Früher war nicht alles besser: Neues aus der südamerikanischen Dürreforschung

Immer wenn es besonders viel oder wenig regnet, ist der öffentliche Aufschrei groß: Der menschengemachte Klimawandel zerstört unsere Lebensgrundlage! Dabei lohnt es sich durchaus, in die Vergangenheit zu schauen, denn dort können wir eine enorme natürliche Niederschlagsvariabilität beobachten, die viele heute verdrängt haben. Um den Sinn für solche Feucht-Trocken-Zyklen bzw. -Schwankungen zu schärfen, berichten wir hier in unregelmäßiger Weise über neue Fachliteratur zu diesem Thema. Heute geht es nach Südamerika. Beginnen wollen wir in der Karibik. Von Antigua im Bereich der Kleinen Antillen beschrieben Berland et al. 2013 die Regen-Historie für den Zeitraum 1770-1890. Das überraschende Fazit: Regenreiche Jahre wechselten mit regenarmen, lange vor der industriellen Phase. Hier die Kurzfassung:

This paper presents the first extensive reconstruction of precipitation variability in the Lesser Antilles using historical documentary sources. Over 13 250 items of documentation pertaining to Antigua from the period 1769–1890 were consulted, including missionary, plantation and governmental papers as well as contemporary scholarly publications. Based on the predominant meteorological conditions observed throughout the island, each “rain-year” (December–November) was assigned one of five classifications (very wet, wet, “normal”, dry and very dry). Local weather references relating to seven plantations in central-eastern Antigua were grouped according to dry (December–April) and wet seasons (May–November), each of which were also categorised in the aforementioned manner. Results comprise individual island-wide and central-eastern Antiguan chronologies of relative precipitation levels, spanning the rain-years 1769–70 to 1889–90 and 1769–70 to 1853–54 respectively. The former is compared with available instrumental data for the years 1870–1890. Significant dry phases are identified in the rain-years 1775–80, 1788–91, 1820–22, 1834–37, 1844–45, 1859–60, 1862–64, 1870–74 and 1881–82, while wet episodes were 1771–74, 1833–34, 1837–38, 1841–44, 1845–46 and 1878–81. Evidence for major wet and dry spells is presented and findings are evaluated within wider historical and palaeoclimatic contexts.

Im Januar 2013 berichtete CBS über Dürren im Amazonasgebiet, die seit 2005 dort gehäuft aufgetreten waren und interpretierte sie als Folge des Klimawandels:

Severe Droughts in Amazon linked to climate change, study says.

War es wirklich der Mensch, der die Dürren selber verursacht hat? Im Februar 2014 kam die teilweise Auflösung in Form eines Papers von Regina Rodrigues und Kollegen. Fazit: Die Dürre 2011/2012 geht auf das Konto von La Nina. Hier die Kurzfassung der Arbeit:

Why did the 2011–2012 La Niña cause a severe drought in the Brazilian Northeast?
The Brazilian Northeast (NE) is strongly affected by El Niño–Southern Oscillation (ENSO). During La Niña events, the precipitation over the NE is generally above average. However, during the last La Niña event in 2011–2012, the NE went through its worst drought in the last 30 years. In this study, observations and numerical simulations are used to determine what made the 2011–2012 event different from other events. We find that eastern Pacific (canonical) La Niña events cause a cooling of the tropical North Atlantic and warming of the tropical South Atlantic that lead to a southward migration of the Intertropical Convergence Zone, which in turn brings rain to the NE. On the other hand, La Niña events with the cooling concentrated in the central Pacific cause the opposite meridional sea surface temperature (SST) gradient in the tropical Atlantic, leading to droughts over the NE. The 2011–2012 event was of the latter type. This study also shows that it is possible to predict the sign of the NE rainfall anomaly during ENSO events using a simple SST index.

Weiter nach Peru. Latina Press schrieb Ende 2013:

Klimawandel bringt Wasserquellen in Peru zum versiegen
Der weltweite Klimawandel beginnt die Wasserversorgung in Arequipa, der Hauptstadt der gleichnamigen Region Arequipa im Süden des südamerikanischen Anden-Staates Peru, zu beeinflussen. Laut Jorge Sánchez Salinas, Koordinator der Fachgruppe über Klimaänderungen in der Region, ist eine Grundwasserquelle am Fuße des Berges Pichu Pichu bereits vollständig ausgetrocknet.

Und da ist sich Herr Salinas ganz sicher. Denn nur wenn es der Klimawandel war, gibt es Hilfsgelder. Hätte sich der Herr Koordinator ein bisschen für die Wissenschaft interessiert, so wäre ihm aufgefallen, dass der Regen in Peru schon immer durch starke natürliche Schwankungen gekennzeichnet war. Apaéstegui et al. (2014) rekonstruierten die Regenhistorie Perus für die vergangenen 1600 Jahre und fanden eine enorme Variabilität, die u.a. von atlantischen und pazifischen Ozeanzyklen getaktet wird. Auch eine längerfristige Komponente trat auf. Während der Mittelalterlichen Wärmeperiode war der regenreiche Monsun besonders schwach, eine Situation die der heutigen Modernen Wärmeperiode offenbar entspricht. Während der Kleinen Eiszeit war der Monsun hingegen stark und regenreich. Hier die Kurzfassung der Arbeit:

Hydroclimate variability of the South American Monsoon System during the last 1600 yr inferred from speleothem isotope records of the north-eastern Andes foothills in Peru
In this paper we explore a speleothem δ18O record from Palestina Cave, North Eastern Peru, at a site on the eastern side of the Andes cordillera, upper Amazon Basin, interpreted as a proxy for South America Summer Monsoon (SASM) intensity. This record allows reconstructing SASM activity with 5 yr time resolution over the last 1600 yr, spanning two major periods of climate variability: the Medieval Climate Anomaly (MCA; 900–1200 AD) and Little Ice Age (LIA 1400–1850 AD) recognized as periods of decrease and increase SASM activity respectively. Time series and wavelet analyses reveal decadal to multidecadal frequencies. Our results suggest that Atlantic Multidecadal Oscillation mode (AMO) plays an important role for SASM modulation on multidecadal scale (~65 yr), especially over dry periods such as observed during MCA. Frequencies of 8 and 25 yr simultaneously with multidecadal signal (65 yr) are found over the LIA. and suggest that those modes could be related to North Atlantic Oscillation (NAO) and Interdecadal Pacific Oscillation mode (IPO). Comparison with other South American Paleoprecipitation records shows that the Atlantic and Pacific decadal to multidecadal variability and their teleconnections play an important role in the intensity and the regional patterns of rainfall distribution during the last 1600 yr.

Schittek et al. (2014) gehen sogar 8600 Jahre zurück in einer Regenrekonstruktion aus Peru. Auch hier eine enorme natürliche Variabilität. Die Mittelalterliche Wärmeperiode war trocken, fanden die Autoren. Auszug aus dem Abstract der Arbeit:

Holocene environmental changes in the highlands of the southern Peruvian Andes (14° S) and their impact on pre-Columbian cultures
The mid-Holocene period of 8.6–5.6 ka is characterized by a series of episodic dry spells alternating with spells that are more humid. After a pronounced dry period at 4.6–4.2 ka, conditions generally shifted towards a more humid climate. We stress a humid/relatively stable interval between 1.8–1.2 ka, which coincides with the florescence of the Nasca culture in the Andean foreland. An abrupt turnover to a sustained dry period occurs at 1.2 ka, which coincides with the collapse of the Nasca/Wari society in the Palpa lowlands. Markedly drier conditions prevail until 0.75 ka, providing evidence for the presence of a Medieval Climate Anomaly. Moister but hydrologically highly variable conditions prevailed again after 0.75 ka, which allowed the re-expansion of tussock grasses in the highlands, increased discharge into the Andean foreland and the re-occupation of the settlements in the lowlands during this so-called Late Intermediate Period.

Morales et al. (2012) untersuchten die Niederschläge des Altiplano in den Anden für die vergangenen 700 Jahre. Auch sie fanden eine bedeutende natürliche Variabilität der Niederschläge, die einerseits an El Nino / Südliche Oszillation (ENSO) gekoppelt war. Andererseits gab es immer wieder trockene Phasen im Jahrhundert-Maßstab, so dass der beobachtete Austrocknungstrend in der Region seit den 1930er Jahren nichts Ungewöhnliches ist. Abstract der Arbeit:

Precipitation changes in the South American Altiplano since 1300AD reconstructed by tree-rings
Since the 1930s to present, a persistent negative trend in precipitation has been recorded in the reconstruction, with the three driest years since 1300AD occurring in the last 70 yr. Throughout the 707 yr, the reconstruction contains a clear ENSO-like pattern at interannual to multidecadal time scales, which determines inter-hemispheric linkages between our reconstruction and other precipitation sensitive records modulated by ENSO in North America. Our reconstruction points out that centuryscale dry periods are a recurrent feature in the Altiplano climate […].

Und wie sieht es in Patagonien in Argentinien aus? Mundo et al. 2012 haben die Regenhistorie für die vergangenen 650 Jahre rekonstruiert und fanden wiederum eine große Variabilität, die u.a. durch Ozeanzyklen gesteuert wurde. Kurzfassung:

Multi-century tree-ring based reconstruction of the Neuquén River streamflow, northern Patagonia, Argentina
The reconstruction covers the period 1346–2000 AD and was developed from a network of 43 tree-ring chronologies, grouped in composite series, using a nested principal component regression approach. Analyses of the frequency, intensity, and duration of droughts and pluvial events indicate that the 20th century contains some of the driest and wettest annual to decadal-scale events in the last 654 yr, but longer and more severe events were recorded in previous centuries. Blackman-Tukey and singular spectral analyses identified quasiperiodic oscillations from 3.5 to 17.5 yr. A dominant 6.8-yr cycle explains ca. 23.6% of the total variance in the Neuquén River streamflow reconstruction. Correlation analyses showed that discharges of the Neuquén River are related to variations in the Southern Annular Mode (SAM), a measure of air mass exchanges between middle and high latitudes in the Southern Hemisphere. This association is consistent with previous studies that indicate a strong correlation between rainfall in northern Patagonia and SAM variations.