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

Abstract

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 Nino-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 Nino 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.