Climate variability in the southern hemisphere

South America is an especially important landmass in the southern hemisphere, for it spans a range of climates that are influenced by multiple drivers such as the El Niño Southern Oscillation, Antarctic Climate and the high Andes, for example. Although proxies with good age control are sparse in tropical South America, the southern half of the continent provides many more proxies that we can use to infer past climatic fluctuations. Ice cores from Andean glaciers and ice fields, drilled at up to 6100 metres above sea level, record snow accumulation and the chemical composition of the atmosphere over time. Trees living up to 3500 years and growing at altitudes up to 5000 metres above sea level respond to variations in temperatures and/or drought, recording this information in the width of their annual growth rings. We derived additional information from lake and marine sediments. Furthermore, we used historical documents from the time of the Spanish colonisation, now stored in many different archives in Europe and the Americas, that report on agricultural yields and the climatic (and non-climatic) causes of yield fluctuations.

It is one thing to have proxy records, though, and another to pool together available information for specific locations and then infer the palaeoclimatic history of half a continent. In the area of investigation, proxies were not distributed evenly in space and time. Some proxies were more suited to estimating summer temperatures than winter ones. And some were actually from areas outside of southern South America but were known to have the same set of controls on their climate. All of this meant that we needed an elaborate statistical methodology to reconstruct the annual history of summer and winter temperatures, and of precipitation (rain and snow) of the region. We now have summer temperatures stretching back more than 1000 years. But winter temperatures could be reconstructed only for the past 300 years or so due to the more limited number of proxy data that resolve winter temperature conditions. We constructed summer and winter precipitation to the late 15th and late 16th centuries respectively.

Comparisons and contrasts
The new records now allow us to compare the climate evolution of both hemispheres, leading to some interesting observations. Consider the comparison between the summer and winter 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 regions 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 phenomenon. Other periods do not show a synchronicity for summer temperatures, and the winter temperatures generally do not seem to vary in consort. It is likely that the effects of the global forcing mechanisms were superimposed with and perturbed by strong regional to hemispheric-scale influences during these periods. To pinpoint the causes of these variations, we will require reconstructions from other regions and climate-model simulations.

In contrast to the more muted warming, we find that in recent decades precipitation has changed substantially in some areas of southern South America. For example, the patterns of annual rainfall during the past four centuries in the catchment of the Laguna Mar Chiquita, a large lake in northern Argentina, are rather different from those in central Chile (Figure 2). In the former region, a large jump in rainfall amounts (an increase of more than 100 millimetres per year on average) occurred in the 1970s, signalling a shift from a relatively dry regime to the presently wet one. In contrast, central Chile currently suffers from a prolonged drying; modern conditions are probably drier than at any time over the last four centuries. In general, our data and analysis suggest that summers in many parts of southern South America have become progressively wetter, whereas winters have become drier.

Climate-model simulations for the 21st century project up to 50 percent reduction in precipitation relative to the present day conditions, mainly in the central Chile area. In combination with future melting of Andean glaciers, this may lead to critical reductions in water availability, which may strongly affect agriculture, freshwater supply and hydropower generation in some areas. In the northern and southern parts of the study area, models project rather wetter conditions, which may benefit the agricultural sector in the highly populated area between Buenos Aires and Rio de Janeiro.

The southern hemisphere has not received the kind of attention from climate researchers that it deserves. Our research is the first to reconstruct the regional climate in any part of this hemisphere at a high temporal resolution. Although the results raise more questions than they answer at this stage, we hope they will provide a foundation for further regional studies in South America in particular and the southern hemisphere in general. More importantly, they are expected to refine our understanding of how and why climate changes at local and regional scales, and thereby guide our responses to future change.

References
Falvey M and Garreaud R (2009) Journal of Geophysical Research DOI:10.1029/2008JD010519.

Luterbacher J et al. (2007) Geophysical Research Letters, DOI: 10.1029/2007gl029951.

Neukom R et al. (2010) Climate Dynamics, DOI: 10.1007/s00382-010-0793-3.

Neukom R et al. (2010) Geophysical Research Letters, DOI: 10.1029/2010glo43680.