Linking hurricanes to climate change is not based on empirical science and threatens to misdirect policies.
Hurricane Harvey was the first major hurricane to strike the USA since 2005.
The storm made landfall in Houston, causing widespread damage, mostly from flooding.
And then Harvey was followed by Hurricane Irma, which landed in Florida.
The return of hurricanes has excited many climate change campaigners. And even some scientists have blamed both storms on climate change.
But was there anything unusual about these extreme weather events? And can we detect a human influence on these storms, as has been widely reported?
Climatologist professor Judith Curry, says no.
“These aren’t particularly unusual as far as hurricanes go. They’re top-20 kind of storms, but they’re not record-breaking in any way, apart from the overall rainfall from Harvey, which was really more of a fluke from the weather situation that allowed the storm to sit in one place for a very long time. There is nothing particularly unusual about this hurricane season or about Harvey or Irma. The US had incredibly lucky run of 12 years without a major landfall during this active phase of the hurricane cycle. So we were incredibly lucky. You know our luck is now broke. But you know, it’s totally expected.”
The coastal region of the south western US has a long history of hurricanes, the most deadly claiming 8,000 lives in Galveston, Texas, in 1900.
Since then, and in spite of climate change, the human costs caused by hurricanes have fallen.
But around the world, the news media carried stories about climate change worsening hurricane frequency and intensity.
But these claims, too, have no foundation in science.
Prof Judith Curry:
“Well we only have good satellite data back to maybe 1980. We have some satellite date going back to 1970. But it’s of lesser quality. So we don’t have long global records. But in the Atlantic, we have pretty good historical records, at least for the land-falling hurricanes — not necessarily for the total number in the basins. So, for the satellite record globally, there’s no trend in the numbers or overall, erm, accumulated cyclone energy.”
According to the observational record the number of hurricanes of all categories landing on the US have reduced in frequency. And there is no convincing evidence of hurricanes or tropical cyclones increasing in intensity, frequency, or accumulated energy.
In spite of the science, however, the story of increasing extreme weather persists.
At the centre of these stories is the way hurricanes are born. They form over large bodies of warm water drawing energy from evaporation and the Earth’s rotation.
But, says Judith Curry, sea surface temperature is not the whole story:
“Sea surface temperature is only one ingredient for hurricane development and intensification, and it doesn’t just seem to be absolute sea surface temperature, either. So it’s more relative sea surface temperatures and the overall dynamics of the atmosphere that are arguably the key ingredients – not just absolute sea surface temperature itself. I mean you can go back and there were really strong hurricanes in the nineteenth century for example, where surface temperatures were significantly cooler. And there were some horrendous hurricanes in the Atlantic in the early part of the twentieth century, when sea surface temperatures were noticeably cooler.”
This is a more complex picture of what causes hurricanes than the one presented by many commentators.
And many have been keen to draw a direct link between human activity and hurricanes.
But even according to mainstream climate science, as represented by NOAA and the IPCC, global warming may only increase the intensity of storms by as little as 2% to 11 per cent by the end of the century.
Claims that the effects of climate change can be seen in hurricanes are premature given the high degree of natural variability in these events.
Prof Judith Curry:
“We’re probably looking at a shift in the Atlantic to the cool phase of the Atlantic Multidecadal Oscillation. In 1995, after a relatively quiet period in the Atlantic, we flipped to the warm phase of the Atlantic Multidecadal Oscillation, and that like really juiced-up the hurricanes. And so, at some time, probably in the order of ten years, we’ll see a flip to the cool phase again, and presumably much quieter situation in the Atlantic for hurricanes.”
The danger is that claims about global warming and extreme weather are a diversion from the real issues that affect people’s lives.
Indeed, such strong statements about hurricanes and other extreme weather events are not based on the science as we understand it and they may misdirect policies.
Prof Judith Curry:
“In terms of trying to figure out how to manage extreme events and reduce our vulnerability, what’s causing it is almost a secondary concern. I mean, we’re not preparing for the evens we have now, or the events we have seen in the 20th century, let alone the events we might see in the later part of the 21st century.”
Disaster resilience and preparedness are the best policies against hurricanes, regardless of the influence of carbon dioxide or climate change.