In recent years the media has been full of hyped-up stories about increasing extreme events, such as storms, floods and droughts, and keen to attribute them to climate change, along with claims of imminent catastrophe. To what extent is this supported by the IPCC, and has there been a climb-down in IPCC claims about this? Roger Pielke Jr suggested (see point 5 of his post) that there has been a ‘walking back’ of claims regarding extreme events, and says ‘kudos to the IPCC’ for this. In a follow-up post he gives a number of direct quotes from AR5 chapter 2, all of which use phrases like “limited evidence”, “no significant trend”, “low confidence”, but he laments that this is unlikely to slay the storm-caused-by-global-warming zombie.
There are two relevant tables in the AR5 report. Regarding the chance of some catastrophic event (such as the collapse of the Gulf Stream) there is Table 12.4 in Chapter 12 of the main report. At Bishop Hill, Katabasis has already discussed Table 12.4, pointing out that almost all of the catastrophes listed in Table 12.4 are either described as “very unlikely” or assessed with “low confidence”. Judith Curry has also discussed this, “Did the AR5 take the ‘dangerous’ out of AGW?”
Possible increases in events such as storms and floods are described in Table SPM.1, page 23 in the SPM. It is interesting to compare Table SPM.1 with the corresponding one from AR4. Fortunately we don’t have to compare two tables side by side because this comparison is included in the table. The text in black is the AR5 assessment, the text below in blue is from the 2012 SREX report, and the last line in red (orange) is from the 2007 AR4 (irritatingly, although Table SPM.1 is on page 23, its caption is on page 4!)
The table shows that for tropical cyclones (which includes hurricanes) there has been a reduction in confidence (for example from ‘likely’ to ‘more likely than not’ in some regions). And similarly for droughts, where the phrase ‘low confidence’ appears three times. On the other hand, risk of extreme high sea levels in the future seems to have gone up, from ‘likely’ to ‘very likely’. Similarly, likelihood of human contribution to heat waves has gone up.
So overall, is it a climbdown or a ratcheting up of extreme event risk?