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Global warming signals still unidentifiable in extreme rainfall over India

Of late, the extreme rainfall events have increased, but the present climate model simulations are not enough to conclusively attribute the trend to human induced climate change, a study has found. This is an important result because it shows that the country needs to fine tune its models to simulate hydroclimatic variables at the regional level. The study was conducted by the researchers from the Indian Institute of Science, Bengaluru.

“Studies like this are essential for framing policies for different eventualities under a changing climate. The results of our study can also influence how we manage our water resources”, says Dr Arpita Mondal, now an Assistant Professor at IIT Bombay, Mumbai. She carried out the study when she was a research scholar at the IISc working with Prof. P. P. Mujumdar in the Department of Civil Engineering.

The Intergovernmental Panel on Climate Change (IPCC) mandates that every country should try to quantify how human activities are influencing the climate in the country. Arpita Mondal has used a mathematical technique called 'Detection and attribution', to ascertain how anthropogenic activities are influencing extreme rainfall in India.

Different parts of India receive different amounts of rainfall. Some regions of Thar desert have to wait for more than 117 years to receive the quantity of rainfall received by Chirrapunji in one year. Every year, the monsoons lash the coastal regions and help Western Ghats bubble with life. In addition to this, there are vast regions of the country that lie in between: neither as wet as Chirrapunji, nor as dry as the Thar desert. Such large variations in the quantity of received rainfall poses a unique challenge to scientists studying climate of the Indian subcontinent – climate models that operate at large spatial scales can't capture the variety that defines the region.

There are other challenges too. As Arpita says, “Thanks to disputes in water sharing among neighbouring states, important datasets related water resources are classified. Also, we don't have enough rain gauges that provide high quality, high resolution datasets”.

Arpita has been studying how extreme rainfall events in India are undergoing a shift in the last century. It is clear that they have become more common in the last century, but it's not clear how much of this could be attributed to human induced climate change. Her recent study shows that the present climate models are not able to solve this puzzle.

Using sophisticated mathematical techniques, the researchers painstakingly analysed rainfall data obtained from 1800 rain gauges scattered all across the country by the India Meteorological Department. They particularly looked at very heavy rainfall received in one day and in a span of five days, because such rainfalls are most likely to cause pluvial floods. Using what is called the 'detection and attribution analysis', they searched for clues to better understand how human activities are influencing extreme rainfall events in the country. As it turned out, there were no clear clues.

The link between raising temperature and extreme rainfall is well understood: high temperatures result in greater moisture holding capacity of the atmosphere, which, in turn, leads to extreme precipitation. However, many other factors including large scale natural circulation patterns, glacial mass balance, evaporation, evapotranspirations and even urbanization may also have a say.

“The international disaster mitigation funding agencies might soon want to quantify human causes behind loss and damage involved in individual hazardous events. Therefore, in future, we would like to investigate how best we can integrate available techniques and modelling tools in both physics and statistics to understand how hydroclimatic extremes change and how anthropogenic activities might possibly influence them”.

About the authors:Arpita Mondal, the first author of the paper, is an Assistant Professor in IIT Bombay, Mumbai. She carried out the study when she was a research scholar at the IISc, Bengaluru. Pradeep Mujumdar, the corresponding author, is a Professor in the Department of Civil Engineering, IISc.

Contact: Arpita Mondal can be contacted at +91 22 2576 9305.

Link to the paper:http://dx.doi.org/10.1016/j.jhydrol.2015.09.030