Measuring and predicting the effects of global warming on the biota of western North America are among the most challenging tasks facing the science of ecology because of the enormous climatic variation within the region, in which the current plant and animal life functions. The latitudinal span from the U.S.-Mexican border to Point Barrow, Alaska is approximately 38 degrees. There are continuous temperature, day-length, and growing season gradients along this span. Additionally, precipitation changes from aridity in the south, to high levels in the middle, to low levels again in the far north. Moreover, temperatures and growing season lengths grade from high to low, and precipitation from low to high, along the elevational spans from valleys and foothills to the tops of the numerous mountain ranges and Cordilleras in the region. Nevada alone has 120 mountain ranges.
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