Wie haben sich die Dürren in Nordamerika in den letzten 1000 Jahren entwickelt? Sind sie häufiger oder seltener geworden? Gibt es vielleicht natürliche Zyklen? Wie sind die letzten Jahrzehnte im Vergleich zum letzten Jahrtausend einzuordnen? Wir begeben uns auf paläoklimatologische Spurensuche.
Im Sommer 2012 schlug in den USA eine heftige Dürre zu. Der Focus griff das Thema damals auf und sprach mit dem Greenpeace-„Experten“ Karsten Smid über die Dürre. Der Untertitel des Beitrags gibt zunächst Hoffnung:
Dürren gab es schon immer in den USA oder Südeuropa.
Sollte Greenpeace wirklich seine Hausaufgaben gemacht haben und den historischen Dürrekontext endlich in die Argumentation aufgenommen haben? Leider nein. Gleich im ersten Satz des Beitrags enttäuschen der Focus und Greenpeace:
Dürren gab es schon immer in den USA oder Südeuropa. Nicht aber so oft.
Das ist nachweislich falsch. Im gestrigen Beitrag analysierten wir die US-Dürregeschichte der letzten 100 Jahre, in der kein Anstieg der Dürrehäufigkeit zu erkennen ist. Focus und Greenpeace liegen falsch. Vermutlich wissen sie dies sogar und hoffen, dass es die Leser nicht merken. Dazu passt auch die klimareligiöse Wortwahl im Haupttitel des Beitrags:
,,Die Prophezeiungen bewahrheiten sich“
Was sagt die seriöse Wissenschaft dazu? Als das US-Dürrejahr 2012 vorbei war, veröffentlichten Cook et al. im Journal auf Climate eine Studie, die man dem Focus gerne als Lektüre empfehlen würde. Die Forscher zeigen in ihrem Artikel, dass es während der Mittelalterlichen Wärmeperiode in den südlichen USA gehäuft zu Mega-Dürren gekommen ist, die sich über mehrere Jahrzehnte hinzogen. Hier die Kurzfassung:
Regional droughts are common in North America, but pan-continental droughts extending across multiple regions, including the 2012 event, are rare relative to single-region events. Here, the tree-ring-derived North American Drought Atlas is used to investigate drought variability in four regions over the last millennium, focusing on pan-continental droughts. During the Medieval Climate Anomaly (MCA), the central plains (CP), Southwest (SW), and Southeast (SE) regions experienced drier conditions and increased occurrence of droughts and the Northwest (NW) experienced several extended pluvials. Enhanced MCA aridity in the SW and CP manifested as multidecadal megadroughts. Notably, megadroughts in these regions differed in their timing and persistence, suggesting that they represent regional events influenced by local dynamics rather than a unified, continental-scale phenomena. There is no trend in pan-continental drought occurrence, defined as synchronous droughts in three or more regions. SW, CP, and SE (SW+CP+SE) droughts are the most common, occurring in 12% of all years and peaking in prevalence during the twelfth and thirteenth centuries; patterns involving three other regions occur in about 8% of years. Positive values of the Southern Oscillation index (La Niña conditions) are linked to SW, CP, and SE (SW+CP+SE) droughts and SW, CP, and NW (SW+CP+NW) droughts, whereas CP, NW, and SE (CP+NW+SE) droughts are associated with positive values of the Pacific decadal oscillation and Atlantic multidecadal oscillation. While relatively rare, pan-continental droughts are present in the paleo record and are linked to defined modes of climate variability, implying the potential for seasonal predictability. Assuming stable drought teleconnections, these events will remain an important feature of future North American hydroclimate, possibly increasing in their severity in step with other expected hydroclimate responses to increased greenhouse gas forcing.
Das Fachmagazin Nature fand die Studie so interessant, dass auch sie über die Arbeit im Konkurrenzblatt berichtete.
Eine weitere Studie zur nordamerikanischen Dürregeschichte erschien Mitte 2013 in PNAS von Asmerom et al. Interessanterweise beschreiben diese Autoren eine langanhaltende Mega-Dürre, die sich über drei Jahrhunderte in der Kleinen Eiszeit abspielte. Yemane Asmerom und Kollegen sehen einen Zusammenhang mit der geringen Sonnenaktivität zu dieser Zeit, die den Monsun verändert habe. Hier die Kurzfassung:
Late Holocene climate in western North America was punctuated by periods of extended aridity called megadroughts. These droughts have been linked to cool eastern tropical Pacific sea surface temperatures (SSTs). Here, we show both short-term and long-term climate variability over the last 1,500 y from annual band thickness and stable isotope speleothem data. Several megadroughts are evident, including a multicentury one, AD 1350–1650, herein referred to as Super Drought, which corresponds to the coldest period of the Little Ice Age. Synchronicity between southwestern North American, Chinese, and West African monsoon precipitation suggests the megadroughts were hemispheric in scale. Northern Hemisphere monsoon strength over the last millennium is positively correlated with Northern Hemisphere temperature and North Atlantic SST. The megadroughts are associated with cooler than average SST and Northern Hemisphere temperatures. Furthermore, the megadroughts, including the Super Drought, coincide with solar insolation minima, suggesting that solar forcing of sea surface and atmospheric temperatures may generate variations in the strength of Northern Hemisphere monsoons. Our findings seem to suggest stronger (wetter) Northern Hemisphere monsoons with increased warming.
Im März 2013 war in den Geophysical Research Letters zudem eine Arbeit von Griffin et al. erschienen, in der anhand von Baumringen die Dürreentwicklung in den südwestlichen USA untersucht wurde. Ähnlich wie Asmerom et al. fanden auch Daniel Griffin und sein Team in den letzten 5 Jahrhunderten etliche Dürreperioden, die sich jeweils über mehrere Jahrzehnte hinzogen. Sowohl der sommerliche Monsunregen als auch der Winterniederschlag war damals zurückgegangen schreiben die Autoren. Die historischen Dürren aus der Kleinen Eiszeit waren dabei heftiger als sämtliche Dürren der heutigen modernen Mess-Ära. Hier die Kurzfassung der Arbeit (siehe auch dazugehörige Pressemitteilung):
The North American monsoon is a major focus of modern and paleoclimate research, but relatively little is known about interannual- to decadal-scale monsoon moisture variability in the pre-instrumental era. This study draws from a new network of subannual tree-ring latewood width chronologies and presents a 470-year reconstruction of monsoon (June–August) standardized precipitation for southwestern North America. Comparison with an independent reconstruction of cool-season (October–April) standardized precipitation indicates that southwestern decadal droughts of the last five centuries were characterized not only by cool-season precipitation deficits but also by concurrent failure of the summer monsoon. Monsoon drought events identified in the past were more severe and persistent than any of the instrumental era. The relationship between winter and summer precipitation is weak, at best, and not time stable. Years with opposing-sign seasonal precipitation anomalies, as noted by other studies, were anomalously frequent during the mid to late 20th century.
Einige Monate später publizierte dieselbe Forschergruppe im Fachblatt Climatic Change eine weitere Dürrestudie, die sich diesmal auf das nordöstliche Arizona beschränkte. Das Autorenteam um Faulstich et al. 2013 zeigte erneut, dass die Dürren aus der vorindustriellen Phase um einiges schlimmer ausfielen als in den letzten Jahrzehnten. Die Dürre des letzten Jahrzehnts im Reservat der Hopi und Navajo Indianer muss aus wissenschaftlicher Sicht in diesem langfristigen Kontext gesehen werden, sagen die Autoren. Hier die Kurzfassung der Arbeit:
For over a decade, the Hopi Tribe and Navajo Nation of northeastern Arizona have suffered the effects of persistent drought conditions. Severe dry spells have critically impacted natural ecosystems, water resources, and regional livelihoods including dryland farming and ranching. Drought planning and resource management efforts in the region are based largely on the instrumental climate record, which contains a limited number of severe, sustained droughts. In this study, a new network of moisture-sensitive tree-ring chronologies provides the basis for evaluating the longer-term temporal variability of precipitation in the Four Corners region. By analyzing the earlywood and latewood components within each annual tree ring, we are able to generate separate, centuries-long reconstructions of both cool- (October-April) and warm-season (July-August) precipitation. These proxy records offer new insights into seasonal drought characteristics and indicate that the instrumental record fails to adequately represent precipitation variability over the past 400 years. Through the use of two different analysis techniques, we identify multiyear and decadal-scale drought events more severe than any in the modern era. Furthermore, the reconstructions suggest that many of the historically significant droughts of the past (e.g., 17th century Puebloan drought) were not merely winter phenomena, but persisted through the summer season as well. By comparing these proxy records with historical documents, we are able to independently validate the reconstructions and better understand the socioeconomic and environmental significance of past climate anomalies on the tribal lands of northeastern Arizona.
Stöbern wir weiter in der umfangreichen Literatur zur nordamerikanischen Dürrehistorie, die den deutschen Medienvertretern offenbar gänzlich unbekannt ist. Im Jahr 2012 erschien in den Environmental Research Letters eine Dürrstudie von Pederson et al., in der das Geschehen für den Südosten der USA für die vergangenen 400 Jahre auf Basis von Baumringen rekonstruiert wird. Auch diese Gruppe fand lang anhaltende Dürren während der Kleinen Eiszeit, wobei das späte 20. und frühe 21. Jahrhundert zu den feuchtesten Episoden der letzten vier Jahrhunderte gehörten. Hier die Kurzfassung der Arbeit:
The depth of the 2006–9 drought in the humid, southeastern US left several metropolitan areas with only a 60–120 day water supply. To put the region’s recent drought variability in a long-term perspective, a dense and diverse tree-ring network—including the first records throughout the Apalachicola–Chattahoochee–Flint river basin—is used to reconstruct drought from 1665 to 2010 CE. The network accounts for up to 58.1% of the annual variance in warm-season drought during the 20th century and captures wet eras during the middle to late 20th century. The reconstruction shows that the recent droughts are not unprecedented over the last 346 years. Indeed, droughts of extended duration occurred more frequently between 1696 and 1820. Our results indicate that the era in which local and state water supply decisions were developed and the period of instrumental data upon which it is based are amongst the wettest since at least 1665. Given continued growth and subsequent industrial, agricultural and metropolitan demand throughout the southeast, insights from paleohydroclimate records suggest that the threat of water-related conflict in the region has potential to grow more intense in the decades to come.
Weiter geht‘s mit Woodhouse et al. 2013 im Fachblatt Water Resources Research. Mithilfe von Baumringen wird in dieser Arbeit die Dürregeschichte des Rio Grande Beckens in den südlichen USA rekonstruiert. Wiederum werden langanhaltende Dürren aus der Kleinen Eiszeit berichtet. Die Autoren zeigen, dass die Dürre der letzten 12 Jahre im Rio Grande Becken Neumexikos in diesem historischen Kontext keineswegs ungewöhnlich sei. Hier die Kurzfassung der Arbeit:
Agriculture and ranching in semiarid regions often rely on local precipitation during the growing season as well as streamflow from runoff in distant headwaters. Where snowpack and reservoir storage are important, this pattern of reliance leads to vulnerability to multiseason drought. The lower Rio Grande basin in New Mexico, used as a case study here, has experienced drought conditions over the past 12 years characterized both by low local summer monsoon precipitation and by reduced availability of surface water supplies from the upper Rio Grande. To place this drought in a long-term context, we evaluate the covariability of local warm-season and remote cool-season hydroclimate over both the modern period and past centuries. We draw on a recently developed network of tree-ring data that allows an assessment of preinstrumental warm-season variations in precipitation over the southwest. Both instrumental and paleoclimatic data suggest that low runoff followed by a dry monsoon is not unusual, although over the full reconstruction period (1659–2008), years with wet or dry conditions shared in both seasons do not occur significantly more often than unshared conditions. Low flows followed by dry monsoon conditions were most persistent in the 1770s and 1780s; other notable periods of shared seasonal droughts occurred in the 1660s and 1950s. The recent drought does not yet appear to be unusually severe in either the instrumental or paleoclimatic context.
Anhand von Seensedimenten untersuchten Lascu et al. 2012 die Dürren der letzten 1000 Jahre in Minnesota. Die Arbeit erschien in Palaeogeography, Palaeoclimatology, Palaeoecology. Ioan Lascu und Kollegen fanden eine Reihe von Dürrephasen, die schwerste und langanhaltendste davon während der späten Kleinen Eiszeit. Hier die Kurzfassung der Arbeit:
Droughts are recurrent features in sedimentary records at the prairie–forest ecotone of northern U.S.A. It is therefore important to understand the duration of such events, as well as their severity and consequences beyond the century-scale instrumental record. The existing drought records can be complemented with additional proxies that clearly document the interactions between external factors and the depositional environment. Here we attempt to reconstruct the last millennium drought history of Deming Lake, a small lake at Minnesota’s prairie–forest ecotone, using sediment magnetism. In particular, this approach allowed us to explore the interactions between lake and catchment, and test the hypothesis that periods of moderate aridity result in decreased hydrologic sediment transport from the catchment to the lake, as a consequence of reduced erosion and overland flow. Concordantly, during dry episodes we find that the mass fractions of in-lake produced organic matter and biogenic magnetic particles increase relative to allochthonous component fractions. We identify several episodes of aridity at Deming Lake consistent with existing regional drought reconstructions. The most prolonged dry event occurred at the end of the Little Ice Age, suggesting that the second half of this cold interval was drier in the region than previously thought.
Nun zu Booth et al. 2012 im Fachblatt Ecology der Ecological Society of America. Die Autoren gingen der Frage nach, wie sich die Mittelalterliche Wärmperiode auf das Dürregeschehen an den Großen Seen Nordamerikas auswirkte. Robert Booth und Kollege fanden für diese Zeit signifikante Dürreepisoden, die damals massive ökologische Schäden in der Region anrichteten. Hier die Kurzfassung der Arbeit:
Climate variability, particularly the frequency of extreme events, is likely to increase in the coming decades, with poorly understood consequences for terrestrial ecosystems. Hydroclimatic variations of the Medieval Climate Anomaly (MCA) provide a setting for studying ecological responses to recent climate variability at magnitudes and timescales comparable to expectations of coming centuries. We examined forest response to the MCA in the humid western Great Lakes region of North America, using proxy records of vegetation, fire, and hydroclimate. Multi-decadal moisture variability during the MCA was associated with a widespread, episodic decline in Fagus grandifolia (beech) populations. Spatial patterns of drought and forest changes were coherent, with beech declining only in areas where proxy-climate records indicate that severe MCA droughts occurred. The occurrence of widespread, drought-induced ecological changes in the Great Lakes region indicates that ecosystems in humid regions are vulnerable to rapid changes in drought magnitude and frequency.
Laird et al. untersuchten die Mittelalterliche Wärmeperiode in Ontario. Das Paper erschien 2012 in Global Change Biology. Kathleen Laird und Kollegen fanden für diese Zeit 900-1400 n. Chr. schwere und langanhaltende Dürreserien. Hier die Kurzfassung der Arbeit:
Multi-decadal to centennial-scale shifts in effective moisture over the past two millennia are inferred from sedimentary records from six lakes spanning a ~250 km region in northwest Ontario. This is the first regional application of a technique developed to reconstruct drought from drainage lakes (open lakes with surface outlets). This regional network of proxy drought records is based on individual within-lake calibration models developed using diatom assemblages collected from surface sediments across a water-depth gradient. Analysis of diatom assemblages from sediment cores collected close to the near-shore ecological boundary between benthic and planktonic diatom taxa indicated this boundary shifted over time in all lakes. These shifts are largely dependent on climate-driven influences, and can provide a sensitive record of past drought. Our lake-sediment records indicate two periods of synchronous signals, suggesting a common large-scale climate forcing. The first is a period of prolonged aridity during the Medieval Climate Anomaly (MCA, c. 900-1400 CE). Documentation of aridity across this region expands the known spatial extent of the MCA megadrought into a region that historically has not experienced extreme droughts such as those in central and western north America. The second synchronous period is the recent signal of the past ~100 years, which indicates a change to higher effective moisture that may be related to anthropogenic forcing on climate. This approach has the potential to fill regional gaps, where many previous paleo-lake depth methods (based on deeper centrally located cores) were relatively insensitive. By filling regional gaps, a better understanding of past spatial patterns in drought can be used to assess the sensitivity and realism of climate model projections of future climate change. This type of data is especially important for validating high spatial resolution, regional climate models.
Steinman et al. untersuchten die Dürregeschichte der nordwestlichen USA an der Pazifikküste für die vergangenen 1500 Jahre mithilfe von Isotopen in Seensedimenten. Die Studie erschien 2012 in PNAS, zudem gibt es eine Pressemitteilung. Fazit: Die Mittelalterliche Wärmeperiode war feucht, während sich die Dürren während der Kleinen Eiszeit häuften. Zudem zeigten die Autoren einige Diskrepanzen mit Daten aus Baumringen aus der gleichen Region auf, denen nachgegangen werden sollte. Hier die Kurzfassung der Arbeit (siehe auch Beitrag auf Terra Daily):
Multiple paleoclimate proxies are required for robust assessment of past hydroclimatic conditions. Currently, estimates of drought variability over the past several thousand years are based largely on tree-ring records. We produced a 1,500-y record of winter precipitation in the Pacific Northwest using a physical model-based analysis of lake sediment oxygen isotope data. Our results indicate that during the Medieval Climate Anomaly (MCA) (900–1300 AD) the Pacific Northwest experienced exceptional wetness in winter and that during the Little Ice Age (LIA) (1450–1850 AD) conditions were drier, contrasting with hydroclimatic anomalies in the desert Southwest and consistent with climate dynamics related to the El Niño Southern Oscillation (ENSO) and the Pacific Decadal Oscillation (PDO). These findings are somewhat discordant with drought records from tree rings, suggesting that differences in seasonal sensitivity between the two proxies allow a more complete understanding of the climate system and likely explain disparities in inferred climate trends over centennial timescales.
Zwei Jahre später ergänzen Steinman et al. 2014 in den Geophysical Research Letters ihre Ergebnisse und erhärten die bereits zuvor geäußerte Schlussfolgerung: Die Mittelalterliche Wärmeperiode war in den nordwestlichen USA feucht, während die Kleine Eiszeit trocken ausfiel. Hier die Kurzfassung der Arbeit:
Reconstructing centennial timescale hydroclimate variability during the late Holocene is critically important for understanding large-scale patterns of drought and their relationship with climate dynamics. We present sediment oxygen isotope records spanning the last two millennia from 10 lakes, as well as climate model simulations, indicating that the Little Ice Age was dry relative to the Medieval Climate Anomaly in much of the Pacific Northwest of North America. This pattern is consistent with observed associations between the El Niño–Southern Oscillation (ENSO), the Northern Annular Mode, and drought as well as with proxy-based reconstructions of Pacific and Atlantic ocean-atmosphere variations over the past 1000 years. The large amplitude of centennial variability indicated by the lake data suggests that regional hydroclimate is characterized by longer-term shifts in ENSO-like dynamics and that an improved understanding of the centennial timescale relationship between external forcing and drought is necessary for projecting future hydroclimatic conditions in western North America.
