Using Trees to Understand Long-term Variations in the Indian Monsoon

A small holding paddy field in Palakkad, Kerala, India.

By Mukund Rao,
Climate and Society ’13

The Indian Monsoon is the primary source of water for approximately one-fourth of the world’s population. Also, close to two-thirds of the Indian population is either directly or in-directly dependent on agriculture for their livelihood. In the Indian context, rainfall is particularly important for agriculture as it is primarily rainfed. Most of this rain in India occurs during the months of June and September during the southwest monsoon.

To accurately inform decision making using climate science it is important that we have a long time series of data. This is where the study of tree rings to deduce the past climate is particularly useful. Trees can be long-lived and their growth is dependent on the climate around them, making them a perfect model to peek into the climate of the past.

In an effort to provide a better climate context for farmers in India, I spent the summer on a reconnaissance mission in the southwestern state of Kerala. It is also known as “God Own Country” because of how beautiful the countryside is. With a lush green landscape, tropical climate and rainfall of approximately 3500mm a year Kerala is an ideal place to find thick forests and old trees.

Field sites in the Indian state of Kerala

Tree-ring scientists usually head out into the field searching for old trees. However, we do not just go from one forest to another hoping to find old trees. As the saying goes, usually the “locals know best.” So, throughout the summer I spent time with researchers from the Lamont-Doherty Tree Ring Lab interacting with people in the region, ecologists and foresters at other research institutions and forest department officials to try and figure out where in the state we might expect to find the oldest trees. After much debate and discussion we were finally able to come up with close to 10 forest sites where we intend to sample trees later this year. We believe that we might find some teak trees which might be older than seven hundred years at a few of these sites. As all of these sites are along the Western Ghat mountain range and some of these are in relatively remote regions, conducting field work will definitely be an exciting adventure.

Western Ghat Mountains and paddy fields

Most weather station data on goes back roughly 50 years in the region. That can give use a sense of how climate patterns such as El Niño-Southern Oscillation and the North Atlantic Oscillation, which occur on interannual timescales, interact with the Indian Monsoon. Having tree-ring data will help improve our understanding of how these oceanic and climate patterns can affect the Indian Monsoon on centennial timescales, though.

Tree-ring data also provides a much better insight the monsoon’s connection with phenomenon like the Pacific Decadal Oscillation, which changes on a frequency of 20-30 years. Where station data might only be able to capture two or three phases of such decadal oscillations, tree-ring data arms us with a chronology of more than 500 years, making it much easier to examine the effect of some of these decadal oscillations.

A canal from the Malampuza Dam in Palakkad district for irrigation. The owners and users of the canal are one potential end user of climate information.

At the end of the day, science is about helping improve people’s lives and climate science must attempt to do so. From this project conducted in collaboration with our Indian partner institutions, we at the Tree Ring Lab hope to be able to improve the understanding and predictability of the Indian Monsoon from one year to the next. We also hope that using this information, we might be able to help inform agricultural and water management decision making at a seasonal level.