Is climate change to blame for Montréal’s messy winter?

(Cover photo: Radio-Canada/Simon-Marc Charron ;Ce billet est aussi disponible en français)

This winter has been a difficult one in Montréal, with several rainstorms followed by a rapid freeze leading to sidewalks that resemble glaciers. Many are wondering if this might be the new normal. Back in November, I wrote a post discussing how the start of “wintry” weather (snow, freezing cold) has been getting later and later over the last 50 years or so, with the winter season getting shorter. Here, I’ll examine a range of climate variables to demonstrate how our winters have already changed with global warming, and how this year’s winter fits into these changes.

Weather vs. climate

The first thing we might assume when we hear “global warming” is that winters will just get warmer. That actually sounds nice, doesn’t it? We first need to keep in mind the difference between weather and climate. When I spoke recently to the Montreal Gazette about how winters are changing, I got an email from a reader questioning whether I was living in the same city as him, since we’ve had plenty of snow this winter, and November was one of the coldest on record in Montréal! So what about global warming?

Remember that weather is what we observe in a given month, week, or day. Climate is the average of all of those months/weeks/days over a certain period (typically at least 30 years). We’re always going to have really cold days and really warm days, but as the climate changes, what constitutes “really warm” and “really cold” is changing. One sign of the warming climate is that we’re seeing far more record hot days than record cold ones. And our “extreme” cold is getting less extreme. For example, through the 1980s, -30°C was observed at Montréal at least once every few years. The last time we hit -30°C (at Trudeau Airport) was in 1994!

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How does weather impact bike traffic in Montreal?

(Cover photo: Michel Rathwell ; Ce billet est aussi disponible en français)

Bixi Montréal was the first major bike share in North America when it opened in 2009. The technology was sold and the familiar Bixi bikes can now be seen in Toronto, New York, Chicago, Boston, Washington, D.C., London, and many other cities around the world. Bixi’s open data contains information on all trips since 2014 that last longer than 1 minute and shorter than 2 hours, including start/end times and docking stations. There’s a lot to analyze. In particular, I’m interested in how the weather dictates the usage of Bixi. In addition, the City of Montreal has about 20 bicycle counters set up along various bike routes in the city. While this data is a bit less reliable than Bixi’s (several counters have had outages that are not always easy to detect in the data), it’s useful in that Bixi only operates from April 15-November 15. Bike counter data will allow us to see how cyclists react when temperatures drop during the winter.

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When does Montréal start seeing wintry weather?

https://fr.wikipedia.org/wiki/Fichier:Montreal_Skyline_winter_panorama_Jan_2006.jpg

After a particularly hot and humid summer, temperatures have finally cooled off in Montréal. Weather models have started showing light snow approaching the area in the next couple of weeks, and it looks like our first 0°C temperatures could arrive in the next few days. So, when do we typically start seeing this wintry weather in Montréal?

To answer this, I’ve looked at Environment and Climate Change Canada data for Montréal’s Trudeau airport (CYUL), which has observations since 1942. We’ll focus on two components of wintry weather – cold temperatures and snow.

First “freezing” temperatures

First, let’s look at the first autumn day on which the minimum daily temperature reaches at least 0°C. In the plot below, the date for each year is plotted, with a 10-year centered running average shown in the blue line. Prior to around 1990, the first 0°C day almost always occurred between the last week of September and the third week of October. Since 2005, however, the first 0°C day has always occurred between the second week of October and the first week of November. The 10-year average first 0°C day is now October 25, an increase of more than 2 weeks since 1990, when the 10-year average was around October 8.

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