The climate system is a fundamental natural resource of the earth. It is driven by the sun and contains the gases necessary for photosynthesis, and is thereby the foundation of all food chains necessary for human life. It keeps the temperatures on the earth's surface within the narrow range tolerated by life. It drives the biogeochemical cycles that distribute nutrients and water about the biosphere. It delivers the water for shipping, irrigation, municipal consumption, and hydroelectric generation. It generates wind to turn windmills and makes snow for skiers. It also provides warm, sunny days that please the senses. In short, climate is thoroughly involved in virtually every aspect of the environment and human activity.
Human beings and societies have always had to cope with variations in weathershifts of wind, temperature and precipitation that can be extreme and that are experienced on the time scales of minutes, hours, and days. Humanity has also always coped with variations in climateaverages of weather on longer time scales. Seasonal variations affect the need for clothering and the availability of food and water, and people have responded by varying their diets and clothing and developing systems of building construction and food and water storage. And, at least since biblical times, the potential to experience years of plenty followed by years of famineinterannual climate variabilityhas been a major issue for societies. Climatic variations have contributed to the rise and fall of societies throughout human history.
People can respond to climate in several ways. At the most general level, people adapt to the average or mean climate of the region in which they live, on the assumption that the average of past experience is the best guide to the future. Thus, people in desert regions develop irrigation, design housing, and adapt their lifestyles to cope with the hot, dry conditions they routinely expect. Farmers choose crops appropriate to the average local climate and its usual variability and develop agricultural calendars that give a recommended day for planting. People also respond to observed conditions of climate and weather after the fact. Farmers wait to plant until the rains actually begin or apply more irrigation on hot days. Households adjust home heating and air conditioning in response to observed temperature and humidity. And people respond to forecasts, both of weather and of climate, with a range of anticipatory actions that depend on the lead time and reliability of the forecast. A farmer may decide not to plant at all if a drought is forecast; a water manager may adjust plans for reservoir control.
In responding to climate, people may act both to minimize the risk of hazardous climate and to capitalize on climatic opportunities. Flood-