1 The Tethys Suture
Earth is a dynamic living planet with a churning heat engine that continually reshuffles its materials and repaves its surface. The Indian region is a spectacular result of this ongoing metamorphosis. We are lucky to learn about these changes through our sojourns at sea and on land, and in the incredible diversity of landscapes, ecosystems, and cultures in the region.
Thousands of miles below our feet, Earth’s iron core and mineral mantle drive heat to the surface through a slow churn of solid rock over millions of years. This convective conveyor belt gradually pushes the continents and ocean basins around in a process of continental drift also known as plate tectonics.
Over the past 200 million years, the continents of Earth’s Eastern Hemisphere have been assembled through a series of progressive collisions as landmasses moved from south to north. The deep south-to-north conveyor pushed continents together and dragged heavier ocean crust downward into the underlying mantle, leaving the largest landmasses in today’s world.
Siberia and Scandinavia hosted Europe and Central Asia and then Africa and India docked somewhat later. Each of these great gatherings consumed ocean plates, which now dangle thousands of miles below like tattered draperies. Each suture or seam is a crumple zone, where solid rock was smushed together into massive folds and gigantic west-to-east mountain ranges from the Atlantic to the Pacific.
Most of our voyage will be an exploration of this great suture zone where once the Tethys Ocean lapped the shores of Laurassia and Gondwana. From the Atlas Mountains of Morocco, along the Alps, to the roof of the world in Central Asia, to the volcanic ridges of Indonesia, the Tethys Suture divides the Old World into northern and southern domains. The Mediterranean is a tiny remnant of the former glory of the Tethys Ocean.
The west-east vastness of the Old World was built in long stages, forming globe-spanning ecozones that allowed evolving plants and animals to flourish across broad latitude zones. This in turn encouraged emerging humanity to spread pastoral and farming practices and our associated cultures along the natural grain of Earth’s surface. Boreal forests stretch from Norway across Siberia. Dappled summer sunlight filters through leafy deciduous canopies from the Lime trees of Ireland across to Shinto shrines of Japan. Vast grasslands are home to herds of hoofed mammals, nomadic pastoralists, and horse-riders across 12 time zones from Spain to Manchuria. Sun-scorched sands stretch from the Sahara to the Gobi, with seasonal savannas and teeming tropical forests further south.
Conversely, the closing of the Tethys Suture produced nearly insurmountable east-west mountain barriers that separate life zones, ecosystems, and human cultures to this day. Unlike the north-south mountain ranges that split the Americas, catastrophically crumpled mountain chains of Central Asia wall off the frigid and parched interior from the hot wet Indian subcontinent. India and China host nearly a third of humanity, and their histories and cultures have interacted for millennia. Yet the very distinct cultures of these close neighbors owe much to the mountain fastness that divides them, combined with the forbidding deserts to the west and jungles to the southeast.
India was a late arrival to the Tethys Suture. The Indian Plate swept rapidly up from the deep south, with subduction swallowing the intervening ocean starting around 70 million years ago. By 55 million years ago, compression had formed huge chains of volcanos and mountain belts as the ocean basin closed. India “docked” with the Asian mainland around 10 million years ago but the slow catastrophe in ongoing. The greatest mountain ranges on Earth wrap around the leading edge where India plows deep into central Asia. These ranges wrap around Iran, Afghanistan, Pakistan, Tibet, Nepal, and Southeast Asia. They structure climate, hydrology, vegetation, food supply, and culture over a vast region affecting billions of people.
2 Roof of the World
The Himalayas form the southernmost and highest boundary of the great continental crumple zone that resulted from India’s collision with Asia. The name is derived from Sanskrit hima “snow” alaya “abode.” The chain is composed of many smaller ranges and reaches above 8000 meters (26,000 feet, 5 miles) elevation in places.
The Himalaya and related mountain chains drastically alter the climate of Asia. The highest mountains are indeed
covered with snow and ice, hosting enormous glaciers. Monsoon winds push tropical moisture up from the south where it falls in seasonal torrents, yet to the north of the great mountain barrier the land is parched and bare.
The Himalayan foothills are lush and wet with seasonal rains, eroded into complex systems of rugged canyons and valleys. They were densely forested in ancient times, but large areas have been cleared for farms and pastures.
3 The Indian Monsoon
Westerners often naively misunderstand the word “monsoon” to mean torrential rain, and indeed truly prodigious rains do fall during the rainy season in parts of India. But the monsoon is better understood as a vast system of seasonally-reversing winds. The English word monsoon is derived from the Arabic word mausin meaning “season.” Rather than cold and warm seasons experienced in North America and Europe, south Asia has very pronounced wet and dry seasons.
The equatorial waters of the western Pacific and Indian Oceans are like nowhere else on Earth. Warm water is pushed westward by Trade Winds across thousands of miles, accumulating in a great wedge nearly 1000 feet deep called the Warm Pool. An area larger than Russia is always around 30 ºC (86 ºF). This huge pool of deep warm water evaporates like crazy, so that the region is perpetually both very hot and very humid. The juxtaposition of this humongous heat reservoir with the high cold Himalayas and Tibetan Plateau forms the heat engine for the great Asian Monsoon.
During the summer or southwest monsoon, solar heating of the high mountain terrain in the Himalayas and Tibetan Plateau causes the air to rise over a huge region. The rising air draws hot moist air up from the Indian Ocean and drives it into the mountains, wringing out phenomenal amounts of precipitation over south and southeast Asia. In the winter or northwest monsoon, bitter cold air sinks from the mountains and flows southward across the subcontinent.
Seasonally reversing monsoon winds control the cycles of productive vegetation, crops, and livestock across much of Asia. Billions of people depend on monsoon rains for food production. Farmers and pastoralists structure their lives around the seasonal winds and rains.
Monsoon rains are geographically structured by the hemispheric flow of moisture and the rugged topography of the Indian subcontinent. Rainfall in excess of 10 meters per year is typical in some areas of the Himalayan foothills!
Since ancient times, seafarers used the reliable reversal of the monsoon winds to travel across the Indian Ocean between East Africa and South Asia. Thousands of years before Mediterranean mariners dared sail out of sight of land, sailors plied a robust trade in goods across the Indian Ocean and Arabian Sea.
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| Seasonally-reversing monsoon winds |
4. Rivers of India
Rivers, Deltas, Rice, and a Whole Lot of People!
The incredible flow of water from the tropical Indian Ocean into the Himalayas -- and the resulting torrent of monsoon rains in summer – produces an enormous outflow of water via gigantic river systems. The Ganges and Brahmaputra drain the high terrain to the northeast of India, flowing together to form an enormous delta at the Bay of Bengal. The Indus River, for which India is named, drains the northwest which is now Pakistan. Due to the topographic barrier of the Western Ghats, most rivers in central and southern India drain to the east.
5 It’s Both the Heat AND the Humidity!
The Warm Pool region of the Indian Ocean and West Pacific is the place on the planet with sea surface temperatures constantly at or above 30 ºC. Evaporation from the warm ocean also contributes a lot of humidity in the overlying atmosphere.
Evaporation is a critical way to transfer heat energy from place to place. Globally, evaporation of water moves more than three times as much heat from Earth’s surface to the atmosphere as rising warm air. As you’d expect, evaporation is much more effective at removing heat when the air is dry than when it’s humid.
Scientists define wet-bulb temperature as the coolest air can become by evaporation. It’s measured by wrapping a thermometer in wet gauze and then swinging it around so that it loses heat by evaporation from the gauze. When the air is very dry, wet-bulb temperature is lower than “regular” (dry-bulb) temperature. When the air is saturated (100% relative humidity), the wet-bulb and dry-bulb temperatures are equal.
Evaporation from our skin is a critical mechanism that helps us regulate our body temperature. Internal human body temperature is 37 ºC (98.6 ºF), and the heat released by our metabolism always tends to raise it higher. To conduct metabolic heat away from our core, the skin must be cooler than about 35 ºC. Perspiration and evaporation allow the skin to remain cool enough to eliminate metabolic heat even when the air is hotter than our bodies.
If the wet-bulb temperature exceeds about 34 ºC, evaporative cooling can’t keep our skin cool enough to conduct heat away and metabolic heat can’t escape. Under these circumstances, people die of the heat in a matter of hours. Luckily, such extreme heat and humidity are very rare on Earth!
In hot desert regions near very warm ocean waters, wet-bulb temperatures occasionally approach lethal thresholds. The Persian Gulf and Red Seas are especially prone to extreme heat and humidity, but these areas are very sparsely populated. The Indus, Ganges, and Brahmaputra valleys are slightly less likely to experience lethal wet-bulb conditions, but are home to more than far more people. Four or five ºC of warming would put hundreds of millions of people at risk for heat-related catastrophe.
6 The Scourge of Poisonous Air
Air is a mixture of around 21% oxygen (O2) and 78% inert nitrogen (N2). Toxic gases in air include carbon monoxide (CO, from incomplete combustion); nitrogen oxides (NO and NO2, from burned air); and ground-level ozone (O3, formed in the air from combustion byproducts exposed to sunlight). Tiny particles in the air (PM10 and PM2.5 from smoke, dust, and other sources) also contribute to toxic air pollution.
India has some of the worst air pollution in the world due to industrial coal combustion, heavy manufacturing, poorly-enforced regulations on outdoor burning, and agricultural dust in its hot dry and sunny environment. Mortality from indoor air pollution has decreased almost 65% since 1990, but deaths from outdoor air pollution have more than doubled in the same period!
The following is reprinted from https://www.bc.edu/bc-web/bcnews/nation-world-society/international/air-pollution-in-inda.html
Air pollution in India resulted in 1.67 million deaths in 2019—the largest pollution-related death toll in any country in the world—and also accounted for $36.8 billion in economic losses, according to a new study led by researchers from the Global Observatory on Pollution and Health at Boston College, the Indian Council of Medical Research, and the Public Health Foundation of India.
The 2019 death toll attributed to air pollution in India accounted for 17.8 percent of all deaths in the country in 2019, according to the study’s findings, published in the journal Lancet Planetary Health.
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