Traces of industry in Asia’s water tower Study: How greenhouse gases and air pollution are altering the ecosystem on the Tibetan Plateau
The Tibetan Plateau is often referred to as the ‘water tower of Asia’. It stores and regulates water that supplies almost two billion people with fresh water. Its numerous lakes respond sensitively to climate change and preserve long-term environmental information in their sediments. A study by an international research team led by Technische Universität Braunschweig now shows that greenhouse gas emissions and industrial air pollution have significantly altered the sensitive ecosystem of the lakes over the past millennium. To this end, the scientists analysed, amongst other things, sediments from Lake Nam Co. The findings have now been published in the journal “Nature Communications”.
Lake Nam Co, with the Nyainqêntanglha Mountains in the background. Picture credits: Wengang Kang
The Tibetan Plateau, together with the Hindu Kush–Karakorum–Himalaya region, has more snow and ice than any other region on Earth, apart from the polar regions. As a result, this high-altitude region is particularly sensitive to climate change, making it of special importance when analysing the impacts of climate change. In recent years, researchers from the DFG Research Training Group TransTiP have been investigating changes in the region’s geo-ecosystems.
“Our results decipher the complex interplay of natural and anthropogenic climate drivers, to which even lake ecosystems in remote regions react highly sensitively, and whose impacts are archived in lake sediments. Furthermore”, says Professor Antje Schwalb, head of the Institute of Geosystems and Bioindication and spokesperson of the DFG Research Training Group TransTiP. “I am pleased that the curiosity and perseverance with which our former doctoral researcher Wengang Kang advanced this topic have now been rewarded with a publication in Nature Communications, and that Beijing remains connected with Braunschweig.”
Reconstructing past climate and environmental changes
Sediment sampling on Lake Nam Co. Picture credits: Weng Kang
To investigate how natural and human-induced climatic factors have shaped the lake ecosystems on the Tibetan Plateau, the researchers used lake sediments to reconstruct environmental and ecological changes in Nam Co, the third-largest lake in the region. The lake, which covers an area of around 2,000 square kilometres, is situated at an altitude of 4,720 metres above sea level. Geochemical indicators, including titanium, were used to reconstruct fluctuations in South Asian monsoon rainfall, whilst fossilised diatoms and pigments documented the long-term ecological responses within the lake.
“This approach provided a detailed overview of how the lake’s ecosystem responded to past climate and environmental changes,” says Dr Wengang Kang, a former doctoral researcher in the TransTiP Research Training Group and lead author of the study.
In addition, our international research team employed a so-called ‘climate fingerprinting’ approach, led by Céline Bonfils at Lawrence Livermore National Laboratory, USA. To this end, the researchers analysed simulations from several Earth system models that replicate climate trends over the past 1,000 years. This enabled them to distinguish natural climate fluctuations from changes caused by external influences. This made it possible to determine the extent to which factors such as volcanic eruptions, changes in the Earth’s orbit, greenhouse gases and man-made air pollution have contributed to the observed climate fluctuations.
Less ice, more glacial meltwater
The study shows that two major climatic processes in particular have had a decisive influence on the water balance of the Tibetan Plateau and the lake ecosystems over the past one thousand years.
The first process relates to temperature. Before industrialisation, volcanic eruptions cooled the climate repeatedly, thereby influencing the ecological conditions of the lakes. Since the 19th century, however, global warming caused by greenhouse gases has fundamentally altered this mechanism: the lake is covered by ice for shorter periods, and more glacial meltwater flows into it. This has triggered ecological changes: for example, a significant restructuring of diatom communities and the emergence of ecological conditions that were not observed during earlier warm periods.
The second process focuses on precipitation. Changes in the position of the Intertropical Convergence Zone (ITCZ) – an elongated low-pressure system near the equator – determined how much monsoon rainfall reached the Tibetan Plateau. In doing so, they influenced the salinity of the lake and the composition of the diatoms, which are highly sensitive to changes in salinity. In the pre-industrial era, this mechanism was primarily driven by volcanic eruptions and changes in the Earth’s orbit, which affected the climate in different ways. During the industrial era, human-induced sulphate aerosols from Europe and North America – produced, for example, by the combustion of coal and oil – caused this zone to shift exceptionally far south. In the mid-20th century, this triggered the most severe drought that the researchers were able to identify in their reconstruction. Stricter air pollution control regulations and the warming caused by greenhouse gases, which has persisted since the 1970s, ensured that precipitation levels recovered rapidly and the salinity of the lakes fell sharply.
From the past, through the present, to the future
“Our findings provide evidence that human activities are fundamentally reshaping some of the world’s most remote and climate-sensitive ecosystems. Furthermore, our findings suggest that these changes are likely to persist as global warming, glacier loss and hydroclimatic shifts intensify,” emphasises Dr. Wengang Kang.
Future research
Using deep drill cores from Nam Co obtained as part of an international drilling campaign, the researchers can now further investigate how climatic influences and lake ecosystems interacted over several glacial–interglacial cycles. At the same time, future research will aim to link the reconstruction of past ecosystems with current observations and climate projections. In doing so, the scientists are drawing a line from the past through the present into the future in order to better understand ecosystem changes. This could also improve predictions of how Tibetan lakes might respond to ongoing warming, glacier retreat and changing monsoon dynamics.