Even if we do know with certainty that the Earth is warming, and even if we do know with certainty that humans are responsible (whether in part or in whole) for that, we still don’t know with certainty what the effects will be.
Today we set a record in the U.S. for days between major hurricanes.
The previous record was in the early 20th century, between 1900 and 1906.
While Kevin Trenberth (2005) did note that we couldn’t yet say anything about how warming would affect the number of hurricanes, he did pretty unequivocally say there would be an increase in intensity.
[O]nce a tropical storm has formed, the changing environmental conditions provide more energy to fuel the storm, which suggests that it will be more intense than it would otherwise have been, and that it will be associated with heavier rainfalls. Groisman et al. found no statistically significant evidence that precipitation associated with hurricanes increased along the southeastern coast of the contiguous United States during the 20th century; however, their analysis did not include years after 2000, and there was a distinct increase in hurricane precipitation after 1995. Groisman et al. found a linear upward trend in precipitation amount by 7% in the 20th century in the contiguous United States; the increases in heavy precipitation (the heaviest 5%) and very heavy precipitation (the heaviest 1%) were much greater at 14% and 20%, respectively. Such trends are likely to continue.
Thus, although variability is large, trends associated with human influences are evident in the environment in which hurricanes form, and our physical understanding suggests that the intensity of and rainfalls from hurricanes are probably increasing, even if this increase cannot yet be proven with a formal statistical test. Model results suggest a shift in hurricane intensities toward extreme hurricanes.
Of course we’re dealing with short-range data here (and as Pielke notes, this record streak will end sometime and won’t be repeated anytime soon, a pretty safe bet when dealing with once-in-a-century events), but Trenbarth was also dealing with short-range data (his mention of finding a trend “in the 20th century” is somewhat disingenuous, given that he clearly indicates it is found only because of data from the years 1996-2000).
Will hurricanes get more intense in the future? Maybe. But obviously we don’t yet know really know.