FIGURE 5.9 Illustration of the impacts of gradual climate change with adaptation and abrupt climate change without adaptation (Reilly, submitted). Global welfare losses from agricultural impacts for different climate and CO₂ fertilization scenarios. Upper plots (o) represent global welfare loss associated with abrupt climate change as modeled by an assumption of no adaptation and no CO₂ fertilization. The lower plots (x) allow for full adaptation and include CO₂ fertilization. The three different results are three alternative global yield scenarios from Rosenzweig and Parry (1994) as simulated by Reilly, Hohmann, and Kane (1993) or briefly reported in Reilly and Hohmann (1993). Note that all scenarios go through the origin as a normalization of no welfare loss with no climate change.

more centuries after climate settles down to a new level. Ecologists disagree on how quickly ecological systems will adjust and what they will look like in the process of adjustment. Some ecologists predict vast and rapid losses of forests and grasslands to fires and insects. Others predict slow and gradual change as reproductive competition dominates the dynamics. Both may be correct in different locations, but it seems safe to conclude that there is little secure knowledge about how these new ecosystems will react to climate change.

Abrupt climate change would increase the required adjustment and the uncertainty for forests. Ecosystems may collapse more rapidly, with forests disappearing in vast fires and grasslands dying and turning lands into dust bowls. Such events have occurred in historical times, earlier during the