Continental-Scale Partitioning ofFire Emissions During the 1997to 2001 El Nin˜o/La Nin˜a PeriodGuido R. van der Werf,1* James T. Randerson,2†G. James Collatz,3Louis Giglio,4Prasad S. Kasibhatla,5Avelino F. Arellano Jr.,5Seth C. Olsen,2Eric S. Kasischke6During the 1997 to 1998 El Nin˜o, drought conditions triggered widespreadincreases in fire activity, releasing CH4and CO2to the atmosphere. We eval-uated the contribution of fires from different continents to variability in thesegreenhouse gases from 1997 to 2001, using satellite-based estimates of fireactivity, biogeochemical modeling, and an inverse analysis of atmospheric COanomalies. During the 1997 to 1998 El Nin˜o, the fire emissions anomaly was2.1 ⫾ 0.8 petagrams of carbon, or 66 ⫾ 24% of the CO2growth rate anomaly.The main contributors were Southeast Asia (60%), Central and South America(30%), and boreal regions of Eurasia and North America (10%).Global atmospheric CO2and CH4levels areincreasing, but at variable rates from year toyear (1–5). Because both are greenhouse gases,our ability to predict future changes in climatedepends, in part, on our understanding of thesources of this variation (6). Atmospheric CO2has been systematically measured since 1958, andduring this time, the annual growth rate hasranged from nearly zero to roughly the rate atwhich fossil fuels were being combusted (1, 3). Acombination of atmospheric measurements (3),inverse modeling (7), and ocean observations andmodeling (8) has attributed most of this variabilityto changes within the terrestrial biosphere.Proposed mechanisms to explain the ob-served variability in CO2growth rate have fo-cused primarily on the balance between terres-trial photosynthesis and ecosystem respiration(7, 9–11). A recent study by Langenfelds et al.(12), however, provided evidence that parallelchanges in atmospheric CO2, CO, CH4, andother trace gas species emitted by fires implythat a large part of the CO2variability in the1990s was a result of variability in global fireemissions. This result is consistent with reportsof increased fire activity during the 1997 to1998 El Nin˜o in Indonesia (13), Central Amer-ica (14), parts of Amazonia (15, 16 ), multiplecountries in Africa (14), and boreal regions ofNorth America and Eurasia (17, 18). Althoughinverse studies have connected the fires in In-donesia and Southeast Asia with the spatial-temporal pattern of global atmospheric CO2anomalies during this period (19, 20), a quan-titative partitioning of global fire emissionsamong the continental regions has not yet beenattempted. This information may provide in-sight into the biogeochemical and socioeco-nomic processes that regulate biosphere-atmosphere exchange of CO2and CH4.Here we present a means of isolating thecontributions of fire emissions from differentcontinents to atmospheric CO2and CH4con-centration anomalies for the 5-year periodfrom January 1997 through December 2001.This time period includes a strong El Nin˜o(1997 to 1998) and a La Nin˜a (1999 to 2000)(21). Our analysis consisted of two steps.First, we combined satellite observations offire activity over this period from the Tropi-cal Rainfall Measuring Mission–Visible andInfrared Spectrometer (TRMM-VIRS), Euro-pean Remote Sensing Satellite–Along TrackScanning Radiometer (ERS-ATSR), and Ter-ra MODerate resolution Imaging Spectrora-diometer (Terra-MODIS) sensors with theCarnegie-Ames-Stanford Approach (CASA)biogeochemical model (22) to estimate inter-annual fire emissions at a 1° by 1° spatialresolution and with a monthly time step. Weconverted TRMM-VIRS (1998 to 2001) andERS-ATSR (1997 to 2001) fire activity datato time series of burned area using MODISburned area estimates (available for limited10° by 10° areas starting in 2001) in thetropics, and using country-level burned-areastatistics in the northern extratropics (23).The CASA model used this burned-area timeseries to estimate carbon emissions fromfires, taking into account local variations infuel type, fuel density, and combustion com-pleteness (22, 23). Carbon emissions fromCASA were multiplied by biome-dependentemission factors (24 ) to obtain CO, CO2, andCH4fire emissions (25). We then used theGoddard Earth Observing System Atmo-spheric Chemistry Transport Model (GEOS-CHEM) atmospheric chemistry transportmodel (26) to predict the temporal and spatialdistribution of the emitted CO, CO2, and CH4during the 1997 to 2001 period, separatelytracking the unique space-time pattern of theemitted trace gases from seven continental-scale regions (listed in Table 1).In the second step of our analysis, using aleast squares inversion, we solved for thelinear combination of the seven patterns ofCO concentration anomalies generated fromour forward model that minimized the dif-ference with observed CO concentrationanomalies at flask stations from the NationalOceanic and Atmospheric AdministrationClimate Monitoring and Diagnostics Labora-tory (NOAA/CMDL) (27). In this step, weassumed that fires caused all of the observedmonthly anomalies of atmospheric CO from1997 through 2001. In addition to fire emis-sions, other major sources of troposphericCO include emissions from fossil fuel andbiofuel combustion, CH4oxidation by OH,and oxidation of volatile organic compounds(28). Although these sources contribute sub-stantially to mean latitudinal and seasonalvariations of atmospheric CO (28), our as-sumption that these sources influenced theanomalies only minimally over the 1997 to2001 period was predicated by the largevariations in fire emissions during this period[e.g., (27 )]. Because satellite and biogeo-chemical model information was used toconstruct the form of the basis function ineach region, no additional a priori constraintswere applied in the inversion. We assumed a5 parts per billion (ppb) error for the monthlymean CO concentrations from each station onthe basis of observed variations and measure-ment precisions (27). The seven scalars thatwe solved for in the inversion were then usedto separately adjust the regional time series ofcarbon, CO2, and CH4emissions that weobtained from our forward model (23).Widespread increases in fire emissionsoccurred across multiple continents duringthe August 1997 to September 1998 period(Fig. 1A) and appeared to be linked in manyregions with El Nin˜o–induced drought (29).Over the entire 1997 to 2001 period, contri-butions to mean annual emissions were great-est from southern South America (23%),northern Africa