Climate, drought

Part of the concern about global climate change stems from the human tendency to seek meaniiig in events that may or may not be more than simply a random event. A particularly cold winter, a particularly hot summer, an especially rainy season, or an especially severe drought will all send people off on a search for the greater meaning of the phenomenon. Is it a pattern, or a one-time event Must we build a dike, or has the danger passed Since the summer of... 

Obviously, one looks for causes. That declines in one or another species have natural factor etiologies is unequivocal. The demise of American elms and of the chestnut were due to natural factors. Insect infestations, bacterial and fungal diseases, hurricanes, floods, freezes, droughts and many other stresses can cause extensive tree death (5). But in such declines typically only a single species is affected or climatic events caused decline in a delimited area. In almost all declines caused by natural events, the causal factors can be identified we know their precise etiologies. Natural events are always part of the natural environment and must be factored in when evaluating forest declines (Table I). 

Climatic change may occur as fluctuations in means and/or extremes of climatic variables. Most studies of climatic change, however, focus almost exclusively on trends in mean values of climatic variables (usually temperature). In fact, a climatic shift also may be manifest as an increase or decrease in the frequency of occurrence of extremes. That is, episodes of drought or excessive rainfall or record high or low temperatures may become more or less frequent. Changes in frequency of extremes may occur with little or no concurrent change in means. 

The possibility that the climatic future might feature a change in frequency of weather extremes underscores the desirability for climate models capable of predicting the future spectrum of prevailing circulation patterns. Weather extremes such as drought or excessive cold are the products of certain persistent (blocking) planetary wave patterns. In fact, it is reasonable to assume that past climatic... 

A positive feedback between vegetation and atmospheric CO2 will occur if biomass declines. This will happen to the extent that climatic warming causes increased water stress, either through decreased precipitation or increased evap-otransporation, particularly on soils of low water-holding capacity. Decreases in soil nutrient availability, either directly caused by drought or indirectly caused by replacement with taxa with more recalcitrant litter, may further decrease the net release of carbon from the biosphere to the atmosphere. Positive feedback will also arise if the current standing biomass of trees is replaced by small trees, shrubs, and herbs that store less carbon. 

Water resources decision making in many areas, particularly arid and semi-arid climates such as the American West, depends on interannual to decadal variations in surface water availability. In addition to more predictable seasonal differences, runoff tends to exhibit long-term trends alternating between flood and drought periods. Figure 6-10 shows historical wet and dry periods based on streamflow records for 50 world rivers. For the most part, these periods are consistent on a regional basis, though they appear to alternate on a hemispheric scale. 

Nevertheless, the response of water resources will be more complex, as human activities will also change in response to altered climates. The intensity of the pressure put on water resources and aquatic ecosystems by external drivers is related to higher economic income (e.g. expressed by electricity production and consumption) of human societies [5]. The limitation of resources can be qualified by a diversity of terms, varying somewhat in intensity drought, temporality, and... 

Fig. 4 Maximum salinity intrusions into the Delta at five year time intervals from 1945 to 1990 are shown. Drought conditions and rising sea level from climate change are predicted to increase salinity intrusions eastward into the Delta in the future...

Water scarcity is a structural, persistent drought affecting resources and aquatic ecosystems, with implications in water quality and societal needs. Scarcity results in repeated drought episodes. While drought is a temporary (and often normally associated to climatic patterns) decrease in water resources, water scarcity occurs when water demand exceeds the water resources exploitable under sustainable conditions. 

Rainfall is an important criterion of climatic suitability, but precise requirements cannot be stated since its distribution pattern, atmospheric humidity, temperature, and altitude all affect plant requirements. If drainage is adequate, there does not seem to be an upper limit since tea is grown successfully in some parts of Sri Lanka where rainfall exceeds 500cm/yr.19 Long periods of drought are harmful, but are mitigated to some extent by high atmospheric humidity and lower temperatures. 

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