The Indian summer monsoon represents a significant component of the global climate system, specifically its role in the hydrological cycle during the summer monsoon months of June through to September when vast amounts of rainfall are released over the Indian subcontinent. Coupled with its role in Earth’s internal climate system, transporting heat and moisture across the tropics and subtropics, the Indian Summer Monsoon exerts a significant influence on the socio-economies of the densely populated Indian subcontinent. An anomalous monsoon season can result in flooding and widespread infrastructure damage associated with too much rainfall, and a season with below normal rainfall can result in crop failure and famine. Under anthropogenic induced climate change anomalous monsoon behaviour is expected to increase. We, therefore, need to have a comprehensive understanding of the Indian Monsoon in order to assess its sensitivity to different climatic scenarios to help better understand its response to a changing climate.
Late Pleistocene: Time of the Ice Ages
The climate of the past ~1.2 million years has been characterised by fluctuations between warm interglacial periods and subsequent growth of ice-sheets during glacial periods recurring every 100,000 years. These cycles are thought to be regulated by variations in the amount of solar insolation received on Earth due to changes in the Earth’s position relative to the sun. Coupled with this external climate forcing, internal climate feedbacks (such as greenhouse gas concentrations) act to amplify the climate response to insolation. Assessing the behaviour of the monsoon during the penultimate deglaciation (~140-130 thousand years ago), subsequent warmth of the last interglacial (~130-118 thousand years ago) and return to glacial conditions, we intend to unlock the behaviour of the monsoon to ‘natural’ (i.e. non-anthropogenically influenced) climate changes and assess its relationship with other aspects of Earth’s internal climate system (e.g. ice volume).
Tiny fossils, big information
Changes in rainfall, river runoff and temperature forced by past Indian Summer Monsoon variability can be revealed in the shell chemistry of tiny (about the size of a sand grain) ocean-dwelling organisms called foraminifera. These organisms build their shells from calcium carbonate, soaking up the chemistry of the seawater surrounding them as they do this. The chemistry of the seawater that they lock into their shells can record the temperature of the seawater that they grew in and how fresh or salty the water was. At the end of their life cycle, these calcium carbonate shells remain intact, sink and are buried on the ocean floor. A wealth of information on past climate can be accessed by drilling into the ocean floor to retrieve marine sediment cores in which these fossils can be found and subsequently analysed using geochemical techniques. Applying these techniques to foraminifera that once lived in the waters of the northern Bay of Bengal, receiving both monsoon rainfall and induced river runoff from the Ganges-Brahmaputra systems, provides clues to unlocking how the monsoon reacted to past climate changes.