Globally, the frequency of tropical cyclones has not increased since the last century, and in fact the number may have decreased
Awaiting the arrival of Hurricane Milton in Florida, which will be the ninth of the 2024 Atlantic season and the second within two weeks to hit the area after Hurricane Helene, the question raised and analyzed in a BBC article is whether climate change causes or influences extreme weather events of this kind.
What are hurricanes?
Hurricanes are strong storms that develop in warm tropical ocean waters.
In other parts of the world, they are known as cyclones or typhoons. Generally, these storms are referred to as “tropical cyclones”.
Tropical cyclones are characterized by strong winds, heavy rainfall and storm surges, and short-term sea level rise. The phenomenon often causes widespread damage and flooding.
How do hurricanes form?
Hurricanes and cyclones begin as atmospheric disturbances—like, for example, a tropical wave, an area of ​​low pressure where storms and clouds develop.
As warm, moist air rises from the surface of the ocean, the eye of a hurricane forms, which are strong, rotating winds. The process is linked to the rotation of the earth which affects the winds in tropical regions just off the equator.
For a hurricane to develop and continue to rotate, the sea surface must generally be at least 27 C to provide enough energy, and the winds must not vary much with height.
If all these factors come together, a severe hurricane can form, although the exact causes of individual storms are complex.
Are they getting worse?
Globally, the frequency of tropical cyclones has not increased since the last century, and in fact the number may have decreased – although long-term data is limited for some regions.
However, it is “likely” that a higher proportion of tropical cyclones around the world will reach category three or above, meaning winds blow at higher speeds, according to the UN’s climate watchdog, the IPCC.
The IPCC reports that there is likely to have been an increase in the maximum rate of rainfall associated with tropical cyclones.
The frequency and size of “rapid intensification events” in the Atlantic have also increased. This means that maximum wind speeds increase very quickly, which can be particularly dangerous.
There also appears to have been a slowdown in the speed at which tropical cyclones move over the Earth’s surface. This usually brings more rainfall to the specific areas. For example, in 2017 Hurricane Harvey lingered over Houston resulting in 100cm of rain falling in three days.
In some places, the average location where tropical cyclones reach their maximum intensity has shifted poleward—for example, the western North Pacific. This can put new areas at risk.
At the same time, there is some evidence that the increased intensity of hurricanes in the US is causing more damage.
Is climate change affecting hurricanes?
Assessing the precise impact of climate change on individual tropical cyclones is challenging. Storms are relatively localized and short-lived and can vary greatly from case to case.
But rising temperatures are affecting these storms in many measurable ways.
First, warmer ocean waters mean storms can pick up more energy, leading to higher wind speeds.
Record high sea surface temperatures were a key reason US scientists predicted a longer-than-normal Atlantic hurricane season for 2024.
The high temperatures are mainly due to long-term greenhouse gas emissions.
Second, a warmer atmosphere can hold more moisture, leading to heavier rainfall.
Climate change made the extreme rainfall events from Hurricane Harvey in 2017 about three times more likely, according to one estimate.
Finally, sea levels are rising, mainly due to a combination of melting glaciers and ice sheets and the fact that warmer water takes up more space. Local factors can also play a role. This means that the storms occur on top of already raised sea levels, exacerbating coastal flooding.
For example, it is estimated that the height of the flood from Hurricane Katrina in 2005 – one of America’s deadliest storms – was 15-60% higher than it would have been under 1900 climate conditions.
Overall, the IPCC concludes that there is an estimate that humans have contributed to an increase in the rainfall associated with tropical cyclones and to a tropical cyclone being more intense.
How might hurricanes change in the future?
The number of tropical cyclones worldwide is unlikely to increase, according to the IPCC.
But as the climate gets warmer it is “very likely” that we will see higher rainfall rates and stronger winds. This means that a greater percentage will reach the more intense categories, four and five.
The more global temperatures rise, the more extreme these changes will tend to be.
Source: Skai
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