Long-term trends of temperature variations across the southern Andes (37-55 • S) are examined using a combination of instrumental and tree-ring records. A critical appraisal of surface air temperature from station records is presented for southern South America during the 20th century. For the interval 1930-1990, three major patterns in temperature trends are identified. Stations along the Pacific coast between 37 and 43 • S are characterized by negative trends in mean annual temperature with a marked cooling period from 1950 to the mid-1970s. A clear warming trend is observed in the southern stations (south of 46 • S), which intensifies at higher latitudes. No temperature trends are detected for the stations on the Atlantic coast north of 45 • S. In contrast to higher latitudes in the Northern Hemisphere where annual changes in temperature are dominated by winter trends, both positive and negative trends in southern South America are due to mostly changes in summer (December to February) temperatures. Changes in the Pacific Decadal Oscillation (PDO) around 1976 are felt in summer temperatures at most stations in the Pacific domain, starting a period with increased temperature across the southern Andes and at higher latitudes. Tree-ring records from upper-treeline were used to reconstruct past temperature fluctuations for the two dominant patterns over the southern Andes. These reconstructions extend back to 1640 and are based on composite tree-ring chronologies that were processed to retain as much low-frequency variance as possible. The resulting reconstructions for the northern and southern sectors of the southern Andes explain 55% and 45% of the temperature variance over the interval 1930-1989, respectively. Cross-spectral analysis of actual and reconstructed temperatures over the common interval 1930-1989, indicates that most of the explained variance is at periods >10 years in length. At periods >15 years, the squared coherency between actual and reconstructed temperatures ranges between 0.6 and 0.95 for both reconstructions. Consequently, these reconstructions are especially useful for studying multidecennial temperature variations in the South American sector of the Southern Hemisphere over the past 360 years. As a result, it is possible to show that the temperatures during the 20th century have been anomalously warm across the southern Andes. The mean annual temperatures for the northern and southern sectors during the interval 1900-1990 are 0.53 • C and 0.86 • C above the 1640-1899 means, respectively. These findings placed the current warming in a longer historical perspective, and add new support for the existence of unprecedented 20th century warming over much of the globe. The rate of temperature increase from 1850 to 1920 was the highest over the past 360 years, a common feature observed in several proxy records from higher latitudes in the Northern Hemisphere.

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In most of Argentina, the warming since 1901 was a little lower than the global average, although with strong trends in temperature extremes and in heat waves during the most recent decades. There was a remarkable increase in precipitation over most of subtropical Argentina, especially since 1960. This has favored agriculture yields and the extension of crop lands into semiarid regions, but this increase also came with more frequent heavy rainfalls and consequent flooding of rural and urban areas. Since the early 1970s, the main rivers of the Plata Basin have increased their mean flows, but this was attributable not only to increased precipitation, but also to land use changes. In contrast, over the Andes Mountains, reduced rainfall and increased temperature has led to glaciers receding and reduced river flows. Climate projections for the first half of this century maintain observed trends and raise additional concerns that in most cases can be dealt with timely adaptation policies. However, by the end of this century, under an extreme emissions scenario, the projected warming reaches 3.5°C in the north of the country with respect to present‐day conditions. There is insufficient knowledge to assume that this warming would not create severe damages to the people and the economy of Argentina. Because of the damages and casualties that heat waves and extreme precipitation events are already producing, the first and most urgent adaptation required is to reinforce early warning systems and contingency planning to cope with climatic extremes and their consequences on health. WIREs Clim Change 2015, 6:151–169. doi: 10.1002/wcc.316 This article is categorized under: Trans‐Disciplinary Perspectives > National Reviews

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