“hurricanes and tornadoes show no changes attributable to human influences”
This is the claim Steven Koonin makes in his 2021 book Unsettled: What Climate Science Tells Us, What It Doesn't, and Why It Matters. By the time I got to this statement I had read enough of his book to expect some skepticism of an impending hurricane disaster, but I was still surprised by the strength of this statement. He says people have been misled by assessment reports that “present summary ‘spin’ inconsistent with their own findings.” Not only that, but future hurricane projections don’t look that bad either. He says that while hurricanes might get worse, it’s nothing that could be described as catastrophic.
While reading his book, I kept wondering how well his claims would stand up if I laboriously went through his references and looked at the primary literature myself for contradicting information. I wouldn't be able to do this for every topic, but I thought I could do a deep dive into the scientific literature on hurricanes as a sort of approximation for how well his claims stand up overall. So, here’s the deep dive into the science of hurricanes.
Background on Hurricanes
Before we get into it, we should start with a clarification of terminology. It’s important to know that hurricanes, typhoons, cyclones, and tropical cyclones are all essentially the same thing — they’re just called different things in different places. “Tropical cyclone” is the most generic term (e.g. hurricanes and typhoons are both tropical cyclones) so that’s the term you’ll see in a lot of the climate studies that I quote here. They’re called “typhoons” in the western Pacific, “cyclones” in the Indian Ocean, and “hurricanes” in the Atlantic Ocean and eastern Pacific Ocean. That’s the term Koonin uses so I do the same. At different times I’ll be citing research that uses one term or the other — just know that they’re mostly interchangeable.
Disentangling Human-caused Climate Change from Natural Variability
As we move forward, we have to be careful of how we evaluate the evidence. It’s important to remember that hurricane behavior is always changing through natural cycles and variations. For example, chapter 6 of the 2019 Special Report on the Ocean and Cryosphere in a Changing Climate (SROCC) says:
Patterns of storm activity across TC [tropical cyclone] basins show variations through time that appear to be correlated with El Niño-Southern Oscillation (ENSO), North Atlantic Oscillation (NAO), and changes in atmospheric dynamics related to changes in precession of the sun (Toomey et al. 2013; Denommee et al. 2014; Denniston et al. 2015).
To dig into one such cycle for a moment, take a look at the Atlantic Multidecadal Oscillation (AMO) in the figure below. It shows that there is a natural warming and cooling cycle in the Atlantic Ocean with a period of 60–80 years. The AMO and other natural cycles have significant effects on hurricane activity. This means that we can’t just look for changes in hurricane data and attribute them to human-caused climate change. We’ll have to look for patterns of behavior that are outside of natural variability.
Investigating The Claim
Background
Koonin says when he was researching climate change in 2016, he “turned to the (then most recent) National Climate Assessment” of 2014. He found the following summary of hurricane behavior, Key Message 8, quite alarming:
The intensity, frequency, and duration of North Atlantic hurricanes, as well as the frequency of the strongest (Category 4 and 5) hurricanes, have all increased since the early 1980s. The relative contributions of human and natural causes to these increases are still uncertain. Hurricane-associated storm intensity and rainfall rates are projected to increase as the climate continues to warm.
However, he dug deeper into the report and found contradicting statements “buried in the text of Appendix 3,” which he quotes:
There has been no significant trend in the global number of tropical cyclones nor has any trend been identified in the number of U.S. landfalling hurricanes.
Noting the apparent inconsistency, he got curious and dug into what he considered the NCA’s main paper on the topic, a 2010 paper titled Tropical cyclones and climate change. He “found it stated quite explicitly that there are no significant trends beyond natural variability in hurricane frequency, intensity, rainfall, or storm surge flooding.”
This is where I started my foray into the scientific literature. In the study he referenced, Koonin seems to be keying off of this sentence:
Therefore, it remains uncertain whether past changes in tropical cyclone activity have exceeded the variability expected from natural causes.
However, the study goes on to discuss model-based projections as well. The next two sentences are:
However, future projections based on theory and high-resolution dynamical models consistently indicate that greenhouse warming will cause the globally averaged intensity of tropical cyclones to shift towards stronger storms, with intensity increases of 2–11% by 2100. Existing modelling studies also consistently project decreases in the globally averaged frequency of tropical cyclones, by 6–34%.
I think it’s important to note that the authors do expect human-caused climate change to affect hurricanes, though they hadn’t found a clear signal at the time of that writing.
In total, I think Koonin’s assessment of this report is accurate. My main objection is that this study was in 2010. Since then, we’ve continued to learn more about hurricanes. What does the latest research say?
Aspects of Hurricanes
There are a lot of different aspects of hurricanes that humans could potentially impact — I guess as many as one could measure. But I saw the same three coming up in study after study, so those are the three I will focus on: frequency, intensity, and amount of precipitation. Because he referenced both current data and future projections, I wanted to look at both.
I find evaluating the quality of scientific research very difficult and time-consuming, even in my own field. And climate science is not my field. For topics that are outside my field, I often look to meta-analyses and survey papers. Fortunately, one of Koonin’s sources was just such a survey that I relied on to help me understand expert opinions on hurricane activity.
The survey, “Tropical Cyclones and Climate Change Assessment: Part I: Detection and Attribution,” was co-authored by 11 hurricane experts and published in 2019. From my outsider’s perspective, it seems to be an excellent study. It’s done by 11 climate scientists looking at all the evidence in two different ways: One through the lens of trying not to overstate any anthropogenic effect (type I errors) and then a second time through the lens of trying not to understate any anthropogenic effect (type II errors). This approach is important because many times scientists try to “reject a null hypothesis,” which is a good approach but has its flaws. The "null hypothesis" approach is like saying if they don't have evidence to be sure of something, they will default to a "nothing is happening" explanation. The problem is that with weak evidence, they might fail to conclusively reject a null hypothesis even if the balance of evidence suggests an alternative hypothesis is more likely. But looking at it through both lenses gives both a conservative and aggressive bound for the problem.
In addition, the same authors published “Tropical Cyclones and Climate Change Assessment: Part II: Projected Response to Anthropogenic Warming” in 2020, which contains projections of future activity. This research looks at model projections of hurricane behavior assuming 2°C of anthropogenic global warming. To get a sense of the timeline that these projections would be relevant for, we can use a study published in January of 2022 that looked at this and put their conclusions right in the headline: “Plausible 2005–2050 emissions scenarios project between 2 and 3 degrees C of warming by 2100.” They found a median of 2.2°C of warming by 2100, so the projections from the hurricane survey could be said to roughly correspond to a little under 80 years away. I'll be referring to these studies throughout this post as Part I and Part II of the 11-expert assessment.
Frequency — All Hurricanes
Present
Part I of the 11-expert assessment surveyed the literature for changes in global hurricane frequency. They concluded:
global TC and hurricane frequency since 1970 … [shows] no strong evidence for trends.
I wanted to look at the data myself and fortunately, an author cited by the assessment maintains a site, climatlas.com, which keeps an up-to-date graph of global hurricane frequency over time. Here’s what it shows:
In this graph, “Major Hurricanes” refers to categories 3–5 on the Saffir–Simpson scale (5 is the highest possible score), and “All Hurricanes” contains categories 1–5. It’s pretty clear that there are cyclical patterns in the data, but I don’t see any obvious overall upward or downward trend.
Future Projections
What about future projections? Many reports have made projections for future changes in hurricane frequency. The 2012 IPCC Special Report of Extreme Events states that (italics in the original):
It is likely that the global frequency of tropical cyclones will either decrease or remain essentially unchanged.
Following on that, the 2019 SROCC says (my bolding):
IPCC AR5 concluded that there was low confidence in any longterm increases in TC activity globally and in attribution of global changes to any particular cause (Bindoff et al. 2013; Hartmann et al. 2013). Based on process understanding and agreement in 21st century projections, it is likely that the global TC frequency will either decrease or remain essentially unchanged
We can also look at part II of the 11-expert assessment for their projection. Here’s Figure 1 from that paper:
The first thing I noticed is that the ranges are incredibly large. The range in every basin overlaps with 0% change, indicating that it’s possible absolutely nothing is changing at all in terms of frequency. And the uncertainties for the North Atlantic and North Indian basins are incredibly large. The ranges go all the way from a halving to a doubling — the scientific way of saying “we really don’t know.” It’s also interesting that the median value for global percent change is noticeably negative, somewhere around -15%. So it seems likely that hurricanes will decrease globally, but there’s a good chance they remain essentially unchanged.
Frequency — Intense Hurricanes
Present
What about if we focus solely on the most intense (and therefore destructive) hurricanes — are they becoming more frequent? It seems the evidence here is more mixed. I found a study from 2005 called “Changes in Tropical Cyclone Number, Duration, and Intensity in a Warming Environment” (full text available in this homework assignment) stating that:
hurricanes in the strongest categories (4 + 5) have almost doubled in number (50 per pentad [five year period] in the 1970s to near 90 per pentad during the past decade) and in proportion (from around 20% to around 35% during the same period).
However, it doesn’t seem like this analysis has held up very well. The 2010 paper Tropical cyclones and climate change mentions this study and states that “Other studies contested this finding, based on concerns about data quality and the short record-length relative to multidecadal variability in the northwest Pacific” (references to these disputes are provided in the 2010 paper).
Looking again at the graph of Global Major Hurricane Frequency from above and focusing on the lower line for major hurricanes, I don’t see any clear upward or downward trend.
Future Projections
There has been some evidence from modeling that there could be an increase in the frequency of the most intense hurricanes. For example, from the 2018 IPCC Special Report on Global Warming of 1.5 °C (SR15):
Tropical cyclones are projected to decrease in frequency but with an increase in the number of very intense cyclones (limited evidence, low confidence).
Looking at part II of the 11-expert assessment again, it seems there’s a greater chance that these are increasing than there was for all hurricanes, but the median value is basically right at 0%. Looking at all the regions, it seems plausible that there’s no change anywhere.
It was also interesting to see how the experts from the survey were split on this issue. The paper said:
In summary, author opinion was divided on whether the global frequency of very intense (e.g., category 4–5) TCs will increase or not, with the confidence in an increase ranging from low (three authors), to low-to-medium (two), to medium (one), to medium-to-high (four) to high (one).
My takeaway is that it seems more likely than not that there will be a small but detectable increase in very intense hurricanes, although this is quite uncertain.
Intensity
Present
A 2013 study on “Trend Analysis with a New Global Record of Tropical Cyclone Intensity” looked at changes in hurricane intensity. They found that the evidence was suggestive of an increase in global hurricane intensity, but the data wasn’t statistically significant. However, they followed it up in 2020 with more recent data and found that there was a statistically significant increase.
But remember, we’re not just looking for change over time; we’re looking for change due to anthropogenic factors. Although it’s consistent with the theory that human-caused climate change has caused this, there is a myriad of other possible causes, and the study did not distinguish between them (my bolding):
Ultimately, there are many factors that contribute to the characteristics and observed changes in TC intensity, and this work makes no attempt to formally disentangle all of these factors. In particular, the significant trends identified in this empirical study do not constitute a traditional formal detection, and cannot precisely quantify the contribution from anthropogenic factors. From a storyline, balance-of-evidence, or Type-II error avoidance perspective (e.g., refs. 6 and 45), the consistency of the trends identified here with expectations based on physical understanding and greenhouse warming simulations increases confidence that TCs have become substantially stronger, and that there is a likely human fingerprint on this increase. Given the well-understood impacts and risk that increasingly powerful TCs carry with them, strict adherence to Type-I error avoidance could be considered overly conservative.
So it seems like hurricanes might be getting more intense, but the effect is small. It’s not clear that this is caused by climate change, but experts think it is likely.
Future Projections
The 2010 Tropical cyclones and climate change paper stated the following:
future projections based on theory and high-resolution dynamical models consistently indicate that greenhouse warming will cause the globally averaged intensity of tropical cyclones to shift towards stronger storms, with intensity increases of 2–11% by 2100
The lead author on that paper works at NOAA’s Geophysical Fluid Dynamics Laboratory and keeps this excellent site up-to-date on global warming and hurricanes. On it, he said the following:
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.
Part II of the 11-expert assessment (of which the above author was a co-author) looked at 15 global estimates for hurricane intensity based on 2°C of anthropogenic global warming and found them all to be between 0–10% increases.
The modal value is a 4% increase. My takeaway is that a 4% or some other single-digit percentage increase in intensity over the next 80 years seems like our best guess.
As far as Koonin’s claim goes, he was referring to changes “attributable to human influences,” which the study that found an increase in hurricane intensity did not find. Everything I read about hurricane intensity was consistent with his claim.
The Atlantic Exception
But given all of the above, how did we get the alarming statement from the 2014 NCA? I printed it at the top, but I’ll repeat it here:
The intensity, frequency, and duration of North Atlantic hurricanes, as well as the frequency of the strongest (Category 4 and 5) hurricanes, have all increased since the early 1980s. The relative contributions of human and natural causes to these increases are still uncertain. Hurricane-associated storm intensity and rainfall rates are projected to increase as the climate continues to warm.
It was paired with this alarming graph of increasing Power Dissipation Index (PDI). PDI is an aggregate of hurricane frequency, intensity, and duration into a single index — you can think of it as total hurricane power over a season.
Koonin says that the graph shown in the 2014 NCA is misleading. The graph of increasing PDI is just a subset of the graph from this paper, except that it has zoomed in on a specific region to highlight the increase. With the historical context, the growth in PDI seems to be within natural variability.
When I first read the NCA statement, I thought they focused on North Atlantic hurricanes because it’s a U.S.-based report and North Atlantic hurricanes have the largest impact on the U.S. I assumed that global trends were roughly following a similar trajectory. I realized now that that assumption is not correct — Atlantic hurricane behavior is quite anomalous compared to global trends. Part I of the 11-expert assessment split hurricane activity into regions, and you can see the different behavior for North Atlantic hurricanes here:
You can see the increase in North Atlantic hurricane intensity here, but the various trends don't match my intuition for an effect driven by global warming. My thought is that if global warming were causing more intense hurricanes, they would be more intense everywhere (within some variability), not just in the Atlantic. But it seems that hurricane maximum intensity is decreasing in other basins and that there’s no clear overall trend. In terms of isolating human-caused climate change, this doesn’t seem like a clear signal to me.
There is a section in the full text of the Sixth IPCC AR, which came out in 2021 (after Koonin published his book), that specifically addresses hurricanes in the Atlantic:
there is still no consensus on the relative magnitude of human and natural influences on past changes in Atlantic hurricane activity, and particularly on which factor has dominated the observed increase (Ting et al., 2015) and it remains uncertain whether past changes in Atlantic TC activity are outside the range of natural variability
The contrast between this sentence (which I felt was rather hard to find on page 2862 of the full report) and the headline from the 2014 NCA is striking. And it’s not that the evidence has gotten weaker, because the statement from the 2021 AR6 is nearly identical to the statement from Tropical cyclones and climate change nearly a decade prior that was mentioned before: “Therefore, it remains uncertain whether past changes in tropical cyclone activity have exceeded the variability expected from natural causes.” As Koonin points out, if we don’t know that hurricane activity has changed beyond natural variability, then how can we even begin to say that it’s being caused by human influences?
It does seem like hurricanes in the North Atlantic basin got more intense from the 1980s to 2010. (I haven’t been able to find the same graph that includes the last decade). However, I don’t think it is clear that this is the result of human-caused climate change. The fact this increase is only happening in the Atlantic basin makes me think it’s more likely to be associated with natural variability, although I have low confidence in all of this. This also means it’s hard to say what will happen in the future.
Precipitation
Present
Unlike some of the other effects, there’s a relatively simple mechanism behind the climate’s effect on hurricane precipitation. As the air is warmer, it holds more water vapor, and an atmosphere with more water vapor has more water to precipitate. In addition, the amount of precipitation that falls on a single area is a function of how long a hurricane is above that area. Thus, slower hurricanes dump more water on a specific region than faster ones. So there are multiple reasons precipitation from hurricanes may change as the climate changes.
There has been a lot of research recently about climate change and hurricane precipitation from Hurricane Harvey, so I’m going to start there. Hurricane Harvey was a Category 4 hurricane famous for flooding Houston, Texas in 2017. It produced record-breaking rainfall over three days in Houston. The National Hurricane Center Tropical Cyclone Report on Hurricane Harvey says that Harvey was responsible for “at least 68 direct deaths.” Fortunately for us, it was also a well-studied hurricane, and many researchers looked into how much human-caused climate change affected it.
As we’ve seen, it is really hard to determine how much of a change in hurricane behavior is due to human-caused climate change versus other factors. However, studies that link disasters directly to human-caused climate change have started appearing. These are known as “attribution studies.” They rely on counterfactual models of the world. That is, asking the question “What would have happened in a world without human-caused climate change?” Attribution studies are hard — Koonin says this, climatologists say this, everyone seems to agree about this. As Koonin says, the whole basis of science is the ability to check your results against real data. We’re not able to do that in counterfactual models. That doesn’t mean we throw them all out, but it does mean we should be less sure of them. As Koonin points out, even the World Meteorological Organization warns against attributing an event to human-caused climate change:
any single event, such as a severe tropical cyclone, cannot be attributed to human-induced climate change.
These attribution studies are trying to do just that. For example, one study on the economic costs of Hurricane Harvey estimated the impact of the anthropogenic component of climate change. Here’s what they found:
In this paper, we use a probabilistic event attribution framework to estimate the costs associated with Hurricane Harvey that are attributable to anthropogenic influence on the climate system. Results indicate that the “fraction of attributable risk” for the rainfall from Harvey was likely about at least a third with a preferable/best estimate of three quarters. With an average estimate of damages from Harvey assessed at about US$90bn, applying this fraction gives a best estimate of US$67bn, with a likely lower bound of at least US$30bn, of these damages that are attributable to the human influence on climate.
A study on Attributable Human-Induced Changes in the Likelihood and Magnitude of the Observed Extreme Precipitation during Hurricane Harvey found the following:
We find that human-induced climate change likely increased the chances of the observed precipitation accumulations during Hurricane Harvey in the most affected areas of Houston by a factor of at least 3.5.
Another study looking at Hurricane Harvey said this:
we conclude that global warming made the precipitation about 15% (8%–19%) more intense, or equivalently made such an event three (1.5–5) times more likely.
Another study had this to say:
The 60 member ensemble simulations suggest that post-1980 climate warming could have contributed to the extreme precipitation that fell on southeast Texas during 26–29 August 2017 [that’s Hurricane Harvey] by approximately 20%, with an interquartile range of 13%–37%.
I don’t know how much weight to put into these studies. The ranges of increased precipitation seem to overlap between the studies, so that gives me higher confidence in these results. I’m sure the exact numbers can be debated, but this is evidence that human-caused climate change has changed hurricane behavior in a way that has led to billions of dollars of additional damage. I expect that because they are attribution studies is part of the reason Koonin doesn’t mention them.
But if we don’t put any weight into model-based studies like these, I’m not sure how we would ever tease out how much impact human influences are having in any useful timescale. If better data were available, we could throw out the weaker data and just rely on the good stuff. But other than waiting 100 years for enough data, I don’t think there is better data available. So we’ve got to accept model-based data, and we’ll have to weigh it appropriately (which is not 0 but also not 100%).
I did look for studies that modeled Hurricane Harvey and found no effect from human-caused climate change, but I didn’t find any.
All of these studies fed into the Sixth IPCC Assessment Report (AR6), which concluded:
There is high confidence that anthropogenic climate change contributed to extreme rainfall amounts during Hurricane Harvey (2017) and other intense TCs.
Given what I had read and the fact that AR6 stated they had “high confidence” in this, I was feeling like the case was pretty strong at this point. So I started looking for counter-evidence from other methods, or at least other opinions. And I found some. In particular, the 11-expert assessment reviewed the evidence that human-caused climate change increased Hurricane Harvey’s precipitation as well. Here is the conclusion from their section on Hurricane Harvey:
In summary, from a type I error perspective, the author team had low confidence that anthropogenic influence specifically on hurricane precipitation rates has been detected. Alternatively, from the perspective of reducing type II errors, all authors concluded that the balance of evidence suggests that there has been a detectable long-term increase in occurrence of Hurricane Harvey–like extreme precipitation events in the eastern Texas region, and that anthropogenic forcing has contributed to this increase.
That’s what we’re left with. The “balance of evidence suggests” anthropogenic effects. So it seems like it is the case that we have enough evidence to suggest Hurricane Harvey was worse because of human-caused climate change, but how much worse is not possible to quantify, and even this evidence isn’t a slam dunk.
Future Projections
We can also look at future projections from part 2 of that survey in this graph.
Similar to the present case for increased precipitation, this is the strongest evidence for a future projection we’ve seen so far.
Other reports, such as the 2019 SROCC, also expect precipitation rates to increase (italics in the original):
There is medium confidence … that average TCs precipitation rates (for a given storm) will increase by at least 7% per degree Celsius SST [Sea Surface Temperature] warming, owing to higher atmospheric water vapour content
Because the evidence for increased precipitation seems the strongest, I was curious about how much it mattered. How significant is precipitation in hurricane damage? So I found a study on the causes of death in the United States directly attributable to Atlantic tropical cyclones, 1963–2012. They show that storm surges, which are the temporary rises in water levels generated by a storm, are the most significant factor. But right behind them is rain, causing 27% of total deaths in the U.S. from hurricanes.
So any change in either total precipitation or the distribution of the precipitation should be taken very seriously.
The Long Tail
As well as understanding the most likely outcomes, we need a sense of what the worst outcomes could be and how we’ll know if we’re on or near that path. We have to be concerned about long-tail outcomes. We need to understand how bad they are and how to know if we’re getting too close to them. One thing we've seen is that the concern about increasingly destructive hurricanes stems from model-based projections of their behavior, not changes we’ve seen so far (remember, there’s not even consensus that hurricane behavior has changed beyond natural variability). This is important to keep in mind for several reasons. One is that the projections of worse hurricanes are not guaranteed to happen. Another is that it means we are possibly headed into a world where hurricanes are far more destructive, so the current data may not be useful for predicting the future.
Even though the bad scenarios are unlikely, we still have to consider them and continuously evaluate their probability. To see what the bad scenarios would look like, take a look at (a) in the figure below. This is from the website Global Warming and Hurricanes, which is maintained by the lead author of the 11-expert assessment and the 2010 article “Tropical cyclones and climate change.” This is the type of thing that worries me.
The top graph shows hurricane PDI dramatically increasing, ending up with a 300% increase by 2100. How concerned should we be about this? Here are two paragraphs from the site that discuss this:
Model-based climate change detection/attribution studies have linked increasing tropical Atlantic SSTs to increasing greenhouse gases, but proposed links between increasing greenhouse gases and hurricane PDI or frequency has been based on statistical correlations. The statistical linkage of Atlantic hurricane PDI to Atlantic SST suggests at least the possibility of a large anthropogenic influence on Atlantic hurricanes. If this statistical relation between tropical Atlantic SSTs and hurricane activity is used to infer future changes in Atlantic hurricane activity, the implications are sobering: the large increases in tropical Atlantic SSTs projected for the late 21st century would imply very substantial increases in hurricane destructive potential–roughly a 300% increase in the PDI by 2100 (Figure 1a).
On the other hand, Swanson (2008) and others noted that Atlantic hurricane power dissipation is also well-correlated with other SST indices besides tropical Atlantic SST alone, and in particular with indices of Atlantic SST relative to tropical mean SST (e.g., Figure 1b from Vecchi et al. 2008). This is in fact a crucial distinction, because while 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, the alternative statistical relationship between the PDI and the relative SST measure shown in the lower panel of Figure 1 would imply only modest future long-term trends 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, even with substantial Atlantic warming projections from climate models, because, crucially, the warming projected for the tropical Atlantic in the models is not very different from that projected for the tropics as a whole.
Ultimately, it comes down to the question of how much increased greenhouse gases will affect Atlantic hurricane PDI. We know there is a tight connection between greenhouse gases and global mean temperature. The underlying physics is (relatively) straightforward and the evidence is solid. But that’s not the case for greenhouse gases and Atlantic hurricane PDI. Take a look at the graph below taken from the same site and compare how Global Mean Temperature changes over time to that of Adjusted Hurricane Counts (it’s adjusted to correct for changes in our ability to detect hurricanes):
Global Mean Temperature is clearly rising, but to my eyes, the Adjusted Hurricane Counts isn’t. It was at a high point recently, but that looks entirely within natural variability to me. The website goes into more detail, but the summary is (bolded in the original):
In short, the historical Atlantic hurricane frequency record does not provide compelling evidence for a substantial greenhouse warming-induced long-term increase.
This drives the point home for me. The current amount of climate change has not clearly changed hurricane frequency.
Conclusion
Let’s go back to Koonin’s statement, which was “hurricanes and tornadoes show no changes attributable to human influences.” I would say that this is not entirely accurate. In particular, there is fairly strong evidence that precipitation has increased because of human-caused climate change, with research on Hurricane Harvey providing the bulk of the evidence. As precipitation is a significant factor in the number of deaths and destruction a hurricane causes, I think Koonin was mistaken to overlook it.
Looking ahead, the median projections for hurricane changes with 2°C of anthropogenic global warming (which might occur around the year 2100) are no increase in frequency, around a 4% increase in intensity, and just below a 15% increase in precipitation. Although these are important, I think Koonin is right to say the projected changes are not catastrophic.
But for me, there’s a larger point that I learned about hurricanes and climate change. It’s interesting to reflect on what I thought was true before I did this research. Based on the constant drumbeat of alarming climate news, I had assumed that there was overwhelming evidence that hurricanes and other extreme weather events were becoming many times worse than they have been in the pre-industrial world. I didn’t have specific facts or figures in my head, like “hurricanes will be X times more intense.” It was just a general, poorly-defined notion that they were going to be much worse in the future. I had also assumed that their destructiveness today was far beyond their historical range.
But after researching it, I was surprised by how weak or non-existent the evidence for changes in hurricane behavior due to anthropogenic climate change is, especially for catastrophic changes. Saying that hurricane changes are still within natural variability sounds like the “climate denier” position, but I’m not sure what else to conclude when a 2018 study on “Anthropogenic influences on major tropical cyclone events” opened with “There is no consensus on whether climate change has yet affected the statistics of tropical cyclones, owing to their large natural variability and the limited period of consistent observations.” The Sixth IPCC Assessment Report, which is the most recent version of the most important climate report, stated, “it remains uncertain whether past changes in Atlantic TC activity are outside the range of natural variability.”
I do think that hurricanes will continue to get worse. They will be more dangerous in the future, primarily because rising sea levels will lead to worse storm surges, and increased precipitation will cause more flooding. But a projected 4% increase in intensity is nowhere near the catastrophe that I feared was awaiting us. I wonder how many other people would be in the same situation when confronted with the data.