Increased Precipitation in High Mountain Asia Anticipated for 21st Century – Affecting Water Supply for Billions

Humidity from the Indian Ocean funneled through the Yarlung Tsangpo Grand Canyon to the Tibetan Plateau, captured in Medog, China. Attribution: Weibiao Li

Despite present drying tendencies, scientists forecast a more humid 21st century for High Mountain Asia due to changes in aerosol emissions and stable greenhouse gas influences, posing consequences for the water supply of billions of people.

High Mountain Asia (HMA), which includes the Tibetan Plateau and neighboring Hindu Kush, Karakoram, and Himalayan mountain ranges, contains the globe’s third-largest glacial ice reserve. This region serves as the origin of over 10 major rivers in Asia and is crucial for the water needs of nearly 2 billion individuals.

In recent years, there has been a bipolar shift in HMA’s precipitation levels, marked by rising levels in the northern area but diminishing ones in the southeast. Such alterations carry substantial ramifications for water security and ecological balance in both proximate and downstream zones.

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Research Outcomes and Projections

A collaborative team from the Institute of Atmospheric Physics (IAP) of the Chinese Academy of Sciences (CAS), the Pacific Northwest National Laboratory in the United States, Germany’s Max Planck Institute for Meteorology, and Ocean University of China have elucidated the factors governing these changes in precipitation.

Of particular note, the team anticipates that, owing to pollution abatement initiatives, the presently arid Himalayan area will experience heightened humidity by the 2040s under medium to elevated greenhouse gas emission frameworks.

The research is slated for publication today (October 11) in the scientific journal Nature.

Key Factors Influencing Precipitation Shifts

The primary focus of the research was on enduring changes in summer precipitation in HMA over a decade-long period, as opposed to annual variances. According to Dr. Jie Jiang from IAP, the principal author of the study, changes in summer precipitation in HMA are primarily regulated by two overriding patterns: one associated with westerly winds and another tied to the monsoon. The former elevates precipitation in the northern HMA and reduces it in the southeast, whereas the latter shows an inverse relationship between South Asia and the southeastern HMA.

Through climate model simulations, the researchers identified that irregular anthropogenic aerosol emissions across Eurasia have weakened the jet stream since the 1950s, thereby bolstering the westerly wind-associated precipitation pattern. Conversely, the monsoon-related pattern is affected by the Interdecadal Pacific Oscillation (IPO), a natural variability that alternates approximately every 20 to 30 years. The recent cycle of the IPO, initiated in the late 1990s, has brought about enhanced monsoon rainfall in South Asia and decreased rainfall over the southeastern HMA.

Projections and Future Consequences

Impacted by these two main patterns, there has been an accelerated drying trend in the southeastern Himalayas for the past two decades. Nevertheless, long-term climate models suggest an overall increase in moisture levels across HMA during the 21st century, including the areas currently experiencing drying. Understanding the factors and timing of this transition from a drying to a wetter state is essential.

The scientists ascertained that cutbacks in anthropogenic aerosol emissions due to cleaner air regulations, along with the rise in greenhouse gas levels, account for the impending increase in HMA humidity. The pivotal change in precipitation regimes, from a “South Drying-North Wetting” model to an all-encompassing humid condition, will primarily hinge on shifts in human-caused aerosol emissions. Greenhouse gas impacts remain consistent over the past seven decades and into the future, advocating a general rise in precipitation levels.

Dr. Jiang concluded that variations in HMA precipitation result from a nuanced interplay between human-induced external triggers and natural internal factors such as the IPO.

Climate model simulations indicate that by the 2040s, anthropogenic influences will supersede internal climatic variations in terms of their effect on southeastern Himalayan precipitation. This will occur alongside global temperature increases ranging from 0.6–1.1 °C compared to present conditions, under medium to high emission scenarios.

Prof. Tianjun Zhou emphasized that future shifts in HMA precipitation will add considerable intricacy to water resource projections for the region. He thus posited that understanding the role of aerosol reductions in shaping the area’s climatic and water conditions is imperative.

Reference: “Alterations in High Mountain Asia Precipitation Driven by Air Quality Improvement,” 11 October 2023, Nature.
DOI: 10.1038/s41586-023-06619-y

Frequently Asked Questions (FAQs) about High Mountain Asia precipitation

What is the main focus of the study published in Nature?

The study focuses on forecasting increased precipitation in High Mountain Asia (HMA) during the 21st century. This change is attributed to shifts in aerosol emissions and consistent greenhouse gas effects. The study examines the impact of these changes on the water resources that nearly 2 billion people rely on.

Who conducted the research?

The research was conducted by a collaborative team from the Institute of Atmospheric Physics (IAP) of the Chinese Academy of Sciences, the Pacific Northwest National Laboratory in the United States, Germany’s Max Planck Institute for Meteorology, and Ocean University of China.

What geographical regions are primarily affected?

The geographical regions primarily affected are High Mountain Asia, encompassing the Tibetan Plateau and the surrounding Hindu Kush, Karakoram, and Himalayan mountain ranges. These regions are vital sources for more than 10 major Asian rivers.

What are the primary factors driving changes in precipitation?

The main factors driving changes in precipitation are anthropogenic aerosol emissions and greenhouse gas concentrations. The study also identifies two dominant patterns affecting precipitation: a westerly-associated pattern and a monsoon-associated pattern.

When are these changes expected to occur?

The changes are expected to transition the currently drying Himalayan region to wetter conditions by the 2040s, under medium to high greenhouse gas emission scenarios.

What are the implications of these changes?

The implications are significant for water resource security and ecological balance. Nearly 2 billion people depend on the water resources originating from this region. The research emphasizes the need to understand the impact of aerosol reductions and greenhouse gas concentrations on the region’s climate and water resources.

What is the role of aerosol emissions in these changes?

The role of aerosol emissions is pivotal. Reductions in anthropogenic aerosol emissions due to cleaner air policies are responsible for the emerging wetter trend in High Mountain Asia.

How do greenhouse gas effects contribute?

The greenhouse gas effects remain consistent over the past seven decades and into the future, advocating for a general increase in precipitation across the region.

What does the term “bipolar shift” mean in the context of this study?

The term “bipolar shift” refers to the recent trend in HMA’s precipitation levels, where there has been an increase in the north but a decrease in the southeast. This change has implications for both local and downstream water security.

Is the study peer-reviewed?

Yes, the study is peer-reviewed and is slated for publication in the scientific journal Nature.