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growing at altitudes up to 5000 metres above level respond to variations in temperatures and/or drought, recording this information in the width of their annual growth rings. We derived additional information variability from lake and marine sediments. Furthermore, we used historical documents from the time of the Spanish colonisation, now stored in the in many different archives in and the , that report on agricultural yields and the climatic (and non-climatic) southern causes of yield fluctuations. It is one thing to have proxy records, though, and another to pool together available hemisphere information for specific locations and then infer the palaeoclimatic Most studies that reconstruct the climatic conditions of the past history of half a . In the area of investigation, proxies centuries to millennia tend to focus on the . were not distributed evenly in But now an intriguing multicentennial record of temperature and space and time. Some proxies were more suited to estimating precipitation in southern America is available. Raphael temperatures than Neukom and Jürg Luterbacher elaborate on its significance. ones. And some were actually from areas outside of southern but were known to have the same set of e need to know about These results, synthesised by controls on their climate. All of past climate because it Neukom et al. (2010), provide this meant that we needed an Wis important both for a high-resolution record of elaborate statistical methodology understanding how the climate temperature and precipitation to reconstruct the annual system works and for improving in South America going back history of summer and winter the accuracy of projections of several hundred years. temperatures, and of precipitation future climate change. But the South America is an especially ( and ) of the . We instrumental climate record important landmass in the now have summer temperatures covers just the past 150 years or southern hemisphere, for it stretching back more than 1000 so. To overcome this limitation, spans a range of that are years. But winter temperatures reconstructions of the climate influenced by multiple drivers could be reconstructed only further back in time rely on such as the El Niño Southern for the past 300 years or so due what are termed as “proxies” – Oscillation, Climate to the more limited number of historical documents or natural and the high , for example. proxy data that resolve winter archives that allow indirect Although proxies with good temperature conditions. We estimates of variables such as age control are sparse in tropical constructed summer and winter temperature and rainfall. Yet, South America, the southern half precipitation to the late 15th and much of this research has focused of the continent provides many late 16th centuries respectively. on the northern hemisphere; the more proxies that we can use to southern hemisphere mostly South America infer past climatic fluctuations. Comparisons remains terra incognita. Now, is an especially Ice cores from Andean glaciers and contrasts results of an international and ice fields, drilled at up to The new records now allow us research initiative mean we can important 6100 metres above sea level, to compare the climate evolution begin to quantify continental- landmass in record snow accumulation and of both hemispheres, leading to scale climate variations in the southern the chemical composition of the some interesting observations. southern South America before atmosphere over time. Trees Consider the comparison the instrumental record began. hemisphere. living up to 3500 years and between the summer and winter

26 ❚ Global Change ❚ Issue 76 ❚ 2011 Polylepis tarapacana trees are highly sensitive to climate changes in the Andean , and store information about such changes in their annual growth rings. © Mariano Morales

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temperature trends for southern South America and Europe, for which seasonally resolved temperature reconstructions are available (Figure 1). During some periods, the summer temperatures in the two seem to have fluctuated quite synchronously, for example in the 17th and 20th centuries. This co-variation could arise from global controls such as changes in solar irradiation, large volcanic eruptions or decadal-scale changes in the behaviour of globally relevant climate phenomena such as the El Niño-Southern Oscillation. It could just as well be a chance Figure 1. Decadal-scale summer (top) and winter (bottom) temperature fluctuations in southern South America phenomenon. Other periods do (Neukom et al. 2010) and Europe (Luterbacher et al. 2007) over the past 500 years, expressed as anomalies with respect not show a synchronicity for to the 1901-1995 reference period. Shaded areas represent the reconstruction uncertainties expressed as two standard summer temperatures, and the errors. The black lines extend the temperature curves to the present using instrumental data. winter temperatures generally do not seem to vary in consort. It is likely that the effects of The new records hemisphere thus reacts with a from a relatively dry regime the global forcing mechanisms now allow us certain lag to global warming to the presently wet one. In were superimposed with and as compared to the northern, contrast, central currently perturbed by strong regional to compare land-dominated hemisphere. This suffers from a prolonged to hemispheric-scale influences the climate could be why the temperatures drying; modern conditions are during these periods. To pinpoint in southern South America are probably drier than at any time the causes of these variations, evolution not yet extraordinary compared over the last four centuries. we will require reconstructions of both with previous centuries. In In general, our data and from other regions and hemispheres. contrast, comparable northern- analysis suggest that climate-model simulations. hemisphere reconstructions do in many parts of southern Notably, in South America the show 20th-century temperatures South America have become 20th-century warming was less that are markedly different from progressively wetter, whereas pronounced in the context of past centuries. In fact, changes in have become drier. the preceding centuries than for atmospheric dynamics have even instance in Europe, especially led to cooling over a small area Societal during the summer (Figure 1). in coastal Chile in recent decades implications For example, we found that the (Falvey and Garreaud 2009). We know that key historical average surface temperature In contrast to the more muted events in this region have in southern South America warming, we find that in recent coincided with changes in rose 0.44°C in the 20th century. decades precipitation has climate. Earlier work has shown, European records representing changed substantially in some for example, that periods of northern hemisphere conditions areas of southern South America. warfare and migration to fortified show a more pronounced For example, the patterns of sites in the Andean Altiplano rise of 0.79°C. The lower rate annual rainfall during the past in the 14th and 15th centuries of warming in the southern four centuries in the catchment coincided with and may have hemisphere, which is also of the Laguna Mar Chiquita, a resulted from severe drought. evident in climate models, can large lake in northern , Climate reconstructions of the be explained by the role played are rather different from those sort discussed in this study by the waters in this hemisphere. in central Chile (Figure 2). In can help provide a far more The Southern has a large the former region, a large jump detailed context for these events. capacity to take up heat and store in rainfall amounts (an increase They also have implications for it in its waters, which leads to a of more than 100 millimetres contemporary societies. The smaller warming at the surface. per year on average) occurred economies on both sides of the The southern, water-dominated in the 1970s, signalling a shift Andes are highly dependent on

28 ❚ Global Change ❚ Issue 76 ❚ January 2011 Figure 2. 400 years of annual precipitation history at Laguna Mar Chiquita (northern Argentina; upper panel) and Central Chile (lower panel). Precipitation was reconstructed using a probabilistic “ensemble reconstruction” approach. The median represents the most probable values. The dotted line represents 0-millimetre anomalies (reference period 1901-1995). Modified after Figure 3 from Neukom et al. (2010).

the fresh water that stems from it deserves. Our research is the Precipitation This research was conducted under the auspices of the LOTRED-SA the mountains for irrigation and first to reconstruct the regional and associated hydropower generation. Recent climate in any part of this project, an initiative of IGBP’s Past changes in the total amount hemisphere at a high temporal uncertainties Global Changes (PAGES) project and the seasonal distribution of resolution. Although the results has changed and the recently launced PAGES 2K precipitation have significant raise more questions than they initiative. This initiative aims to consequences for these sectors. answer at this stage, we hope substantially in reconstruct the climatic variation Our reconstructions show that they will provide a foundation some areas of of all at high temporal and spatial resolution over the such changes have occurred in the for further regional studies in southern South past and should be incorporated South America in particular past two millennia. into future economic planning and the southern hemisphere America. http://www.pages.unibe.ch/ and adaptation strategies. in general. More importantly, science/last2millennia.html Climate-model simulations for they are expected to refine our the 21st century project up to 50 understanding of how and why References percent reduction in precipitation climate changes at local and Falvey M and Garreaud R (2009) relative to the present day regional scales, and thereby guide ❚ Journal of Geophysical Research conditions, mainly in the central our responses to future change. DOI:10.1029/2008JD010519. Chile area. In combination with future melting of Andean Raphael Neukom is a Luterbacher J et al. (2007) glaciers, this may lead to critical Postdoctoral Research Fellow Geophysical Research Letters, DOI: reductions in water availability, at the Oeschger Centre for 10.1029/2007gl029951. which may strongly affect Climate Change Research agriculture, freshwater supply (OCCR), University of Bern, Neukom R et al. (2010) Climate and hydropower generation in Switzerland and at the School Dynamics, DOI: 10.1007/s00382- some areas. In the northern and of Sciences, University of 010-0793-3. southern parts of the study area, , . E-mail: Neukom R et al. (2010) models project rather wetter [email protected]. Geophysical Research Letters, DOI: conditions, which may benefit 10.1029/2010glo43680. the agricultural sector in the Jürg Luterbacher is highly populated area between Professor for Physical Geography and . in the Department of Geography, The southern hemisphere has Justus Liebig University of not received the kind of attention Giessen, Germany. E-mail: Juerg. from climate researchers that [email protected].

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