The hypothesis that we will test in this interdisciplinary science investigation is that the interannual variations in the fire regime in the boreal forest are responsible for a significant portion of the interannual variations in the seasonal amplitude of the atmospheric CO₂ record at high northern latitudes. By quantifying the biomass burning component of the seasonal amplitude, we can begin to distinguish among different mechanisms, including changes in the length of the growing season, the effects of interannual variability in snow and ice cover, and the direct effects of temperature and soil moisture on ecosystem carbon fluxes. The goal of this IDS project is to use information derived from satellite obervations available through NASA's Earth Science Enterprise to reduce uncertainties in the constraints on fire carbon emissions estimates for the boreal forest.  

Background
During the past three decades, the phase and amplitude of the seasonal cycle of atmospheric CO₂ has changed at high northern latitudes. The phase has advanced by approximately 1 week (Keeling et al. 1996), and the peak-to-trough amplitude has increased by approximately 0.5 ppmv yr^-1 (Manning 1993; Keeling et al. 1996; Randerson et al. 1999) (Figure 1). A combination of mechanisms may account for these changes:

• Anomalously warm temperatures during late winter and spring have accelerated snow melt and enhanced photosynthesis (Keeling et al. 1996). Increases in NDVI during the 1980s provide additional support to this hypothesis (Myneni et al. 1997).
• Warming during the growing season has led to an increase in the active layer depth at the end of the growing season. The increase in thaw depth exposes soil organic matter to decomposition and increases soil respiration during late summer and fall (Goulden et al. 1998). Increased CO₂ fluxes during winter and spring will lead to a higher CO₂ maximum during the following spring.
• Disturbance from humans and fires has changed the successional, and consequently the phenological composition of northern forests (Zimov et al. 1999).
• An increase in fire activity over the past three decades in the boreal region has increased emissions from biomass burning.