Due to all the agro-ecosystem processes described above, it is fairly certain that agricultural regions will experience some changes, and that these changes will evolve continuously through the coming decades. Shifts in crop zonation are likely to occur, with some crop types expanding their ranges and others contracting. Given the range of projected temperature and precipitation changes from global climate models, and the unknown degree of manifestation of direct CO2 effects on crops growing in farmers' fields, however, the magnitudes and rates of these changes are uncertain.
The interactions between beneficial and detrimental agro-ecosystem processes are likely to change over time for several reasons. First, as biophysical responses move through their temperature-response curves, responses to change in temperature may shift from positive to neutral, and then to negative (Figure 10.3). Another reason that climate change effects are likely to be transformed over time is the potential for decadal shifts in the hydrological cycle. While it is difficult to predict the direction of change in any specific agricultural region, global climate models do show increased decadal variability in hydrological regimes. Finally, as crop breeding and pest species evolve in the coming decades under changing climate conditions, new agro-ecosystem weeds, insects, and diseases are likely to emerge, and the adjustment to these may be costly.
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Figure 10.4 Simulated percentage changes in U.S. regional agricultural production, with adaptation, under the Canadian Climate Center scenario. (From Reilly, J., F. Tubiello, B. McCarl, et al. 2003. Climatic Change, 57:43-69. With permission.)
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