Across the Himalaya mountains, in the inner land of the Tibet Plateau, you find the source region of the Yangtze River, the origin of the third-longest river in the world. It is a cold, barren, isolated area in the mountains where it is difficult for plant, animal, human or even observational equipment to survive. Climate change is raising more and more problems in this area, such as unstable conditions of grassland, decreasing number of plants, melting of glaciers, and so on. The local government struggles to balance economic development and environmental protection. To be able to take the right actions, they first need to know key factors for climate change: rainfall. This is where our study can help.
How will rainfall change from the past to the future? First, we detected a trend of climate variables for the past 50 years and found that rainfall amount in this area increased significantly. Water storage was constant despite a continuous small negative trend. To investigate the future, climatologists provide us simulation results under different greenhouse gas emission scenarios. After comparing and analysing the future rainfall in this area, huge increase was revealed by our analysis. If we take intermediate control of the greenhouse gas emission, the rainfall may increase by 25% by the end of 21st century. If we take no control of the greenhouse gas emission, the increase will double. A far-reaching plan is needed on the top list of the local governments.
Beside the long-term change, the rainfall varies year to year. As rainfall comes from water vapor in the atmosphere, wind and temperature of land or ocean surface can influence the rainfall by changing the water vapor circulation, sometimes even at a considerable distance, which we call teleconnection. We found that two teleconnection patterns show a close relation with summer rainfall in our study area: El Niño–Southern Oscillation (ENSO) and Pacific Decadal Oscillation (PDO). These patterns are indices for the surface thermal condition of the Pacific Ocean. We found that more rainfall occurs in our study area during cold PDO years, particularly when cold PDO and cold ENSO coincide. This means 6% more rainfall than usual. In contrast, warm PDO brings dryness to the area, and warm ENSO makes it even drier, with up to 10% less than average rainfall. When cold PDO occurs with warm ENSO or the opposite happens, warm PDO with cold ENSO, their influence compensates, and the change in rainfall is marginal. The reason behind is, the combination of ENSO and PDO affects the strength of the wind that transports moisture to our study area and the intensity of ascending movement. Such information is extremely useful since it offers us a way of predicting a changing future. According to many studies, a cold PDO phase may continue until 2050. Also, extreme El Niño (warm ENSO) and La Niña (cold ENSO) events both tend to increase due to greenhouse gas emissions. This means that we can expect more extreme ENSO years combined with a cold PDO phase in the future. In this scenario, more rainfall is likely to fall in the study area, especially during extreme La Niña years.
Our findings provide insights to improve the understanding of rainfall change. The considerable increase of future rainfall provides local government with guidance for adaptive solutions. The rainfall variation in relation to teleconnections can help to improve rainfall forecasting at a low-cost level. With such information, the local authorities can prepare the study area for rainfall change with higher accuracy, lower budget, and localized suitability.