Climate change not only worsens cyclones, but also floods: research

Super cyclones, known as hurricanes or typhoons in different parts of the world, are among the most destructive weather events on our planet.

Although wind speeds in these storms can reach 270 km/h, the greatest loss of life comes from the flooding they cause – called “storm surges” – when seawater is pushed ashore. Climate change is expected to make these floods worse, puffing up hurricane clouds with more water and causing sea levels to rise that allow storm surges to travel further inland.

People take refuge on a sports field following the flooding caused by Cyclone Idai in Mozambique. DFID/Flickr, CC BY-SA

In May 2020, Super Cyclone Amphan hit the India-Bangladesh border bringing heavy rains and strong winds and affecting over 13 million citizens. The cyclone also caused storm surges of 2 to 4 meters, flooding the coastal regions of the Bay of Bengal.

Over the ocean, this category five storm – the highest possible rating for a storm – became the strongest cyclone to form in the Bay of Bengal since 1999. , reaching wind speeds of up to 260 km/h. Although it became a Category 2 storm after making landfall, it remained the strongest cyclone to hit the Ganges Delta since 2007.

Amphan had serious consequences for people, agriculture, the local economy and the environment. It tragically claimed more than 120 lives, as well as damaged or destroyed homes and power grids: leaving millions of people without power or communication amid an ongoing pandemic.

Relief and aid efforts have been hampered by flood damage to roads and bridges, as well as coronavirus restrictions. Large areas of crops, including rice, sesame and mangoes, were damaged and fertile soil washed away or contaminated by salty seawater. Overall, Super Cyclone Amphan was the costliest event on record in the northern Indian Ocean, resulting in more than $13bn (£10bn) in damage.

In Kolkata, India, Super Cyclone Amphan caused extensive damage. Indrajit Das/Wikimedia

In a recent study conducted by the University of Bristol and drawing on research conducted in Bangladesh and France, we investigated how the effects of storm surges like the one caused by Amphan on the populations of India and Bangladesh could change under different future climate and demographic scenarios. .

Amphan: Mark II

Rising sea levels – largely due to melting glaciers and ice caps – appear to be driving the greatest increase in future risk of cyclonic flooding, as they allow storm surges to reach further inland. It is therefore essential to understand and predict how sea level rise could exacerbate storm-induced flooding, in order to minimize loss and damage in coastal regions.

Our research used climate models from CMIP6, the latest in a series of projects aimed at improving our understanding of climate by comparing simulations produced by different modeling groups around the world. We first modeled future sea level rise under different future emission scenarios, then added this data to storm surge estimates from a super cyclone Amphan model.

We ran three scenarios: a low emissions scenario, a business as usual scenario, and a high emissions scenario. And in addition to modeling sea level rise, we also estimated future populations across India and Bangladesh to assess how many additional people storm surges might affect. In most cases, we have found that populations are likely to increase: especially in urban areas.

Our findings were clear: exposure to flooding from cyclonic storm surges is extremely likely to increase. In India, the increase in exposure ranged from 50-90% for the lowest emissions scenario to a 250% increase for the highest emissions scenario. In Bangladesh, we found a 0-20% increase in exposure for the lowest emission scenario and a 60-70% increase for the highest emission scenario. The difference in exposure between the two countries is mainly due to the decline of coastal populations due to inland urban migration.

Imagine it’s now the year 2100. Even in a scenario where we managed to keep global emissions relatively low, the local population exposed to storm surge flooding from an event like Amphan will have jumped by around 350,000 people. . Compare that to a high emissions scenario, where an additional 1.35 million people will now be exposed to flooding. And for flood depths of more than a meter – a depth that poses an immediate threat to life – almost half a million additional people will be exposed to storm floods in a high emissions scenario, for example. compared to a low emission scenario.

A satellite image shows Amphan approaching the coasts of India and Bangladesh. Pierre Markuse/Wikimedia

This research provides even more support for rapidly and sustainably reducing our greenhouse gas emissions to keep global warming to 1.5°C above pre-industrial levels.

Although we have focused on storm surge flooding, other cyclone risks are also expected to increase, including deadly heat waves from land-striking cyclones. And in Amphan’s case, the interplay between climate change and the coronavirus likely made things worse for people on the ground. As the world heats up, we must not avoid the reality that pandemics and other climate-related crises will only get worse.

Urgent action on emissions is vital to protect highly climate-vulnerable countries from the fatal effects of extreme weather. Amphan Mark II need not be as destructive as we anticipated if the world’s governments act now to meet the climate goals of the Paris Agreement.

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