Climate change, warmer oceans could have made Cyclone Nivar more intense

Climate change is contributing to the increasing frequency and intensity of devastating cyclonic storms such as Ockhi and Nivar, say experts.

Cyclone Nivar brought with it heavy to extremely heavy rainfall across the coastal districts of Tamil Nadu with winds going upto speeds of 130 km/h as it made landfall near Puducherry on the night of November 25th.

The cyclone rapidly gained strength in its last few hours and was categorised as a “severe cyclonic storm”. With repeated warnings from the Indian Meteorological Department (IMD) to brace for the cyclone, many districts shut down completely and suspended transportation services to ensure safety. Cities in the region, including Chennai, experienced heavy rain and flooding. 

Link with climate change

With the cyclone having made landfall, it is worthwhile to examine how climate change may have heightened the threat from cyclone Nivar in several ways.

Sea surface temperature and storm strength: Warm ocean temperatures brought about by global warming, caused by human greenhouse gas emissions, favour the formation and rapid intensification of tropical cyclones. There has been a global increase in the intensity of the strongest storms over recent decades: a study published in June found that the proportion of the strongest storms is increasing at the rate of about 8% a decade. 

A recent study concluded that “ocean surface and subsurface conditions played a vital role in the genesis and intensification of cyclone Ockhi,” a very similar cyclone that hit the same region almost exactly three years ago, causing 844 deaths. Similar to other parts of the world, sea surface temperatures in the Bay of Bengal have been steadily increasing over the last decades.

Changes in sea surface temperature in the Bay of Bengal (BOB, red line) and the Arabian sea (AS, black line) between 1880 and 2010. From Kumar et al (2016). 

Rapid intensification: A growing proportion of tropical cyclones are developing quickly, known as rapid intensification, according to multiple studies – these changes are linked with climate change. Warm ocean waters are one factor driving rapid intensification so higher ocean temperatures, caused by human greenhouse gas emissions, make it more likely. Rapid intensification is a threat because it makes it harder to forecast how a storm will behave and so to prepare before it makes landfall. 

Warmed atmosphere and more intense rainfall: The planet’s atmosphere is warming as a result of carbon emissions. A warmer atmosphere can hold more water, driving extreme rainfall during cyclones, which increases the threat of flooding. Scientists have directly linked the increase in atmospheric moisture with human-caused climate change. The number of record-breaking rainfall events globally has significantly increased in recent decades, as a result of global warming, and scientists predict that rainfall from cyclones will increase with continued climate change. 

Higher sea levels and increased storm surges: The potential storm surge from cyclones are often the most dangerous threats from the storm. Increases in storm surge related to climate change can be due to rising sea levels, increasing size, and increasing storm wind speeds. Global sea levels have already increased about 23 cm as a result of human carbon emissions – dramatically increasing the distance that storm surges can reach. 

In addition, tropical cyclones in the Bay of Bengal are affected by the El Niño–Southern Oscillation (ENSO), a meteorological phenomenon that affects wind patterns and sea surface temperatures in some regions of the Pacific Ocean with consequences in different parts of the world. 

Scientists have found a correlation between the cooler phase of ENSO, known as La Niña, and increased tropical cyclone activity in the Bay of Bengal. Because we are currently experiencing a La Niña period, this could be one of the underlying causes for the formation of cyclone Nivar. 

Experts weigh in

Dr Roxy Mathew Koll, Scientist at the Indian Institute of Tropical Meteorology and Lead Author on the IPCC Oceans and Cryosphere report pointed out that of the last six severe cyclones to hit Tamil Nadu, five of them coincided with La Nina like conditions in the Pacific. So that means to some extent the emergence of cyclone Nivar was not a surprise. 

On how climate change plays a part, she said, “Now coming to the local conditions, that is where the role of climate change is visible. The case of Cyclone Nivar is similar to that of Cyclone Ockhi in several ways. We found that unusually warm ocean temperatures favoured the evolution of Ockhi from a depression to cyclone in 9 hours and then to a very-severe-cyclone in 24 hours.”  

Dr Roxy added that the high temperatures in the Bay of Bengal meant additional energy for the cyclone to maintain and develop. Warm conditions might have favoured the rapid intensification of Cyclone Nivar. 

Dr Anjal Prakash, Research Director and Adjunct Associate Professor at the Bharti Institute of Public Policy, Indian School of Business, and Coordinating Lead Author of IPCC’s Special Report on Oceans and Cryosphere highlighted that out of these eight recorded incidences of cyclones in 2020, Amphan and Nisarga were super cyclones while Gati and Nivar are considered very severe cyclonic events. There were four relatively smaller depressions this year that also caused heavy rains and increased wind velocity. 

“IPCC scientists have been warning about such events. The most recent report that covered Oceans and Cryosphere, clearly warned that if global warming is not stopped, there would be a rise in these events both in number and in the severity of cyclones,” said Dr Anjal.

On our adaptability, Dr Anjal says, “The larger point is that we must make our infrastructure tuned towards such climatic events and plan to adapt for future events.This is how we can save the lives of millions of people especially the poor and the marginalised.”

(This article is based on a briefing note prepared by the Delhi-based communications initiative, Climate Trends and has been republished here with some edits.)

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