Multiproxy Analysis of Climate Variability for the Last Millennium in the Tropical Andes
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Multiproxy analysis of climate variability for the last millennium in the tropical Andes V. Jomelli 1,2, J. Argollo', D. Brunstein', V. Favier", G. Hoffmann", M.P. Ledru", lE. Sicarr 1. CNRS Laboratoire de Geographie Physique de Meudon UMR8591 France. 2. IRD UR GREAT ICE MSE Montpellier, France 3. IGEMA, University Mayor San Andres, La Paz, Bolivia 4. CEAZA, Benavente 980, La Serena, Chile 5. CEA Laboratoire des Sciences du Climat et de I'Environnement, Gif sur Yvettes, France Vincent, some ofthis paper is in British English and some in American, which changes the spelling modelling/modeling etc etc and you should have the same spelling throughout Abstract Reconstructing climate over the last millennium is one major target of climate research. The extent of climate variability over recent centuries sets the scale on which ongoing climate changes can be measured. The quality of reconstructions of millennial temperatures in the northern hemisphere has greatly improved over the last decade (IPCC, 2007). Nevertheless there is still a lack of precisely dated, high-quality climate reconstructions in the tropics, a lack which is particularly disturbing since this region is a major climate engine for the global water cycle. In recent years, a significant effort has been made to improve our knowledge on climatic variability in the tropical Andes by developing new proxy records, new transfer functions and better time resolution of available proxy records. Hence the aim of this paper is to review the different proxies with multi-decadal to annual resolution available in the tropical Andes (Bolivia, Peru, Ecuador) such as glacial moraines, pollens, isotopic variations in ice core records, and tree rings. For each proxy, we focus on recent major methodological progress and on new results revealing climatic shifts over the last millennium. 1- Introduction The last millennium is obviously the period which most resembles (and actually includes) the pre-industrial period in the early 19th century. The amplitude of ongoing anthropogenic climate change should therefore be compared with this most recent period. Presumably "natural" climate variability -like that human societies experienced from the medieval period up to the first years of industrialisation also defines the range of climate to which humans have successfully adapted. Early meteorological observations are available since about the late 18th century but mainly in Western Europe and North America (Auer, et al., 2007). South America offers a wealth of historical information and natural climate proxies that have only recently been discovered and analysed. The last millennium enables us to investigate the influence ofdifferent forcing factors. It includes a relatively cold period, called the Little Ice Age (LIA, ~ 1500-1900 yr AD), when solar activity was low and volcanoes were active, and a relatively warm Medieval Warm Period (MWP: before ,. 1200 yr AD) both of which were characterised by relatively stable greenhouse gas forcing. Hence, the last 1000 years serve as a "Iaboratory" where the effect of c1imate mechanisms can be tested, and provide a basis for improving our understanding of c1imatic change. Indeed, temperature changes that occurred during the LIA are in the same order of magnitude as those estimated by sorne Global Circulation Models for the middle of the 21 st century. On the other hand, despite a small amplitude temperature change during the last millennium (Iess than 1°C), considerable environmental consequences have been observed. Numerous catastrophic events related to temperature and hydrological cycles (freezes, floods, droughts, heat waves, etc.) as weIl as dramatic event with serious consequences for human societies (diseases, famines, etc.) have been documented (Ladurie, 1987; Grove, 1988). Consequently, long c1imatic series are needed to validate ocean-atmosphere models over a long period of time and to study the impact of change on pre-modern societies. Another important aspect concerns the heterogeneous geographical distribution of available c1imatic data. Most of these data are based on observations or multi-proxy reconstitutions (dendroclimatology, historical documents, weather data, etc.) at mid and high latitudes. However the inter-tropical band, considered a key area for the energetic exchanges that play a key role in global c1imate (Mc Gregor and Nieuwolt, 1998), is also the region where data are the most rare (Jones, et al., 2001). The surface of tropical oceans is the main source of heat for atmosphere ocean circulation (Broecker and Denton, 1989; Wang et al., 2004) and, the impacts of tropical c1imate reach far beyond the limits of the tropics (Mc Gregor and Nieuwolt, 1998). In addition, intra-annual and abrupt changes in tropical c1imate (Iike ENSO) have a major influence beyond the actual tropics, including in mid-latitudes. Why focus on the tropical Andes? • The first reason is geographicai. The Tropical Andes coyer about 3800 km from lOON to 23°S. The shape and setting of the tropical CordiIIeras means they form a barrier for the dominant and persistent easterly atmospheric flow separating the wet Amazon side from the dry Pacifie side and producing pronounced windward and leeward effects (Kaser and Georges, 1997). Many summits reach elevations of over 6000 m asi. For these reasons, the tropical Andes are characterized by contrasted c1imates and steep climatic and ecological gradients over short spatial distances that significantly influence biological, geomorphological, and hydrological processes and form a system that is extremely sensitive to environmental changes. • Second, climatic changes are frequently inferred to explain either the demise of sorne historic or older civilisations and their economy, or the development of subsistence strategies to ensure their survivai. For example, the decline of Tiwanaku field agriculture from 950 AD (Middle Horizon) in the Titicaca Basin (Ortloff and Kolata, 1993; Binford et al., 1997) has been attributed to a failure of the corresponding societies to respond to short-term climatic deterioration. Hence, the better our knowledge of past c1imatic changes the better we understand the relationships between old civilisations and c1imate changes. In addition, c1imate variability analysed either using observed data or output modelling (Aceituno, 1988; Vuille et al., 1998b; Vuille and Bradley, 2000; Garreaud et al., 2003) has a strong impact on the availability ofwater resources in most ofthe Andean capitals such as Lima, Arequipa or La Paz, which are experiencing a high migratory flux 2 from the countryside (Kaser et al., 2003; Mark and Seltzer, 2003; Pouyaud et al. 2005). All these cities are located in dry areas where drinking water and the dam water that supplies electric power is expected to run short in the near future. • The last reason is the recent improvements in our understanding of the relationships between the climate and the proxies via transfer functions, and to the discovery of new proxies that help to better document past climatic conditions. The last question is why do we need a multiproxy approach? The answer is that each proxy has a specific relationship with climatic conditions, for example sorne dendrochronological records are more sensitive to changes in precipitation than others. On the other hand, sorne proxies do not enable high-frequency climate variability (less than a decade) to be documented either because of their low time resolution or to their specific relationship with climatic shifts. In addition, each of the proxies has its own specific problems that can mostly be overcome by combining approaches. In this context, the aims ofthis paper are to 1) highlight recent improvements in the continental proxies in the tropical Andes, 2) document climatic conditions during the last millennium inferred from different proxies in the tropical Andes. This paper is organized as follows: in the next section, we describe high resolution proxies i.e. ice core and tree ring records; in section three, we describe low resolution proxies i.e. pollen and moraine records; and in section 4, we compare climatic conditions inferred from these different proxies 2- High resolution proxies 2.1 Ice core records Over the last 30 years, numerous high-altitude ice cores have been drilled and successfully analysed. In the Andes, these ice cores were located within a huge latitudinal strip that reaches between tropical Ecuador and the dry central Cordillera Real in Bolivia. Ongoing drilling activities are extending the geographical range even farther towards the south of Chile and Patagonia. Here we give only a short overview of centennial scale variability (basically during the last Millennium) archived in these cores. In addition, we refer to two other recent review papers (Thompson and Davis, 2006; Vimeux, et al., 2007) that raised sorne of the questions we discuss here. All together, five main deep ice core sites have been explored in the Andes and many short and deep cores have been drilled and analysed (figure 1) from North to South: in Chimborazo (1.3°S, Ecuador), Huascaran (9 0 S, Cordillera Blanca, Peru), Quelccaya (l3.5°S, Peru), Illimani (I6.4°S, Cordillera Real, Bolivia) (figure 2), and Sajama (ISoS, Bolivia). At least two additional cores (Corupuna, Peru; and San Valentin, Chile) have been drilled and partly analysed, but results are not yet published. For the time period of interest here, i.e. the last millennium, the Illimani and the Quelccaya core will be analysed in most detail. When analysing low-latitude ice cores, many practical and theoretical difficulties have to be overcome, most of which are due to their exceptional location and due to the specific impact of tropical climate on the various proxies archived in the ice. In contrast to polar ice cores, the preservation and transport of the ice is a major issue in the tropics. The ice is typically carried down from the glaciers in commercial ice boxes and afterwards put into boxes cooled with dry 3 ice. Due to the rapidly rising temperatures during the descent, the time the ice is preserved without additional cooling must be kept as short as possible.