of flux measurements, like each new tower, will go up. —Carolyn Krause

Contact: Lianhong Gu

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