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The vast diversity of present vegetation and environments that occur throughout South America (12°N to 56°S) is the result of diverse processes that have been operating and interacting at different spatial and temporal scales. Global factors, such as the concentration of CO2 in the atmosphere, may have been significant for high altitude vegetation during times of lower abundance, while lower sea levels of glacial stages potentially opened areas of continental shelf for colonisation during a substantial portion of the Quaternary. Latitudinal variation in orbital forcing has operated on a regional scale. The pace of climate change in the tropics is dominated by precessional oscillations of c. 20 kyr, while the high latitudes of the south are dominated by obliquity oscillations of c. 40 kyr. In particular, seasonal insolation changes forced by precessional oscillations must have had important consequences for the distribution limits of species, with potentially different effects depending on the latitude. The availability of taxa, altitude and human impact, among other events, have locally influenced the environments. Disentangling the different forcing factors of environmental change that operate on different timescales, and understanding the underlying mechanisms leads to considerable challenges for palaeoecologists. The papers in this Special Issue present a selection of palaeoecological studies throughout South America on vegetation changes and other aspects of the environment, providing a window on the possible complexity of the nature of transitions and timings that are potentially available. [ABSTRACT FROM AUTHOR]

The so-called ‘Little Ice Age’ (LIA) of the 15th to 19th centuries ad is well-attested from much of Europe and from some other parts of the Northern Hemisphere. It has been attributed to solar forcing, associated with reduced solar activity, notably during the Spörer, Maunder and Dalton solar minima, although other causes have also been proposed and feature strongly in recent papers. Detection of the LIA in some proxy-climate records from the Southern Hemisphere is less clear, leading to suggestions that the LIA was perhaps not a global phenomenon. Resolving this issue requires more data from the Southern Hemisphere. We present proxy-climate data (plant macrofossils; peat humification) covering the past three millennia from an ombrotrophic mire (peat bog) in Tierra del Fuego, southern South America, but focus our discussion on the period traditionally associated with the LIA. During parts of this time, the mire surface was apparently relatively dry compared with much of its 3000-year record. It was reported earlier that a particularly dry episode in the mire coincided with the 2800 cal. BP ‘solar’ event (since identified as a Grand Solar Minimum), which was attributed to solar-driven changes in atmospheric circulation, and more specifically to a shift in position of the Westerlies. Parts of the LIA record show a similar shift to dryness, and we invoke a similar cause. The shifts to and from dry episodes are abrupt. These new data support the concept of a global LIA, and for at least the intense dry episodes might reinforce the claim for solar forcing of parts of the LIA climate. [ABSTRACT FROM AUTHOR]

The ombrotrophic peat bogs of Tierra del Fuego are located within the southern westerly wind belt (SWWB), which dominates climate variability in this region. We have reconstructed late-Holocene water-table depths from three peat bogs and aimed to relate these records to shifts in regional climate. Water-table depths were quantified by the analysis of testate amoeba assemblages, and a regional transfer function was used to infer past water-table depths. During the last 2000 years, testate amoeba assemblages have been relatively stable, with a dominance of Difflugia pulex and Difflugia pristis type, and an increase in Assulina muscorum and other Euglyphida at the top of each section. Multivariate analyses show that water-table depth remained the main environmental variable explaining assemblages along the TiA12 core, but reconstructions were not significant for the two other cores. In line with the low variability in assemblages, water tables were relatively stable during the last 2000 years. Slightly wetter conditions were found between ~1400 and 900 cal. BP and a pronounced recent dry shift was reconstructed in all of the three peat profiles. Considering the regional climatic context, this recent shift may have been forced by a decrease in precipitation and warmer conditions linked to an increase in the importance of the SWWB. Nevertheless, we cannot exclude the influence of higher UV-B radiation resulting from the local degradation of the ozone layer since the late 1970s, which may have had an additional effect on the relative presence of A. muscorum in the southern Patagonian region. [ABSTRACT FROM AUTHOR]

In the existing 276 international river basins, the increase in water variability projected by most climate change scenarios may present serious challenges to riparian states. This research maps the institutional resilience to water variability in transboundary basins and combines it with both historic and projected variability regimes, with the objective of identifying areas at potential risk of future hydropolitical tension. To do so, it combs existing international treaties for sources of institutional resilience and considers the coefficient of variation of runoff as a measure of past and future water variability. The study finds significant gaps in both the number of people and area covered by institutional stipulations to deal with variability in South America and Asia. At present, high potential risk for hydropolitical tensions associated with water variability is identified in 24 transboundary basins and seems to be concentrated mainly in northern and sub-Saharan Africa. By 2050, areas at greatest potential risk are more spatially dispersed and can be found in 61 international basins, and some of the potentially large impacts of climate change are projected to occur away from those areas currently under scrutiny. Understanding when and where to target capacity-building in transboundary river basins for greater resilience to change is critical. This study represents a step toward facilitating these efforts and informing further qualitative and quantitative research into the relationship between climate change, hydrological variability regimes, and institutional capacity for accommodating variability. [ABSTRACT FROM PUBLISHER]

Conspicuous grey, clay-rich flood deposits in the predominantly olive, varved sediment in the central Santa Barbara Basin, California, were dated via varve-counting to c. AD 212, 440, 603, 1029, 1418 and 1605. The ~200-yr quasi-periodicity and the timing of individual floods match time periods of known supraregional climatic changes and the documented ~200-yr periodicity in palaeoclimate records elsewhere. The floods of c. AD 440 and AD 1418 occurred during global reorganizations of atmospheric circulation. Cold spells seem to have accompanied the floods of c. AD 1029 and AD 1605. California floods and other palaeoclimate records suggest that expressions of regional climates are modulated by solar variability with a ~200-yr periodicity. Our compilation of palaeoclimatic and Mesoamerican and South American archaeological records provides evidence that palaeoenvironmental and cultural changes were often coincident within their respective dating uncertainties and occurred in ~200-yr steps. The matching pattern supports the hypothesis that manifestations of changing climates, such as drought or flooding, served as destabilizing cofactors in transitions of precolumbian cultures. Extrapolation of the ~200-yr recurrence pattern of Santa Barbara flooding suggests that catastrophic flooding in southern California may be expected for the early part of this century. [ABSTRACT FROM AUTHOR]

We present a palaeoclimate record for northern South America, extending from the latest Pleistocene (|12600 14 C yr BP) to present. Climate reconstruction for the Valencia Basin, Venezuela, was based on sediment geochemistry and d 18 O records from ostracod and gastropod shells in a 568 cm sediment core. Sediment chronology was established by AMS 14 C dating of terrestrial wood fragments. From |12600 to |10000 14 C yr BP the Valencia basin was drier than present and the coring site, now under 9.4 m of water, was only intermittently wet. After |10000 14 C yr BP, moisture availability increased and lake level rose, permanently covering the core site. From |10000 to |8200 14 C yr BP, Lake Valencia was hydrologically closed and the isotope records reflect pronounced variability in the ratio of evaporation to precipitation. During the wetter early to middle Holocene (|8200 to |3000 14 C yr BP), lake level was high and water was lost to outflow. Greater moisture availability at this time may have been caused by increased intensity of the annual cycle (with wetter wet seasons and drier dry seasons), a result of large, orbitally driven differences in seasonal insolation. Two brief periods of lower lake level, at |7000 and |3300 14 C yr BP, are indicated in the oxygen isotope and calcium carbonate records. Since |3000 14 C yr BP, water level in Lake Valencia has been dropping. [ABSTRACT FROM AUTHOR]