Toronto Extreme Rainfall Trends - 100-Year Daily Rainfall in Engineering Climate Datasets

Previous posts have reviewed trends in extreme rainfall across Canada, in various regions including southern Ontario, and in the Greater Toronto Area (GTA), including Toronto and Mississauga where long-term climate data is available for review.

Projections of future extreme rainfall increases are commonly made as part of climate change studies. A review of past trends in extreme rainfall was made in the 2021 National Research Council flooding cost benefit guidelines, as summarized in a previous post. The following chart was included in those guidelines and shows the trends in 100-year daily rainfall at two GTA climate stations in downtown Toronto and at Pearson International Airport in the adjacent municipality of Mississauga.

The chart shows the 100-year rainfall depth using data records up to 1990 and then adding more recent data up to 2017. The chart shows that the 100-year rainfall at Pearson Airport/Mississauga has been decreasing slightly when recent data is added after 1990. Meanwhile the Toronto rainfall has been increasing slightly (see dotted and dashed black lines on the chart above for the trends).

Several climate studies have projected that the 100-year daily rainfall would increase over coming decades as shown on the chart. The Toronto's Future Weather & Climate Driver Study by SENES projected a doubling of this rainfall statistic by 2040-2049, relative to a 2000-2009 baseline value (see the orange dashed line on the chart above, where the 2000-2009 value is shown at 2005 and the 2040-2049 value is shown at 2045).

Some additional data has been analyzed by Environment and Climate Change Canada for the Toronto climate station, now including data up to 2021. This allows the 100-year daily rainfall statistic to be updated with a few more years of data. The chart below shows the additional Toronto data point circled in yellow at 2021.

While the Toronto rainfall statistic up to 2017 was 97.5 mm, the value up to 2021 decreased slightly to 97.3 mm. The value up to 2017 reflected the prior July 8, 2013 extreme event, creating a jump after 2007 when the value was slightly lower at 94.7 mm. As more data is observed below the 2013 extreme, the statistic should continue to decrease as more data is added and analyzed.

The take-away? Observational data, including data up to 2021, does not support the projected significant increases in 100-year daily rain in climate studies. The Toronto data is available over the period of 1940 to 2021.

How far off are the projected increases in extreme rainfall? The Toronto Future Weather & Climate study projected a theoretical 31mm/decade increase over 40 years - that was for Pearson Airport climate station. Actual data at Pearson Airport shows an observed increase of only 3.1mm/decade.  This considers a value of 115.1 mm in the middle of the 1950-2003 period and a value of 125.5 mm for 2003-2017 - that later value is estimated to generate the current value of 117.3 mm by using a weighted average across all years from 1950 to 2017. For Toronto the actual increase is only 2.0 mm/decade.

On average the GTA (Toronto and Pearson/Mississauga) increase is about 2.5 mm/decade, or less than a tenth of almost 31mm/decade projected in the SENES climate/future weather study.

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Further reading in previous posts on extreme rainfall trends:

1) Rainfall intensity trends in Canada:

a) 226 long term climate stations in the Engineering Climate Dataset are used to show actual trends between rain intensity statistics up to 2007 and then up to 2017: https://www.cityfloodmap.com/2020/12/design-rainfall-trends-in-canada.html

b) more of the above plus observed annual maxima rainfall trends as reported in the 2021 National Research Council of Canada (NRC) "National Guidelines on Undertaking a Comprehensive Analysis of Benefits, Costs and Uncertainties of Storm Drainage and Flood Control Infrastructure in a Changing Climate": https://www.cityfloodmap.com/2022/02/nrc-national-guidelines-on-flood.html

2) Rainfall intensity trends in Southern Ontario:

a) ECCC's Engineering Climate Dataset Intensity Duration Frequency (IDF) trends for long-term southern Ontario climate stations, comparing statistics up to 1990 and current values (v3.3 datasets with some station data up to 2021): https://www.cityfloodmap.com/2023/05/southern-ontario-extreme-rainfall.html

3) Rainfall extreme reporting (?) in the media (including Toronto, Mississauga trend review):

a) Thinking Fast and Slow About Extreme Weather and Climate Change, inspired by the late Daniel Kahneman (RIP good sir), exploring the cognitive biases in extreme rainfall reporting in the media: https://www.cityfloodmap.com/2015/11/thinking-fast-and-slow-about-extreme.html

b) my paper with "Thinking Fast and Slow" themes published in the Journal of Water Management Modelling with the title "Evidence Based Policy Gaps in Water Resources: Thinking Fast and Slow on Floods and Flow": https://www.chijournal.org/C449

4) Local studies that observed no increases in design rainfall when updating IDF values:

Many studies: https://www.cityfloodmap.com/2018/03/extreme-rainfall-and-climate-change-in.html

While media and the insurance industry has repeated that climate change has been responsible for increased flood damages and insurance claims over past decades, the lack of increases in extreme rainfall means that other factors are at play. These include fundamental changes in hydrology in urbanized communities, e.g., increased watershed development and intensification. See previous posts for some examples of expanding urbanization in Ontario communities over previous decades: https://www.cityfloodmap.com/2016/08/land-use-change-drives-urban-flood-risk.html

When fact checkers look into media statements regarding extreme rainfall trends, including the CBC and Radio Canada Ombudsmen offices, data shows no overall increase in extreme rain across Canada. This post shares corrections made by the CBC over recent years: https://www.cityfloodmap.com/2019/06/cbc-correcting-claims-on-extreme.html

Data on Climate Change Impacts excluded due to "Cabinet Confidences" in Requests under Access to Information Act

Our readers will be familiar with previous posts reviewing extreme rainfall and temperature trends, focusing on reliable, official datasets - in many cases this is to respond to claims made in the media that are not supported by data. This post lists many corrections on extreme rainfall trends made by the CBC and Radio Canada over the years: https://www.cityfloodmap.com/2019/06/cbc-correcting-claims-on-extreme.html. In 2015 the CBC wrote: ""Environment Canada verified that there has been no significant change in rainfall events over several decades". In 2019 the Environment Minister Catherine McKenna  wrote that "For Canada as a whole, observational evidence of changes in extreme precipitation amounts, accumulated over periods of a day or less, is lacking."

Despite this, federal documents continue to suggest otherwise. As a result I made an Access to Information Act request on this statement in the 2021 budget:

"Communities across Canada now face once-in-a century floods every few years due to climate change. These devastating deluges are damaging homes, businesses, and infrastructure."

As show below I requested records of flood frequency for communities across Canada referred to in the 2021 Budget statement that show once-in-a-century floods, e.g., 1-in-100-year floods are already occurring every few years. The response below cited exclusions in the Act related to cabinet confidences:

Others have made similar inquiries regarding statements on climate change impacts made in the 2022 federal budget. For example the statement: "Canada is already experiencing an increase in heat waves, wildfires, and heavy storms. These impacts—and the economic and health repercussions that come with them—will continue to accelerate if we do not act now.", was made in the budget published on April 7, 2022.

A reader's inquiries for information to support the above statement (ATIP File A-2022-00126) was also met with the same exclusions. Below are links to a couple of the reader's follow up documents including:

1) A complaint to the Office of the Information Commissioner on the cited exclusion: https://drive.google.com/file/d/1T_9NbtUMjzQrR97MlFsF84sdOgTFfs4u/view?usp=sharing

and

2) A summary of factual data on temperature, wildfire and rainfall trends:  https://drive.google.com/file/d/18z8wZgT_Q1DlIEILG8dXAal2hg4BGtZN/view?usp=sharing

The later cites data showing that heat waves in Canada have not increased, the area of wildfires has not increased, and heavy storms have not increased either. This is all contrary to the budget statement, for which data is protected by 'cabinet confidence'. 

Good policy and budget decisions should be guided by good fundamental data - that data should not be a secret, protected by cabinet confidences. The current lack of openness to provide data to back-up statements (claims?) on extreme weather trends in the 2021 and 2022 budgets calls into question the validity of the strategic directions advanced in these recent budgets.

Southern Ontario Extreme Rainfall Intensity Trends - Update From Environment Canada Engineering Climate Datasets

Environment and Climate Change Canada has updated and extended the Engineering Climate Datasets as noted in the last post. This post shows the updated trends in extreme rainfall intensities across long-term southern Ontario climate stations - the good news is that intensities have not increased. This means that infrastructure built in the last few decades is not undersized considering current rainfall design intensities.

Previously trends in some southern Ontario intensity-duration-frequency (IDF) values and annual maximum series were evaluated in my paper "Evidence Based Policy Gaps in Water Resources: Thinking Fast and Slow on Floods and Flow" in the Journal of Water Management Modeling: https://www.chijournal.org/C449

Further analysis of trends in long-term stations has been presented in this blog and in the National Research Council of Canada's "National Guidelines on Undertaking a Comprehensive Analysis of Benefits, Costs and Uncertainties of Storm Drainage and Flood Control Infrastructure in a Changing Climate" - that guideline included trends in southern Ontario up to Version 3.10, as presented in the earlier post: https://www.cityfloodmap.com/2022/02/nrc-national-guidelines-on-flood.html

The southern Ontario IDF trend data has now been updated based on the Version 3.3 dataset released in May 2023 and includes some station updates to 2021- 9 of the 21 stations were updated. The following table and charts show the trends in 2-year to 100-year design rainfall intensities.

The table below shows changes in average intensity - decreases since 1990 are shaded in green and increases are in red. Note the the trends are weighted by record length. Across all durations and return periods the average decrease is - 0.33 %.  That is a slight decrease from the Version 3.20 datasets, meaning less intense rainfall when more recent data has been included. On average 30 statistics decreased while 20 statistics increased.

It is noteworthy that none of the 2-year intensities increased and the largest increase was 0.8% for 100-year intensities at one station for durations of 30 minutes and 1 hour. Overall for 21 stations 100-year intensities were virtually unchanged with the average intensities decreasing 0.1% after 30+ years, and the median increasing 0.2%. Skewed data statistics should increase over time with longer records - check out this post for more on that: https://www.cityfloodmap.com/2016/02/ontario-climate-change-trends-going.html

The following chart shows the range of changes for each return period as well as the average change. The decreases are greater than any increases for the 5 to 100-year events.

(note: above chart average intensity change dashed line corrected Nov. 10, 2024)

The following chart provides more of a breakdown by duration. One can see the red 5 minute intensities decreased on average for all return periods. The 2 hour to 24 hour intensities decreased for most return periods and where there were increase they were minor compared to other decreases. For the 5-year to 100-year return periods the 15 minutes to 1 hour intensities increased, but by no greater than 0.8%. These increases and decreases are basically insignificant in terms of impacts on infrastructure design.

This last table is annotated to show how various statistics are used in design. Infrastructure that has been designed considering short duration intensities like local sewer systems are now subject to virtually the same 2 to 10-year design intensities that existed over 30 years ago. Ponds designed for long duration higher return periods (e.g., 100-year events) are now subject to virtually the same intensities, or design event volumes, they were subject to decades ago as well.