Left: Map from from Liu et al. 2020. Black arrows: present-day wind pattern. Insert: The Asian Monsoon system. Circled numbers: the three strategic cave sampling sites along the monsoon trajectory. The stars mark caves from which previous δ18O records have been sampled.
The Asian monsoon is the world’s largest weather system and affects the lives and livelihoods of about half the world’s population. Current human-induced climate change is affecting the Asian monsoon and causing it to change in various ways, with potentially huge socioeconomic consequences. The key to understanding and predicting its future behaviour lies in historical records of past responses of the Asian monsoon to natural climate variability, like cycles of glaciation. In the latest issue of Science Advances, current and past ASE/EOS researchers Dr Liu Guangxin, Dr Lu Yanbin, Dr He Shaoneng, Dr Yuan Shufang, Dr Chiang Hong-Wei, Dr Lin Thu Aung, Mr. Phyo Maung Maung and Assoc Prof Wang Xianfeng (PI of the Isotope Geochemistry Lab ) shed new light on a previously mysterious lack of response to past ice ages in Asian monsoon climate records. Through creating new records from strategic locations in Myanmar, Thailand and Southwestern China, they show that as a monsoon proxy, the oxygen isotope ratio (δ18O) is a) more sensitive to glacial climate in coastal areas compared to inland; and b) the temporal change in coastal areas corresponds better to real monsoon change. They term the spatial sensitivity the ‘Moisture transport pathway effect’, and use the temporal changes from the coastal record to calculate more accurate Asian Monsoon changes in Southeast Asia.
Oxygen isotope ratios (δ18O) in the layers of speleothem or cave deposits are a looking glass into the past, revealing rainfall patterns, in the case of this study, as far back as the time of the first fossils of our species, or two ice ages ago (to be more precise, 180,000 years ago). During glacial periods in the earth’s history, multiple records show signs of lower global average temperature and lower sea level as glaciers and polar ice caps advance and hold more water. During inter-glacial periods, temperatures and sea levels rise. Historic oxygen isotope records from cave deposits in several locations in South China show some expected variability of the Asian monsoon but little change in response to the last glacial maximum about 20,000 years ago, and this has puzzled climate scientist.
Dr Liu and co-authors measured oxygen isotopes in speleothems at sites that follow the course of the Indian summer monsoon (an important branch of the Asian monsoon) from water source (coast) to inland areas. They found that the records show decreasing δ18O values inlandwards due to progressive rainout effect. More importantly, they found that records close to the monsoon moisture source show large glacial-interglacial variability, which then decreases landward. A larger temperature gradient from coast to inland could be an explanation, so could suppression of water release through plant transpiration due to colder climate and less vegetation. Either way, this ‘moisture transport pathway effect’ detected by Liu and co-authors counteracts the effect of an ice age on the Indian summer monsoon and needs to be incorporated into climate models for more accurate future predictions.