According to its website, GFDL "works cooperatively with the [National Oceanic and Atmospheric Administration] to advance its expert assessments of changes in national and global climate through research, improved models, and products."
It is apparent from reading the report that Mr. Knutson is very supportive of the anthropogenic (man made) global warming theory. He seemed very eager to prove that global warming had caused an increase in Atlantic hurricane activity. In conducting his study, he used data to go beyond the typical 50 year historical record of Atlantic hurricanes. To Mr. Knutson's credit, he reported the data honestly:
To gain more insight on this problem, we have attempted to analyze much longer (> 100 yr) records of Atlantic hurricane activity. If greenhouse warming causes a substantial increase in Atlantic hurricane activity, then the century scale increase in tropical Atlantic SSTs since the late 1800s should have produced a long-term rise in measures of Atlantic hurricanes activity.
Existing records of past Atlantic tropical storm numbers (1878 to present) in fact do show a pronounced upward trend, which is also correlated with rising SSTs.... However, the density of reporting ship traffic over the Atlantic was relatively sparse during the early decades of this record, such that if storms from the modern era (post 1965) had hypothetically occurred during those earlier decades, a substantial number would likely not have been directly observed by the ship-based "observing network of opportunity." We find that, after adjusting for such an estimated number of missing storms, there is a small nominally positive upward trend in tropical storm occurrence from 1878-2006. But statistical tests reveal that this trend is so small, relative to the variability in the series, that it is not significantly distinguishable from zero.... In addition, a new study by Landsea ... notes that the rising trend in Atlantic tropical storm counts is almost entirely due to increases in short-duration (less than 2 days) storms alone. Such short-lived storms were particularly likely to have been overlooked in the earlier parts of the record, as they would have had less opportunity for chance encounters with ship traffic. In short, the historical tropical storm count record does not provide compelling evidence for a substantial greenhouse warming induced long-term increase.
If we instead consider Atlantic basin hurricanes, rather than all Atlantic tropical storms, the result is similar: the reported numbers of hurricanes were sufficiently high during the 1860s-1880s that again there is no significant positive trend in numbers beginning from that era (Figure 4, black curve, from CCSP 3.3 (2008) ). This is without any adjustment for "missing hurricanes". (Emphasis is in the original document.)
The evidence for an upward trend is even weaker if we look at U.S. landfalling hurricanes, which even show a slight negative trend beginning from 1900 or from the late 1800s (Figure 4, blue curve)....As a scientist Knutson had set out a hypothesis, i.e. that global warming had led to an increase in Atlantic hurricane activity, and found that, upon a fair assessment of the data, the hypothesis could not be proven. The report should have ended there. Instead Knutson changes the focus of his report to making predictions about the future of hurricanes incorporating into those predictions all kinds of scary projections about rising temperatures. Knutson concludes:
So in a nutshell, Knutson set out to prove that global warming had resulted in increased hurricane activity. Failing to find data to support that, Knutson switches to making guesses about the future of hurricanes based on "model simulations of greenhouse warming."climate warming will cause hurricanes in the coming century to be more intense globally and to have higher rainfall rates than present-day hurricanes. In my view, there are better than even odds that the numbers of very intense (category 4 and 5) hurricanes will increase by a substantial fraction in some basins, while it is likely that the annual number of tropical storms globally will either decrease or remain essentially unchanged.
More evidence of politicization of his role of a scientist is evident from his Knutson's conclusions which actually leads off the report:
- It is premature to conclude that human activities--and particularly greenhouse gas emissions that cause global warming--have already had a detectable impact on Atlantic hurricane activity. That said, human activities may have already caused changes that are not yet detectable due to the small magnitude of the changes or observational limitations, or are not yet properly modeled (e.g., aerosol effects).Knutson devotes only one paragraph, the first one, to the conclusion he reached after studying the hurricane data. In the paragraph though h all but apologizes for concluding the data do not show a correlation between global warming and increased hurricane activity and even suggests the reader disregard that conclusion. He then proceeds to spend three paragraphs talking about predictions about future hurricane activity based on computer models of increased global warming, which was not at all the express purpose of his report.
- Anthropogenic warming by the end of the 21st century will likely cause hurricanes globally to be more intense on average (by 2 to 11% according to model projections for an IPCC A1B scenario). This change would imply an even larger percentage increase in the destructive potential per storm, assuming no reduction in storm size.
- There are better than even odds that anthropogenic warming over the next century will lead to an increase in the numbers of very intense hurricanes in some basins—an increase that would be substantially larger in percentage terms than the 2-11% increase in the average storm intensity. This increase in intense storm numbers is projected despite a likely decrease (or little change) in the global numbers of all tropical storms.
- Anthropogenic warming by the end of the 21st century will likely cause hurricanes to have substantially higher rainfall rates than present-day hurricanes, with a model-projected increase of about 20% for rainfall rates averaged within about 100 km of the storm center.
It is important to note that these conclusions, inserted at the beginning of his report, would be the only thing that many media outlets, politicians and other lay people would read. The headlines would be that Mr. Knutson researched the issue and found that global warming increased hurricane activity when in fact his review of the data proved just the opposite.
As I have said before, the politicization of our science is one of the most dangerous trends today.