The biosphere causes a seasonally cyclic oscillation in atmospheric carbon dioxide concentrations, with a drawdown in the active growing season and increase during periods when vegetation is decaying back to carbon dioxide. Weissert and Bernasconi (2004) point out that the Indonesian fires of 1998 were widespread enough to cause a clearly measured increase in atmospheric CO2 above the natural seasonal cycle. They point out that this mechanism might have been responsible for a rather abrupt rise in global temperatures about 55 million years ago, due to a subsequent "super-greenhouse effect" of such a large increase. This well-known Palaeocene-Eocene thermal maximum was formerly thought to have arisen from emissions of methane-bearing gas hydrates for the ocean floor, providing elevated atmospheric levels of methane, also a strong greenhouse gas.
Reference
Weissert, H., and S.M. Bernasconi, 2004: An Earth on fire. Nature, 428, 130-132.