Monat: August 2017

Bei jeder Dürre dasselbe: Stets gibt es einen Klimaktivisten, der einen Zusammenhang mit dem menschengemachten Klimawandel sieht und eine Abkehr vom frevelhaften Tun fordert. Dies begründet sich allerdings vor allem auf einer klimahistorischen Kurzsicht. Würden sich die Damen und Herren Aktivisten ein wenig mehr für die Geschichte des Klimas interessieren, würde ihnen ihr Denkfehler sofort auffallen.

Im heutigen Blogbeitrag wollen wir uns um die Dürregeschichte Chinas kümmern, die aufgrund einer Vielzahl von neuen Studien immer besser bekannt wird. Beginnen wir im Norden Chinas. Chen et al. (2014) beschrieben in Global and Planetary Change die Dürren auf dem Löss Plateau während der vergangenen 250 Jahre auf Basis von Baumringen. Das Resultat: Trocken- und Feuchtphasen wechselten stets ab. Ein stabiles Hydroklima hat es nie gegeben und ist auch für die Zukunft nicht anzunehmen. Antrieb der Schwankungen ist der PDO-Ozeanzyklus sowie Änderungen asiatischen Sommermonsun. Abstract:

A tree-ring based drought reconstruction (AD 1760–2010) for the Loess Plateau and its possible driving mechanisms
We have developed a 272-year ring-width chronology of Chinese pine (Pinus tabulaeformis) growing in the Huanglong Mountains, North China. Climatic response analyses revealed that mean January–July Palmer drought severity index (PDSI) has positive effect on the radial growth of pine trees. Based on the relationships, the mean January–July PDSI was reconstructed for the period from 1760 to 2010. The percentage of variance in the data explained by the reconstruction was 41% during the calibration period of 1950–2010. Spatial correlation analyses between the PDSI reconstruction and gridded PDSI data show that the PDSI reconstruction captures regional drought variations over the environmentally-sensitive area linked to the East Asian summer monsoon. Relatively wet periods are identified for AD 1766–1781, 1795–1804, 1811–1821, 1838–1859, 1884–1889, 1909–1914, 1937–1977 and 2003–2008. Dry conditions prevailed during AD 1760–1765, 1782–1794, 1805–1810, 1822–1837, 1860–1883, 1890–1908, 1915–1936, 1978–2002 and 2009–now. There is a reasonable agreement with dry/wet periods previously estimated from tree-ring data of the Kongtong Mountains of the Loess Plateau. Spatial correlation analyses with sea surface temperature in the Pacific Ocean and tropical Indian Ocean indicated that the Asian summer monsoon circulations play a role in modulating drought variations in the study area whereas the effects of the Pacific Decadal Oscillation are relatively strong.

Bleiben wir in Nordchina. Li et al. 2016 veröffentlichten in Dendrochronologia eine ähnliche Baumringstudie zum Fen River. Wiederum scheinen Ozeanzyklen im Pazifik verantwortlich zu zeichnen für die natürliche Variabilität der Regenfälle:

Tree-ring-based reconstruction of drought variability (1792–2011) in the middle reaches of the Fen River, North China
We developed a tree-ring chronology based on 52 ring-width series from 25 Pinus tabulaeformis trees at Tianlong Mountain (TLM) using the signal-free method. TLM is located in the middle reaches of the Fen River, North China, and is influenced by the East Asian monsoon system. Tree growth was highly correlated (0.789) with the Palmer Drought Severity Index (PDSI) from May to July and indicated a drought-stress growth pattern. Therefore, we developed a robust May-July PDSI reconstruction for 1792–2011 that explained 62.3% of the instrumental variance for 1951–2005. Severe drought years determined by the reconstruction are consistent with conditions reported in historical documents. The TLM PDSI reconstruction was consistent with other tree-ring-based hydroclimate reconstructions in North China; thus, it may accurately represent dry/wet changes that occur over a large area. Cyclical spectral peaks at 2–8 years in the reconstructed PDSI may indicate ENSO activity, as suggested by the positive correlation with the western Pacific sea-surface temperatures (SSTs) and the negative correlation with the eastern Pacific SSTs on the inter-annual scale.

Weiter mit Cai et al. 2015 in Paleo3, ebenfalls in Nordchina. Wieder das gleiche Bild: Zyklen im tropischen Westpazifik bestimmen das Dürregeschehen:

Reconstruction of drought variability in North China and its association with sea surface temperature in the joining area of Asia and Indian–Pacific Ocean
Using tree-ring data from the northernmost marginal area of the East Asian summer monsoon (EASM) in North China, May–July mean Palmer drought severity index (PDSI) was reconstructed back to 1767 AD. The reconstruction captured 52.8% of the variance over the calibration period from 1945 to 2005 AD and showed pronounced pluvial periods during 1850–1905, 1803–1811 and 1940–1961 and dry periods during 1814–1844, 1916–1932 and 1984–2012. These anomalous periods have previously been reported in other parts of North China. Spatial correlation analyses and comparisons with other hydroclimatic indices in North China indicated that our new PDSI reconstruction could represent spatial and temporal drought variability in this region well. Our work also suggested that the drying tendency currently observed in the northern part of North China (including the study area) is consistent with the weakening of the EASM. Meanwhile the drying trend was seemingly restrained at present in the southern part of North China. Spatial correlation patterns with global sea surface temperature (SST) indicated that the regional hydroclimatic variability in North China was tightly linked to SST over the joining area of Asia and Indian–Pacific Ocean (AIPO), especially over the tropical western Pacific. When SST from prior November to current July (NJ-SST) in the AIPO area was anomalously high (low), the thermal contrast between Asian land and ocean was weakened (strengthened), and the EASM was correspondingly weakened (strengthened), thereby causing droughts (pluvials) in North China. The results of this study do not only provide useful information for assessing the long-term climate change in North China, but also suggest that abnormal variability in NJ-SST over the AIPO area could be used to forecast hydroclimatic conditions in North China.

Und nun zu Yang et al. 2015, immer noch in Nordchina. Die Gruppe dokumentierte eine intensive Dürrephase vor 4200 Jahren, die es in sich hatte. Pressemitteilung der Baylor University vom 16. Februar 2015:

Study Sheds Light on How Populations Respond and Adapt to Climate Change

Using a relatively new scientific dating technique, a Baylor University geologist and a team of international researchers were able to document—for the first time—a drastic climate change 4,200 years ago in northern China that affected vegetation and led to mass migration from the area.

Steve Forman, Ph.D., professor of geology in the College of Arts & Sciences, and researchers—using a dating technique called Optically Stimulated Luminescence—uncovered the first evidence of a severe decrease in precipitation on the freshwater lake system in China’s Hunshandake Sandy Lands. The impact of this extreme climate change led to desertification—or drying of the region—and the mass migration of northern China’s Neolithic cultures. Their research findings appear in the January 2015 issue of the Proceedings of the National Academy of Sciences and are available online.

“With our unique scientific capabilities, we are able to assert with confidence that a quick change in climate drastically changed precipitation in this area, although, further study needs to be conducted to understand why this change occurred,” Forman said. Between 2001 and 2014, the researchers investigated sediment sections throughout the Hunshandake and were able to determine that a sudden and irreversible shift in the monsoon system led to the abrupt drying of the Hunshandake resulting in complications for the population. “This disruption of the water flow significantly impacted human activities in the region and limited water availability. The consequences of a rapid climatic shift on the Hunshandake herding and agricultural cultures were likely catastrophic,” Forman said.

He said these climatic changes and drying of the Hunshandake continue to adversely impact the current population today. The Hunshandake remains arid and even with massive rehabilitation efforts will unlikely regrow dense vegetation. “This study has far-reaching implications for understanding how populations respond and adapt to drastic climate change,” Forman said. Forman is the director of the Geoluminescence Dating Research Lab in the department of geology.

Nochmal Nordchina, diesmal Li et al. 2014 in den Geophysical Research Letters. Sie fanden, dass die Wüstengebiete damals viel kleiner waren als heute, weil die Temperaturen höher als heute waren und der ostasiatische Sommermonsun verstärkt wurde:

Distribution and vegetation reconstruction of the deserts of northern China during the mid-Holocene
Desertification is potentially a serious threat to society, and therefore, it is critical to understand how deserts may respond to future climate change. The mid-Holocene (6 ± 0.5 ¹⁴C ka) was warmer than present, and the distribution of deserts at this time may have implications for understanding their response to future warming. Here we reconstruct the distribution of deserts in northern China during the mid-Holocene by combining data on vegetation type and the sedimentary facies of aeolian deposits. The results demonstrate that during the mid-Holocene, the deserts retreated northwestward to the location of the modern 300 mm isohyet. Most of the Eastern Desert was stabilized with steppe or forest-steppe vegetation, whereas the Western Desert exhibited no significant change and remained mobile, occupied by desert vegetation. The deserts in northern China were greatly reduced during the mid-Holocene because of the enhancement of the East Asian summer monsoon in a warmer climate than today.

Jetzt ein wenig weiter nach Westen, nach Nordwest China. Lee et al. 2015 beschrieben in The Holocene den engen Zusammenhang der dortigen Dürren mit den pazifischen Ozeanzyklen:

Reconstruction of the geographic extent of drought anomalies in northwestern China over the last 539 years and its teleconnection with the Pacific Ocean
Recent paleo-climatic/environmental studies have resulted in several high-resolution paleo-precipitation/moisture reconstructions in Northwestern (NW) China over extended periods. Nevertheless, those reconstructions are mostly about the climatic history of individual sites, while fine-grained portrayal and analysis of the geographic extent of drought anomalies across the entire NW China are still missing. We based our study on the dryness/wetness grade series of 19 sites in NW China, which are primarily derived from historical documents, to reconstruct the annual geographic extent of drought anomalies in NW China in AD 1470–2008. Our reconstruction reveals the following periods of drought in NW China: the AD 1470s–1490s, 1620s–1640s, 1700s–1720s, 1770s–1790s, 1860s–1870s, and 1910s–1930s. The most extremely dry years were AD 1928 and 1929. In addition, we found that the influence of El Niño Southern Oscillation (ENSO) on the geographic extent of drought anomalies in NW China was non-stationary at the inter-annual to multi-decadal timescale and that the correlation switched from positive to negative since the late ‘Little Ice Age’. We propose that this non-stationary relationship is attributable to the variance of ENSO and the strength of Asian Summer Monsoon. To conclude, we discuss the implications of the above findings within the context of global warming.

Auch in Nordostchina war der Monsunregen nicht stabil, wie das Lamont-Doherty Earth Observatory (Columbia University) am 6. Februar 2017 mitteilte:

Shifting Monsoon Altered Early Cultures in China, Study Says

The annual summer monsoon that drops rain onto East Asia, an area with about a billion people, has shifted dramatically in the distant past, at times moving northward by as much as 400 kilometers and doubling rainfall in that northern reach. The monsoon’s changes over the past 10,000 years likely altered the course of early human cultures in China, say the authors of a new study.

Hier noch der Abstract der Studie von Goldsmith et al. 2017:

Northward extent of East Asian monsoon covaries with intensity on orbital and millennial timescales
The magnitude, rate, and extent of past and future East Asian monsoon (EAM) rainfall fluctuations remain unresolved. Here, late Pleistocene–Holocene EAM rainfall intensity is reconstructed using a well-dated northeastern China closed-basin lake area record located at the modern northwestern fringe of the EAM. The EAM intensity and northern extent alternated rapidly between wet and dry periods on time scales of centuries. Lake levels were 60 m higher than present during the early and middle Holocene, requiring a twofold increase in annual rainfall, which, based on modern rainfall distribution, requires a ∼400 km northward expansion/migration of the EAM. The lake record is highly correlated with both northern and southern Chinese cave deposit isotope records, supporting rainfall “intensity based” interpretations of these deposits as opposed to an alternative “water vapor sourcing” interpretation. These results indicate that EAM intensity and the northward extent covary on orbital and millennial timescales. The termination of wet conditions at 5.5 ka BP (∼35 m lake drop) triggered a large cultural collapse of Early Neolithic cultures in north China, and possibly promoted the emergence of complex societies of the Late Neolithic.

Jetzt nach Zentralchina und Yin et al. 2014 (Climate of the Past). Ein Höhlentropfstein registrierte dort starke Schwankungen in den Niederschlägen während der letzten drei Jahrhunderte. Regenreiche Phasen ereigneten sich während Zeiten hoher solarer Aktivivität, die den Sommermonsun intensivierten:

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Das grönländische Inlandeis schmilzt derzeit. Das ist auch keine große Überraschung angesichts der Erwärmung der letzten 150 Jahre nach Ende der Kleinen Eiszeit. Was allerdings nur wenige wissen ist, dass der grönländische Eisschild vor ein paar tausend Jahren viel kleiner war als heute. Eine Arbeit von Briner et al. 2016 in den Quaternary Science Reviews fasste die Entwicklung des Grönlandeises schön zusammen:

Holocene climate change in Arctic Canada and Greenland
This synthesis paper summarizes published proxy climate evidence showing the spatial and temporal pattern of climate change through the Holocene in Arctic Canada and Greenland. Our synthesis includes 47 records from a recently published database of highly resolved Holocene paleoclimate time series from the Arctic (Sundqvist et al., 2014). We analyze the temperature histories represented by the database and compare them with paleoclimate and environmental information from 54 additional published records, mostly from datasets that did not fit the selection criteria for the Arctic Holocene database. Combined, we review evidence from a variety of proxy archives including glaciers (ice cores and glacial geomorphology), lake sediments, peat sequences, and coastal and deep-marine sediments. The temperature-sensitive records indicate more consistent and earlier Holocene warmth in the north and east, and a more diffuse and later Holocene thermal maximum in the south and west. Principal components analysis reveals two dominant Holocene trends, one with early Holocene warmth followed by cooling in the middle Holocene, the other with a broader period of warmth in the middle Holocene followed by cooling in the late Holocene. The temperature decrease from the warmest to the coolest portions of the Holocene is 3.0 ± 1.0 °C on average (n = 11 sites). The Greenland Ice Sheet retracted to its minimum extent between 5 and 3 ka, consistent with many sites from around Greenland depicting a switch from warm to cool conditions around that time. The spatial pattern of temperature change through the Holocene was likely driven by the decrease in northern latitude summer insolation through the Holocene, the varied influence of waning ice sheets in the early Holocene, and the variable influx of Atlantic Water into the study region.

In der Arbeit wird auch eine Kurve der Flächenveränderungen des grönländischen Eisschildes während der vergangenen 10.000 Jahre gezeigt, basierend auf einer Modellierung von Larsen et al. 2015 (Abb. 1). Schön zu erkennen das Schrumpfen des Eises nach der letzten Eiszeit. Vor etwa 5000 Jahren erreichte das Eis dann seine geringste Ausdehnung und wuchs in der Folge bis heute wieder an. Das heutige Abschmelzen wirkt dagegen eher untergeordnet.

 

Abbildung 1: Flächenveränderungen des grönländischen Eisschildes während der vergangenen 10.000 Jahre. Graphik: Briner et al. 2016 nach einer Modellierung von Larsen et al. 2015.

 

Jason Briner war auch Teil einer Eiskernstudie von Thomas et al. 2016, die interessante Hintergrundinformationen liefert. Als das Eis vor 6000-4000 Jahren besonders stark abschmolz und die globalen Temperaturen ein Maximum erreichten, nahm auch der Schneefall zu. Warme Zeiten sind also schneereiche Zeiten in Grönland. Hier die Pressemitteilung der University at Buffalo vom 23. Mai 2016:

A history of snowfall on Greenland, hidden in ancient leaf waxes
A surprising trove of data yields indications of increased Arctic snowfall in times of warming

The history of Greenland’s snowfall is chronicled in an unlikely place: the remains of aquatic plants that died long ago, collecting at the bottom of lakes in horizontal layers that document the passing years. Using this ancient record, scientists are attempting to reconstruct how Arctic precipitation fluctuated over the past several millennia, potentially influencing the size of the Greenland Ice Sheet as the Earth warmed and cooled. An early study in this field finds that snowfall at one key location in western Greenland may have intensified from 6,000 to 4,000 years ago, a period when the planet’s Northern Hemisphere was warmer than it is today. While more research needs to be done to draw conclusions about ancient precipitation patterns across Greenland, the new results are consistent with the hypothesis that global warming could drive increasing Arctic snowfall — a trend that would slow the shrinkage of the Greenland Ice Sheet and, ultimately, affect the pace at which sea levels rise.

“As the Arctic gets warmer, there is a vigorous scientific debate about how stable the Greenland Ice Sheet will be. How quickly will it lose mass?” says lead researcher Elizabeth Thomas, PhD, an assistant professor of geology in the University at Buffalo College of Arts and Sciences who completed much of the study as a postdoctoral fellow at the University of Massachusetts Amherst. “Climate models and observations suggest that as temperatures rise, snowfall over Greenland could increase as sea ice melts and larger areas of the ocean are exposed for evaporation. This would slow the decline of the ice sheet, because snow would add to its mass,” Thomas says. “Our findings are consistent with this hypothesis. We see evidence that the ratio of snow to rain was unusually high from 6,000 to 4,000 years ago, which is what you would expect to see if sea ice loss causes snowfall to increase in the region.” The research was published on May 23 in Geophysical Research Letters, a journal of the American Geophysical Union.

Aquatic plant leaf waxes as a record of snowfall

Das Schmelzen des Grönlandeises läuft wie nicht anders zu erwarten in jahreszeitlichen Zyklen ab. Im Sommer schmilzt das Eis und im Winter wächst es dann wieder. Allerdings verschiebt sich das Niveau momentan jedes Jahr nach unten. Der winterliche Eiszuwachs kann die sommerliche Schmelze nicht voll ausgleichen. Harig & Simons haben 2016 die Entwicklung detailliert dokumentiert:

 

Abb 2. Veränderung der grönländischen Eismasse während der letzten 15 Jahre. Quelle Harig & Simons 2016.

 

Hier der Abstract der Arbeit:

Ice mass loss in Greenland, the Gulf of Alaska, and the Canadian Archipelago: Seasonal cycles and decadal trends
Over the past several decades mountain glaciers and ice caps have been significant contributors to sea level rise. Here we estimate the ice mass changes in the Canadian Archipelago, the Gulf of Alaska, and Greenland since 2003 by analyzing time-varying gravimetry data from the Gravity Recovery and Climate Experiment. Prior to 2013, interannual ice mass variability in the Gulf of Alaska and in regions around Greenland remains within the average estimated over the whole data span. Beginning in summer 2013, ice mass in regions around Greenland departs positively from its long-term trend. Over Greenland this anomaly reached almost 500 Gt through the end of 2014. Overall, long-term ice mass loss from Greenland and the Canadian Archipelago continues to accelerate, while losses around the Gulf of Alaska region continue but remain steady with no significant acceleration.

Das grönländische Inlandeis hat sich in den letzten Jahrzehnten zunehmend verdunkelt, so dass die Rückstrahlkraft (Albedo) abgenommen hat. Hierdurch verstärkt sich die Schmelze, wie Columbia University’s Lamont-Doherty Earth Observatory am 3. März 2016 per Pressemitteilung bekanntgab:

Greenland’s Ice Is Getting Darker, Increasing Risk of Melting

Greenland’s snowy surface has been getting darker over the past two decades, absorbing more heat from the sun and increasing snow melt, a new study of satellite data shows. That trend is likely to continue, with the surface’s reflectivity, or albedo, decreasing by as much as 10 percent by the end of the century, the study says. The culprits are two feedback loops that are created by the melting itself. One of those processes isn’t visible to the human eye, but it is having a profound effect.

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