The frequency and intensity of hurricanes making landfall in the United States is being carefully monitored, as scientists look for patterns in extreme weather. According to an article in the Independent, a new study has found “support for the controversial idea that global warming is causing more frequent and destructive hurricanes” – but the authors say that’s not what their research concludes.
The study was intended to create a record of hurricane activity which could look further back than the satellite record. It shows that during warm periods over the last 90 years, hurricane activity along the southeast coast of the United States was higher. It also found the chances of a major storm doubled compared to cool years. But this isn’t quite the same as the Independent’s headline, which says that “Global warming is ‘causing more hurricanes”’
Linking climate change and hurricane activity is incredibly complex, and at the moment there isn’t enough scientific evidence to definitively make the link. So how did the Independent come to this conclusion? We take a look at the study in more detail, and investigate the difficulties of attributing the longer trend.
More activity in warmer years
The study found evidence that hurricane activity increased in warmer years in tide gauge records, taken from 6 locations along the coastline. The records document times when the sea level rose rapidly due to the storm surges accompanying hurricanes. By looking at this, the researchers were able to determine how the activity of landfalling hurricanes changed between 1923 and present day.
Contrary to the Independent‘s headline, the study doesn’t show a significant change in the total number of hurricanes that hit the coast. Instead, it finds evidence that the number of major (severe) hurricanes increased. The authors suggest that if hurricane activity continued on this upward trend, the number of major events could increase further by 2100.
This is where the authors’ conclusions stop. They don’t suggest a reason for the past rise, or indicate why it might in the future. The Independent, however, was less cautious about what the reason might be, pointing the finger at manmade global warming.
When we asked lead author, Dr. Aslak Grinsted, about the Independent’s conclusion, he explained:
“It is possible [that global warming is increasing hurricane activity], but that’s not what the study shows”
So while climate change might be one feasible explanation for the increase in hurricane activity, this study in itself isn’t evidence of a link between the two. The ways that climate change might affect the strength and frequency of hurricanes is still the subject of a lot of research, and climate models struggle to agree on how that will change in the future.
More hurricane activity in warmer years
A closer look at the study highlights an obvious pattern, that hurricane activity was higher in warmer years. This seems logical when considering that high water temperatures, along with the right wind conditions, are needed for hurricanes to form.
Surface waters must be at least 27 degrees Celsius, and winds from different directions must converge, forcing moisture upwards. These winds must remain constant at height, so moisture can be channelled high into the atmopshere.
With higher sea surface temperatures in warmer years, this critical condition is fulfilled. So it may logically follow that hurricane activity would increase. It’s also possible that warmer seas provide more energy for storms, which could explain why major storms were more likely in warmer years.
What about the longer trend?
Looking at the changes observed beyond what happens from one year to the next is much more complicated. It is possible that climate change could affect hurricane activity, but whether it does is as yet unclear – and it’s also beyond the scope of this study.
Over the past few decades, the surfaces of most tropical oceans have warmed by between 0.3 and 0.5 degrees Celsius, which several studies link to increasing greenhouse gases. Since it is known that warm water is a condition needed for hurricanes to form, this could potentially influence hurricane activity.
At the same time, however, a growing body of work suggests the threshold water temperatures for hurricanes formation is rising at about the same rate as seas are warming – so the man made impact is so far negligible.
Ultimately, the many ways climate change could affect the formation of hurricanes are poorly understood. Climate change might lead to warmer seas, but it might also affect the other conditions critical to hurricane formation. Until these mechanism are well understood, it incredibly difficult to pinpoint climate change as a cause of change.
The general consensus from scientists at the moment suggests the biggest factor determining hurricane activity over a number of years is natural variability – brought about for example by ocean and atmosphere circulations the like El NiÃ±o Southern Oscillation.
What about climate models?
Climate models aren’t yet certain about how climate change might alter hurricane activity in the future.
A 2010 review of climate modelling concluded the global frequency of hurricanes may either decrease or remain unchanged. The intensity of hurricanes globally may increase, and the number of very severe hurricanes is likely to increase. But working out how the frequency and intensity of hurricanes will change in individual ocean basins is less certain still.
Climate models are limited in their ability to recreate tropical storms, so it is difficult to simulate how climate change might affect the critical conditions needed for hurricanes to form.
Given the limitations of current knowledge and modelling, a lot more caution is needed before drawing definitive links between studies like this and climate change. Warm water is just one of the conditions needed for hurricanes to form, and how other factors like wind will change is uncertain.
It is possible that climate change plays a role in hurricane activity, but that isn’t the conclusion of this study.
Updated 23/10/12 14.00 - To include reference to study on climate modelling as indicated in comment below.