“Ten years ago, Webster et al. documented a large and significant increase in both the number as well as the percentage of category 4 and 5 hurricanes for all global basins from 1970 to 2004, and this manuscript examines whether those trends have continued when including 10 additional years of data.

“In contrast to that study, as shown here, the global frequency of category 4 and 5 hurricanes has shown a small, insignificant downward trend while the percentage of category 4 and 5 hurricanes has shown a small, insignificant upward trend between 1990 and 2014. Accumulated cyclone energy globally has experienced a large and significant downward trend during the same period.

“The primary reason for the increase in category 4 and 5 hurricanes noted in observational datasets from 1970 to 2004 by Webster et al. is concluded to be due to observational improvements at the various global tropical cyclone warning centers, primarily in the first two decades of that study.”

“Our paper found that the large increasing trends in Category 4-5 hurricanes observed in Webster et al. (2005) were primarily due to changes in observational technology at the various warning centers. Most model projections predict a slight increase (on the order of 5-10%) in storm intensity, with perhaps fewer storms, over the next century.

“SSTs correlate tightly with Atlantic hurricane activity, due to other large-scale climate features such as sea level pressure and vertical wind shear. In the tropical Atlantic, warm sea surface temperature anomalies result in lower tropical and subtropical Atlantic pressure. The associated weaker pressure gradient results in weaker trade winds, reducing vertical wind shear (since upper level winds blow out of the west in the tropical Atlantic). The weaker trade winds cause less mixing, evaporation and upwelling of the sea surface, which then feed back into reinforcing the warm SST anomalies in the tropical Atlantic.

This wind-evaporation-SST feedback process in the Atlantic has been shown to be critical for the Atlantic Meridional Mode. Generally, positive values of the Atlantic Meridional Mode are associated with warm SSTs, low sea level pressure, and reduced vertical wind shear. The actual impact of the SST anomalies themselves is shown to be relatively small in partial correlation analysis. This was first demonstrated two decades ago in “Atlantic Sea Surface Temperatures and Tropical Cyclone Formation” by Lloyd J. Shapiro and Stanley B. Goldenberg in Journal of Climate, April 1998.

These types of relationships do not necessarily occur in other basins. For example, the NW Pacific has just had its 2nd Cat. 3+ typhoon this year, while the average to date is 4.5. All of this despite record warm SST anomalies in their Main Development Region. Circulation features are a far more critical driver of typhoon activity than SSTs, since they are always plenty warm to support intense activity.”

See “Global Warming and Hurricane Harvey” by Cliff Mass at his website. He gives a rebuttal to those articles asserting a clear link between Global Warming and Hurricane Harvey. Opening…

“Before the rains had ended, dozens of media outlets had published stories suggesting that global warming forced by humans (mainly by emissions of CO2 into the atmosphere) played a significant role in producing the heavy rainfall and resulting flooding associated with Hurricane Harvey.

“Most of the stories were not based on data or any kind of quantitative analysis, but a hand-waving argument that a warming earth will put more water vapor into the atmosphere and thus precipitation will increase. A few suggesting that a warming atmosphere will cause hurricanes to move more slowly.

“This blog will provide a careful analysis of the possible impacts of global warming on Hurricane Harvey. And the results are clear: human-induced global warming played an inconsequential role in this disaster. …”

“Model projections of hurricane frequency and intensity are based on climate models. However, none have shown skill at predicting past (as hindcasts) variations in hurricane activity (or long term change in their behavior) over years, decades, and longer periods. Thus, their claim of how they will change in the future remains, at most, a hypothesis (i.e. speculation). When NOAA, IPCC and others communicate to the media and public, to be scientifically honest, they should mention this.”

Dr. Pielke Sr. is a Senior Research Scientist in CIRES and Emeritus Professor of Atmospheric Science at Colorado State U. See his bio.

“Ever since Hurricane Harvey, the global warming – hurricane hysteria has ratcheted up to levels I haven’t seen since 2006. NOAA GFDL has written a good article on Global Warming and Hurricanes. {See below.} …I much prefer {NOAA’s} model-based quantitative estimates (but they need some serious uncertainty estimates, including structural uncertainty), relative to hysterical arm waving by Mann and Trenberth using undergraduate basic thermodynamics reasoning. There is nothing basic or simple about hurricanes. …

“{See} my 2010 post Hurricane Katrina – 5 years later, particularly relevant given the cool SST values that Irma formed and intensified.”

Dr. Curry is a professor emeritus at the Georgia Institute of Technology and President of Climate Forecast Applications Network (CFAN). See her bio.

Summary.

“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 or global tropical cyclone activity. …”

A. Statistical relationships between SSTs and hurricanes.

… The Power Dissipation Index (PDI) …is an aggregate measure of Atlantic hurricane activity, combining frequency, intensity, and duration of hurricanes in a single index. …

This is in fact a crucial distinction, because the statistical relationship between Atlantic hurricanes and local Atlantic SST shown in the upper panel of Figure 1 would imply a very large increases in Atlantic hurricane activity (PDI) due to 21st century greenhouse warming, while the statistical relationship between the PDI and the alternative relative SST measure shown in the lower panel of Figure 1 would imply only modest changes of Atlantic hurricane activity (PDI) with greenhouse warming. In the latter case, the alternative relative SST measure in the lower panel does not change very much over the 21st century in global warming projections from climate models, because the warming projected for the tropical Atlantic in the models is not very different from that projected for the tropics as a whole. …

B. Analysis of century-scale Atlantic tropical storm and hurricane records.

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 or hurricane numbers (1878 to present) in fact do show a pronounced upward trend, which is also correlated with rising SSTs (e.g., see blue curve in Fig. 4 or Vecchi and Knutson 2008). 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 (Figure 2).

In addition, Landsea et al. (2010) note that the rising trend in Atlantic tropical storm counts is almost entirely due to increases in short-duration (<2 day) 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. …

“While major hurricanes show more evidence of a rising trend from the late 1800s, the major hurricane data are considered even less reliable than the other two records in the early parts of the record. Category 4-5 hurricanes show a pronounced increase since the mid-1940s (Bender et al., 2010) but again, we consider that these data need to be carefully assessed for data inhomogeneity problems before such trends can be accepted as reliable.”

E. Summary for Atlantic Hurricanes and Global Warming.

In summary, neither our model projections for the 21st century nor our analyses of trends in Atlantic hurricane and tropical storm counts over the past 120+ yr support the notion that greenhouse gas-induced warming leads to large increases in either tropical storm or overall hurricane numbers in the Atlantic. …

“{G}lobal area burned appears to have overall declined over past decades, and there is increasing evidence that there is less fire in the global landscape today than centuries ago. Regarding fire severity, limited data are available. For the western USA, they indicate little change overall, and also that area burned at high severity has overall declined compared to pre-European settlement.”

“We don’t even plan for the past.” — Steven Mosher (of Berkeley Earth), a comment posted at Climate Etc.