Climate change could spark a new wave of ‘mega-hurricanes’ so powerful that scientists say a new category is needed to measure them.
Since the 1970s, the National Hurricane Center has used the Saffir-Simpson Windscale to measure hurricanes on a scale of one to five.
But now, scientists from the Lawrence Berkeley National Laboratory say that the scale should be extended to include category six hurricanes.
With sustained winds of more than 192 mph (309km/h) the hurricanes in this new category have wind speeds as fast as a Lamborghini Gallardo.
Over the last decade, only five storms would have been included in this new category – but researchers say that the risk will increase as the climate warms.
Researchers say that storms are becoming so powerful due to climate change that massive hurricanes like Hurricane Patricia, pictured here over the Atlantic Ocean in 2015, should have a new category made for them
The researchers propose a new hypothetical category six for measuring hurricanes. These storms have sustained winds of more than 192 mph (309km/h), as fast as a Lamborghini Gallardo
Hurricanes, otherwise called tropical storms or typhoons, are currently categorised according to their sustained wind speeds.
Category One hurricanes have windspeeds of 74mph (119km/h).
At Category Three, which is considered to be a risk of severe damage to life and property, there are sustained windspeeds between 111 and 129 mph (178-208 km/h).
Meanwhile, Category Five, which is reserved for the most devastating storms, begins at 157 mph (252 km/h) but is open-ended.
The reason the scale stops here is that the it is intended to reflect property damage and it was believed that a category five hurricane would completely destroy any structure in its path.
However, in their paper, published in Proceedings of the National Academy of Sciences, Dr Wehner and his co-author suggest that the scale should be extended to include a hypothetical category six.
He says: ‘Our motivation is to reconsider how the open-endedness of the Saffir-Simpson Scale can lead to underestimation of risk, and, in particular, how this underestimation becomes increasingly problematic in a warming world.’
By following the pattern of gaps between the previous categories, the researchers set the lower bounds for category six and looked through past weather data to see if any hurricanes would qualify.
In an analysis of data from 1980 to 2021, they found that five storms would have been classified as category six and that all of these occurred in the last nine years of records.
Those five storms include some of the most powerful and destructive hurricanes in recent history.
This graph shows how the Saffir Simpson Category relates to wind speed. The coloured triangles show the storms which the researchers would classify as category six. As you can see they are far beyond the limit of category five (shown as a grey triangle)
This diagram shows Hurricane Patricia as it approaches the coastline of Mexico. At the centre of the storm, shown in dark red, wind speeds were at 198 mph (320 km/h) which would make this a category six hurricane
Hurricane Patricia, for example, had windspeeds of 201mph before making landfall in the Gulf of Mexico.
Typhoon Haiyan meanwhile killed thousands when it struck the Philippines in 2013.
The authors note that even at the time, Typhoon Haiyan was so powerful that many called for it to be classed as category six.
According to study co-author Dr James Kossin of the First Street Foundation, the Saffir-Simpson Windscale is far from perfect as a means of communicating hurricane risk.
This is because much of the damage and fatalities caused by hurricanes are not due to the wind speed but the storm surge, flooding, and heavy rainfall.
Dr Kossin says: ‘Changes in messaging are necessary to better inform the public about inland flooding and storm surge, phenomena that a wind-based scale is only tangentially relevant to.
‘Our results are not meant to propose changes to this scale, but rather to raise awareness that the wind-hazard risk from storms presently designated as Category 5 has increased and will continue to increase under climate change.’
Category five storms like Hurricane Dorian, seen here from the International Space Station, are already extremely dangerous. But the researchers warn that storms even more powerful are likely to become more common in the future as the planet warms
This diagram shows how many days wind speeds were in excess of ‘category six’ levels between 1979 and 2019. The darker red regions had more days of category six wind such as the North West coast of Australia which had as many as 45 days in some areas
And, as the climate continues to warm, storms that would fall into category six are likely to become more frequent.
Typhoons, hurricanes, and tropical storms, which are all essentially the same weather phenomena, are caused when warm moist air rises from the ocean.
As human-induced climate change raises the Earth’s temperature, the seas and oceans become hotter in regions where these storms typically form.
This means that there is more heat energy to drive the storm, leading to more powerful and dangerous storms.
According to the researchers’ simulations, a rise in temperature of only 2°C above pre-industrial levels could lead to storms becoming significantly more common.
At these levels of warming the risk of a category six storm increases by up to 50 per cent near the Philippines and doubles in the Gulf of Mexico.
Dr Wehner says: ‘Even under the relatively low global warming targets of the Paris Agreement, which seeks to limit global warming to just 1.5°C above pre-industrial temperatures by the end of this century, the increased chances of Category 6 storms are substantial in these simulations.’