Extreme Rainfall and Climate Change in Canada and Ontario - Compiled Engineering Reports, Research Papers and Presentations by Esteemed Hydrologists Indicate No Change

We have sent a request to the Globe and Mail regarding a recent article on climate change and extreme rainfall and flooding. Specifically we have requested that a correction be made to this statement:

"Moreover, extreme weather related to climate change appears to be increasing the frequency and severity of flooding events."

The article was entitled "How we can better mitigate flood risk in Canada", January 15, 2018, by Glen Hodgson, a senior fellow at the Conference Board of Canada. This is a link to the article .. or should we say the "Opinion Piece":

Certainly everyone is entitled to their own 'opinion', by not their own 'data'. With that in mind we have asked the Globe for a correction based on the following data and analysis:


The following engineering reports, thesis documents, peer-reviewed papers, presentations by Canada's most esteemed hydrologists and comments by Environment and Climate Change Canada staff clearly refute the Globe article statement that the frequency and severity of extreme weather is increasing and that "extreme weather related to climate change appears to be increasing":

i. City of Guelph – Ward 1, Frequency Analysis of Maximum Rainfall and IDF Design Curve Update, Earthtech, 2007. Concludes that city would “prefer to retain the existing curves and higher values” as updated rainfall intensities were lower.

ii. Updates of Intensity-Duration-Frequency (IDF) Curves for the City of Waterloo and the City of Kitchener, University of Waterloo, 2012: indicates “new IDF curves tend to be lower .. for rainfall durations up to one to two hours”.

iii. Rainfall Intensity-Duration-Frequency (IDF) Curves RFP11-080 Engineering Design Criteria and Standards Update (DRAFT), fabian papa & partners inc., 2012. Found that “City’s existing criteria ... are more conservative than recently compiled statistics” and “up-to-date Environment Canada IDF Curves will result in lower storm intensities”.

iv. Trends in Canadian Short‐Duration Extreme Rainfall: Including an Intensity–Duration–Frequency Perspective, Environment Canada, Atmosphere Ocean 2014: indicates “no detectable trend signal” across Canada an no regional increases in Ontario.

v. Precipitation Intensities for Design of Buried Municipal Infrastructure, Yi Wang, University of Guelph, Ph.D Thesis 2014: identifies 24 significant increases and 41 significant decreases.

vi. Waterloo Sanitary Master Plan, Volume 1, Appendix A Climate Change, Stantec, 2015. The study did not identify any rainfall intensity changes but adopted a different rainfall pattern to be more conservative in design.

vii. Changes in Rainfall Extremes in Ontario, International Journal of Environmental Research, University of Guelph, 2015. The paper that identified “results of this study indicate that the +ve and -ve changes in annual rainfall extremes are similar in the order of magnitude.”

viii. Short Duration Frequent Rainfall Show No Change in Southern Ontario IDF Design Intensities - No Change in Averages Suggests No Change in Extremes, Muir, 2018. This analysis of Environment and Climate Change Canada’s IDF statistics from 1990 to the current Version 2.3 Engineering Climate Datasets shows no change in 2-year to 10 year 5-minute to 2-hour rainfall.


ix Floods in Southern Ontario Have Changed, University of Guelph, MNRF Floodplain Technical Workshop, Vaughan, March 7, 2018. Professor Emeritus Dr Trevor Dickinson presented  last week that:

"In fact:
- the number of rainfall events has not increased,
- the total amount of rainfall occurring over the growing season has not increased, &
- to date, there is no evidence that rain storms are more severe."

x Environment and Climate Change Canada (ECCC) has commented to the CBC/Ombudsman to dispute insurance industry statements that we have more storms (see letter to me at this link):


That was in response to this story that had no fact-checking on extreme weather frequency:


And which had this correction made based on ECCC and real data: "However, Environment Canada says it has recently looked at the trends in heavy rainfall events and there were "no significant changes" in the Windsor region between 1953 and 2012." 

xi Canadian Underwriter editors dispute insurance industry statement on more frequent / severe storms after fact-checking with ECCC: 


"Associate Editor’s Note: In the 2012 report Telling the Weather Story, commissioned to the Institute for Catastrophic Loss Reduction by the Insurance Bureau of Canada, Professor Gordon McBean writes: “Weather events that used to happen once every 40 years are now happening once every six years in some regions in the country.” A footnote cites “Environment Canada: Intensity-Duration-Frequency Tables and Graphs.” However, a spokesperson for Environment and Climate Change Canada told Canadian Underwriter that ECCC’s studies “have not shown evidence to support” this statement."

Please issue a correction to the article. Thank you.

Robert J. Muir, M.A.Sc., P.Eng. 

BONUS - the City of Ottawa has also reviewed IDF curves in October 2015 and found the following:


"The IDF curves used for sizing of storm sewers and stormwater management designs that are currently in use at the City and provided in the Ottawa Sewer Guidelines (OSG) is based on rainfall data collected at the Ottawa McDonald Airport from 1967 to 1997 by Environment Canada (further referred to OSG IDF Curves).  Environment Canada updated the IDF curves using data to 2007 (further referred to as 2007 IDF Curves).  Over the recent past years, the update was brought to ISD’s attention for adopting into the OSG.

 A comparison of the OSG and the 2007 IDF curves revealed that for short durations the intensities from the 2007 IDF Curves are less than the OSG IDF Curves on average by 5% and 10% for the 5 year and 100 year curves respectively.  However, for the longer durations the intensities for the 2007 IDF Curves are actually greater than the OSG IDF Curves on average by 11% and 17% for the 5 year and 100 year curves respectively.  The intensities at the shorter durations would influence storm sewer sizing while the longer durations will influence stormwater pond sizing.  The lower intensities at the short durations will tend to result in smaller storm sewer sizes while the larger intensities at higher durations will tend to increase stormwater management pond requirements.  


Given that the percentage differences in intensities between the IDF curves is within the margin of error associated with data collection and hydrologic assessments, it was ISD’s opinion not to update the OSD IDF curves.  As part of the Stormwater Levels of Service review, the need to revisit the IDF curve selection was identified."

Because stormwater ponds are not designed with IDF curve statistics alone, i.e,. they are designed by simulating temporal patters of storms in hydrologic/hydraulic models, it is questionable if an IDF shift in long durations alone would alter the design of a stormwater management pond. That is the design storm (or hyetograph) pattern would influence the pond performance more than the input IDF used to adjust storm volume. Also, some municipalities assess pond performance assuming the outlet is completely blocked, as an operational worst case scenario, such that all collected runoff is routed through the emergency spillway. In this extreme operating condition, a small change in rainfall volume may not affect overall performance (i.e., spillways operate efficiently as weirs, able to accommodate additional flow release with limited increase in operating level (pond water surface elevation).

ANOTHER BONUS - the City of Hamilton has also reviewed IDF curves in 2015 and found no change that warranted updates in their current standards (R.Muir personal communication with Hamilton engineering staff May, 2018).


Update: The Globe and Mail responded by 'working their Google' and citing examples of flood damages increasing, diverting from the question on extreme weather frequency. We have explained that flood damages can increase for many reasons and that they have fallen prey to 'attribute substitution'. Check out this recently published paper on heuristic biases and challenges in framing and solving problems related to extreme weather and flooding:

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