Global climate has changed through time and continues to change and to display variability (Burroughs, 2001; Lockwood, 2001). Both climate change and variability can be a response to internal or external forcing of the climate system. Passive forcings involve modulation of faster responding components by slower response time components. Active forcings result from variations and instabilities of the climate system dynamics and by coupled interactions between climate system components. Passive forcings are known as stochastic forcings due to their random evolution while active forcings are called dynamic forcings because a strong dynamical response is required in the slower component (Bigg et al., 2003). It is now becoming evident that one component of climate change and variability is related to natural processes and another component is attributed to human activities (IPCC, 2001). Aside from concerns for the causal factors, the important point is that an abundance of evidence shows that climate is dynamic and not static. This has significant applied ramifications because it warns us that past experience provides a limited view of potential future conditions.
Climate change and variability in both space and time have significant practical importance related to climate forcing of the hydrologic cycle. Energy and moisture are the active factors of climate whose coincidence is managed through the hydrologic cycle. The partitioning of precipitation among competing environmentally determined options within the hydrologic cycle includes runoff. Runoff is the water that appears in rivers, lakes, and streams and along with groundwater represents the water supply available for human use. Recurring droughts and declining water supplies are powerful reminders of human dependence on climate as the source of an essential natural resource.
While drought emphasizes a short-term climate variation, the potential for long-term climate change represents an additional need for comprehensive knowledge of the climate system and hydroclimatology. Many scientists expect that human alteration of atmospheric trace gases will produce global warming by the middle of the twenty-first century that will cause climates to be different from those of the present (Burroughs, 2001; IPCC, 2001). The present climate serves as a benchmark for identifying and assessing the magnitude of climate change. State-of-the-art general circulation models do not provide precise indications of regional climate change expected to accompany global warming, and this contributes additional confusion to the already complex issue of understanding hydroclima-tology. The large-scale averaging in GCMs has the effect of smoothing regional-scale processes of greatest concern for understanding hydroclimatic problems. However, several techniques are available for subgrid-scale parameterizations that permit the GCM areal averages to be downscaled (Shelton, 2001). Improvements in representing land surface hydrological processes in coupled atmosphere-land-ocean GCMs provide another avenue for achieving better representation of regional hydroclimatic processes.
Intuitively, we expect a warmer Earth to be wetter as a result of accelerated evaporation serving as a forcing to increase precipitation within the hydrologic cycle. However, regional climate differences can be expected to produce broad geographical variations in the temperature and precipitation response. An additional complication suggested by model simulations is that the relationship between warmer climates and the intensity of the hydrologic cycle may result from the sensitivity of sea surface temperatures to a warmer climate. Consequently, warmer climates can be associated with either an increase or a decrease in the intensity of the hydrologic cycle (Yang et al., 2003), and there may be latitudinal and seasonal differences related to vegetation's role in recycling water (Dirmeyer and Brubaker, 2006). The present climate must be understood before the ramifications of climate change can be fully appreciated.
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