Karakorum Gletscher widersetzen sich dem Schmelzbefehl: Wegen Vortex oder Bewässerung?

Die Gletscher der Erde schmelzen. Viele, aber nicht alle. Und früher gab es auch schon einmal Schmelzphasen, bevor die Gletscher dann während der Kleinen Eiszeit stark wuchsen. Diesen historischen Kontext gilt es im Auge zu behalten. Im heutigen Blogartikel schauen wir uns die neuesten Ergebnisse zur Gletscherforschung in Asien an.

Wir beginnen im Karakorum. Tobias Bolch und Kollegen dokumentierten im zentralen Teil des Gebirges ein Gebiet, in dem die Gletscher seit den 1970er Jahren weitgehend stabil sind:

Brief communication: Glaciers in the Hunza catchment (Karakoram) have been nearly in balance since the 1970s
Previous geodetic estimates of mass changes in the Karakoram revealed balanced budgets or a possible slight mass gain since  ∼  2000. Indications of longer-term stability exist but only very few mass budget analyses are available before 2000. Here, based on 1973 Hexagon KH-9,  ∼  2009 ASTER and the SRTM DTM, we show that glaciers in the Hunza River basin (central Karakoram) were on average in balance or showed slight insignificant mass loss within the period  ∼  1973–2009. Heterogeneous behaviour and frequent surge activities were also characteristic of the period before 2000. Surge-type and non-surge-type glaciers showed on average no significantly different mass change values. However, some individual glacier mass change rates differed significantly for the periods before and after  ∼  2000.

Insgesamt scheinen die Gletscher im Karakoram aber zu wachsen. Wie kann das sein? Die Newcastle University meint es herausgefunden zu haben. Es ist der Vortex!

Researchers crack the ‘Karakoram anomaly’

A summer ‘vortex’ of cold air over the Karakoram mountain range is causing the glaciers in the region to grow in spite of global warming, scientists have shown.

Publishing their findings today in Nature Climate Change, the team from Newcastle University, UK, have identified a large scale circulation system — or vortex — centred over the Karakoram, a large mountain range spanning the borders of Pakistan, India, and China. In winter, the vortex affects the temperature over the whole 2,000 kilometre mountain range, but in the summer the vortex contracts and has an effect only over the Karakoram and western Pamir. This induces an anomalous cooling in summer which is different to the warming seen over the rest of the Himalaya.

Co-author Professor Hayley Fowler, says this Karakoram vortex goes some way to explaining why the glaciers in this region are behaving differently to those in most other parts of the world. “While most glaciers are retreating as a result of global warming, the glaciers of the Karakoram range in South Asia are stable or even growing,” explains Professor Fowler, Professor of Climate Change Impacts at Newcastle University. “Most climate models suggest warming over the whole region in summer as well as in winter.

“However, our study has shown that large-scale circulation is controlling regional variability in atmospheric temperatures, with recent cooling of summer temperatures. This suggests that climate models do not reproduce this feature well. “We don’t know how climate change will affect this circulation system and what the effect of sudden shifts might be. “But the circulation system is currently providing a dampening effect on global warming, reducing glacial melt in the Karakoram region and any change will have a significant effect on ice melt rates, which would ultimately affect river flows in the region.”

The Karakoram anomaly

Usually, glaciers oscillate between growth and retreat. Snow falls on the peaks and gradually compacts and turns to ice while lower down the glaciers lose ice to melting and evaporation. If snowfall equals snowmelt, the glacier is in equilibrium but global warming has tipped the balance so that most of the world’s glaciers are shrinking. The Karakoram anomaly was first described in 2005 and since then, scientists have been trying to determine what might be causing the expansion of glaciers in the region – which includes the world’s second largest mountain K2.

Acting like a counter-weighted temperature control, the unique summer interaction of the Karakoram vortex and the South Asian Monsoon causes temperatures in the Karakoram and Pamir to cool while those in the Central and Eastern Himalaya are warming, and vice versa. Over recent decades, these vortex-monsoon interactions have resulted in stormier conditions over the Karakoram. “This vortex provides an important temperature control,” explains Newcastle University’s Dr Nathan Forsythe, lead author of the study.

“It is therefore important to look at how it has changed and influenced temperature over the last century so we can better understand how a change in the system might affect future climate. “This is of huge importance in terms of food security because of the large populations that rely on water resources from snow and ice melt from the mountainous catchments to grow their irrigated crops in the Indus Plains of the Sindh and Punjab states and provinces of Pakistan and India.”

Andere Forscher haben andere Theorien. Eine Gruppe um Remco de Kok erklärte im Februar 2018, dass vielleicht die künstliche Bewässerung im Tarimbecken zusätzliches Wasser in die Atmosphäre einbringt, die dann als Schnee die Gletscher wachsen lässt:

Irrigation as a Potential Driver for Anomalous Glacier Behavior in High Mountain Asia
Many glaciers in the northwest of High Mountain Asia (HMA) show an almost zero or positive mass balance, despite the global trend of melting glaciers. This phenomenon is often referred to as the “Karakoram anomaly,” although strongest positive mass balances can be found in the Kunlun Shan mountain range, northeast of the Karakoram. Using a regional climate model, in combination with a moisture‐tracking model, we show that the increase in irrigation intensity in the lowlands surrounding HMA, particularly in the Tarim basin, can locally counter the effects of global warming on glaciers in Kunlun Shan, and parts of Pamir and northern Tibet, through an increase in summer snowfall and decrease in net radiance. Irrigation can thus affect the regional climate in a way that favors glacier growth, and future projections of glacier melt, which may impact millions of inhabitants surrounding HMA, will need to take into account predicted changes in irrigation intensity.

Bereits Wang et al. 2017 hatten verstärkte Regenfälle als Antrieb des Gletscherwachstums in den Pamir- und Hindukusch-Bergen ausgemacht.

Im September 2017 erinnerte die Times of India, dass die Gletscherschmelze kein einzigartiges Phänomen der letzten 70 Jahre ist, sondern bereits vor 400 Jahren einsetzte:

Himalayan glaciers melting for 400 years, finds BSIP study[...] the phenomenon of Himalayan glaciers melting is not a recent one. In fact, it has been happening for 400 years. It was during the Little Ice Age, a period of cold conditions from 1645 CE-1715 CE, that Arctic and sub-Arctic glaciers were expanding but Himalayan glaciers were melting.

Weiterlesen in der Times of India

Die neuesten Gletscherbilanzen aus der Himalaya-Region ab 2000 haben Brun et al. 2017 veröffentlicht:

We calculate a total mass change of −16.3 ± 3.5 Gt yr−1 (−0.18 ± 0.04 m w.e. yr−1) between 2000 and 2016, which is less negative than most previous estimates.