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IPCC AR6: Event attribution for tropical cyclones, unspun edition

29 Sep 2021 | Science Notes

“Event attribution” is the latest fad in climatology in which scientists point to events after they have happened and try to prove greenhouse gases must be at fault. These studies have been criticized for their biased and unreliable methodology, but they seem to be here to stay. So, to find out what the fine print reveals regarding event attribution for major tropical cyclones, we turn to the IPCC AR6 Section As always, we sometimes skip lists of references using (---) and spell out occasional short forms, but otherwise what follows is the AR6 text verbatim.

There is general agreement in the literature that anthropogenic greenhouse gases and aerosols have measurably affected observed oceanic and atmospheric variability in Tropical Cyclone (TC)-prone regions (see Chapter 3). This underpinned the SROCC assessment of medium confidence that humans have contributed to the observed increase in Atlantic hurricane activity since the 1970s (---). Literature subsequent to the AR5 lends further support to this statement (---). However, there is still no consensus on the relative magnitude of human and natural influences on past changes in Atlantic hurricane activity, and particularly on which factor has dominated the observed increase (---) and it remains uncertain whether past changes in Atlantic TC activity are outside the range of natural variability. A recent result using high-resolution dynamical model experiments suggested that the observed spatial contrast in TC trends cannot be explained only by multi-decadal natural variability, and that external forcing plays an important role (---). Observational evidence for significant global increases in the proportion of major TC intensities (---) is consistent with both theory and numerical modeling simulations, which generally indicate an increase in mean TC peak intensity and the proportion of very intense TCs in a warming world (---). In addition, high-resolution coupled model simulations provide support that natural variability alone is unlikely to explain the magnitude of the observed increase in TC intensification rates and upward TC intensity trend in the Atlantic basin since the early 1980s (---).

… Event attribution is used in case studies of TCs to test whether the severities of recent intense TCs are explained without anthropogenic effects. In a case study of Hurricane Sandy (2012), Lackmann (2015) found no statistically significant impact of anthropogenic climate change on storm intensity, while projections in a warmer world showed significant strengthening. On the other hand, Magnusson et al. (2014) found that in [European model] simulations, the simulated cyclone depth and intensity, as well as precipitation, were larger when the model was driven by the warmer actual SSTs than the climatological average SSTs. In super typhoon Haiyan, which struck the Philippines on 8 November 2013, Takayabu et al. (2015) took an event attribution approach with cloud system-resolving (~1km) downscaling ensemble experiments to evaluate the anthropogenic effect on typhoons, and showed that the intensity of the simulated worst case storm in the actual conditions was stronger than that in a hypothetical condition without historical anthropogenic forcing in the model. However, in a similar approach with two coarser parameterized convection models, Wehner et al. (2018) found conflicting human influences on Haiyan’s intensity. Patricola and Wehner (2018) found little evidence of an attributable change in intensity of hurricanes Katrina (2005), Irma (2017), and Maria (2017) using a regional climate model configured between 3 and 4.5 km resolution. They did, however, find attributable increases in heavy precipitation totals. These results imply that higher resolution, such as in a convective permitting 5-km or less mesh model, is required to obtain a robust anthropogenic intensification of a strong TC by simulating realistic rapid intensification of a TC (---), and that whether the intensification of TCs can be attributed to the recent warming depends on the case.

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