Is climate change making Florida's flooding worse?

almost certainly yes

In the past few days, you may have seen headlines like this:

Whenever extreme weather strikes, an immediate argument arises: Is this a consequence of climate change? Those who are in favor of climate action will connect the event to climate change. Those who are not in favor will claim that there’s no connection. In this post, I’ll explain the connection between extreme rainfall and climate change.

why does it rain?

Rainfall occurs when air rises. For example, consider a low pressure system:

from NOAA SciJinks

Remembering that nature abhors a vacuum, air converges into a low, meaning it flows inward toward the low-pressure center. Because of conservation of mass, as air flows inward at the surface, air above the low must be ascending.

As this air ascends, it cools. As the air cools, the water vapor in the air condenses, forming liquid water droplets. These liquid droplets can grow large enough to begin falling out of the atmosphere — it is raining!

When the air reaches the top of its ascent and detrains into the upper troposphere, it has had the vast majority of the water vapor wrung out from it, leaving just a percent or two¹.

I sometimes refer to this process as “nature’s air conditioner” because it works like the one in your house: moist air is exposed to cold temperature — in your house, a cold coil, in the atmosphere, a cold upper troposphere — and the water is converted to liquid and removed, leaving very dry air.

rainfall intensity increase with global warming?

Let me emphasize up front that climate change doesn’t cause rain events. Rather, the role of climate change is like steroids for the weather — it injects an extra dose of intensity into existing weather patterns. So the right question is whether climate change has amped up this event.

Warmer air can hold more water vapor — about 7% more for every degree Celsius increase in temperature². Consequently, the air converging into a storm system in a warmer climate carries more water vapor. Since most of the water vapor entering the storm’s updraft will fall out as rain, everything else the same, more water in the air flowing into the storm will lead to more intense rainfall. That’s it. Not terribly complicated.

Obviously, not everything else has to stay the same. Things like the vertical velocity and the temperature profile can both change and that could modify the rate at which intense rainfall is getting more intense. See this paper for a complete discussion of these other factors. Considering all of the factors yields a very similar conclusion.

can we see this in observations? yes!

Fischer and Knutti, Nature Climate Change, 2016

When it comes to flooding, the rain events we care about are the most intense. And this plot shows the observed frequency of occurrence of these rain events in two periods, the mid-20th century and the late-20th/early-21st century. As you can see, the heaviest events (above 99th percentile) are indeed becoming more intense.

what does the scientific community conclude?

Because of this, the IPCC’s synthesis report concluded:

The frequency and intensity of heavy precipitation events have increased since the 1950s over most land areas for which observational data are sufficient for trend analysis (high confidence), and human-caused climate change is likely the main driver.

They go on to say that:

The projected increase in frequency and intensity of heavy precipitation (high confidence) will increase rain-generated local flooding (medium confidence).

what about this particular event in Florida?

of course he does

As discussed above, the physics of precipitation suggests a clear mechanism for climate change to be intensifying rainfall in Florida. This means that our default assumption at this point is that climate change made this rain event more intense. If you want to argue that it did not, show me your analysis.

That said, we cannot quantify the impact without detailed attribution analysis. Thus, the right response from the Florida State Government is: “Climate change almost certainly made this event worse. How much worse cannot be determined without detailed attribution analysis, which I'm sure will be carried out.”

But that, of course, was not their response. Instead, they politicize the issue, converting a physics problem into a culture war issue to rally their supporters. Anyone who disagrees with them is, by definition, part of the opposition, e.g., a woke activist.

This will not end well for Florida. It is literally in the bulls-eye for climate impacts — e.g., hurricanes, sea level rise, extreme heat, extreme rainfall. This is already affecting human systems we rely on. For example, we can see cracks forming in Florida’s insurance market, with companies raising premiums or pulling out of high-risk areas altogether.

The Florida government’s position might be good politics, but it’s not scientifically accurate. Ultimately, it will be the citizens of Florida who will suffer.

Related posts

Anatomy of a heat wave

Pinning down climate change’s role in extreme weather

1. I’m referring here predominantly to very deep convection, the type associated with the most intense rain events.

2. Although, for technical reasons, the increase in rainfall will be slower than 7%/degree.