Alpha, beta, gamma...
Alpha, beta, gamma...
Is the record-breaking Atlantic hurricane season a sign of climate change?
On a good day, the Basin Reserve is one of the loveliest places to watch cricket, with a sunny embankment, graceful pohutukawa trees and nothing but a white picket fence between players and spectators. But there’s not much to be done about the fact that it’s in Wellington, and good days are far from common. When a test match is scheduled in December, I expect to be watching that match wrapped up against the cold.
I was not disappointed on Sunday when I went to see New Zealand play the West Indies. I expected cold weather and I got cold weather. But I couldn’t help feel for the West Indian players – the weather was miserable for someone used to Wellington weather, and it must have been much worse for people used to tropical climates.
It’s easy to be flippant on days like that, and say that a world which is, on average, a degree or two warmer doesn’t seem like a bad thing. It doesn’t seem like much when you are wrapped in polar fleece and yet are still freezing in early summer.
But talking about average temperature is deceptive. The world is not warming evenly, and it’s easy, in a place like New Zealand, to forget how serious climate change really is. Overall, New Zealand has warmed less than many other parts of the globe, and that trend is likely to continue. As I discussed in my previous article, El Niño is likely to intensify, leading to cooler oceans around New Zealand. Quite literally, we are insulated against the full impacts of climate change by the ocean around us.
On the other hand, the West Indian cricket team comes from a region which has suffered more than most under climate change’s uneven impacts this year. The Atlantic hurricane season has been the worst on record, with the greatest number of named storms ever reported. As a result, the World Meteorological Organisation resorted to a naming convention needed only once before – using letters of the Greek alphabet instead of human names.
Tropical storms are named to make communication about those storms easier. Every year, there is a pre-set list of human names, which are used for any storm above a certain size – that size varies because different regions have different criteria. In the Atlantic region, all storms with sustained winds over 39 miles per hour get a name. The list isn’t completely new each year, rather the lists are reused every few years. But if there is a notable storm that people will remember by its name, like the infamous Hurricane Katrina, then that name is removed from the list and not used again.
At the start of the season, meteorologists have a list of twenty or so names to be used for that season (the exact number varies by region, as most lists leave out less common letters like Q, X or Z). But in 2005, the year of Hurricane Katrina, there was a problem – there were so many storms that they ran out of names for the first time.
Now the same problem has happened again. By the 18th of September, an area of disturbed weather which had travelled from the coast of Africa had been given the name Wilfred – the last name on the list for Atlantic hurricanes in 2020. The following day, there were already two storms named from the Greek alphabet. By the time the season officially ended on the 30th of November, there had been nine storms named from the Greek alphabet, bringing the total number of named storms to thirty, exceeding the record set in 2005 by two.
In my last article, I discussed how New Zealand’s unusually warm spring was linked to both climate change and the La Niña climate pattern. The Atlantic hurricane season, as it turns out, is also linked to these two factors, but it’s a much more complex picture than that.
The La Niña impacts I discussed last time related to differences in ocean temperature, but in the Atlantic it isn’t the water affected by El Niño and La Niña; it’s the wind. Under El Niño conditions, the warm water in the eastern Pacific warms the air above it. The warm air rises, increasing winds in the upper atmosphere over Central America, the Caribbean Sea and the Atlantic Ocean. The increased wind at high altitudes makes it harder for storms to form over the Caribbean and Atlantic, thereby reducing hurricanes. El Niño leads to a range of negative impacts, but it does also have a positive side.
In La Niña, however, less warm air rises, slowing the winds above the Caribbean and Atlantic. The more stable weather results in more and larger hurricanes. Forecasters knew that the 2020 hurricane season would be bad well before they started using names from the Greek alphabet.
The other part of the story is Atlantic sea surface temperature, which has been higher in the last decade than any other time in the last 3000 years. We tend to think of global warming in terms of air temperatures, but most of the increased heat over the last few decades has actually been absorbed by the oceans. Ocean warming has a range of impacts, from coral bleaching to reduced oxygen levels. And it increases tropical cyclones.
Tropical cyclones form in the ocean over areas of particularly warm water – at least 26.5oC. The air above this water then becomes warm and rises, creating an area of low pressure. Air flows into the area of low pressure, creating wind. The rotation of the earth bends the wind direction, causing it to spiral. The rising of the air and the wind spiralling into the area of low pressure creates a feedback loop, with more air rising and winds blowing in even faster. At the same time, the warm air is evaporating water from the ocean surface and carrying it upwards, where it condenses into huge clouds.
The best explanation I’ve found is in this video, so if you want a better understanding than I can give with words on a page, have a look.
Although climate change means warmer oceans, and it is given as one of the reasons for the record Atlantic hurricane season, there’s another reason that the Atlantic hurricane season was so bad, and it’s a surprising one. Over the last 40 years, there has been an increase in Atlantic hurricanes corresponding with a decrease in air pollution. In the latter part of last century, cyclone activity was reduced by the cooling effect of the pollution in the atmosphere over the Atlantic ocean. As pollution levels have been reduced, due to regulation and better technology, cyclone activity has increased.
The picture is still far from clear, however. Pollution, climate change, El Niño – all of these factors affect cyclones, but exactly what effects they will have, in combination, is highly uncertain. One important point is that, just as with global temperatures, averages are deceptive. The global average number of tropical cyclones has held steady for the last forty years, unlike global temperatures. But what has changed is where those cyclones occur.
This image, from the US National Oceanic and Atmospheric Administration, shows that while there have been more tropical cyclones in the Atlantic and Caribbean, there have been fewer in the western Pacific.
As well as changing the distribution of tropical cyclones, climate change is predicted to increase their intensity. But it’s far from a simple matter to know if this is actually happening. We don’t have detailed data on cyclones for as long as we have, for example, temperature records. We do know that cyclones are becoming more damaging, but damage is measured in relation to people and property. That is, the more people and property in the path of a cyclone, the more damage that cyclone causes.
However, recent evidence based on 39 years of satellite data suggests that the models are correct, and cyclones are becoming more intense.
Now that the Atlantic hurricane season has ended, and the Pacific season is beginning, I have to wonder what we can expect to see here. Will the Pacific have as many storms and see as much damage as the Caribbean?
The answer, mostly, is “no”. La Niña might have made things worse for the Atlantic, but similar La Niña years in the Pacific haven’t been so bad. Most Pacific nations have an average or below-average tropical cyclone risk for the coming season. There are two places which are exceptions, though, New Caledonia and New Zealand. Unfortunately, for us, the warm water pushed down to us from the western Pacific is likely to bring trouble. For New Zealand, a La Niña cyclone season is not good news.
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