We use a fully coupled climate-biosphere model, CCM3/IBIS, to illustrate how vegetation dynamics may be capable of producing long-term variability in the climate system, particularly through the hydrologic cycle and precipitation. We conduct two simulations of the global climate, with fixed climatological sea-surface temperatures: one including vegetation as a dynamic boundary condition, and the other keeping vegetation cover fixed. A comparison of the precipitation power spectra over land from these two simulations shows that dynamic interactions between climate and vegetation enhances precipitation variability at timescales from a decade to a century, while damping variability at shorter timescales.

In these simulations, the two way coupling between climate and dynamic vegetation cover introduces persistent precipitation anomalies in several ecological transition zones: between forest and grasslands in the North American Midwest, in Southern Africa, and at the southern limit of the tropical forest in the Amazon basin, and between savanna and desert in the Sahel, Australia and portions of the Arabian Peninsula. These regions contribute most to the long-term variability of the climate-vegetation system. Slow changes in the vegetation cover, resulting from a “red noise” integration of high frequency atmospheric variability, are responsible for generating this long-term variability. Lead and lag correlation between precipitation and vegetation leaf area index (LAI) shows that LAI influences precipitation in the following years, and vice-versa. A mechanism involving changes in LAI resulting in albedo, roughness and evapotranspiration changes is proposed.