Because Latin America spans a vast range of latitudes and contains important high-elevation mountain ranges, it is hardly surprising that it has a wide variety of climates. Although it is the only southern continent to reach high latitudes, its broadest extent is in the equatorial zone; thus, tropical conditions prevail over the larger portion of the region. The large-scale climatic features of Central and South America are defined by the region's predominant atmospheric circulation patterns and geomorphological features. The main circulation features are relatively low pressure at the equatorial belt (10�N-10�S), quasi-permanent high-pressure cells over the north and south Atlantic and southeast Pacific Oceans, and a belt of low pressure defining the westerly flow on the southern portion of the South American subcontinent. Mexico and Central America are affected by the penetration of cold fronts and tropical cyclones over the Atlantic and Pacific Oceans, whereas the Atlantic coast of South America is mostly free of high-intensity tropical storms.
The development of a thermal low located between 20�S and 30�S over the high, dry lands to the east of the Andes introduces a monsoonal circulation pattern, bringing seasonal precipitation on the high Andean Plateau and influencing the positioning of the Inter-Tropical Convergence Zone (ITCZ) in the region.
Latin America is characterized largely by humid, tropical conditions. However, important areas (e.g., northeastern Brazil) are subject to droughts and floods, and others are affected by freezes-all of which have negative impacts on agricultural production (Magalh�es and Glantz, 1992).
Atmospheric circulation and cold ocean currents have remarkable influence on the weather and climate in the southern part of the region-giving origin to the Peruvian, Atacama, and Patagonian deserts, which receive less than 100 mm of mean annual precipitation. The cold Humboldt ocean current-which flows northward along the west coast of South America-brings to the coasts of Ecuador, Peru, and Chile large masses of phytoplankton that originate in the Antarctic Ocean, supporting one of the world's richest fisheries. This process is interrupted by the western displacement of the Humbolt current and the irruption (advection) of warmer waters caused by the weakening of the easterly surface winds (El Ni�o phenomenon), resulting in adverse conditions for fishing.
Climate trends over the past century have been investigated in the majority of countries that have the required amount of information (i.e., measured data and reliable proxy data). Studies are available on average and extreme values of temperature, humidity, and precipitation for different regions of Latin America and different periods of time.
Some of these studies reveal significant warming in southern Patagonia east of the Andes, with increases in maximum, minimum, and daily mean temperatures of more than 1�C. According to some researchers, no warming has been observed north of about 42�S. These observations are consistent with changes in vapor pressure and precipitation-which have increased (north of 40�S) since 1940 (Hoffman et al., 1996).
Similar studies peformed in Chile indicate that mean surface temperatures showed no increasing trend before 1900-but that, during the period 1900-90, the temperature in the Southern Hemisphere increased by a total of 0.4�C, at a fairly constant rate (Rosenbl�th et al., 1997). These authors have reported a significant cooling in the southern half of Chile in 1991 and 1992, coinciding with the eruptions of the Pinatubo and Hudson's volcanoes (June and August 1991, respectively), as a result of the effect of the sulfate aerosols emitted into the stratosphere.
A trend analysis of 81 series of precipitation data in Central America (Brenes-Vargas and Trejos, 1994) shows that changes in general circulation influence precipitation rates and consequently the availabilty of water on the isthmus, with related problems in hydroenergy production (see Section 6.3.2).
Further, an analysis of precipitation trends in the southern portion of South America, east of the Andes cordillera, indicates that the mean annual precipitation in the humid Pampa has increased by about 35% in the past half-century (Forte-Lay, 1987; Casta�eda and Barros, 1996).
Trends in annual precipitation (in percentages), mean annual temperatures (in degrees centigrade per hundred years), and variations in annual precipitation and temperatures are provided in Annex A.
Other reports in this collection