The river, important for wildlife on the plains of Kenya, has ceased to flow in this October 2009 aerial photograph.
Source: NASA Earth Observatory
Drought in the preceding post was called an absence of adequate wet season water to meet dry season demand, and severe drought follows several years of inadequate precipitation. Seasonal drought is common in many climates. Equally common are climate cycles with a series of wet years followed by a series of dry years. Early humans were acutely aware of these cycles, witness the biblical seven fat years and seven lean years. Not a little effort was expended on trying to foresee conditions in the coming season. There is some evidence that Inca seers used stargazing conditions to predict crop conditions for the coming season. If the sky was clear enough to see certain stars, then the season was likely to be a good one, while years when those stars could not be seen meant bad crops. While the record is difficult to interpret, especially as the Inca had no recorded language as far as we know, it is quite possible these observations were based on El Niño conditions off the nearby coast.
While they are considerably more difficult to prepare for than annual variations in precipitation, longer term climatic cycles have also led to human efforts to minimize drought effects. In Kenya water from more humid higher elevations can take the place of local precipitation in drought periods. If there is advance warning of drought conditions, then the crops planted can be adjusted and the number of stock grazed on land can be reduced. Large storage dams are another response, storing water in a wet period to use in a dry one. All of the responses are premised on the expectation that the dry period will be limited to only a few years. When the dry spell is extended over many years, responses become much more difficult.
Climatic cycles occur over various time periods from a single year to five years or so in the case of the El Niño to a decade even to centuries and perhaps even millenia. The essence of a cycle is the expectation that at some point conditions will return to "normal," that is to levels of precipitation suitable for agriculture and human settlement. Generally humans can only adjust to cycles of a decade or less with temporary measures. Longer term cycles can lead to complete changes in activity or even abandonment of land and settlements.
Now there is evidence that some recently observed climatic shifts that are unlikely to be cyclic but rather are probably permanent. Overall this is often called "global warming," for the general movement is toward higher temperatures across the earth's surface. Related to those warmer temperatures are higher rates of evapo-transpiration and in many, but not all, arid and subhumid areas diminished precipitation. Some marginal areas where precipitation has been barely adequate for agriculture are likely to become unusable.
