Based on model projections, climate alarmists contend that global warming should lead to both more frequent and more extreme weather phenomena, such as droughts and floods. It is important, therefore, to search for real-world evidence of these phenomena, especially over the past century when temperatures and CO2 concentrations rose to levels that climate alarmists claim are unprecedented over thousands of years (temperature) to millions of years (CO2 concentration). In the present summary, we thus examine what researchers have found in this regard for South America.
Morengo (2009) worked with hydrometeorological indices for the Amazon basin and its several sub-basins in an effort designed "to explore long-term variability of climate since the late 1920s and the presence of trends and/or cycles in rainfall and river indices in the basin," which analyses were based on northern and southern Amazonian rainfall data that were originally developed by Marengo (1992) and Marengo and Hastenrath (1993), and which were subsequently updated by Marengo (2004). According to the Brazilian researcher, the results of this effort indicated that "no systematic unidirectional long-term trends towards drier or wetter conditions [were] identified." Instead, he found that "the rainfall and river series show variability at inter-annual scales." And of the patterns he uncovered, Morengo writes that they are "characteristic of decadal and multi-decadal modes," which he describes as "indicators of natural climate variability" that are linked to the El Niņo Southern Oscillation, "rather than any unidirectional trend towards drier conditions (as one would expect, due to increased deforestation or to global warming)."
In another paper, Minetti et al. (2010) evaluated the annual occurrence of droughts and their persistence in what they describe as "an attempt to determine any aspects of the impact of global warming." This they did by examining a regional inventory of monthly droughts for the portion of South America located south of approximately 22°S latitude -- which was divided into six sections (the central region of Chile plus five sections making up most of Argentina).
The results of this effort indicated, in the words of the authors, "the presence of long favorable tendencies [1901-2000] regarding precipitations or the inverse of droughts occurrence are confirmed for the eastern Andes Mountains in Argentina with its five sub-regions (Northwest Argentina, Northeast Argentina, Humid Pampa, West-Centre Provinces and Patagonia) and the inverse over the central region of Chile." From the middle of 2003 to 2009, however, they report "an upward trend in the occurrence of droughts with a slight moderation over the year 2006." However, they additionally note that the driest single year periods were 1910-11, 1915-16, 1916-17, 1924-25 and 1933-34, suggesting that 20th-century global warming has not promoted an abnormal increase in droughts over the southern third of South America.
Taking a longer view of the subject were Mundo et al. (2012), who employed 43 new and updated tree-ring chronologies from a network of Araucaria araucana and Austrocedrus chilensis trees in reconstructing the October-June mean streamflow of Argentina's Neuquen River over the 654-year period AD 1346-2000. According to the eight researchers who conducted this study, in terms of the frequency, intensity and duration of droughts and pluvial events, "the 20th century contains some of the driest and wettest annual to decadal-scale events in the last 654 years." However - and it's a very big however - they report that "longer and more severe events were recorded in previous centuries," the significance of which becomes apparent when it is recognized that the bulk of the 554 years that preceded the 20th century were part of the much colder Little Ice Age. Therefore, it would appear that, if anything, the "unprecedented" global warming of the past century has brought Argentina's Neuquen River less extreme streamflow conditions, which is just the opposite of what the world's climate alarmists contend should have happened.
In one final study that examines drought over an even longer period of history, Masiokas et al. (2012) developed the first reconstruction and quantitative analysis of variations in snow accumulation of the past eight-and-a-half centuries in the Andes between 30° and 37°S. The record was based on "instrumental rainfall and streamflow data from adjacent lowlands, a variety of documentary records, and century-long tree-ring series of precipitation-sensitive species from the western side of the Andes," representing "the first attempt to reconstruct annually-resolved, serially complete snowpack variations spanning most of the past millennium in the Southern Hemisphere," which record "allows testing the relative severity of recent 'extreme' conditions in a substantially longer context."
Based on their findings, the eight researchers who conducted this study report that "variations observed in the last 60 years are not particularly anomalous when assessed in a multi-century context," noting that both extreme high and low snowpack values "have not been unusual when assessed in the context of the past eight centuries." Indeed, they say that "the most extreme dry decades are concentrated between the late 16th century and the mid-18th century," and that there were "decade-long periods of high snowpack levels that equaled or probably surpassed those recorded during the past six decades."
In light of the results of the several studies described above, it would appear that contrary to the claims of the world's climate alarmists, the supposedly unprecedented warming of the 20th and early 21st centuries has brought nothing unusual, unnatural or unprecedented in the way of trends in drought frequency and severity for the studied areas of South America.
References
Marengo, J.A. 1992. Interannual variability of surface climate in the Amazon basin. International Journal of Climatology 12: 853-863.
Marengo, J.A. 2004. Interdecadal and long term rainfall variability in the Amazon basin. Theoretical and Applied Climatology 78: 79-96.
Marengo, J.A. 2009. Long-term trends and cycles in the hydrometeorology of the Amazon basin since the late 1920s. Hydrological Processes 23: 3236-3244.
Marengo, J. and Hastenrath, S. 1993. Case studies of extreme climatic events in the Amazon basin. Journal of Climate 6: 617-627.
Masiokas, M.H., Villalba, R., Christie, D.A., Betman, E., Luckman, B.H., Le Quesne, C., Prieto, M.R. and Mauget, S. 2012. Snowpack variations since AD 1150 in the Andes of Chile and Argentina (30°-37°S) inferred from rainfall, tree-ring and documentary records. Journal of Geophysical Research 117: 10.1029/2011JD016748.
Minetti, J.L., Vargas, W.M., Poblete, A.G., de la Zerda, L.R. and Acuņa, L.R. 2010. Regional droughts in southern South America. Theoretical and Applied Climatology 102: 403-415.
Mundo, I.A., Masiokas, M.H., Villalba, R., Morales, M.S., Neukom, R., Le Quesne, C., Urrutia, R.B. and Lara, A. 2012. Multi-century tree-ring based reconstruction of the Neuquen River streamflow, northern Patagonia, Argentina. Climate of the Past 8: 815-829.
Last updated 12 December 2012