Klimamodellierer stehen im Regen: Das unerwartete Comeback des Indischen Monsunregens

Waren die Niederschläge in Indien und Nachbargebieten in vorindustrieller Zeit stets ausreichend verfügbar, pünktlich und zuverlässig? Wenn man die Geschichten der Klimahardliner hört, könnte man fast auf den Gedanken kommen, vor 1850 hätten allerorten klimatisch himmlische Zustände geherrscht, die der Mensch dann mutwillig zerstört habe. Wir schauen heute in diese vermeintlich so paradiesische Zeit hinein.

Ergebnisse der University of Cambridge aus dem Januar 2017 lassen Zweifel aufkommen. Forscher studierten alte Indus-Kulturen vor 4000 Jahren und stellten heftige natürliche Klimaschwankungen fest, die in einigen Zeitabschnitten zu schlimmen Dürrephasen führten. Hier die Pressemitteilung aus Cambridge:

The Ancient Indus Civilization’s Adaptation to Climate Change

Published in Current Anthropology, a new article explores how an ancient culture dealt with variable environments.

With climate change in our own era becoming increasingly evident, it’s natural to wonder how our ancestors may have dealt with similar environmental circumstances. New research methods and technologies are able to shed light on climate patterns that took place thousands of years ago, giving us a new perspective on how cultures of the time coped with variable and changing environments.

A new article in the February issue of Current Anthropology explores the dynamics of adaptation and resilience in the face of a diverse and varied environmental context, using the case study of South Asia’s Indus Civilization (c.3000-1300 BC). Integrating research carried out as part of the Land, Water and Settlement project — part of an ongoing collaboration between the University of Cambridge and Banaras Hindu University — that worked in northwest India between 2007 and 2014, the article looks at how Indus populations in north-west India interacted with their environment, and considers how that environment changed during periods of climate change. Lead author, Dr. Cameron Petrie of the Division of Archaeology, University of Cambridge notes that “for most ancient complex societies, water was a critical factor, and the availability of water and the way that it was managed and used provide critical insight into human adaptation and the resilience of subsistence practices.”

Most early complex societies developed in regions where the climatic parameters faced by ancient subsistence farmers were varied, but not especially diverse. The Indus Civilization developed in a specific environmental context, where the winter and summer rainfall systems overlapped. There is now evidence to show that this region was subject to climate change during the period when the Indus Civilization was at its height (c.2500-1900 BC). The Indus Civilization therefore provides a unique opportunity to understand how an ancient society coped with diverse and varied ecologies and change in the fundamental and underlying environmental parameters.

In the early Holocene, the Indus Civilization was situated in proximity to Kotla Dahar, a deep lake, implying regular and consistent rainfall input to offset evaporation, which given its location, would have been primarily monsoonal. The lake showed evidence for two dramatic decreases in monsoon rainfall and a progressive lowering of the lake level. The second of these shows Kotla Dahar becoming completely ephemeral ca. 2200-2000 BC as a result of an abrupt weakening of the monsoon, and the weakening of the monsoon is visible in speleothem records in Oman and northeast India. The proximity of the Kotla Dahar record to the area occupied by Indus populations shows that climate must be formally considered as a contributing parameter in the process of Indus deurbanization, at least in the context of the plains of northwest India.

It has long been hypothesized that there was variation in the subsistence practices used by Indus populations and this fits with the theme of coping with diverse environments. Petrie comments that “we argue that rather than being forced to intensify or diversify subsistence practices in response to climatic change, we have evidence for the use of millet, rice, and tropical pulses in the pre-urban and urban phases of the Indus Civilization. This evidence suggests that local Indus populations were already well adapted to living in varied and variable environmental conditions before the development of urban centers. It is also possible that these adaptations were beneficial when these populations were faced with changes to the local environment that were probably beyond the range of variation that they typically encountered.”

Paper: Cameron A. Petrie, Ravindra N. Singh, Jennifer Bates, Yama Dixit, Charly A. I. French, David A. Hodell, Penelope J. Jones, Carla Lancelotti, Frank Lynam, Sayantani Neogi, Arun K. Pandey, Danika Parikh, Vikas Pawar, David I. Redhouse, Dheerendra P. Singh. Adaptation to Variable Environments, Resilience to Climate Change: Investigating Land, Water and Settlement in Indus Northwest India. Current Anthropology, 2017; 1 DOI: 10.1086/690112

Im Juli 2017 folgte dann eine Studie von Singh und Kollegen zu Dürren aus dem indischen Himalayagebiet anhand von Baumringen. Auch hier wird wieder die große natürliche Variabilität bei den Regenmengen in der Region deutlich:

Tree ring drought records from Kishtwar, Jammu and Kashmir, northwest Himalaya, India
Droughts in semi-arid and arid regions of the northwest Himalaya are very common causing distress to socioeconomic systems. Our understanding on natural variability in droughts in the northwest Himalaya in long-term perspective is limited largely due to paucity of observational and high-resolution proxy records. We developed a 275-years (A.D. 1740–2014) long Standardized Precipitation Index (eight months SPI of May, SPI8-May) reconstruction using ring-width chronology of Himalayan cedar (Cedrus deodara (Roxb.) G. Don) from Kishtwar, Jammu and Kashmir in the northwest Himalaya, India. The most conspicuous feature of reconstruction is pluvial 1950s, 1990s and dry 1970s. The wettest phase of 1990s is followed by a distinct drying since 2000s in Kishtwar. The reconstructed SPI8-May series showed very good consistency with tree–ring-based upper Indus basin discharge and gridded summer (June–July–August) PDSI data of the northwest Himalaya–Karakoram region. Such a consistency in SPI8-May, Indus discharge and summer PDSI in westerly dominated region of the Himalaya–Karakoram region underscores potential utility of SPI reconstructions in understanding climate change over the region in long-term perspective.

Und wie steht es momentan um den Indischen Sommermonsunregen? Überraschenderweise hat der sich seit 2002 wieder verbessert, wie Jin & Wang im Juli 2017 in Nature Climate Change berichteten. Die Autoren stellen zudem nüchtern fest, dass die gängigen Klimamodelle das jüngste Erstarken der Regenfälle nicht nachvollziehen können, wobei sie die Prognosefähigkeit dieser Modelle in Frage stellen.

A revival of Indian summer monsoon rainfall since 2002
A significant reduction in summer monsoon rainfall has been observed in northern central India during the second half of the twentieth century, threatening water security and causing widespread socio-economic impacts. Here, using various observational data sets, we show that monsoon rainfall has increased in India at 1.34 mm d−1 decade−1 since 2002. This apparent revival of summer monsoon precipitation is closely associated with a favourable land–ocean temperature gradient, driven by a strong warming signature over the Indian subcontinent and slower rates of warming over the Indian Ocean. The continental Indian warming is attributed to a reduction of low cloud due to decreased ocean evaporation in the Arabian Sea, and thus decreased moisture transport to India. Global climate models fail to capture the observed rainfall revival and corresponding trends of the land–ocean temperature gradient, with implications for future projections of the Indian monsoon.

Die Variabilität der ostasiatischen Monsunregenfälle war im Oktober 2017 auch Thema bei Chiang und Kollegen.