SINGAPORE - Within the span of about a month, two tropical cyclones pummelled Japan and an island chain in the Caribbean, leaving widespread destruction to human lives and property.
Last weekend, Typhoon Hagibis - reportedly the most powerful typhoon to hit Japan - killed more than 70, overwhelmed river banks and damaged bullet trains.
It followed another storm last month, when Hurricane Dorian slammed into the Bahamas, leaving at least 50 dead and tens of thousands of people homeless.
While it may be difficult to link individual storms to climate change, Dr Xie Shang-Ping, a climate scientist at the United States' Scripps Institution of Oceanography, said warming oceans can increase the probability of their occurrence.
"It's no coincidence that new records have recently been set in tropical cyclone intensity," said Dr Xie. "I think we are seeing the climate change effect. The warmer the ocean gets, the stronger tropical cyclones will become."
Dr Xie pointed also to Typhoon Haiyan, which devastated the Phillipines in 2013, and Hurricane Patricia, which in 2015 blitzed across Mexico and south-western US.
Typhoon Haiyan was the most intense tropical cyclone at landfall in the world, while Hurricane Patricia was the strongest tropical cyclone in the north-eastern Pacific, Dr Xie said.
In comparison, Dorian was the most intense tropical cyclone at landfall in the North Atlantic.
Hagibis too had approached the Category 5 storm category - the highest in the scale - but otherwise, it was not that extreme, Dr Xie said. It was the heavy rains that caused huge damage in Japan, he added.
Hurricanes and typhoons are different names for the same type of storm.
The storms that rage across the western Pacific Ocean are called typhoons, while the ones that blitz the Atlantic and eastern Pacific are called hurricanes.
They are all known as tropical cyclones.
These form when warm ocean water evaporates - forming an air parcel that is both warm and moist - away from the Equator, where Earth's rotational force can be felt.
Water vapor evaporating from the warm water surface raises the energy of the atmosphere near the surface, explained Dr Xie, an expert in ocean-atmosphere interactions.
"The rising air cools, causing water vapor inside to condense. The latent heat released during the condensation warms the air," he said.
Due to the Coriolis force, which is the force that can be felt because of Earth's rotation, the whole system of clouds and wind spins and grows as it is fed by the ocean's heat and water evaporating from the surface.
Said Dr Xie: "Tropical cyclones are often viewed as heat engines that can convert heat energy released from the condensation of warm, moist ocean air, into the kinetic energy of the storms."
With human activity pumping more emissions into the atmosphere and trapping heat on the planet, the ocean heat content will also go up, said Singapore Management University’s Associate Professor Winston Chow, a lead author for an upcoming report by the Intergovernmental Panel of Climate Change (IPCC). This could provide additional “fuel” for tropical cyclones and increase their intensity, said Prof Chow.
The climate change link
Weather scientist Koh Tieh Yong from the Singapore University of Social Sciences said there is no fixed duration between two high-intensity tropical cyclones.
So while Hurricane Dorian and Typhoon Hagibis had occurred one after the other, it was not possible to directly link their occurrence to climate change, said Associate Professor Koh.
Last month, the United Nations' Intergovernmental Panel on Climate Change (IPCC) had noted that if global temperatures go up by 2 deg C, the average intensity of tropical cyclones, the proportion of Category 4 and 5 tropical cyclones, and the associated average rainfall rates are projected to increase as well.
Limiting global warming to 2 deg C above pre-industrial levels is a threshold used by global leaders in setting policies that aim to limit heat-trapping emissions.
But Prof Koh noted that the IPCC had only cited limited evidence linking rising global temperatures with an increased number of Category 4 or 5 tropical cyclones per year.
More research is needed to confirm this aspect, he said.
A Singapore perspective
Tropical cyclones are unlikely to hit Singapore due to the Republic's position near the Equator, where the Coriolis force, which arises from Earth's rotation, is practically absent.
However, there was an exception in December 2001: Typhoon Vamei.
It was the first recorded tropical cyclone formation within 1.5 degrees of the equator, noted a 2003 scientific paper.
Prof Koh said it was a rare occurrence, with some scientists estimating that such events tend to happen only once every 100 to 400 years, at random.
During that event, two key "ingredients" required for the formation of a tropical cyclone were present - a rotational momentum; and warm, moist air.
Explained Prof Koh: "In that month, there were several strong surges of monsoon winds in the South China Sea."
So even though the Coriolis force is absent on the equator, the strong monsoon winds carried rotational momentum from the higher latitudes into our neighbourhood, said Prof Koh.
Warm, moist air was also present in 2001, noted Prof Koh.
"Coincidentally, in the day preceding the formation of Vamei, severe storms had formed off west Borneo in the South China Sea," he said.
As rain fell, it released latent heat that encouraged the rising, twisting motion in the surrounding air.
"These storms organised and intensified the ambient rotating winds into an intense cyclone," said Prof Koh.
Asked whether tropical cyclone activity near Singapore could increase with climate change, Prof Koh said it was too soon to say, pointing to insufficient observational data.