In a study of the history of Asian monsoons, Hai Cheng and others conclude that insolation (incoming solar radiation) had a demonstrable effect on monsoons during the past 640,000 years.* We think of monsoons as seasonal wind and rain associated with semi-permanent low-pressure systems. Those rain events over India, for instance, occur when the Northern Hemisphere counterclockwise rotation around a Low sits over the Arabian Sea and Indian Ocean, spinning moist air inland. The maritime air mass, moist and warm, rises to get over the Deccan Plateau, affecting western India, or over the Himalayas, affecting eastern India and Bangladesh. As it rises, maritime air loses its moisture to condensation, and rains its moisture onto the landscape. The influx of rain frequently causes devastating floods.
Floods caused by monsoons in June, 2016, killed more than 100 people. In the states of Arunachal Pradesh, Assam, Bihar, Madhya Pradesh, Maharashtra, Odisha, Uttar Pradesh and Uttarakhand the rising water also killed more than 50,000 cattle. Flooding during the event destroyed about 3,000 homes and damaged more than 300,000. The effects of such flooding are obviously local; the cause is distant.
That the sun has influenced Asian monsoons for more than a half million years is an important discovery. It shows us how our planet’s hydrologic cycle, distribution of surface energy, and the breakdown or redistribution of materials are sun-dependent. Forces external to Earth can exert influences with both beneficial and dire consequences. We need the sun’s energy because we convert it through plants to food. That same energy can, in the absence of rain, dry out and destroy crops and desertify a landscape. We need the sun to evaporate ocean water so that the atmosphere can carry it to the land, but too much water presents its own set of problems.
Each of us is an Earth analog. We can’t exist in a vacuum. External influence, though intermittent, alters the topography of our lives, at times for good and at other times for bad. The reality of human life is that external human influences are like the sun’s energy: Inescapable, necessary at times, and somewhat variable. Each of us bears some mark of past influence, of ideas that have washed over us. Inculcation suffuses who we are and, like floodwater, permeates our mental homes. Our only partial escape is to head to high ground. It’s only above a floodplain where we can find individuality and independence.
All of us are products of past floods caused by monsoons no more in our personal control than seasonal rains are in the control of people in the Indian subcontinent. Fortunately, we know that although we have no ability to alter the sun’s activities past or present, we do have the ability to move off the floodplain and to build a flood-control system. Personal levees and dams lessen the potential for damage or negative influence.
Monsoons are historically seasonal, and that’s good news. Though less regular, human influences, like the sun’s effect on monsoons, might be traceable into our personal pasts. That, too, is good news. We might not be able to stop completely such influence, but we do ourselves some good by understanding what has caused either flooding or drought in our lives. The knowledge prepares us for seasons to come.
* Hai Cheng, R. Lawrence Edwards, Ashish Sinha, Christoph Spötl, Liang Yi, Shitao Chen, Megan Kelly, Gayatri Kathayat, Xianfeng Wang, Xianglei Li, Xinggong Kong, Yongjin Wang, Youfeng Ning & Haiwei Zhang, The Asian monsoon over the past 640,000 years and ice age terminations. Nature 534, 640–646 (30 June 2016) doi:10.1038/nature18591. The authors published a revision in January, 2016, showing more exact data for the last 400,000 years and extrapolated data for the period between 400,000 and 640,000 years ago, but they argued that their conclusions are still valid for the entire period in question. http://www.nature.com/nature/journal/v534/n7609/full/nature18591.html