Cyclone Fani and global climate change

The temperature of the Earth changes over geologic time. During periods of glaciation, it was about five degrees Celsius cooler and in the interglacial period about five degrees warmer. The last glaciation period was 100,000 years ago. Since then, there have been fluctuations of a few degrees, the period of 1430 to 1850 being one of particularly low temperatures in Europe. Although there were fluctuations from year to year, it seems evident that there has been a steady increase in average global temperature since the Industrial Revolution. According to the World Meteorological Organization, average global temperatures will reach a new milestone this year—one whole degree higher than temperatures before industrialisation.

In the early 1990s, when concern about climate change caused by the rise in temperature became widespread, the "signal" of anthropogenic effects hadn't unambiguously emerged from the "noise" of natural climate variability. However, we now know that most of the climate-related changes observed over the past 50 years is attributable to human activities. In fact, by burning prodigious amounts of fossil fuels that emit carbon dioxide, which is the principal greenhouse gas, we humans have taken Earth's atmosphere in general and global temperature in particular into a regime that our planet hasn't seen for millions of years.

Although the interplay between carbon dioxide and temperatures is complex and not necessarily 100 percent predictive, nevertheless, the obvious correlation between the two variables suggests that we might expect a significant adverse climatic response to the industrial-era surge in fossil fuel derived atmospheric carbon dioxide. Undeniably, the effects of this interplay are manifested in the increase in the ferocity of storms, floods of biblical proportions, spike in the number of unusually hot days, melting of the glaciers, drought, desertification and deforestation, polar vortex, uncontrollable forest fires, degradation of the coral reefs, habitat loss and rise in the sea level, to mention a few.

Today, because of global warming, intense storms are occurring in many parts of the world. If they form in the Atlantic or Caribbean, they are known as hurricanes, and in the Pacific or China Sea as typhoons. If they develop off the coast of Indian Ocean or the Bay of Bengal, we call them cyclones. These storms are one of the most awe-inspiring displays of the raw power of nature. They are also among the deadliest and costliest natural disasters we have to contend with routinely.

After churning through the Bay of Bengal for several days, gathering immense amount of energy along the way, cyclone Fani roared through Bangladesh on May 4, 2019, leaving behind a massive trail of destruction—killing more than a dozen people, knocking out power, shredding roofs and leaving hundreds of thousands homeless. Classified by meteorologists as the equivalent of a Category 4 hurricane, it was one of the most intense cyclones in 20 years in the region.

Cyclones batter Bangladesh at regular intervals, mainly in April/May or October/November, when weather conditions align in a manner most favourable for storm origination and sustenance. As examples, cyclones Aila struck southern Bangladesh on May 27, 2009 and Sidr made landfall on November 14, 2007. The occurrence of these and other cyclones in close succession is a reminder of the country's extreme vulnerability to the devastating effects of human-induced climate change.

The 1970 cyclone that hit Bangladesh on November 12 and raged the strongest on November 13 was the worst natural disaster we have witnessed so far. The resulting storm surge, more than 20 feet high and topped by huge tidal waves, washed over offshore islands and carried water from the sea many miles inland. The cyclone and flood destroyed the entire infrastructure of the country's southern coast and killed an estimated half a million people, though some researchers estimate that the death count was close to a million. The failure of the Pakistani government to respond quickly to the crisis, among other things, contributed to the political turmoil that led to an independent Bangladesh in 1971.

Tropical cyclones are influenced by many factors, but the role of warm sea-surface temperatures is the primary source of energy for cyclones. In particular, a cyclone gets most of its energy from the latent heat of condensation and the moisture generated from the sea. Thus, for the genesis of cyclones, temperature of water near the surface of the sea must be higher than 27 degrees to a depth of at least 150 feet. Additionally, heat from the sea and Earth's counter-clockwise rotation conspire to create the cyclone's spin and propulsion. Furthermore, rising sea levels mean that surges produced by cyclones are much more powerful, thereby increasing the risk of inland flooding.

Moreover, cyclones need to be at least 300 miles from the equator, where a deflective force known as Coriolis force resulting from Earth's rotation begins to take effect. When cyclones reach land, or cooler water, they lose energy as the conditions necessary to reinforce them are no longer present.

As a result of global warming, temperature near the surface of the Bay of Bengal varies from 27 degrees in January to more than 31 degrees in May. The unusually warm water, together with geographical and environmental factors, make the Bay of Bengal a hot spot for cyclonic activity.

Can changes in frequency and intensity of cyclones observed so far be attributed solely to anthropogenic global warming as against long-term periodic natural variations? Cyclones are affected by natural fluctuations too, driven by external factors, such as solar variability and volcanic eruptions, natural internal variations of the complex physical, chemical, and biological systems of Earth.

Additionally, research has shown that urbanisation significantly contributes to the amount of rainfall dumped, as evidenced by over 130 centimetres of rain that fell on the Houston region during hurricane Harvey in 2017. This is because the "roughness" of the city—as in the buildings and infrastructure—creates a drag on the storm system, causing it to slow down, resulting in more rain over the city area.

Climate models predict that global warming could spawn more bizarre and violent weather, notably cyclones and severe floods in the future. Indeed, while people are trying to come to grips with the effects of Fani, meteorologists have warned that Bangladesh is likely going to experience another cyclonic storm called Vayu some time later this month.

The models also predict that by the end of this century, global warming effects could increase a cyclone's intensity by about 20 percent, making them more destructive than ever. The amount of rainfall would also increase substantially. Other estimates predict that a doubling of carbon dioxide concentration would result in a 40-50 percent increase in destructive cyclones.

So, what should we do to keep our planet in the so-called Goldilocks zone of the solar system? We have to make a concerted effort to end our dependence on fossil fuels. We have to replace them with non-polluting, renewable sources of energy. We have to develop more carbon-free energy technologies. We have to sequester carbon dioxide emissions using the available technology. More importantly, we have to shun the "business as usual" attitude. In short, we will have to build a sustainable future. Otherwise, climate change will cause our civilisation to collapse.

Quamrul Haider is a Professor of Physics at Fordham University, New York.