on the role of vegetation (or lack of vegetation) in modulating the climate of the Sahara are classic foundations for the modern science of atmosphere-ecosystem interactions. Later studies on the role of vegetation in the climate of the Amazon basin (Salati and Vose, 1984; Shukla et al., 1990; Dickinson and Henderson-Sellers, 1988; Lean and Warrilow, 1989) began to bring human actions into the science of atmosphere-ecosystem interactions. At about the same time, analyses of deforestation indicated its potentially large contribution to climate forcing through the carbon cycle (Woodwell et al., 1983). Also around this time a series of breakthroughs established the role of chemicals released from plants and from human processes in modulating the chemistry of the atmosphere (ozone hole, biogenic volatile organic components).

Since these early discoveries, understanding the nature and implications of atmosphere-ecosystem interactions has been one of the central goals in earth system science. It is also increasingly clear that understanding atmosphere-ecosystem interactions is one of the fundamental prerequisites for designing a path to a sustainable future.

CONTEXT AND IMPACTS

The earth, oceans, atmosphere, and human actions need to be considered as a single, coupled system for a thorough understanding of climate, ecosystems, hydrology, or atmospheric chemistry. At small spatial and temporal scales, the coupling ceases to be of first-order importance. But at larger scales of space and time, the coupling between the atmosphere, land ecosystems, and oceans is always relevant and often dominant.

In the coupled earth system, components respond differently to different forcings. Responses are often nonlinear and often have threshold-type characteristics, with little response over a wide range of forcing, followed by a transition to a fundamentally new state over a short time or a narrow range of forcing. Understanding the locations of these thresholds and the mechanisms controlling them is among the most important challenges in earth system science. The lack of an obvious response to initial forcing can lead to the incorrect conclusion that a component of the system is insensitive to the altered environment.

Many of the behaviors of parts of the earth system have clear threshold responses. Wildfires, for example, almost never occur until temperature, humidity, fuel load, and fuel moisture enter the permissive range. But when all the environmental conditions are compatible with sustaining a wildfire, risks increase rapidly. This wildfire threshold could have important implications for Amazon rainforests if the future is warmer and drier (Nepstad et al., 1999). It could also interact in an important way with anthropogenic burning given the recent evidence that aerosols from Amazon fires can decrease rainfall (Andreae et al., 2004). And in a clear example of a feedback, reduced rainfall over tropical land masses during El Niño events has been shown to encourage more biomass burn-



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