Urban Flooding – It’s Not About Climate Fact Check
We used to call it rain. Now it’s climate change, and that means it’s your fault.
I’m John Robson, and this is a Climate Discussion Nexus Fact Check on urban flooding.
There’s nothing new about rain. And there’s nothing new about floods either.
Throughout history we’ve had to contend with the destructive powers of nature, including regular floods that can sometimes cause enormous destruction.
[Potomac River 1913]
[Johnstown PA 1936]
But now people call it a “climate emergency” as if it’s something new and unusual. Has anything really changed?
When you listen to the slogans of politicians they keep coming back to flooding as an example of the harm being done by greenhouse gas emissions. And they’re not the only ones. The insurance industry has also been pushing the idea that flooding is getting worse and worse due to extreme rainfall connected to climate change.
They recently claimed that major flood events that used to happen every 50 to 100 years now happen every 6 years. But when the CBC ran a story on this on January 2019, they were called out. An engineer named Robert Muir, who works in urban flood control, challenged the CBC, and they ended up backing down.
“Although in recent years the news has been full of stories of bigger and more violent storms, and massive rainfall and flooding, there is nothing to prove that this type of precipitation event has been on the rise in Canada. Data compiled by Environment Canada since the 1950s show that there has been no significant change in their frequency.”
A major reason the CBC backed down is that they checked with Environment Canada:
“For Canada as a whole, observational evidence of changes in extreme precipitation is lacking.”
Which in English means, in this country it’s not raining harder than it used to. Remarkably, the UN’s Intergovernmental Panel on Climate Change says the same thing:
“In the United States and Canada during the 20th century and in the early 21st century, there is no compelling evidence for climate-driven changes in the magnitude or frequency of floods”
In fact you’d be hard-pressed to find anywhere in the world where rainfall is trending upwards.
In 2015, scientists William van Wijngaarden and Ali Syed at York University in Toronto examined rainfall records collected at nearly 1,000 locations around the world, extending back as far as the 1700s in some places, to assess if the world is getting wetter.
William van Wijngaarden
And sure enough, precipitation varies from year to year, sometimes there are even decades when things are drier than others, but we wanted to see is there a change when you look at long time spans like a century or longer. We found there was no change anywhere. So if you look at every continent separately, we saw no change when you look at 100 to 150 year time scales. Our conclusion was that people ought to be a bit cautious about saying there has been any change in precipitation when you look at the data that we’ve taken for about 100 to 150 years.
But even if rainfall isn’t changing, is it still possible that urban flooding is getting worse?
Trevor Dickenson is an emeritus professor of civil engineering at the University of Guelph. He’s been studying rainfall patterns and urban flood control in Ontario for decades.
Yes we’re likely to see more urban flooding, but I don’t think it’s primarily climate change, no. The relationship between rainfall and runoff has changed over the years. The question is: what has changed it? And the two things in southern Ontario that have the impact are climate change and urbanization. But interestingly enough, inside Ontario, climate change has reduced flooding because it has reduced the size of our spring floods.
Because we have more snow-melt events during the winter with warmer temperatures, and we have more rain events during the winter than we used to. The precip[itation] hasn’t increased in total, but the proportion of it that is rain has increased. So during the winter, there is more water flowing and so our winter flows have gone up, not quite to flooding extent yet. But it means then, with less snowfall in the winter and more snow melt during the winter, there is less snow at the end of the winter, and so the spring freshet is down.
So, in fact, our spring flooding in southern Ontario—in urban watersheds, rural watersheds, all over—has decreased. And so climate change has, in fact, decreased our significant spring floods now, over the years. But our summer flooding has increased, but only in urban areas.
We’ve never had summer floods in rural areas to speak of. We still don’t. But as our urban area has expanded, both in terms of the amount of paving and just the total area, the probability that severe storms hit the urban areas has gone up, just because the target’s bigger. And, in fact, with all the paving, now most of our floods in southern Ontario are in urban areas in the summer.
As we expand the urban footprint, bare land is replaced with hard impenetrable surfaces. Now instead of rain going straight into the ground, it has to run off over the artificial surfaces in search of a drain.
Rain now means run-off, and if there’s enough run-off in a short time, and no place for the water to go, we get an urban flood.
We haven’t managed it as well as we should have. When we look at rainfall amounts, people often say, “Well the reason we’re getting all these summer storms and summer floods is we have more rain events, and many of them or most of them are bigger than they used to be.” The reality is, we are not having more rain events in the summer. And from our look at it here, the extreme rainfall events haven’t increased.
I’m aware that many people have noticed that some extremes have gone up in other parts of the world. I think the jury’s out here in terms of whether they are going to go up or not. But to date, we can’t prove that they’ve gone up. But nonetheless, the amount of water coming off as we continue paving, we just aren’t managing the volumes as well as we should have.
The lower end of the Don Valley Parkway floods rather regularly in the summer in heavy storms. The lower end of the Don River in the valley, is functioning as the bottom end of the storm water system. And it’s then convenient, in a sense, to blame climate change and say, “Well, my gracious, our floods are due to increased rain storms,” rather than to accept the responsibility that we haven’t really done as good a job as we should have in managing the storm water.
Cities from Vancouver to Halifax are jumping on the bandwagon of declaring a “climate emergency”, blaming global warming for causing the floods they now say they have to contend with.
This is foolishness. Instead of fixing the drainage problems they themselves created, problems we’ve understood for decades, it’s so much easier to point the finger of blame at the sky or citizens who just won’t stop emitting CO2. But as we’ve seen, there’s no more rain than there used to be, there’s just nowhere for the water to go, because of poor management of well-understood urban challenges.
So city mayors, quit making excuses, do your job, and fix the dang drains.