While I think it’s foolish to comment too specifically on what the IPCC will say until the report is actually released, I join Chris Mooney and Roger Pielke, Jr. in finding this report interesting:
Global warming has made stronger hurricanes, including those in the Atlantic such as Katrina, an authoritative panel on climate change has concluded for the first time, participants in the deliberations said Thursday.
During marathon meetings in Paris, the Intergovernmental Panel on Climate Change approved language that said an increase in hurricane and tropical cyclone strength since 1970 “more likely than not” can be attributed to man-made global warming, according to Leonard Fields of Barbados and Cedric Nelom of Surinam.
I don’t know if either is considered to have a stake in that scientific argument, so I can’t assess the sources. It would be a substantial break if the IPCC does state that past hurricanes have been influenced by climate change, since the World Meteorological Organization recently produced a consensus report concluding that there was insufficient evidence to justify that claim.
I lean towards Chris Mooney’s assessment that, while this claim will be controversial, it probably is less of a break with the WMO than it might seem. In IPCC-speak “more likely than not” would presumably translate to a probability greater, but not much greater, than 50%, while the WMO’s assessment that:
The scientific debate concerning the Webster et al. and Emanuel papers is not as to whether global warming can cause a trend in tropical cyclone intensities. The more relevant question is how large a change: a relatively small one several decades into the future or large changes occurring today? Currently published theory and numerical modeling results suggest the former, which is inconsistent with the observational studies of Emanuel (2005) and Webster et al. (2005) by a factor of 5 to 8 (for the Emanuel study). The debate is on this important quantification as to whether such a signal can be detected in the historical data base, and whether it is possible to isolate the forced response of the climate system in the presence of substantial decadal and multi-decadal natural variability. This is still hotly debated area for which we can provide no definitive conclusion.
To me, the most important observation here is that there is not a scientific debate about whether global warming will induce more intense hurricanes eventually, merely whether we have empirically detected that signal already. The debate about the past trend is important in terms of demanding action and in terms of estimating the precise consequences of climate change, but I don’t know exactly what difference it makes if we haven’t yet detected the trend in hurricanes that everyone agrees will happen eventually if we continue on the course we have set for ourselves and our planet. Either way we can expect more hurricanes like Katrina.
The IPCC and the WMO agree that warmer sea surface temperatures will cause more intense hurricanes. They agree that sea surface temperatures have risen. And they consider the evidence balanced or nearly balanced about whether we’ve already seen a rise in intensity of hurricanes over the last 20 years.
By my reading of the WMO consensus statement, that scientific debate relates to issues of data quality and natural variability. The development of geosynchronous weather satellites has allowed us to get a much better sense of hurricane status since the 1970s, and some of the participants in the debate argue that the data showing a strong increase in hurricane intensity is an artifact of better hurricane tracking. Others argue that the correlation between sea surface temperature and hurricane intensity is too good to be an artifact, and that the trend persists even when you attempt to control for various conflating factors.
Pielke thinks it would indicate some sort of deep schism between the IPCC and the WMO if the IPCC were more forceful in its statement. But the difference may be a matter of small degrees, with the WMO pegging the probability that the trend has already been observed at 50%, and the IPCC pegging it at 55% (or even 51!).
My opinion is that the difference of a few percentage points ought not to make any grand difference in how we address this possibility. The consequences of allowing that to happen would be dramatic, costly and unavoidable once we cross a threshold. Under those circumstances, even if the argument over the past trend is a push, the certainty about the future trend strongly suggests the necessity of action. Action may be costly, but inaction is almost sure to be deadly and costly.