In Extreme Weather #1 we looked at trends in landfalling tropical cyclones (TCs), where data goes back over 100 years. Way more TCs form over the ocean and don’t hit land, thankfully. Trends on these would be informative – are they getting worse?
There isn’t much quality data before satellites started going up around 1980, so we have good data for over 40 years. More coverage was added around 1990 so we have even better data over the last 30 years.
What does the latest IPCC report say? Chapter 11 of AR6 covers extreme weather.
Here’s the simple version:
There are significant positive global trends in TC intensity.
The actual text, from p. 1585, is in the Notes at the end of this article.
This seems like bad news but it’s actually good news.
The Executive Summary for the chapter includes the “bad news”, p. 1519:
It is likely that the global proportion of Category 3–5 tropical cyclone instances has increased over the past four decades.. The global frequency of TC rapid intensification events has likely increased over the past four decades. None of these changes can be explained by natural variability alone (medium confidence).
I was confused when I read this section of the report and the paper referenced – Kossin et al., “Global increase in major tropical cyclone exceedance probability over the past four decades”, 2020. I’ve read a number of papers on TCs in the satellite era and “getting worse” didn’t seem correct. I found a paper from Klotzbach et al 2022 in my files and reread it. Both Kossin and Klozbach are heavily cited in this field, including by this IPCC report and the previous report (AR5).
Here’s Klotzbach 2022:
This study investigates 1990–2021 global tropical cyclone (TC) activity trends, a period characterized by consistent satellite observing platforms. We find that fewer hurricanes are occurring globally and that the tropics are producing less Accumulated Cyclone Energy—a metric accounting for hurricane frequency, intensity, and duration.
Here’s Kossin 2020:
Here we address and reduce these heterogeneities and identify significant global trends in TC intensity over the past four decades. The results should serve to increase confidence in projections of increased TC intensity under continued warming.
I emailed Phil Klotzbach asking for clarification – different dataset? different time period? looking at a different metric? and he very kindly replied within 24 hours explaining. (I’ve emailed a number of climate scientists during the years of writing this blog and have found them to be exceptionally responsive, courteous and helpful).
Now it’s clear. And I should have figured it out myself. Here is my plain English version:
The number of category 4-5 TCs (the most extreme) hasn’t changed. The number of category 1-3 TCs has reduced.
So this seems like good news. We can express it as “the percentage of the most extreme TCs has increased” but that’s just another way of saying the same thing.
For people still confused, like a couple of friends I explained this to.. suppose the number of murders is flat but the number of other violent offences has reduced. We could say “violent crime is down”, or we could say “extreme violence has increased (as a percentage of overall violent crime)”. The first one is the plain English version.
Now, we’re looking at a short duration – 30-40 years. Is the trend due to climate variables like La Nina? Will the trend continue? Reverse? All good questions, perhaps to be considered in a future article.
This aim of this article is about the simpler question of what has been observed about trends in tropical cyclones out over the oceans. We’ll let Phil have the last word:
We find that fewer hurricanes are occurring globally and that the tropics are producing less Accumulated Cyclone Energy—a metric accounting for hurricane frequency, intensity, and duration
Notes
Text of AR6 on TC trends in the satellite era, from p. 1585:
There are previous and ongoing efforts to homogenize the best-track data (Elsner et al., 2008; Kossin et al., 2013, 2020; Choy et al., 2015; Landsea, 2015; Emanuel et al., 2018) and there is substantial literature that finds positive trends in intensity-related metrics in the best-track during the ‘satellite period’, which is generally limited to around the past 40 years (Kang and Elsner, 2012; Kishtawal et al., 2012; Kossin et al., 2013, 2020; Mei and Xie, 2016; Zhao et al., 2018; Tauvale and Tsuboki, 2019).
When best-track trends are tested using homogenized data, the intensity trends generally remain positive, but are smaller in amplitude(Kossin et al., 2013; Holland and Bruyère, 2014).
Kossin et al. (2020) extended the homogenized TC intensity record to the period 1979–2017 and identified significant global increases in major TC exceedance probability of about 6% per decade.
In addition to trends in TC intensity, there is evidence that TC intensification rates and the frequency of rapid intensification events have increased within the satellite era (Kishtawal et al., 2012; Balaguru et al., 2018; Bhatia et al., 2018). The increase in intensification rates is found in the best-track and the homogenized intensity data.
References
Seneviratne et al, 2021: Weather and Climate Extreme Events in a Changing Climate. In Climate Change 2021: The Physical Science Basis. Contribution of Working Group I to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change
Global increase in major tropical cyclone exceedance probability over the past four decades, Kossin et al, PNAS (2020)
Trends in Global Tropical Cyclone Activity: 1990–2021, Philip J. Klotzbach et al, GRL (2022)
The Science of Doom 