I recently came across a chart on Reddit that showed the average daily mean temperature in Toronto has risen about 3.5% since 1841. This chart is nice and simple to understand.
However, I wanted to dig beneath the surface to get a more detailed picture of what’s actually going on. Specifically, I wondered whether Toronto is experiencing hotter summers or milder winters. So I broke out the data and created my own chart.
To answer my question I needed to add more variables to my chart, unfortunately making it more complicated to understand. However, if you take the time to digest the information the conclusions are pretty clear.
Each data point in the red shaded area in my chart below represents the maximum mean temperature in the previous 12 month period. The blue shaded area represents the minimum mean temperature in the same 12 month period. Maximum mean temperatures occurred during the summers and minimum mean temperatures occurred during the winters. So what this basically shows is the hottest periods of historical summers and the coldest periods of historical winters.
I then layered on a linear trendline to illustrate the broader trend.
As you can see, the trend is up – for both summers and winters. Summers are getting hotter and winters getting milder. What I found interesting, however, was that while mean maximum temperatures during summers has increased by about 2 degrees Celsius since 1840, mean minimum temperatures during winters has risen about 3.5 degrees Celsius.
Simply put, winters in Toronto are getting milder faster than summers are getting hotter, significantly contributing to the city’s overall rise in average temperature.
Note: The data for this particular weather station is only available until June 2003. I suppose the weather station was closed then?