It’s a sore spot for some climate change pseudoskeptics. Any time anyone makes any kind of claim about the effects of a warming planet on tropical storm activity, you can count on a deluge of rejoinders about how shaky the science on the subject really is.
Much is made of Al Gore’s use of an image of hurricane spewing forth from a smokestack on a promotional poster for “An Inconvenient Truth.” And Chris Mooney devoted an entire book to exploring the science, and the battles over and about the science, Storm World. I was commissioned by the Weather Channel a couple of years back to write a feature on the latest scientific developments as several hurricanes tore threw Haiti. The question everyone wants to know is, was Gore right? Will climate change make hurricanes more destructive and/or more frequent?
If you were hoping for a definitive answer, look elsewhere. But if you’re willing to be patient and consider how slowly science moves on such things, then a new paper in Nature Geoscience is worth a read.
“Tropical cyclones and climate change” is remarkable in part because of the authors list. Almost all the big names are there, including Thomas Knutson, Greg Holland, Kerry Emmanuel, Chris Landsea, and six others who haven’t received the same kind of name recognition for one reason or another. The fact that there is now a single document that represents a sort of consensus on the subject isn’t particularly new — a group composed of many of the same scientists issued a World Meteorological Organization statement on the state of the science in late 2006. Back then there was much debate about what warming sea surface temperatures were doing to hurricane trends and the most definitive statement the group could come up with was
…it is likely that some increase in tropical cyclone intensity will occur if the climate continues to warm.
Since then they’ve learned a fair bit. There are still loads of unanswered questions and predictions for specific ocean basins come with much larger degrees of uncertainty than global trends. But here’s the take home message of the new paper, which serves as an update for the 2006 statement, conveniently summarized in a box:
Detection and attribution
It remains uncertain whether past changes in any tropical cyclone activity (frequency, intensity, rainfall, and so on) exceed the variability expected through natural causes, after accounting for changes over time in observing capabilities.
Tropical cyclone projections
Frequency. It is likely that the global frequency of tropical cyclones will either decrease or remain essentially unchanged owing to greenhouse warming. We have very low confidence in projected changes in individual basins. Current models project changes ranging from −6 to −34% globally, and up to ±50% or more in individual basins by the late twenty-first century.
Some increase in the mean maximum wind speed of tropical cyclones is likely (+2 to +11% globally) with projected twenty-first-century warming, although increases may not occur in all tropical regions. The frequency of the most intense (rare/high-impact) storms will more likely than not increase by a substantially larger percentage in some basins.
Rainfall rates are likely to increase. The projected magnitude is on the order of +20% within 100 km of the tropical cyclone centre.
Genesis, tracks, duration and surge flooding
We have low confidence in projected changes in tropical cyclone genesis-location, tracks, duration and areas of impact. Existing model projections do not show dramatic large-scale changes in these features. The vulnerability of coastal regions to storm-surge flooding is expected to increase with future sea-level rise and coastal development, although this vulnerability will also depend on future storm characteristics.
To summarize the summary: There’s a good chance that we might see fewer hurricanes or maybe no change in frequency overall, but there’s also a good chance that we’ll see storm intensity grow and more of the most intense storms.
So it would appear there is justification for the linking of global warming and hurricane threats after all. Was Al Gore justified five years ago? At the time, the science was less conclusive, but that’s the way things were tilting at the time. Between then and now, things were in flux, and they still are and likely will be for a while yet. But arguing that there is no science to support the link is simply not congruent with the published literature.
–Knutson, T., McBride, J., Chan, J., Emanuel, K., Holland, G., Landsea, C., Held, I., Kossin, J., Srivastava, A., & Sugi, M. (2010). Tropical cyclones and climate change Nature Geoscience, 3 (3), 157-163 DOI: 10.1038/ngeo779