All of these developments in recent Earth history ultimately may be attributed to the land masses now known as India and Arabia, which began moving north some 100 million years ago, on a collision course with what is now Eurasia.

Hypothetically, as India smashed into Asia, the world’s tallest mountain ranges were thrust up like the hood of a car in a head-on collision. On the Indian Plate, the Himalaya Mountains were formed, spanning Pakistan, India, Nepal, and Bhutan. The Indian Plate was shoved under the Eurasian Plate, uplifting the Karakorum and Hindu Kush Mountains in Afghanistan and Pakistan, as well as the great Tibetan Plateau—an expanse about 4.5 kilometers high and half the size of the continental United States. The creation of this dramatic continental topography launched a cascade of planetary changes.

The Tibetan Plateau acts like a gigantic exposed brick, absorbing summer heat and heating the atmosphere above it. Hot air rises, and cool, moist air—drawn in from over surrounding oceans—rushes in to replace it. That moist air is the source of monsoon rains.

New evidence suggests that between 22 and 15 million years ago, the Asian monsoons may have begun to strengthen. The onset of the monsoons may have been triggered when the Tibetan Plateau reached a threshold height of 2 to 3 kilometers.

As the mountains rose upward, the land became more exposed to the forces of weather and gravity.

As the monsoon rains increased and the mountains rose, rivers also swelled and cut more deeply into the mountains, increasing erosion and carrying more sediments into the oceans. To give a sense of scale, the Indus River today deposits about 1,000 million tons of mud and sand each year onto the Indus Submarine Fan in the Arabian Sea. Relieved of such massive sedimentary weight, the mountains could be thrust up higher, in a reinforcing cycle that continued to increase monsoons, erosion, and uplift.

Unfortunately, our detailed theoretical understanding of Earth’s climatic evolution is not matched by a sufficiently detailed record of the evolution of Tibetan and Himalayan uplift and erosion.

Fortunately, marine sediments preserve robust, continuous records that can link tectonic and climatic evolution. From deep-sea cores, marine geologists have pieced together a detailed record of environmental change in Asia and Africa. Many of those cores have come from the Arabian Sea, the South China Sea, and the Bay of Bengal, which offer fertile territory for examining the interacting histories of the solid earth and its climate.

The modern Indus River system drains sediments from the high peaks of the Karakoram, Hindu Kush, and western Tibet. It has created the 10-kilometer-thick Indus Fan, which extends 1,000 to 1,500 kilometers into the Arabian Sea.