Decrease in Southern Ontario Design Rainfall IDF Curves Matches Trends in Observed Storms - Decrease in Both Frequent and Rare Short Duration Intensities - Overall Decrease in Small Storms, Large Storms Mixed

Are Ontario Rainfall Trends a Nothing-Burger?
Read This Post and Find Out !
Previous posts reviewed trends in observed maximum series of observed rainfall, showing more decreasing trends in Southern Ontario than increasing trends (see post). That observed trend analysis is part of  Environment and Climate Change Canada's Engineering Climate Datasets, Version 2.3. Design rainfall intensities are derived from these observations to create intensity-duration-frequency (IDF) curves, by fitting a probability distribution to the observations. A sample of the change in design intensities over time was presented at the National Research Council's February 27, 2018 Workshop on adaptation to climate change impact on Urban / rural storm flooding  (see slides 9 and 10):



The sample IDF review showed no change in 2-year to 10-year return period intensities over durations of 5-minutes to 2 hours. The slide content was also featured in a previous post which includes links to the earlier 1990 datasets used in the comparison (for those who have thrown out those old 5 1/4 inch floppy disks with the 1990 data).

This post shows the change in IDF values for these Southern Ontario climate stations for all durations and all return periods. The chart below summarizes the change in IDF values for the 21 stations, each with 30 years of record or more. It shows the range in IDF change for each return period, across all durations. The changes for each station have been weighted by the duration of the climate station record, so that a station with a record of 60 years is given double the weight of a station with 30 years of record.

Ontario IDF Trends for Extreme Rainfall Climate Change Effects
Southern Ontario IDF Trends - Decreasing Frequent Storm Intensity, Mixed Infrequent Storm Intensity, Overall Decrease in Average Rainfall Intensity Values for Engineering Design. 5-Minute to 24-Hour Durations.
Looking into the details, the next chart shows the change in rainfall intensity for each duration within each return period as well.

Ontario IDF Trends for Extreme Rainfall Climate Change Effects Details
Southern Ontario IDF Trends - Decreasing Short Duration Storm Intensity (5 minutes - dark red bars), Decreasing Moderate Duration Storm Intensity (1-2 hours - green bars), Negligible Change in Long Duration Storm Intensity (12-24 hours - dark blue and purple bars).
The take-aways from the IDF update comparison :

i) small frequent storms (2-year, 5-year, 10-year return periods) used to design storm sewers, for example, are consistently smaller now than in the 1990 dataset,

ii) large infrequent storms (25-year, 50-year, 100-year return periods) used to design major drainage systems and infrastructure networks are mixed with some increases and some decreases since 1990 but no appreciable change that would affect design (any changes are less than 1%, which is negligible in engineering design),

ii) there is an overall average decrease in IDF values of 0.2 % across all return periods and durations.

Percentage IDF change values shown in the detailed chart are summarized in the following table for 5-minute, 10-minute, 15-minute, 30-minute, 1-hour, 2-hour, 6-hour, 12-hour and 24-hour durations, and for 2-year, 5-year, 10-year, 25-year, 50-year, and 100-year return periods.

Ontario IDF Update Trends in Rainfall Intensity and Frequency
Southern Ontario Rainfall IDF Trends From 1990 to Current Version 2.3 Engineering Climate Datasets (Average Values for 21 Long-Term Climate Stations Below 44 Degrees Latitude - Individual Station Percentage Changes Factored by Length of Climate Station Record).
Percentage IDF change values for 'unweighted' station changes (i.e., short records are given the same weight as long records) are summarized in the following table - same overall pattern as the record-length-weighted table above.

Ontario IDF Update Trends in Rainfall Intensity and Frequency Unweighted
Southern Ontario Rainfall IDF Trends From 1990 to Current Version 2.3 Engineering Climate Datasets (Average Values for 21 Long-Term Climate Stations Below 44 Degrees Latitude - Individual Station Percentage Changes Factored by Length of Climate Station Record).
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So what are we to make of this? The media, the insurance industry, and those who are exercising their 'availability' bias instead of looking at storm statistics, have regularly reported that storms are bigger, or more frequent, or both, but the local Ontario data shows the opposite (Northern Ontario will be a different story as AMS trends were up in the north, unlike the south). The Ontario government is website is even out of step wit hthe data.

The new Progressive Conservative government in Ontario has just renamed the Ministry of Environment and Climate Change the Ministry of the Environment, Conservation and Parks, taking out 'climate change', but the content under it has not been updated.

Ontario Ministry of the Environment, Conservation and Parks replaces former Ministry of the Environment and Climate Change. New name but content still reflects climate change effects on storms that is inconsistent with data.
If we look at rainfall trends in Southern Ontario it would seem appropriate to now de-emphasize the change in 'climate' or, regarding storms, the change in weather statistics. The current "MOECP" website reflects the earlier MOECC, and indicates that climate change has caused extreme weather issues in the province.

Ministry of the Environment and Climate Change website links extreme weather with climate change.
Specifically, the website indicated (as of July 2, 2018):
"It damages your property and raises insurance premiums:
  • the severe ice storm in December 2013 resulted in $200 million of property damage in OntarioToronto lost an estimated 20% of its tree canopy during the storm
  • Intact Financial, one of Canada's largest property insurers, is raising premiums by as much as 15-20% to deal with the added costs of weather-related property damage
  • Thunder Bay declared a state of emergency in May 2012 after being hit by a series of thunderstorms, flooding basements of homes and businesses due to overwhelmed sewer and storm water system"
While we cannot comment on ice storms, the official datasets for rain storms show no change, and therefore raised insurance premiums must be due to other factors instead of climate change. Blog readers will point to our review of  urbanization, intensification, etc. as a key cause.

KPMG has also commented in "Water Damage Risk and Canadian Property Insurance Pricing" (2014) for the Canadian Institute of Actuaries that prior to 2013, flood insurance pricing was inadequate, so the 15-20% increase by Intact Financial is just catching up to the market pricing for that service. It also reflects the higher value of contents and finishing of basements that are flooded / damaged during extreme weather.

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