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Figure 2: Number of Katrina magnitude surge events per decade (B) hindcast and projected changes in temperatures from climate model BNU-ESM under for RCP4.5 (A). The thick blue line shows the projection using the full spatial gridded temperatures and confidence interval (5–16–84–95%); magenta and black show the projections using only Main Development Region (MDR) and global average surface temperature.
In short, the Grinsted results suggest that by the end of the century, we will see 2 to 7 times more Katrina-like intense hurricanes. Moreover, their storm surges and associated damage will be even larger because sea levels will also be higher. In another important result, Grinsted et al. found that on average, the frequency of Katrina-magnitude storm surges doubles for every approximately 0.4°C average global surface warming. Since human-caused global surface warming over the past century has already exceeded 0.4°C,
“We have probably crossed the threshold where Katrina magnitude hurricane surges are more likely caused by global warming than not.”
Holland and Bruyère
Holland and Bruyère (2013) developed an Anthropogenic Climate Change Index (ACCI) to investigate the potential global warming contribution to current tropical cyclone activity. Their ACCI is the difference between climate model runs including human climate influences (greenhouse gases and aerosols) and runs without those human influences. The study concluded that while they don’t see any human influence in the total number of hurricanes, there is a strong signal with global warming causing more strong (Category 4 and 5) and fewer weak (Category 1 and 2) hurricanes (Figure 3)
“We find an observed change in the proportion of global Cat 4–5 hurricanes (relative to all hurricanes) at a rate of ~40% increase in proportion per °C increase in ACCI … We conclude that since 1975 there has been a substantial and observable regional and global increase in the proportion of Cat 4–5 hurricanes of 25–30% per °C of anthropogenic global warming.”
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This result means more than a doubling of strong hurricanes for every °C of warming, similar to that of Grinsted et al. (2–7 times more Katrina-like events), though a bit lower. The good news is that the model used by Holland and Bruyère anticipates that we are approaching a limit in this trend of increasing proportionality of intense hurricanes.
“An important finding is that the proportion of intense hurricanes appears to initially increase in response to warming oceans, but then approach a saturation level after which no further increases occur. There is tentative evidence that the saturation level will differ across the tropical cyclone basins and that the global proportion of Cat 4–5 hurricanes may already be near it’s saturation level of ~40–50%.”
Figure 3: Human influence on hurricane proportions in the highest (Category 4-5) and lowest (Category 1-2) Saffir–Simpson hurricane categories.
Summary
These two papers add to the growing body of evidence that we are seeing more intense hurricanes as a result of human-caused global warming. The Grinsted paper also notes that the most harmful aspect of hurricanes – storm surges – have become larger over the past few decades.
The future of hurricanes remains an open question. While Grinsted predicts that the most intense hurricanes will continue to become more and more frequent in a warming world, the results of Holland and Bruyère suggest that we may be near the peak of intense hurricane frequency. The Grinsted results are more in line with most previous hurricane modeling research, but for the sake of people living in areas subject to hurricanes, we hope that Holland and Bruyère are correct about the hurricane saturation level.
Figure 2: Number of Katrina magnitude surge events per decade (B) hindcast and projected changes in temperatures from climate model BNU-ESM under for RCP4.5 (A). The thick blue line shows the projection using the full spatial gridded temperatures and confidence interval (5–16–84–95%); magenta and black show the projections using only Main Development Region (MDR) and global average surface temperature.
Figure 3: Human influence on hurricane proportions in the highest (Category 4-5) and lowest (Category 1-2) Saffir–Simpson hurricane categories.
