With a month to go until its official finish, the 2008 Atlantic hurricane season has seen more damage, as measured in dollars, than any other year except the monster 2005 season. Scientists have yet to agree whether human-induced climate change has caused spiking Atlantic hurricane activity since the early 1990s – and while the season has raged on, researchers have continued to go back and forth on whether worse is in store as the ocean keeps warming. Science this week has the latest salvo in the longtime debate: a Perspective (subscription) by Gabriel Vecchi of NOAA, Kyle Swanson of the University of Wisconsin-Milwaukee, and Brian Soden of the University of Miami.
Warming sea surface temperatures (SSTs) in the Atlantic, thought to fuel hurricanes, are correlated with the recent surge in storm activity. The trend is roughly linear, note Vecchi et al., and extending it along with predicted temperature rises implies that by 2100, a hurricane season like 2005’s might be considered mild.
But that’s only true, they caution, when you look at absolute temperatures. Relative warming – the Atlantic heating up even more than other tropical seas – is equally well-correlated. Citing their recent papers in Nature and Geochemistry, Geophysics and Geosystems, and Knutson et al. in Nature Geoscience, the authors make an argument that relative warming is more likely to be the true cause of increasing activity. And relative warming of the Atlantic sea surface, in contrast to absolute warming, doesn’t keep trending upwards in 21st-century climate predictions.
If they’re correct, we’ve already seen more or less the worst of warming-induced hurricanes. They write:
A future where relative SST controls Atlantic hurricane activity is a future similar to the recent past, with periods of higher and lower hurricane activity relative to present-day conditions due to natural climate variability, but with little long-term trend.
Linking recent changes in hurricane activity to relative SST would also preclude linking them to humans, say Vecchi et al. Unlike rises in absolute Atlantic SST – and many other global and regional temperature changes – “the recent changes in relative SST in the Atlantic are not yet distinct from natural climate variability.”
Photo: Flooding in Galveston, Texas, as Hurricane Ike approached in September / Jocelyn Augustino, FEMA