Researchers predict that high mountain regions of Asia will be wetter in the 21st century due to changes in aerosol emissions and the continued impact of greenhouse gases, despite current drying trends, with implications for water resources for billions of people. High Mountain Asia (HMA), which includes the Tibetan Plateau and its surrounding Hindu Kush, Karakoram and Himalayas, is the third most glaciated region in the world. It is the source of more than 10 major rivers in Asia and an important water resource for nearly 2 billion people.
In recent decades, precipitation in high latitudes has shown a bipolar trend, characterized by an increase in the north and a decrease in the southeast. These changes have had a significant impact on water security and ecological balance in local and downstream areas.
Researchers from the Institute of Atmospheric Physics of the Chinese Academy of Sciences, the Pacific Northwest National Laboratory in the United States, the Max Planck Institute for Meteorology in Germany, and the Ocean University of China revealed the driving mechanisms of these precipitation changes.
But more notably, the researchers also predict that the currently dry Himalayan region will transition to wetter conditions in the 2040s under a medium-to-high greenhouse gas emissions scenario due to air pollution control measures.
The research report will be published today (October 11) in the journal Nature.
Main drivers of precipitation changes
This study focuses primarily on long-term changes in summer precipitation over a decade in the Hamat region, rather than year-to-year fluctuations. According to the study's first author, Dr. Jiang Jie of the International Institute of Atmospheric Research, summer precipitation changes at high latitudes are "anchored" to two dominant patterns: those related to westerly winds and those related to monsoons. The former increases precipitation in the northern HMA region while reducing precipitation in the southeastern region. The latter corresponds to non-phase changes between the South Asian and Southeast HMA regions.
The researchers used various evidence from climate model simulations to reveal that uneven emissions of anthropogenic aerosols over Eurasia since the 1950s have weakened the jet stream and strengthened precipitation patterns associated with westerly winds. In contrast, precipitation patterns associated with the monsoons are influenced by the Pacific Decadal Oscillation (IPO), an internal variability that fluctuates every 20 to 30 years. The most recent IPO cycle began in the late 1990s with a transition from warmer-than-normal sea surface conditions in the tropical central and eastern Pacific to cooler-than-normal sea surface conditions, resulting in increased summer monsoon rainfall in South Asia and reduced precipitation in high southeastern latitudes.
Future Forecasts and Impact
Under the combined influence of these two dominant patterns, the southeastern Himalayas have experienced an accelerated drying trend over the past two decades. However, long-term climate model projections paint a different picture, suggesting that there will be a general trend of increasing humidity throughout the 21st century in the Himalayas, including those currently drying out. It is crucial to identify the cause and timing of the transition from dryness to future wetting.
Researchers found that reduced anthropogenic aerosol emissions from clean air policies, combined with increased greenhouse gas concentrations, are responsible for the wetter trend in the Himalayas. The critical point of change in precipitation regime from "dry in the south and wet in the north" to generally wet will be mainly determined by changes in anthropogenic aerosol emissions. In contrast, the impact of greenhouse gas emissions has been the same over the past 70 years and into the future, favoring a general increase in precipitation.
Dr. Jiang said: "Analysis of observed changes in precipitation in high latitudes reveals that changes in precipitation are the result of a delicate balance between anthropogenic external forcing and internal variability (such as IPO)."
Based on climate model simulations, the researchers found that under medium and high greenhouse gas emission scenarios, human-induced moisture in the southeastern Himalayas will exceed precipitation changes caused by internal climate variability in the 2040s, and at the same time, the world will be 0.6-1.1°C warmer than now.
Professor Zhou Tianjun pointed out that future changes in precipitation patterns in high latitudes will add "great complexity" to water resource predictions in high latitudes. Therefore, he suggested it is important to understand the impact of aerosol reduction in shaping the region's climate and water resources.