A new large-scale analysis published on 9 January in Global Change Biology has revealed that plant life is increasing across the Himalayas, one of the fastest-warming regions in the world (1). The increased growth in so-called subnival vegetation — plants growing in the highest zone between the treeline and snowline — could have serious impacts, such as increased flooding.
The researchers from the University of Exeter used satellite data provided by NASA to study increases in plant life — mainly grass and small shrubs — in these remote hard-to-reach ecosystems of the Himalayas from 1993 to 2018. The study was only possible thanks to the Google Earth Engine, a freely accessible collection of government agency satellite data in the cloud.
The database has essentially “revolutionised this kind of work and enables large-scale, long time-series investigations like this to happen”, said lead author Dr Karen Anderson, who is an Associate Professor in Remote Sensing.
Until now, little was known about inaccessible subnival areas of the Hindu Kush Himalayan region. The analysis showed that, incredibly, subnival ecosystems may cover between five to 15 times the area of permanent glaciers and snow. The researchers also discovered small but significant increases in vegetation at altitudes between 4,150 to 6,000 metres above sea level. The largest increases were observed from 5,000 to 5,500 m. Whereas, conditions at the very top are considered close to the limit of where plants can grow.
While the study did not examine why these changes are occurring, the increased plant growth is consistent with models showing significant global warming in the Himalayan region over the same time period. Moreover, other studies have shown that rates of ice melt have doubled in the region since the start of the century (2).
The implications of further warming are not yet known. Scientists still don’t know much about these regions and the role they play in the water supply story. Nevertheless, the impacts could be significant, particularly since subnival vegetation appears to cover a much larger area than permanent snow and ice.
As Anderson explains: “It’s important to monitor and understand ice loss in major mountain systems, but subnival ecosystems cover a much larger area than permanent snow and ice and we know very little about them and how they moderate water supply.
Furthermore, the Hindu Kush Himalayan region — which stretches over 3500 kilometres across eight countries including Afghanistan, Bangladesh, Bhutan, China, India, Nepal, Myanmar, and Pakistan — is arguably the world’s most important ‘water tower’. Importantly, water from the snow, glaciers, and rainfall of the region feeds ten of Asia’s largest river systems, supplying 1.4 billion people with water.
“Snowfalls and melts here seasonally, and we don’t know what impact changing subnival vegetation will have on this aspect of the water cycle — which is vital because this region (known as ‘Asia’s water towers’) feeds the ten largest rivers in Asia”, says Anderson.
Studies on increased vegetation in the Arctic showed knock-on warming effects in surrounding regions. However, Anderson suggests the increased vegetation might have a cooling effect, instead, via the water that evaporates through the leaves of plants.
(1) Anderson, K. et al. Vegetation expansion in the subnival Hindu Kush Himalaya. Global Change Biology (2020). DOI: 10.1111/gcb.14919
(2) Maurer, J.M. et al. Acceleration of ice loss across the Himalayas over the past 40 years. Science advances (2019). DOI: 10.1126/sciadv.aav7266