Obviously, the hydrological cycle controls much of the climate attributes of precipitation and water availability. It also directly influences sea level. If more precipitation falls as snow than is returned by evaporation (or by sublimation, the direct vaporization of snow cover), snow will accumulate, eventually forming glaciers that can account for considerable changes in sea level. In fact, aside from sea level changes associated with plate tectonic spreading rates, the largest changes in sea level result from the waxing and waning of continental ice sheets. During the last ice age, enough fresh water was removed from the oceans and stored in continental ice to lower sea level by more than 100 m. Even today, in our relatively moderate climate conditions, the return of fresh water to the oceans from melting alpine glaciers and possibly ice sheets is thought to be responsible for about one-half of the 20 cm sea level rise observed in this century. Also, runoff (and ice drift, discussed previously) can considerably influence high-latitude polar regions, where ocean surface salinity plays a predominant role in sea ice formation (and thus local albedo and freshwater transport, among other things) and deep/bottom water formation, which are natural avenues through which the hydrological cycle may influence climate over decade to century scales.

The role of vegetation in the hydrological cycle also must be vigorously investigated. Rind et al. (1997) show that, while the role of vegetation is fairly moderate in contributing to a doubled CO2 warming, the impact of the warming on the vegetation itself can be dramatic. In their GCM simulation with interactive vegetation, the impact of increased atmospheric moisture content is particularly enhanced over land, driving considerable evaporation from the vegetation (through transpiration). The vegetation attempts to limit this drying by reducing transpiration through stomata closing. While this may succeed as a short-term survival tactic against dry conditions, in the long run it also reduces productivity and eventually destroys the vegetation (particularly at lower latitudes). This result is not revealed in simple GCM studies that do not include a treatment of vegetation strain response (such as the treatment that was used in the first IPCC assessment60), though it is indicated in impact studies (such as those used in the second IPCC assessment61). Thus, we must address not only the role of vegetation in the hydrological cycle but the responses and feedbacks of the vegetation as well.

Finally, rainfall is poorly and sporadically measured, and evaporation measurements are woefully inadequate. Development of global climatologies indicates that there is disagreement between them, so it is not surprising that regional series of rainfall data of reasonable quality exist for only a few places. It is becoming more and more important that proxy reconstructions of past rainfall data be made to set the climatic context for studying decadal variability of precipitation and evaporation. Such time series are particularly important for hydrological control issues, such as water resource management; present-day levees, dams, and reservoirs are often engineered on the basis of inadequately short records of flood levels, with dramatic examples of inadequate flood protection (e.g., the Folsom Dam and Sacramento River flood controls) owing to undersized levees.

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