Weather is the state of the atmosphere with respect to wind, temperature, cloudiness, moisture, pressure, etc. at a given point in time, while climate refers to the average weather conditions for an area over a long period of time. Or as has often been said, “the climate is what you expect; the weather is what you get”.
Climate is a key factor determining the characteristics and distribution of ecosystems. Many ecosystem processes are affected by climate and weather, such as photosynthesis, respiration and nutrient cycling. The climate of the Southwest is already changing in ways that can be attributed to human-caused emissions of greenhouse gases. These changes include warmer average daily temperatures, unusually severe drought relative to droughts of the last century, and flows in the four major drainages that are lower than their twentieth century average flows. These changes are impacting species distributions, population sizes, and the timing of reproduction and migration events. They are also resulting in an increase in the frequency of pest and disease outbreaks, and altering fire and other disturbance regimes.