Kleine Dürregeschichte der USA der vergangenen 10.000 Jahre: Schon die Indianer mussten immer wieder unter Trockenheit leiden

Dürren kommen und Dürren gehen. Ein steter Wechsel von trockenen und feuchten Bedingungen. Dies gilt auch für die USA. In den letzten beiden Blog-Beiträgen haben wir in die Dürrgeschichte Nordamerikas für die vergangenen 100 und 1000 Jahre geschaut. Heute geht es noch weiter in der Zeit zurück. Wie sah es in den letzten 10.000 Jahren aus?

Zunächst ist hier die Studie von Kirsten Menking und Kollegen von 2012, die in Palaeogeography, Palaeoclimatology, Palaeoecology erschien, die anhand von Pollen und anderen organischen Resten in Seesedimenten die Dürregeschichte der letzten 10.000 Jahre im Bundesstaat New York untersuchte. Das Forscherteam fand drei Feucht-Trocken-Zyklen, wobei in der Zeit von 5700-4100 Jahre vor heute eine lange Dürreperiode herrschte. Hier die Kurzfassung des Papers:

Sediment cores from Lakes Minnewaska and Mohonk in the Shawangunk Mountains of southeastern New York were analyzed for pollen, plant macrofossils, macroscopic charcoal, organic carbon content, carbon isotopic composition, carbon/nitrogen ratio, and lithologic changes to determine the vegetation and landscape history of the greater Catskill Mountain region since deglaciation. Pollen stratigraphy generally matches the New England pollen zones identified by Deevey (1939) and Davis (1969), with boreal genera (Picea, Abies) present during the late Pleistocene yielding to a mixed Pinus, Quercus and Tsuga forest in the early Holocene. Lake Minnewaska sediments record the Younger Dryas and possibly the 8.2 cal kyr BP climatic events in pollen and sediment chemistry along with an ~ 1400 cal yr interval of wet conditions (increasing Tsuga and declining Quercus) centered about 6400 cal yr BP. Both Minnewaska and Mohonk reveal a protracted drought interval in the middle Holocene, ~ 57004100 cal yr BP, during which Pinus rigida colonized the watershed, lake levels fell, and frequent fires led to enhanced hillslope erosion. Together, the records show at least three wet–dry cycles throughout the Holocene and both similarities and differences to climate records in New England and central New York. Drought intervals raise concerns for water resources in the New York City metropolitan area and may reflect a combination of enhanced La Niña, negative phase NAO, and positive phase PNA climatic patterns and/or northward shifts of storm tracks.

Bereits vor einiger Zeit hatte Henri Grissino-Mayer von der University of Tennessee die Dürreentwicklung in New Mexico für die letzten 2000 Jahre untersucht. Auch diese Studie fand eine bedeutende Zyklik in den Niederschlägen, wobei in vorindustrieller Zeit immer wieder Dürren auftraten, die jene aus dem 20. Jahrhundert in ihrer Intensität in den Schatten stellen. Zuletzt kam es während der Kleinen Eiszeit in der zweiten Hälfte des 16. Jahrhunderts zu einer solchen Mega-Dürre. Lesen Sie, was die NOAA zu dieser Studie schreibt:

The 1950s drought was the most severe drought 20th century drought in this region, but when viewed in the context of the past three centuries, it appears to be a fairly typical drought. However, when the 1950s drought is compared to droughts for the entire reconstruction, back to 136 BC (bottom graph), it is clear that the 1950s drought is minor relative to many past droughts. A number of the severe droughts of the past spanned several decades, the most recent occurring in the second half of the 16th century.

Tian et al. 2006 schauten sich die letzten 3000 Jahre im Zentralbereich des nordamerikanischen Kontinents in Minnesota an. Die Studie erschien in den Geophysical Research Letters. Interessanterweise traten in den vergangenen 3000 Jahren immer wieder schwere Dürren auf die in ihrer Intensität und Dauer jene des 20. Jahrhunderts bei weitem übertrafen. Das Auftreten der Dürren wurde nach Interpretation der Autoren durch den Ozeanzyklus der Pazifisch Dekadischen Oszillation (PDO) sowie Sonnenaktivitätsschwankungen gesteuert. Hier die Kurzfassung des Papers:

Paleorecords are key for evaluating the long-term patterns and controls of drought. We analyzed calcite in annually laminated sediments from a Minnesota lake for oxygen-isotopic composition (δ18O). The δ18O record of the past ∼3100 years reveals that droughts of greater severity and duration than during the 20th century occurred repeatedly, especially prior to 300 AD. Drought variability was anomalously low during the 20th century; ∼90% of the variability values during the last 3100 years were greater than the 20th-century average. δ18O is strongly correlated with the index of the Pacific Decadal Oscillation (PDO) during the past 100 years, and periodicities of the late-Holocene δ18O record are similar to those of the PDO. Furthermore, time series of δ18O and atmospheric Δ14C are generally coherent after 700 AD. Both the Pacific climate and solar irradiance probably played a role in drought occurrence, but their effects were non-stationary through the late Holocene.

Abbildungen aus dem Paper können Sie auf The Hockey Schtick finden.

Gehen wir jetzt nach Colorado. Dort haben Yuan et al. anhand von Seensedimenten die Dürregeschichte der letzten 16.000 Jahre studiert. Sie veröffentlichten ihre Studie im Dezember 2013 in Palaeo3. Die Forscher fanden, dass es seit 2600 Jahren in dem Gebiet deutlich trockener geworden ist als es in den Jahrtausenden zuvor. Zwischen 6700-2600 Jahren vor heute gab es einen steten Wechsel zwischen feucht und trocken. Hier die Kurzfassung des Papers:

Large rapid climate changes occurred over the last glacial cycle in the southwestern United States and elsewhere in many regions of the world. Some of these changes were attributed to alternations between stadial and interstadial conditions in the North Atlantic. But intense debate exists on how climate anomalies in the North Atlantic transmit to the southwest. Here we report a sediment record from San Luis Lake in southern Colorado, through analyses of grain size, magnetic susceptibility, Mg/Ca, total inorganic carbon, δ18O and δ13C, to indicate climatic and environmental changes in the southern Rocky Mountains over the last 16.5 ka. We found that San Luis Lake remained hydrologically closed most of the time but overflowed during the second half of the Mystery Interval (the Big Wet: 15.7–14.9 ka) and the latter part of the mid-Holocene (the Neopluvial: 4–3 ka). Over the course of the last deglaciation, San Luis Lake underwent a series of large millennial-scale hydroclimatic changes such as the Big Dry (16.5–15.7 ka), the Big Wet, the Bølling–Allerød dry (14.9–12.7 ka), and the Younger Dryas wet (12.8–11.6 ka), corresponding to warm/cold phases in the high-latitude Northern Hemisphere. The North American monsoon waxed during the Pre-Boreal interval (11.6–10.5 ka) and waned through the Holocene, in phase with northward and southward displacement of the intertropical convergence zone (ITCZ). The San Luis Lake basin was relatively dry in the early Holocene (10.5–6.7 ka), wet and fluctuating in the mid-Holocene (6.7–2.6 ka), and dry and less variable in the late Holocene (2.6–0 ka). We found evidence that extreme pluvial episodes of the southern Rocky Mountains and elsewhere in the American Southwest were coeval with cold phases of the North Pacific. Our results highlight the role of the North Pacific in modulating atmospheric circulations over the region on millennial timescales.

Kirby et al. untersuchten 2012 die Dürren im Südwesten der USA für die vergangenen 9000 Jahre. Die Studie erschien in den Quaternary Science Reviews. Die Autoren fanden neun feuchte Phasen, die zwischen Jahrzehnten bis mehreren Jahrhunderten andauerten. Dazwischen war es trocken. Hier die Kurzfassung des Papers:

A well-dated, 9170 calendar year before present (cal yr BP) paleohydrologic reconstruction is presented from Lower Bear Lake in the San Bernardino Mountains of the coastal southwest United States. This new multi-proxy record is characterized by alternating organic-rich/carbonate-rich sediment units, interpreted to reflect hydrologically-forced changes in the lake’s depositional environment. Our interpretation of the proxy data indicates nine decadal-to-multi-centennial pluvial episodes (PE) over the past 9170 cal yr BP. Of these nine inferred pluvials, five are interpreted as more pronounced based on their combined proxy interpretations: (PE-V) 9170?8250, (PE-IV) 70006400, (PE-III) 33503000, (PE-II) 850700, and (PE-I) 500476 (top of core) cal yr BP. The Lower Bear Lake record indicates that the San Bernardino Mountains, source region for the Mojave River and its terminal playa, was wet during the same periods (within dating errors), to several of the major pluvials proposed from the lakes in the sink of the Mojave River. Our comparison extends north also to Tulare Lake, which drains the southcentral-western Sierra Nevada Mountains. This temporally and spatially coherent signal indicates that a similar climate forcing acted to increase regional wetness at various times during the past 9170 cal yr BP. As originally proposed by Enzel, Ely, and colleagues (e.g., Enzel et al., 1989Enzel, 1992Ely et al., 1994Enzel and Wells, 1997), we too contend that Holocene pluvial episodes are associated with changing the frequency of large winter storms that track across a broad region at decadal-to-multi-centennial timescales. We build upon their hypothesis through the addition of new and better-dated site comparisons, recent advances in the understanding of atmospheric rivers, and improved knowledge of the oceanatmosphere dynamics that caused the early 20th century western United States pluvial.

Was lernen wir daraus? Das Klima stand niemals still. Der Regen schwankte auf allen vorstellbaren Zeitmaßstäben. Es ist naiv anzunehmen, dass in der vorindustriellen Zeit – vor dem anthropogen bedingten Anstieg des CO2-Gehalts in der Atmosphäre – die Niederschläge in genau ausreichenden Mengen das Land bewässert hätten. Dies ist ein schöner Mythos, der jedoch keineswegs der Wirklichkeit entspricht. Das Paradies war stets flüchtig, und immer lungerte das Unheil hinter der nächsten Ecke.