Global biomass variation and its geodynamic effects, 1982-1998

Redistribution of mass near Earth’s surface alters its rotation, gravity field, and geocenter location. Advanced techniques for measuring these geodetic variations now exist, hut the ability to attribute the observed modes to individual Earth system processes has been hampered by a shortage of reliable * Corresponding author address: M. Rodell, Hydrological Sciences Branch, NASA Goddard Space Flight Center, Code 614.3, Greenhelt, MD 20771. E-mail address: [email protected] Earth Interactions • Volume 9 (2005) • Paper No. 2 • Page 2 global data on such processes, especially hydrospheric processes. To address one aspect of this deficiency, 17 yr of monthly, global maps of vegetation biomass were produced by applying field-based relationships to satellitederived vegetahon type and leaf area index. The seasonal variability of biomass was eshmated to be as large as 5 kg m“ .̂ Of this amount, approximately 4 kg m“ ̂is due to vegetahon water storage variahons. The time series of maps was used to compute geodehc anomalies, which were then compared with exishng geodetic observations as well as the estimated measurement sensihvity of the Gravity Recovery and Climate Experiment (GRACE). Eor gravity, the seasonal amplitude of biomass variations may be just within GRACE’S limits of detect­ ability, but it is still an order of magnitude smaller than current observahon uncertainty using the satellite-laser-ranging technique. The conhibution of total biomass variations to seasonal polar motion amplitude is detectable in today’s measurement, but it is obscured by conhibutions from various other sources, some of which are two orders of magnitude larger. The influence on the length of day is below current limits of detectability. Although the nonseasonal geo­ dynamic signals show clear interannual variability, they are too small to be detected. KEYW ORDS: Biogeochemical cycles. Earth rotation variations. Timevariable gravity. Remote sensing