When most people think about climate change, an image of excessive heat and drought comes to mind. In Canada, we may also think about warmer winters and reduced snowfall (this winter excluded). There is also the impression that weather conditions are getting more variable and unpredictable. While some of these impressions make sense based on observations of weather conditions over the past few decades, when it comes to heat, I had the impression that summers are getting less hot in Southern Ontario. Is it possible that we are warming overall but losing summer heat in this area?

Most climate predictions are based on models that examine a broader geographical area and average trends over a whole year or season. This is not very helpful for agriculture because local, monthly trends are much more important for the potential impacts of the weather on the crops. I am not a climatologist, but as a horticulturist, I wanted to see what the local weather trends show for different times of year here to understand how those changes could affect horticultural producers in this area. This analysis only applies to southern and eastern Ontario. Trends in other areas may be very different.

Data Analysis

I analyzed the weather data from six Environment Canada weather stations that had reliable data going back many decades. The six sites were Windsor (1940-2024), London (1883-2024), Delhi (1934-2024), Peterborough (1882-2024), Belleville (1921-2023), and Ottawa (1892-2023). I had to adjust for missing data and remove some data impacted by weather station moves at some locations to ensure they didn’t affect the interpretation of the results. I examined monthly average low, mean, average high, extreme high, and extreme low temperatures, monthly precipitation, the length of the frost-free period (frost = ≤0^oC), and the number of days over 30°C at each location. I did this by graphing the trends over time, determining how well the trend line fit the data (linear regression analysis), and calculating the start and end data based on that trend.

Temperature Trends

Figure 1 shows the temperature changes for London as a good representative of the trends at all locations. This includes the extreme and average high and low temperatures for each month, with the faded bars representing the past and brighter bars representing the present. While all of the trends were consistent, the trend lines for London were generally only statistically significant when the temperature change was >1°C between the beginning and end of the recording period.

Figure 1. Monthly average and extreme temperature changes at London 1883-2024 calculated based on trend lines. Faded bars represent the 1883 values.

The overall trends at all six locations were very similar to London:

1. Average annual temperatures have increased 1 to 2°C at all locations, with the largest increases in the east.
2. Average summer high temperatures have remained steady or declined slightly over time. The extreme high temperatures for most summer months have declined by 0.6 to 2.5°C at all locations. As a result, the number of days over 30°C has also declined at all locations. In London, the average days over 30°C decreased from 23 to 9 days per year over the recording period. Changes in extreme heat are even more dramatic. At Delhi, there were a total of 91 days over 35°C in the last 90 years, with only three of them occurring in the last 30 years combined (the last one in 2012). There was less change for locations right beside a Great Lake, such as Belleville, probably because warmer water temperatures due to annual warming counteract the cooler air temperatures in the summer.
3. Average and extreme low temperatures have increased significantly for every month of the year, but especially in the winter.
4. The most extreme increase in temperature is occurring in the winter, especially December. The extreme low temperatures for that month are up to 8.5°C warmer than the past. Average December temperatures at Ottawa have increased by 4.5°C. 

Analysing the data further revealed that the coldest days of each month are increasing significantly, while the warmest days are not changing in the winter and are getting cooler in the summer. In effect, our temperatures are becoming more moderate: less summer heat, much less winter cold, and fewer cold nights in the summer.

Frost-Free Period

There has been an incredible increase in the length of the growing season at all locations. On average, the last frost in the spring was a full two weeks earlier and the first frost in the fall was three weeks later, resulting in a 35- to 40-day increase in the frost-free period.

Precipitation Trends

Some of the moderation in temperatures in the summer could be explained by changes in rainfall. While rainfall data was extremely variable and there were no significant trends, analysing all stations together suggests an increase of about 80 mm of rainfall during the summer months (Figure 2). On the other hand, precipitation during the winter has decreased by about 50 mm. Wetter summers could mean more humidity and more clouds, reducing extreme heat while increasing temperatures at night. These conditions could mean higher humidex readings, something that may make workers more uncomfortable but does not affect plants.

Figure 2. Changes in monthly precipitation at six weather stations relative to the start of data collection at each station.

Variability

There was no apparent increase in year-to-year variability in any of the data analysed. The weather here has always been variable, and changes from year to year or within a year do not appear to be different than what they used to be, although this was not statistically analysed. The moderation of temperatures has likely reduced the day-to-day variability all year.

Impacts on Specialty Crops

So, what does this mean for specialty crop growers in southern Ontario? It will likely depend on whether you are growing annual or perennial crops.

Perennial Crops

The data is probably not good news for any perennial crop. A longer growing season and moderate temperatures are generally good for the plant itself, potentially allowing for the production of longer-season varieties or even new crops that could not survive our winters in the past. It could also reduce plant stress over the year. However, the increases in winter temperatures are a concern. Due to much less snow cover and frost in the ground, trees may be prone to breaking dormancy prematurely when the first sunny, warm period of spring arrives. Even though the last frost is two weeks earlier than it used to be, the plants may be even more advanced when that frost occurs in some years, resulting in more damage. There is also a risk that increased precipitation in the summer is accompanied by more extreme weather such as hail and wind, data that isn’t as easily analyzed.

Annual Crops

A longer growing season with less extreme cold and extreme heat is better news for annual crop growers. It could allow for a longer harvest season, the potential for new annual crops that couldn’t reach maturity in the past, and more flexibility around multi-cropping over the season. Cool-season vegetables would have fewer heat events that limit yield, and warm-season vegetables may benefit from warmer nights and less extreme cold. More rainfall and more extreme rainfall events may cause problems for annual crops by delaying seeding, preventing timely pesticide application, and affecting harvest.

Pests

Unfortunately, a longer, wetter, and more moderate growing season is also fantastic for insect and disease pests. Insects may have more generations per year and less winter cold to control populations. New invasive species may survive in our area that couldn’t before. Foliar and root diseases have more ideal wet and moderate conditions for development and spread. This all means more pest controls may be needed and for a longer period of the year.

Take Home Message

The climate is clearly warming in Southern Ontario. However, my analysis showed that the warming is not even over the year. Winters and nights are warming much more than summers and days. In fact, extreme heat is declining while precipitation is likely increasing in the summer. This will have a very different impact on farmers in this area than the picture of heat and drought that dominates the news on climate change. The biggest threats for specialty crop growers are overwintering and spring frost risks for perennial crops, more rainfall for annual crops, and increases in insect and disease pressures for all crops.

It is important to remember that the weather has always been variable, so any year can still have extreme heat, drought, or cold winters. The trends just mean that the risks of these occurring go down a little each year. This is all based on the trends up to now. There is also no guarantee that they will continue in the same direction in the long term.