From Watts Up With That?

Part I

Robert Vislocky, Ph.D.

Over the past few years I’ve been perusing around various climate change debate threads on social media. Unfortunately, all too frequently I see the same flawed (but popular) metrics used to support the narrative that certain forms of severe weather are becoming significantly worse due to global warming. In this series, I’ll review some of these misleading or deceptive metrics and offer a critique on why they are flawed.

With hurricane season officially underway, I’ll kick this off this series by dissecting a measure known as the proportion of major tropical cyclones (TCs) to the total number of TCs (hereafter referred to as “the Ratio”). Several authors have used similar metrics in the past to assess the trend in the proportion of intense to total TCs. For example, see the following article links for Webster, Holland, Curry, and Chang (2005), and Klotzbach and Landsea (2015):

Changes in tropical cyclone number, duration, and intensity in a warming environment – PubMed

Extremely Intense Hurricanes: Revisiting Webster et al. (2005) after 10 Years in: Journal of Climate Volume 28 Issue 19 (2015)

However the one most cited lately as evidence of an anthropogenic signal in hurricane intensity is a 2020 PNAS article by Kossin et al. titled “Global increase in major tropical cyclone exceedance probability over the past four decades”. Thus it is this article which will be the main focus.

Global increase in major tropical cyclone exceedance probability over the past four decades | PNAS

In the 2020 article the authors showed that from 1980 to 2017 the Ratio increased by a statistically significant amount which the authors later conflate to mean that there is an increasing trend in TC intensity. Figure 1 below is a graph of the Ratio out to 2025 that I produced using TC summary data from Colorado State University’s real-time tropical meteorology project. Although the authors in the PNAS article used a different dataset (i.e., 6-hour wind speed fixes inferred from a special satellite dataset), the percent changes in the Ratio from 1980 to 2017 are similar between the PNAS study and Figure 1 below.

Of course, upon publication of the PNAS study the media as always jumps on the climate bandwagon. For example, the New York Times story below says that “Climate Change is Making Hurricanes Stronger” and cites the details from the PNAS article:

Climate Change Is Making Hurricanes Stronger, Researchers Find – The New York Times

NOAA also claimed a “significant global increase in hurricane intensity over a four decade period” as a result of the PNAS study:

Link Between Earth’s Heat and Hurricane Strength Grows | News | National Centers for Environmental Information (NCEI)

So, what’s the problem? Simply put, the proportion of major TCs to total TCs is NOT a valid measure of absolute hurricane intensity. It is a relative measure with a moving denominator. It would be like trying to equate a high relative humidity with a high dew point, the two don’t necessarily go hand-in-hand. To show this flaw using an extremely simplified example, consider a season in which there was only one major hurricane and only one total hurricane. The Ratio metric would yield a value of 1, which is the highest value possible supposedly indicating a very intense hurricane season. Yet, by any reasonable measure a season with only one hurricane would be an extremely benign hurricane season.

This is not just a hypothetical example either as glaring discrepancies can be shown over an entire year at the global level using actual data (see table below). As one example, 2018 had 43% more TCs and 43% more major TCs than 2024. It also had a 75% higher ACE value. Yet according to the flawed Ratio statistic, both years were identical!! As another example, 1990 had 53% more total TCs and the same number of major TCs than 2009. Plus 1990 had a 53% higher ACE value than 2009. Yet the Ratio in 2009 was 53% higher than 1990 which is an absolute joke. There are numerous other discrepancies like those apparent in the annual TC data. The bottom line is the Ratio is not a suitable metric for measuring absolute TC intensity or computing the trends thereof, as it can increase with decreasing numbers of total TCs. Despite its flaw the Ratio is frequently referenced whenever a climate scientist or activist needs to cite a trend that shows hurricanes are becoming more intense (mostly because other indicators like global ACE in the post satellite era, or US landfalling hurricanes over the past century, show no significant trend).

Year     # Total TCs     # Major TCs       ACE       Ratio

2018           60                    33                1105       0.55
2024           42                    23                  630       0.55

Year     # Total TCs     # Major TCs       ACE       Ratio

1990           58                    21                 930        0.36
2009           38                    21                 607        0.55   

Data courtesy of Colorado State University Department of Atmospheric Sciences.

Global Statistics compared with climatology

Another interesting observation is that although the authors of the PNAS article were able to show a statistically significant trend in the Ratio statistic from 1980 to 2017, there has been no significant trend over the last 31 years from 1994 to 2025 (see Figure 2 below).

H/T to Charles Rotter

[editor’s note. I identified an error in the original submission~cr]