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)
[…] note that landfalling TCs are only a small subset of TCs that form out over the ocean, and in the next article we’ll look at […]
Global/national trends (or absence of global trends) sometimes mask regional trends. Offsetting wildfire trends in the western and eastern half of the U.S., for example.
Likewise, not all hurricane basins may respond equally to global warming.
In the North Atlantic basin, the trend for frequency of named storms, hurricanes, and major hurricanes is positive at intervals > 30 years. Likewise, the ACE (wind energy) trend.
The science does not claim that climate change causes more hurricanes, though warmer seas and blocking patterns may magnify their severity and impacts. Hurricane frequency is not an indicator of warming.
Warming increases wind shear, which neutralizes hurricanes. Scientists are projecting fewer but more intense hurricanes, and more large precipitation events. Cyclone activity shows high multi-decadal variability. Meaning it will take time for significant trends to emerge from the signal noise.
“Global Warming and Hurricanes” (NOAA 2023):
“Tropical cyclone rainfall rates are projected to increase in the future (medium to high confidence) due to anthropogenic warming and accompanying increase in atmospheric moisture content. Modeling studies on average project an increase on the order of 10-15% for rainfall rates averaged within about 100 km of the storm for a 2 degree Celsius global warming scenario.
“Tropical cyclone intensities globally are projected to increase (medium to high confidence) on average (by 1 to 10% according to model projections for a 2 degree Celsius global warming). This change would imply an even larger percentage increase in the destructive potential per storm, assuming no reduction in storm size. Storm size responses to anthropogenic warming are uncertain.
“The global proportion of tropical cyclones that reach very intense (Category 4 and 5) levels is projected to increase (medium to high confidence) due to anthropogenic warming over the 21st century. There is less confidence in future projections of the global number of Category 4 and 5 storms, since most modeling studies project a decrease (or little change) in the global frequency of all tropical cyclones combined.
http://www.gfdl.noaa.gov/global-warming-and-hurricanes/
“…On a global scale, the frequency of storms hasn’t increased. However, cyclones are now more common in the central Pacific and North Atlantic, and less so in the Bay of Bengal, western North Pacific and southern Indian Ocean, the study said.
There is also evidence that tropical storms are becoming more intense and even stalling overland…”
“Analysis reveals how climate change is influencing extreme weather” (Thomson Reuters: Jun 28, 2022)
https://www.cbc.ca/news/climate/climate-change-extreme-weather-study-2022-1.6504245
The IPCC projects that “average peak TC wind speeds and the proportion of Category 4–5 TCs will very likely increase globally with warming. It is likely that the frequency of Category 4–5 TCs will increase in limited regions over the western North Pacific. It is very likely that average TC rain rates will increase with warming, and likely that the peak rain rates will increase at rate greater than the Clausius– Clapeyron scaling rate of 7% per 1°C of warming in some regions due to increased low-level moisture convergence caused by regional increases in TC wind intensity. It is likely that … the global frequency of TCs over all categories will decrease or remain unchanged.” (AR6 11.7.1.5)
Thanks for this post.
I have also found the IPCC and Kossin’s focus on the *proportion*, rather than the frequency, of major tropical cyclones confusing and somewhat unhelpful. I look at PNAS result in a little more detail here:
I’m migrating to Substack. This article is reproduced here:
https://scienceofdoom.substack.com/p/extreme-weather-2-trends-in-frequency?sd=pf
[…] « Extreme Weather #2 – Trends in Frequency and Intensity of Tropical Cyclones out over the Oce… […]
SOD: When AR6 appeared, I re-analyzed the data in Table 1 of Kossin (2020), its primary source. Suspiciously, the data was not organized in a way that made it easy to subtract the change in the North Atlantic – where I expected the trend analyzed by Kossin to be biased by the AMO – from the global data. In the global homogenized data (ADT-HURSAT), I found:
1) There was a 5% increase in the total number of global TCs between Kossin’s early (1979-1997) and late (1998-2017) periods and a 5% DECREASE in global TCs outside the North Atlantic. The increase in the North Atlantic was 102%! (The next biggest change was a 24% DECREASE in the South Pacific.)
2) There was a 20% increase in the total number of global major TCs and a 4% increase in major TCs outside the North Atlantic. The increase in major TCs in the North Atlantic was 289%! (The next biggest change was a 38% increase in the Southern Indian Ocean.)
3) There was a 4% increase in the percentage of major TC’s globally and only a 2% increase outside the North Atlantic. This increase in major TCs in the North Atlantic was 16% (18% to 34%).
Based on the data in Kossin (2020) and contrary to its conclusions, THERE HAS BEEN NO GLOBAL TREND IN TCs. There is merely one basin, the North Atlantic, which – during the period of this analysis – has behaved radically differently from the other six basins for well-understood reasons. The trend outside the NA appears negligible.
At this point in my research, I got bogged down in trying to understand what is known about long-term changes in TCs in the North Atlantic. Above, SOD cites the lack of a long-term trend in the number of landfalling hurricanes in the NA. For all NA hurricanes, the simplest information I located was from Klotzbach (2015), which shows three active periods in the NA (1880-1900, 1926-1969, and 1995-2012) and two inactive periods (1901-1925 and 1970-1994) with 61% correlation between ACE and the AMO Index. The active periods had ACE (accumulated cyclone energy) 2-3 fold higher (my estimate) than the inactive periods. This is crudely comparable to the 102% increase in NA TCs and 289% increase in NA major TCs in Kossin (2020). Klotzbach summarized the data differently: The 25 years with the highest AMO index mostly from Kossin’s late period) had 3.2-fold more major hurricane days than the 25 years with the lowest AMO (mostly from the early period). There is no reason to believe that the enormous change in the NA between Kossin’s early and late periods has anything to do with global warming.
Click to access ngeo2529.pdf
Thanks Frank, I might dive back into this subject in the near future.
I emailed a copy of my spreadsheet to your scienceofdoom email address.
It is easy to show that the massive changes in the Atlantic between Kossin’s early and late periods aren’t observed in any other ocean basis and contribute most of the “global” increase in TCs. I stumbled trying to demonstrate that all or even most of the increase in the NA can be attributed to the AMO, because the data isn’t likely to be homogeneous over several cycles of the AMO. The effect of Kossin’s homogenization WITHIN the satellite period is large: IIRC, about 30% of hurricanes and major hurricanes are missed when the satellite data is degraded to a common lower resolution. The US began flying planes into hurricanes back in the 50s or 60s, so we have better data on the North Atlantic than the rest of the world.
Kossin’s early and late periods are somewhat out of phase with the AMO. The AMO and hurricanes switched to an active phase around 1995 – a few years before the end of Kossin’s early period – and may have switch to an inactive phase in 2013 and 2014 (the last years available in Klotzbach’s 2015 paper) near the end of Kossin’s late period. I’m curious what has happened since then. In theory, we may have a accumulated a decade of negative AMO and fewer hurricanes beginning in 2013.