Cleaner air brings a wetter high mountain Asia

High Mountain Asia Precipitation Regime Changes Driven by Cleaner Air

High Mountain Asia (HMA), which includes the Tibetan Plateau and the surrounding Hindu Kush, Karakoram, and Himalayan ranges, holds the world’s third-largest amount of glacial ice. It serves as the source of over 10 major Asian rivers and provides vital water resources for nearly 2 billion people.

In recent decades, HMA has experienced a dipolar trend in precipitation, with an increase in the north but a decrease in the southeast. These changes have significant implications for water resource security and ecological equilibrium in both local and downstream regions.

Research Findings

A collaborative study conducted by researchers from the Institute of Atmospheric Physics (IAP) of the Chinese Academy of Sciences (CAS), the Pacific Northwest National Laboratory in the U.S., the Max Planck Institute for Meteorology in Germany, and Ocean University of China has uncovered the mechanisms driving these precipitation alterations.

Furthermore, the researchers predict that due to air pollution control measures, the currently drying Himalayan region will transition to wetter conditions by the 2040s under medium to high greenhouse gas emission scenarios.

The study, titled “Precipitation regime changes in High Mountain Asia driven by cleaner air,” was published in Nature on October 11.

Long-Term Summer Precipitation Changes in HMA

The study primarily focused on long-term summer precipitation changes in HMA over a decade, rather than year-to-year fluctuations. According to Dr. JIANG Jie of IAP, the lead author of the study, summer HMA precipitation changes are influenced by two dominant patterns: a westerly-associated pattern and a monsoon-associated pattern. The former increases precipitation over the northern HMA region while decreasing it over the southeastern region. The latter corresponds to an out-of-phase variation between South Asia and the southeastern HMA region.

The researchers used evidence from climate model simulations to reveal that uneven emissions of anthropogenic aerosols in Eurasia have weakened the jet stream and reinforced the westerly-associated precipitation pattern since the 1950s. On the other hand, the monsoon-associated precipitation pattern is influenced by the interdecadal Pacific oscillation (IPO), an internal variability that fluctuates every 20 to 30 years. The recent IPO cycle, starting in the late 1990s and transitioning from warmer-than-normal to cooler-than-normal sea surface conditions in the tropical central-eastern Pacific, has led to increased summer monsoon rainfall in South Asia and reduced precipitation over the southeastern HMA region.

As a result of these two dominant patterns, a drying trend has accelerated in the southeastern Himalaya over the past two decades. However, long-term climate model projections indicate a different future scenario, suggesting a widespread trend of increased wetness over HMA throughout the 21st century, including the currently drying Himalayan region. Understanding the reasons behind this transition from drying to future wetting, as well as its timing, is crucial.

Impact of Anthropogenic Aerosols and Greenhouse Gases

The researchers found that reductions in anthropogenic aerosol emissions due to clean air policies, combined with increased greenhouse gas concentrations, are responsible for the emerging wetter trend in HMA. The tipping point in precipitation regime changes, shifting from “South Drying-North Wetting” to universal wetting, will primarily be determined by alterations in anthropogenic aerosol emissions. In contrast, the impacts of greenhouse gas emissions remain consistent in the past seven decades and the future, favoring a general increase in precipitation.

Dr. JIANG stated, “Analyzing observed changes in HMA precipitation reveals that variations are the result of a delicate balance between anthropogenic external forcing and internal variability, such as the IPO.”

Based on climate model simulations, the researchers predict that this human-induced wetting over the southeastern Himalaya will surpass the precipitation changes caused by climatic internal variability in the 2040s. This coincides with a global warming of 0.6–1.1 °C compared to the present, under medium to high greenhouse gas emission scenarios.

Prof. ZHOU Tianjun emphasized that future changes in HMA precipitation patterns will introduce “significant complexity” to projections about HMA water resources. Therefore, understanding the impact of aerosol reduction in shaping the region’s climate and water resources is crucial.

SDGs, Targets, and Indicators Analysis

1. Which SDGs are addressed or connected to the issues highlighted in the article?

• SDG 6: Clean Water and Sanitation
• SDG 13: Climate Action
• SDG 15: Life on Land

The article discusses the changes in precipitation patterns in High Mountain Asia (HMA) and the implications for water resources and ecological equilibrium. These issues are directly connected to SDG 6, which focuses on ensuring availability and sustainable management of water and sanitation for all. Additionally, the changes in precipitation patterns are driven by climate change, making it relevant to SDG 13, which aims to take urgent action to combat climate change and its impacts. The article also mentions the importance of understanding the impact of aerosol reduction on the region’s climate and water resources, which aligns with SDG 15, which focuses on protecting, restoring, and promoting sustainable use of terrestrial ecosystems.

2. What specific targets under those SDGs can be identified based on the article’s content?

• SDG 6.4: By 2030, substantially increase water-use efficiency across all sectors and ensure sustainable withdrawals and supply of freshwater to address water scarcity.
• SDG 13.1: Strengthen resilience and adaptive capacity to climate-related hazards and natural disasters in all countries.
• SDG 15.1: By 2020, ensure the conservation, restoration, and sustainable use of terrestrial and inland freshwater ecosystems and their services.

Based on the article’s content, the identified targets are relevant to addressing the issues discussed. Target 6.4 focuses on ensuring sustainable withdrawals and supply of freshwater, which is crucial for managing the changing precipitation patterns in HMA. Target 13.1 emphasizes the need to strengthen resilience and adaptive capacity to climate-related hazards, which includes understanding and mitigating the impacts of changing precipitation patterns. Target 15.1 highlights the importance of conserving and restoring terrestrial ecosystems, including freshwater ecosystems, which are affected by the changes in precipitation.

3. Are there any indicators mentioned or implied in the article that can be used to measure progress towards the identified targets?

• Indicator 6.4.2: Level of water stress: freshwater withdrawal as a proportion of available freshwater resources.
• Indicator 13.1.1: Number of deaths, missing persons, and directly affected persons attributed to disasters per 100,000 population.
• Indicator 15.1.1: Forest area as a proportion of total land area.

The article does not explicitly mention indicators, but based on the identified targets, these indicators can be used to measure progress towards the targets. Indicator 6.4.2 measures water stress, which is relevant for assessing the sustainable withdrawals and supply of freshwater in HMA. Indicator 13.1.1 measures the impact of climate-related hazards and natural disasters on human populations, which is important for assessing resilience and adaptive capacity. Indicator 15.1.1 measures the proportion of forest area, which indirectly reflects the conservation and restoration of terrestrial ecosystems, including freshwater ecosystems.

4. Table: SDGs, Targets, and Indicators

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Source: eurekalert.org

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