"There is limited to medium evidence available to assess climate-driven observed changes in the magnitude and frequency of floods at regional scales because the available instrumental records of floods at gauge stations are limited in space and time, and because of confounding effects of changes in land use and engineering. Furthermore, there is low agreement in this evidence, and thus overall low confidence at the global scale regarding even the sign of these changes."
IPCC notes low confidence in the sign of changes at a global scale, meaning flood magnitudes could be going up or down.
Other factors driving losses? Research shows for some severe weather event types like hurricanes the driver is GDP growth, e.g., "research is robust in concluding that, for many decades into the future, the primary driver behind increasing economic losses related to hurricanes is expected to be societal growth"
"Since its construction between 1914 and 1922, the Keating Channel has been subject to heavy sediment loads, requiring regular dredging to maintain sufficient depths to allow for and maintain shipping activities at the mouth of the Don River. Between 1950 and 1970, widespread development throughout the Don Watershed and the construction of the Don Valley Parkway increased sedimentation rates by up to four times that of the pre-was era. After 1970, decreases in the number of new watershed disturbances and improved sediment control structures likely contributed to the decline in sedimentation in the Keating Channel to levels similar to the pre-war era. A reduction in shipping activities within the Keating Channel, combined with restrictions on the open water disposal of dredgate imposed by the International Joint Commission (IJC) in 1974, resulted in a cessation of dredging in the Keating Channel. In the following five to six years, the Keating channel filled with sediment and debris to the point where it became visible under all but high lake levels, resulting in increased flood risk along the lower Don."
So flood risks increase due to fluviogeomorphology (the transport and deposition of sediments in a watercourse) and hydraulics - when dredging stops, sediment builds up, hydraulic capacity is reduced and flooding is increased along the river.
Yet despite flooding dating back to the 1800's, as reported in the Inquiry for Premier Davis, and despite impacts on rail lines in the Don River floodplain over decades, flooding has been attributed to climate change effects. Even by the Environmental Commissioner of Ontario. The fact is there is no new normal with "wild weather", but the same old issues and extremes:
This certainly contradicts claims made by an insurance industry-funded research group that have indicated there is 'a lot of data to show it' when it comes to bigger storms. A February, 8, 2018 presentation to the Standing Senate Committee on Energy, the Environment and Natural Resources included this statement:
"So when you see in the news and the media people talk about storms seem bigger and more intense and so forth, those perceptions are correct. And there's a lot of data to show it."
But a review in a recent presentation to the National Research Council's 2018 workshop on flooding that showed there is no data to support the statement. Concerns with insurance industry statements on frequency shifts were also expressed by Environment and Climate Change Canada staff in relation to the Telling the Weather Story 40 year to 6 year weather shift. Staff had concerns with statements that could confuse theory and actual changes. Here is an excerpt from communications regarding the Telling the Weather Story normal bell curve theory shift:
"The presentation looks to be a simple conceptual model for communicating the underlying idea – if one assumes a standard normal, then a shift in the mean implies an attendant change in extremes – which is fine as far as it goes. If this is used as the basis for statements about actual changes in extreme rainfall in Canada, then I would have concerns."
Here was the specific question posed:
Here is a graphic showing the theoretical shift in question, an arbitrary 1 standard deviation shift in a standard normal 'bell curve' (probability density function):
The Environment and Climate Change Canada report also speaks to theoretical shifts in probability density functions, like the Weather Story bell curve shift. This is the example showing a shift right in the distribution of extreme events Figure 4.2.1:
The 'green' shift to the left reflects an overall decrease of 0.4% in rainfall design intensities at 21 long term climate stations since 1990, considering durations related to urban flooding, i.e., 5 minutes to 24 hours. That analysis of the new Version 3.0 Engineering Climate Datasets was presented in this post.
There is often a statement that changes in means will lead to changes in extremes in a distribution of probabilities - this makes sense. This concept is reflected in IPCC reports as well:
But data shows that the means, the 2 Year storm rain intensities, the events that we have the most observations of and the most confidence in assessing trends are decreasing the most. The Version 3.0 datsets review for southern Ontario shows on average a drop of -0.8% in those rain intensities, as shown on this table in the first column:
In this region, the extremes can be expected to decrease along with the means - on average that is happening too for the 100 Year rain intensities.
The Environment and Climate Change Canada report notes 'medium confidence' in increases in annual precipitation across the country and "low confidence in quantifying regional or national total amounts of precipitation" - so medium confidence in it going up but low confidence in saying how much, especially at more local spatial scales, or regions.
Since little or no infrastructure is designed to address annual precipitation, the reports limitations on the annual precipitation statistic are irrelevant to cities facing challenges like urban flooding during extreme, short duration events. Based on CatIQ datasets, a higher number of flood claims and a higher value of claim is associated with rare storm volumes falling over duration of minutes and hours and not annual totals.
The key take-away is that extreme rainfall has not been observed to change, whether for higher frequency events like 2 Year storms, or for low frequency, rare events, like 100 Year storms.
It is easy for the media to confuse annual precipitation with rain extremes, and in the case of Canada’s Changing Climate Report, CBC News reported that urban flooding related to intense rain will increase too - CBC has since corrected that article noting the report did not find increased short-duration rainfall linked to basement flooding:
The Environment and Climate Change Canada report cites research that points to land use change having a "key role" in affecting flooding, for example for the southeast Prairies flood in 2014. Here is the excerpt on attribution of flooding to rainfall or other factors, saying "Anthropogenic influence may have influenced rainfall, but landscape modification played a key role in increased runoff":
This is consistent with reporting by the American Society of Civil Engineers who in their Adapting Infrastructure and Civil Engineering Practice to a Changing Climate document state: "It is important to point out that land-use changes (e.g., urbanization) can result in substantial flooding impacts, independent of climatic forcing functions." - see page 12.
Regarding attribution, it is also consistent with a recent report on extreme rainfall event attribution that also identifies a lack of association of extreme convective storms, those responsible for much urban flooding, with anthropogenic climate change effects. For example the National Academies of Sciences, Engineering, and Medicine. 2016 report Attribution of Extreme Weather Events in the Context of Climate Change states (see page 97):
"Studies of trends in the United States find different results depending on the time period and spatial region chosen, but there is no broad agreement on the detection of long-term trends in overall severe
convective storm activity such as might be related to anthropogenic climate change."
Regarding land use influence on runoff and flood risk, this is also consistent with analysis by the University of Guelph's Engineering Department on changes in urban 'runoff coefficients' (the fraction of rain that runs off and can contribute to flood stresses) due to urbanization like in the Don River watershed:
That analysis was intended to 'disentagle' the impacts of climate change and land use change. Green bars are pre-urbanization coefficients showing we had a small fraction of rain becoming runoff, while blue bars show significant increase in runoff potential after 50% urbanaization. Note there is uncertainty in flow monitoring too, just like in precipitation monitoring, but we see a 10 times, 1000% increase in runoff potential in summer months, when we have the highest rain intensities, due to urbanization. The urbanization effects are MASSIVE - the Scarborough Golf court case reiterated this fact over and over referring to "markedly increased flows".
Compared to urbanization effects on flows, meteorologic effects are a big "nothing burger", with no observed changes and just a lot of theory and speculation. We should design for uncertainty in the future, and incorporate cost-effective adaptation considerations or flexibility for future adaptation (ASCE's Observational Method for climate adaptation) however we should not mischaractierize past trends and risk factors driving today's infrastructure performance limitations.
The University of Guelph analysis also indicates that spring peak flow rates will decrease with climate change effects that reduce winter snowpacks and spring melt flood potential. The follow chart shows the decrease in spring peaks in the rural Moira River watershed:
The Environment and Climate Change Canada report recognizes the impacts of temperature on snow patterns in Chapter 4: "As temperatures increase, there will continue to be a shift from snow to rain in the spring and fall seasons.". The report also cites research that "The reduction in spring snow pack and the ensuing reduction in summer streamflow in British Columbia have been attributed to anthropogenic climate change". Other cited research notes "Such a change in the form of precipitation, from snow to rain, has profound impacts in other components of the physical environment, such as river flow, with the spring freshet becoming significantly earlier." - the University of Guelph research shows that the winter period flows increase from November to early March in the Moira River example, and the peaks decrease significantly from late March and April. This decrease in peaks will result in a decrease in spring flood risks in watershed affected by such events.
So there is no new wild weather, or new normal driving flood damages. Case law in Ontario defining the effects of hydrology, or urbanization, findings of inquiries into Don River flooding for Premier Davis, Municipal Class Environmental Assessment studies investigating basement flooding causes and solutions, and Environment and Climate Change Canada's Engineering Climate Datasets that examine trends in observed rainfall intensities show us that hydrology, hydraulics, fluviogeomorphology explain today's flood risks, and there is has been no shift in rainfall intensities, despite median and insurance industry 'weather stories' and claims.