Deciphering Monsoons in a Time of Drastic Changes

As summer settles onto the landmass once ruled by Genghis Khan, a string of events across the globe begins. Temperatures increase, glaciers shrink, and the annual monsoon cycle commences. The annual creation and eventual melt of the glaciers on the Tibetan Plateau birth the monsoon winds. In the summer the Tibetan Plateau heats quickly, drawing in the warm, moist ocean air, beginning the monsoon. In the winter the land of the plateau cools quickly, pushing cool air out into the warmer ocean air, ending the monsoon. Created by the convergence of two continental tectonic plates, the Tibetan Plateau is the highest and largest flat, open space on the globe. No other place on Earth is as large with such seasonal changes in temperature as is the Tibetan Plateau.

Nor does any place influence the global climate as much: The drastic temperature shifts influence most of the world's monsoons. Researchers have studied monsoons since the 16th century, when geographer Richard Hakluyt first coined the term in reference to the seasonal global shift in wind and rain patterns. Yet researchers still do not fully understand what defines and causes a monsoon. This unknowing has direct impact on the daily lives of billions who depend on monsoon rains annually. Without fully understanding the conceptual basis of a monsoon, researchers cannot predict the future behavior of monsoons as the climate changes. Providing accurate climate change information to communities dependent upon a steady monsoon rainfall lies in being able to accurately model future changes of the monsoon. Such modeling depends upon accurate understandings of past and present monsoons, which requires understanding the birthplace of most monsoons: the Tibetan Plateau.

The Tibetan Plateau holds the headwaters of most of Asia's rivers and many thousands of glaciers, stretching 970,000 square miles across central Asia and surrounded by mountain ranges. To the east it runs into the Qilian range of northern China, where snow leopards inhabit conifer forests. To the west the Tibetan Plateau stops at the Hengduan mountains in southwest China, which were never covered by glaciers during the last Ice Age and are inhabited by the iconic endangered Giant Panda. To the north it spreads to the Kunlun mountain range of western China, a region that is a mythically important facet of Taoism. To the south the Tibetan Plateau reaches the Himalayan mountains of northern India, which contain the highest mountain peak in the world, Mount Everest.

Characterized as a high-altitude arid steppe, the Tibetan Plateau is a semi-desert, with some grasslands and few trees. Incredibly hostile to much life, the Tibetan Plateau can reach temperatures of negative 40 degrees Fahrenheit during the deep of winter and can reach altitudes of 16,000 feet. The horses that helped Genghis Khan achieve so much of his success still roam the region and are still a large part of nomadic tribal life, as are yaks, which graze the grasslands that cover permafrost soils of the Tibetan Plateau that abut the looming Himalayan mountains.

When summer begins, hot air over the land of the Tibetan Plateau rises. While the land changes temperature rapidly and responds quickly to the summer sun's heat, the ocean is much slower to change. Water requires much more energy than land does to warm to the same temperature. Lake Michigan's beaches in Chicago are a perfect example of this energy absorption difference: On a hot summer's day the sand can be scalding and the water freezing.

Since oceans change temperature slowly, the air over the ocean is relatively cool compared with air over the land. The coolness condenses the air's molecules. Such air has a higher pressure than the land's air. The low-pressure area over the land draws in high-pressure air over the ocean to fill the space. High-pressure air always seeks out areas of low pressure to balance out its excess pressure. A large area of low pressure exists over the Tibetan Plateau when the summer heat has melted many glaciers. The rush-in and rush-out dance in the lower atmosphere happens annually: Temperatures rise, and temperatures fall. As climate change warms the land at a faster past than the ocean, the behavior of monsoons change.

The winds blowing across the Tibetan plateau directly affect the Asian-Australian monsoon system, the best-known of the three major global monsoon systems, and indirectly affect the global rainfall patterns upon which so many farmers depend. The increasing irregularity of monsoons affects lives around the world. We do not know how climate change will affect monsoon behavior, but the best scientific hypotheses are that we may never return to a steady, dependable monsoon. Climate change may fatally break the ancient relationship between farmer and monsoon.