|Working Group II: Impacts, Adaptation and Vulnerability|
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14.1.2. Climate Variability and Change
There is ample evidence of climate variability at a wide range of time scales all over Latin America, from intraseasonal to long term. In many subregions of Latin America, this variability in climate normally is associated with phenomena that already produce impacts with important socioeconomic and environmental consequences that could be exacerbated by global warming and associated climate change. Signals that can be related to variability and/or change in climate conditions for Latin America have been identified in some of the analyses performed by researchers in the region, particularly for streamflow, precipitation, temperature, glacier oscillations, general circulation, and extreme events. Estimations of potential future climate conditions are based on climate change scenarios studies developed for some subregions of Latin America.
Glaciers in Latin America have receded dramatically in the past decades, and many of them have disappeared completely (Williams and Ferrigno, 1998). In 18 glaciers in the Peruvian Andes, mass balances since 1968 and satellite images show a reduction of more than 20% of the glacial surface, corresponding to 11,300 million m3 of ice (Morales-Arnao, 1969a,b; INAGGA-CONAM, 1999). Significant reductions also have occurred in southern Chile and Argentina (e.g., glacier Sarmiento) (Basso, 1997). Deglaciation may have contributed to observed negative trends in streamflows in that region (Morales-Arnao, 1999). For rivers in arid lands in northwest Peru and northeast and southeastern Brazil, significant negative trends also have been detected, but these variations seem to be related to human water management for irrigation purposes and increases in agricultural areas, rather than climate-induced changes (INRENA, 1994; Marengo, 1995; Marengo et al., 1998).
Between 20°S and 40°S, precipitation around the Andes occurs mainly during the winter. Snow accumulates in the high parts of the cordillera and melts during the summer, becoming the main source of water for rivers in the region. Agricultural activities in central Chile and the Argentinean central western plains are maintained through irrigation. Therefore, it may be said with high confidence that fluctuations in winter precipitation have a strong socioeconomic impact in the region.
The precipitation record for Santiago, Chile, is highly correlated with snow depth in the cordillera. Recorded precipitation exhibited a decreasing trend from the late 19th century through the mid-1970s but has reverted since then. A similar trend has been detected in streamflow in the region (Minetti and Sierra, 1989; Carril et al., 1997; Compagnucci and Vargas, 1998; Compagnucci et al., 2000; Waylen et al., 2000). In southern Chile and the Argentinean cordillera, a negative trend in precipitation and streamflow has been detected (Quintela et al., 1993; Nuñez et al., 1999).
In northwestern Mexico, there is a tendency for more winter precipitation, which has resulted in positive trends in river water levels. However, along with more intense winter precipitation, interannual climate variability has increased (Magaña and Conde, 2000). On the other hand, some parts of southern Mexico and Central America exhibit positive or negative rainfall trends, depending on the orientation of the catchment (Aparicio, 1993; IPCC, 1996; Jáuregui, 1997; TAR WGI Chapter 3).
For Nicaragua, rainfall analysis for 1961-1995 showed negative trends in the north and northwest parts of the country. A systematic increment was detected on the Caribbean coast, and almost no variation was found along the central and the Pacific coastal regions (MARENA, 2000).
In Colombia, weak rainfall trends have been observed for the period 1955-1995, with no preferred sign at a regional level. For central Colombia, rainy seasons have been occurring earlier in recent years than 25 years ago (Mesa et al., 1997). Trends in Colombian river streamflow are mixed, but the main river catchments such as the Cauca and Magdalena Rivers exhibit decreasing trends. Deforestation could account for such decreasing trends in river discharges (Poveda and Mesa, 1997).
For the Amazon region, Marengo et al. (2000) have identified multidecadal variations in rainfall in northern and southern portions of the basin, with opposite tendencies. Perhaps the most important finding is the presence of periods with relatively wetter or drier conditions that are more relevant than any unidirectional trends themselves. For instance, the period 1950-1976 was regionally wet in northern Amazonia, but since 1977 the region has been drier. This dryness does not seem to be related to regional deforestation (see Marengo et al., 1998; Marengo and Nobre, 2000; TAR WGI Chapter 3). Similarly, streamflow series in Amazonian rivers also exhibit multidecadal variations; they do not display significant unidirectional trends (Richey et al., 1989; Marengo, 1995).
In northeast Brazil, multidecadal variations in atmospheric circulation over the tropical Atlantic have been linked to similar time-scale variations in rainfall over the region (Hastenrath and Greischar, 1993; Nobre and Shukla, 1996; Wagner, 1996). On longer time scales, rainfall in northern northeast Brazil exhibits weak positive trends that are consistent with changes in decadal changes in circulation described in Wagner (1996).
Streamflow in the River Plate basinparticularly in the Negro, Paraguay, Paraná, and Uruguay Riversexhibits a negative trend from 1901 to 1970, which reverses after this period. Multidecadal variability also is observed in discharges (Garcia and Vargas, 1998; Genta et al., 1998; Robertson and Mechoso, 1998). Moreover, there are written reports of alternating floods and droughts periods during the 16th-18th centuries, indicating high natural variability (Prieto and Herrera, 1992).
In subtropical Argentina, Paraguay, and Brazil, precipitation exhibits a long-term change, with a sharp increase in the period 1956-1990 after a dry period along 1921-1955 (Castañeda and Barros, 1996). In the Pampa region, there is a positive trend in precipitation during the period 1890-1984. This increase in annual rainfalls was accompanied by a relative increase in precipitation during the spring and summer (Penalba and Vargas, 1996; Hoffman et al., 1997; Krepper and Sequeira, 1998). At high elevations in northwest Argentina, paleoclimatic records suggest an increase in precipitation in the past 200 years (Villalba et al., 1997). In the same region, as well as in Bolivia and southeast Peru, records show that the 17th-century climate was wetter and less variable (fewer floods and droughts ), whereas the 18th century was highly unstable, with a large amplitude in the annual cycle and recurrent wet and dry periods (Prieto and Herrera, 1992).
Variations in precipitation in Latin America have a strong effect on runoff and streamflow, which also are affected by melting of glaciers and snow. Based on available information, there is evidence that these variations and their sign depend on the geographical subregion under consideration.
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