Savannas are a major terrestrial biome, comprising of grasses with the C sub(4) photosynthetic pathway and trees with the C sub(3) type. This mixed grass-tree biome rapidly appeared on the ecological stage 8 million years ago with the near-synchronous expansion of C sub(4) grasses around the world. We propose a new hypothesis for this global event based on a systems analysis that integrates recent advances in how fire influences cloud microphysics, climate and savanna ecology in a low carbon dioxide (CO sub(2)) world. We show that fire accelerates forest loss and C sub(4) grassland expansion through multiple positive feedback loops that each promote drought and more fire. A low CO sub(2) atmosphere amplifies this cycle by limiting tree recruitment, allowing the ingress of C sub(4) grasses to greatly increase ecosystem flammability. Continued intensification of land use could enhance or moderate the network of feedbacks that have initiated, promoted and sustained savannas for millions of years. We suggest these alterations will overprint the effects of anthropogenic atmospheric change in coming decades.
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