Analysis of atmospheric CO2 growth rates at Mauna Loa using CO2 fluxes derived from an inverse model

Carbon dioxide (CO2) growth rates are estimated for a period 1959–2004 from atmospheric CO2 measurements at Mauna Loa by the Scripps Institute of Oceanography. Only during a few short periods, 1965–1966, 1972–1973, 1987–1988 and 1997–1998, in the last 45 yr have growth rates of atmospheric CO2 been of a similar magnitude or higher than that due to the total emission from burning of fossil fuels. Using results from a time-dependent inverse (TDI) model, based on observations of atmospheric CO2 at 87 stations, we establish that El Niño-induced climate variations in the tropics and large-scale forest fires in the boreal regions are the main causes of anomalous growth rates of atmospheric CO2. The high growth rate of 2.8 ppm yr−1 in 2002 can be predicted fairly successfully by using the correlations between (1) the peak-to-trough amplitudes in the El Niño Southern Oscillation (ENSO) index and tropical flux anomaly, and (2) anomalies in CO2 flux and area burned by fire from the boreal regions. We suggest that the large interannual changes in CO2 growth rates can mostly be explained by natural climate variability. Our analysis also shows that the decadal average growth rate, linked primarily to human activity, has fluctuated around an all-time high value of ∼1.5 ppm yr−1 over the past 20 yr. A statistical model analysis is performed to identify the regions which have the maximum influence on the observed growth rate anomaly at Mauna Loa.