Climate change isn’t just making cyclones worse, it’s making the floods they cause worse too—new research

Amphan Mark II need not be as destructive as we’ve projected if the world’s governments act now to meet Paris agreement climate goals.

SOURCEThe Conversation
Image Credit: NASA

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 within these storms can reach 270 km/h, the largest loss of life comes from the flooding they cause—known as a “storm surge”—when sea water is pushed onto the coast. Climate change is predicted to worsen these floods, swelling cyclone clouds with more water and driving rising sea levels that allow storm surges to be blown further inland.

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

While over the ocean, this category five storm—that’s a storm’s highest possible rating – became the strongest cyclone to have formed in the Bay of Bengal since 1999, reaching wind speeds of up to 260 km/h. Although it weakened to a category two storm following landfall, it remained the strongest cyclone to hit the Ganges Delta since 2007.

Amphan had severe consequences for people, agriculture, the local economy and the environment. It tragically resulted in more than 120 deaths, as well as damaging or destroying homes and power grids: Leaving millions without electricity or communication in the midst of an ongoing pandemic.

Relief and aid efforts were hampered by flood damage to roads and bridges, as well as by coronavirus restrictions. Large areas of crops including rice, sesame and mangoes were damaged, and fertile soils were either washed away or contaminated by saline sea water. Overall, Super Cyclone Amphan was the costliest event ever recorded in the North Indian Ocean, resulting in over $13 billion (£10 billion) of damage.

Two people assess a tree that has fallen across a road
In Kolkata, India, Super Cyclone Amphan caused widespread damage. Indrajit Das/Wikimedia

In a recent study led by the University of Bristol and drawing on research from Bangladesh and France, we’ve investigated how the effects of storm surges like that caused by Amphan on the populations of India and Bangladesh might change under different future climate and population scenarios.

Amphan: Mark II

Rising sea levels—thanks largely to melting glaciers and ice sheets—appear to be behind the greatest uptick in future risk from cyclone flooding, since they allow storm surges to reach further inland. It’s therefore key to understand and predict how higher sea levels might exacerbate storm-driven 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 aiming to improve our understanding of climate by comparing simulations produced by different modelling groups around the world. First we modeled future sea-level rise according to different future emissions scenarios, then we added that data to storm surge estimates taken from a model of Super Cyclone Amphan.

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

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

Imagine we’re now in 2100. Even in a scenario where we’ve managed to keep global emissions relatively low, the local population exposed to storm surge flooding from an event like Amphan will have jumped by ~350,000. Compare this to a high emission scenario, where an extra 1.35 million people will now be exposed to flooding. And for flood depths of over one meter—a depth that poses immediate danger to life—almost half a million more people will be exposed to storm surge flooding in a high emission scenario, compared to a low emission scenario.

A composite satellite image of a large white cyclone
A satellite image shows Amphan approaching the coasts of India and Bangladesh. Pierre Markuse/Wikimedia

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

Although we’ve focused on storm surge flooding, other cyclone-related hazards are also projected to worsen, including deadly heatwaves following cyclones hitting land. And in the case of Amphan, interplay between climate change and coronavirus likely made the situation for people on the ground far worse. As the world warms, we mustn’t avoid the reality that pandemics and other climate-related crises are only forecast to increase.

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’ve projected if the world’s governments act now to meet Paris agreement climate goals.

Laurence Hawker, Senior Research Associate in Geography, University of Bristol; Dann Mitchell, Professor of Climate Science, University of Bristol, and Natalie Lord, Honorary Research Associate in Climate Science, University of Bristol

This article is republished from The Conversation under a Creative Commons license. Read the original article.

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Laurence Hawker is a Senior Research Associate in the School of Geographical Sciences at the University of Bristol. His work focuses on developing flood models, supporting disaster response and developing elevation and population datasets. Currently, I work on a number of projects developing and improving hydrodynamic models in data-sparse regions, namely in Africa (with a focus on Congo Basin and Mozambique) and Vietnam. Recently, along with several co-authors, I have published a 30m resolution global map of elevation with forests and buildings removed called FABDEM. In my research I use remote sensing, hydrodynamic models and work closely in an interdisciplinary team to develop a household survey. In my PhD I worked on an intermediate scale flood model of the Mekong Delta, quantifying river-floodplain connectivity and the simulation of plausible versions of global digital elevation models (DEM) in floodplains. Dann Mitchell is a Alan Turing fellow, a NERC fellow, and Professor of Climate Science at the University of Bristol. He is the Met Office co-chair in climate hazards, co-lead of the Cabot Institute for the Environment ‘Environmental Change’ theme, and the head of the Climate Dynamics group. He moved to Bristol in 2017 after a five-year research post in the Department of Physics at the University of Oxford. Since completing his PhD in 2011 in the world-famous meteorology department of the University of Reading, Mitchell has published over 90 peer-reviewed papers, with a strong focus on climate extremes, hazards, and impacts; especially related to health. He is co-founder and coordinator of the Half a Degree additional warming, Prognosis and Projected Impacts (HAPPI) consortium, which is aimed at understanding climate variability under the Paris Agreement climate goals. Natalie Lord is a climate scientist with an interest in how global, regional, and local climates respond to changes over different timescales, along with the potential impacts of these changes on the environment and society. Her current focus is understanding how the climate system may change in the future, what impacts these changes may have on flood risk and tropical cyclones, and how this will affect populations around the world. She holds an Honorary Research Associate position in the School of Geographical Sciences at the University of Bristol, where she also completed her PhD and several research positions. She has been involved in a variety of research, including modelling changes in high temperature-high humidity extremes over the next century and the potential impacts, and comparing how tropical cyclones are represented in climate models with different spatial scales. Her PhD and a subsequent research position were funded by and carried out in connection with various nuclear industry companies, including RWM (UK), Posiva Oy (Finland), and Svensk Karnbranslehantering AB (SKB; Sweden), and National Cooperative for the Disposal of Radioactive Waste (Nagra; Switzerland). The climate projections for the next million years produced as part of her postdoctoral research will be used by the Finnish and Swedish nuclear waste authorities (Posiva Oy and SKB) as an integral part of the Safety Cases for their high-level radioactive waste repositories located at Olkiluoto and Forsmark. The methodology and models developed during her PhD, and the subsequent projections of climate evolution over the next million years, contributed to the report "Development of a Common Framework for Addressing Climate Change in Post-Closure Radiological Assessment of Solid Radioactive Waste Disposal", written by Working Group 6 of the MOdelling and DAta for Radiological Impacts Assessments (MODARIA) international research programme, and published by the International Atomic Energy Agency (IAEA).

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