This is a re-post from the Climate Brink by Andrew Dessler
Because hurricanes are one of the big-ticket weather disasters that humanity has to face, climate misinformers spend a lot of effort muddying the waters on whether climate change is making hurricanes more damaging.
With the official start to the hurricane season in the North Atlantic coming up (June 1), I figured it was time to explain why we can be so confident that hurricanes are indeed more destructive today due to climate change.
Note: from here on out, I’ll refer to hurricanes as tropical cyclones (abbreviated TCs), which is a more general term for this type of storm.
We have 100% confidence that sea level is rising because humans are heating the planet. And higher sea levels mean today’s TCs do more damage than an identical tropical cyclone in a cooler climate because the storm surge is riding on a higher sea level.
As Prof. Adam Sobel said in Congressional testimony a few years ago:
The most certain way in which hurricane risk is increasing due to climate is that, because of sea level rise, coastal flooding due to hurricane storm surge is becoming worse. Storm surge occurs when the winds from a storm push the ocean onto the land. The total flooding is determined by the surge (the part produced by the wind), the tide, and the background average sea level. As sea level has risen … the flooding is exacerbated by that amount.
Climate misinformers will respond that sea level only contributes a small fraction to the total flood depth. But the non-linearity of flood damages means that even a small contribution from sea level rise to total flood depth can increase damages a lot.
For example, in 2011, Hurricane Irene hit the New York region with a storm surge of around 1.5 meters; existing flood infrastructure was able to handle that and there was little damage. Just a year later, Hurricane Sandy hit the New York region with 2.75 meters of storm surge, overwhelming flood infrastructure and leading to damages of $62B.
This is classic non-linearity, where you get zero damage until a threshold is passed (somewhere between 1.5 and 2.75 m) and then damages increase exponentially.
Once you pass the threshold, every additional unit adds more to the misery. It has been estimated that sea level rise due to global warming increased Sandy’s total flood depth by 0.1 meters and this increased the damage from the storm surge by $8B. Thus, the last 4% of the total flood depth generated 13% of the total damage. The next 0.1 meters will add even more damage.
Bottom line: Because of sea level rise, you can say with 100% certainty that climate change is making every TC more destructive.
Climate change will increase TCs’ rainfall due to the following logic chain: 1) much of the water vapor in air flowing into a tropical cyclone’s core will fall out as rain when the air ascends in one of the rain bands, 2) in a warmer climate, the air flowing into a tropical cyclone’s core holds more water vapor. Put points 1 and 2 together and you get more rainfall!
That’s it. You don’t need any fancy physics to understand why we’re so confident that tropical cyclones will produce more rain as the climate warms. In fact, the IPCC says that we are already seeing this1:
There is high confidence that anthropogenic climate change contributed to extreme rainfall amounts during Hurricane Harvey (2017) and other intense TCs.
They also say it’s going to get worse as the climate continues to warm2:
It is very likely that average TC rain rates will increase with warming.
Bottom line: You can be confident that TCs are already producing more rain due to climate change, and this is another way they are increasingly destructive. The future is for more of this.
Basic physics tells us that hurricane should get more intense as the climate warms. Indeed, the IPCC says there is likely a trend3:
It is likely that the global proportion of Category 3–5 tropical cyclone instances … have increased globally over the past 40 years.
The IPCC further says that climate change probably contributed to this, but the evidence is mixed so we should take that conclusion with a grain of salt. Our inability to attribute the historical trend is actually not that surprising given the fact that hurricanes are relatively rare events and good data doesn’t go back that far.
Regardless of whether we can attribute the past trend, there is high confidence that climate change will drive stronger storms in the future. As the IPCC says4:
the proportion of Category 4–5 TCs will very likely increase globally with warming.
Bottom line: TCs are getting more intense, but the evidence on the cause of the historical trend is mixed, although it seems reasonable that climate change is playing some role. For the future, we can have high confidence that climate change will drive more intense hurricanes.
We don’t really know what’s going to happen to TC numbers. We don’t have a good theory for what determines how many TCs form in the world’s oceans every year. Globally, there are around 80 per year of these storms, and we don’t know why it’s that number and not 8 or 800.
Without a theory, we should have little confidence in how the number of TCs will change as the climate warms. That said, models do predict that TC numbers might decline slightly as the climate warms.
Given that we expect TCs to also become more intense (see last section), a decline in overall TC numbers actually means fewer weak TCs.
A few years ago, Prof. Kerry Emanuel sent me some calculations he did to show why. This plot shows the probability of a TC as a function of damage the TC does for two different climates:
The plot shows that most TCs do little damage ($100M levels); the vast majority of damage is done by the rare but very strong TCs that do $100-billion levels of damage.
That plot also shows that, as the climate warms, we get a slight decrease in weak, low-damage storms and a slight increase in strong, high-damage storms. If we multiply probability times damage, we get the expected damage (it’s the area under the curves):
This is very bad news. The reduction in weaker TCs offers zero benefit, as these storms typically cause little damage, while the increase in stronger TCs leads to huge increase in damages.
Bottom line: The impact of a warming climate on the frequency of TCs remains uncertain. However, we anticipate that most of the reduction in their number would likely affect weaker TCs, which offers little benefit.
If there’s anything climate misinformers love love love to talk about, it’s how there’s no trend in the observed (normalized) TC damage. The conclusion they want you to reach is that TCs are obviously not getting more destructive.
I could explain why this is wrong, but Kerry Emanuel already did it and you should just read what he wrote. In addition, how you do the normalization5 of damages matters. There is debate in the literature about the best way to do this. The clear conclusion is that this approach is simply not robust.
A more reliable yardstick of TC destructiveness is to ask the people who have money on the line: insurance companies. If you do that, the verdict is clear: insurance premiums are skyrocketing and companies are fleeing places that are vulnerable to TCs (Florida, Louisiana) — exactly what you would expect in a world where the risk of TC damage was going up.
Insurance turns out to be the place where everyday citizens may finally realize, “holy cow, I’m screwed.” I’ve written about it previously and I strongly recommend Susan Crawford’s Moving Day substack, which talked about insurance here and here and here.
Bottom line: The “no increase in damages” argument is not very good for many reasons. It deserves to be dropped into the dustbin of history, but it’s so useful to climate misinformers that I’m sure it will never disappear.
This post has only touched on the ways that TCs are getting more damaging. There are even more ways, such as changes in the tracks of TCs moving to higher latitudes, more frequent rapid intensification, or the slowdown of TC translation speed, all of which can also increase destructiveness.
When arguing against this, climate misinformers don’t necessarily propagate outright lies. Rather, their method of misinformation lies in the selective emphasis of certain facts that bolster their stance. For example, they will focus on statistics like the number of TCs (or, worse, landfalling hurricanes), emphasizing that we don’t see any trend while conveniently omitting that climate scientists don’t predict an increase.
And they fail to acknowledge the actual factors that are driving destructiveness, such as the increase of storm surge damage caused by sea level rise or the fundamental physics that tells us that TCs will produce more rain as the climate warms. This is classic cherry picking.
Instead of the selective offering of climate misinformers, you should look at all of the data. If you do that, it’s clear that hurricanes and other TCs are getting more destructive.
Posted by Guest Author on Monday, 13 May, 2024
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