Forests replace glaciers in the Himalaya

[caption id="attachment_54080" align="alignnone" width="900"] Panoramic view of Manang Village and the Gangapurna and Annapurna III Glaciers that have retreated dramatically since 1980. Photo: Solundir

When Swiss geologist Toni Hagen arrived in Nepal, the Ranas were still ruling the country. Throughout the 1950s, Hagen traversed the country on foot from east to west, from north to south, examining rock strata to probe the orogenesis of the Himalaya.

Along the way, he took thousands of photographs and film clips that remain a valuable visual archive of a time when Nepal was emerging from historical isolation. Today, the photographs are a treasure trove for climate researchers to measure the extent of glacial retreat in the Himalaya.

[caption id="attachment_54105" align="alignleft" width="250"] Shalik Ram Sigdel, the Nepali scientist who conducted the study of melting glaciers and new forest cover in Manang.

Repeat photography has allowed Nepali climate scientists like Shalik Ram Sigdel of the Institute of Tibetan Plateau Research at the Chinese Academy of Sciences to plot the rapidity of Himalayan melting due to global warming in the past century.

When Toni Hagen trekked through Manang in 1957, he climbed a slope above the village to take photographs of the northern flank of the Annapurnas. Even at that time, the Gangapurna Glacier was already debris-covered, meaning that it had lost most of its surface ice.

But when Sigdel visited the area and compared it to more recent photographs, he found the glacier snout had receded 900m up the mountain, leaving a lake at its terminus bracketed by sharply defined lateral moraines. The Marsyangdi River has found a way around this feature.

Toni Hagen’s photograph in 1957 of the debris-covered Gangapurna Glacier termius with a few supraglacial ponds, and the same view in 2010 by Nabin Baral showing the growing glacial lake and vegetation on the terminal moraine. Photos: Toni Hagen and Nabin Baral/thethirdpole.net

Before-and-after photographs by Nabin Baral from Manang village of the Gangapurna Glacier and the glacial lake in 2007 and 2018 showing the dramatic retreat of the glacier and increased forest cover on the slopes. Photos: Nabin Baral/thethirdpole.net

In a paper titled Retreating Glacier and Advancing Forest Over the Past 200 Years in Central Himalayas published on last week in the Journal of Geophysical Research - Biogeosciences, Sigdel with co-authors Sher Muhammad of ICIMOD, Hui Zhang, Haifeng Zhu and Eryuan Liang present data on the analysis of glacial debris deposits and tree-rings on the Gangapurna and Annapurna III glaciers in Manang.

[caption id="attachment_54081" align="alignright" width="600"] Google Earth images (south is up) of the Annapurna III Glacier (left) and the Gangapurna Glacier (right).

The paper presents the first detailed evidence of fluctuations in Himalayan glaciers over the past 200 years, and concludes that snowlines have been rising and forests advancing higher, and they did so in spurts since 1790s with the process intensifying since 1980.

While glaciers have been shrinking and snowlines have been naturally receding ever since the Last Glacial Period 12,000 years ago and the Little Ice Age 700 years ago, this process has amplified with fossil fuel burning after the Industrial Revolution.

The researchers used remote sensing satellite imagery and field research to find that as the Gangapurna and Annapurna III glaciers shrank and receded, birch, pine and juniper have been moving up the mountain since 1808.

For instance, in 1988 the Gangapurna Glacier was 9km long and 17.74km² in area, and by October 2019 its length had receded to 8.05km and area was down to 17.5km². They also found that the thickness of the ice in the glaciers had also shrunk considerably.

“Just in the past 60 years, the Gangapurna glacier has receded by more than 900m,” Sigdel said in an interview. “And the speed at which they are receding has accelerated.”

[caption id="attachment_54098" align="alignleft" width="500"] Satellite imagery of the Gangapurna Glacier (top) and the Annapurna III Glacier in 1988, 2008 and 2019 showing that the rate of glacial retreat is accelerating.

Indeed, the paper says that while the length of the Gangapurna Glacier reduced by 386m at 17.5m per year between 1957 to 1979, the speed picked up to 24m per year from 1979 to 2016 – receding by a full 903m in that time. The Annnapurna III Glacier receded at a slower pace and was smaller by 500m in the past 65 years due to the different gradient and size of the catchment.

As the ice melted, vegetation started creeping up the mountains. Sigdel, who specialises in understanding treeline response to a changing climate in the Himalaya, examined tree-rings and the vegetation that have colonised the moraines in the past decades.

As shown in the before-and-after photographs of Toni Hagen’s images as well as through remote-sensing data, older moraines and those that were deposited after the Little Ice Age are already covered by dense forest, presenting evidence of forests replacing glaciers in the Himalaya.

[caption id="attachment_54082" align="alignnone" width="1000"]Repeat photography of the Gangapurna Glacier terminus in 1957 by Swiss geologist Toni Hagen, in 1979 by Czech photographer Zdeněk Thoma. And a view of the glacier from Manang village taken by Zdeněk Thoma in 1979 and the same view in 2016. Contour map of Upper Manang showing the location of the glacier terminus in those years.

Being darker, the forests also absorb more sunlight than snowfields that earlier reflected light, and together with transpiration, this feedback loop in turn could amplify the warming trend in the Himalaya, the paper concludes.

While the warming may favour timber and agriculture production in Nepal, Sigdel says they threaten the dry season water flow in Himalayan rivers that will have consequences downstream.

He adds: “Our research confirms that the climate crisis has been seriously impacting water resources in the Himalaya. The Gangapurna and Annpurna III glaciers feed the Marsyangdi River, and their melting will affect the entire basin, endangering water supply and affecting hydropower generation and irrigation downstream.”