Indian monsoon led to global warming 130,000 years ago that ended ice age

The Indian summer monsoon pulled heat and moisture northwards, driving glacial melting in the northern hemisphere and helping tropical wetlands expand their range.

The past may be a surprisingly useful guide for predicting responses to future climate change. This is especially important for places where extreme weather has been the norm for a long time, such as the Indian subcontinent. Being able to reliably predict summer monsoon rainfall is critical to plan for the devastating impact it can have on the 1.7 billion people who live in the region.

The onset of India’s summer monsoon is linked to heat differences between the warmer land and cooler ocean, which causes a shift in prevailing wind direction. Winds blow over the Indian Ocean, picking up moisture, which falls as rain over the subcontinent from June to September.

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The monsoon season can bring drought and food shortages or severe flooding, depending on how much rain falls and in what duration. Understanding how the monsoon responded to an abrupt climate transition in the past can therefore help scientists better understand its behaviour in the future.

When we researched this weather system’s ancient past, we found it was highly sensitive to climate warming 130,000 years ago. Our new study published in Nature Geoscience showed that the Indian summer monsoon pulled heat and moisture into the northern hemisphere when Earth was entering a warmer climate around 130,000 years ago. This caused tropical wetlands to expand northwards – habitats that act as sources of methane, a greenhouse gas. This amplified global warming further and helped end the ice age.

The rate at which today’s climate is changing is unprecedented in the geological record, but our study shows how sensitive the Indian summer monsoon was during a global transition into warming in the past and may still be.

The monsoon rains of yesteryear

Over the last one million years, the climate fluctuated between a cold glacial – known as an ice age – and a warm interglacial as the Earth’s position relative to the sun wobbled in its orbit. The last transition from an ice age into the warm climate of the present interglacial – known as the Holocene – occurred around 18,000 years ago. This period of Earth’s history is relatively well understood, but how Earth system processes responded to these climate changes deeper in time is still something of a mystery.

A recent expedition to drill deep into the ocean floor of the Bay of Bengal gave an opportunity to reconstruct past Indian monsoon behaviour over hundreds of years before the last ice age.

Our study used these deep sea sediments from the northern Bay of Bengal to capture a direct signal of the Indian summer monsoon from 140,000 to 128,000 years ago, hidden in the fossilised shells of tiny microscopic creatures called foraminifera.