John Robson of Climate Discussion Nexus Share Causes of Urban Flooding - Highlights CBC Ombudsman Findings on 100-Year Storms

This blog has examined causes of urban flooding and non-causes of flooding too. A new video by the Climate Discussion Nexus examines causes of urban flooding and references the recent CBC Ombudsman review of 100-year storm trends (i.e., no changes in extreme rainfall).

The video notes that we have always had flooding which is correct - that is something we have noted as well, like in this presentation to the WEAO / OWWA joint climate change committee:

The presentation noted how GO Train flooding, Toronto Island flooding and Toronto basement flooding area note new phenomena as shown in these images:

A review of the inquiry for Premier Davis on Toronto Don River flooding noted flooding since the 1800's as noted in this post: - an Environmental Assessment in 1983 noted this:

This Acres Consulting report noted the influence of hydrologic changes on peak flows and flood damages. We have described these changes as well such as in this JWMM paper:

GTA watershed urbanization changes were summarized as follows:

Here is a wider perspective:
And here are urbanization trends in other southern Ontario municipalities:

We have assessed the trends in extreme rainfall which supported the CBC Ombudsman decision - here are the english-version findings:

Even Minister McKenna has reinforced comments made in Canada's Changing Climate Report stating in a June 2019 letter "the observational record has not yet shown evidence of consistent changes in short-duration precipitation extremes across the country":

Other CBC story corrections about extreme rainfall are summarized here:

As the Climate Discussion Nexus video notes, the insurance industry has claimed that there is an increase in extreme rainfall caused by climate change - this has been reiterated by senior executives as in this op-ed in the Globe and Mail by Charles Brindamour and Dean Connor "Climate resilience must be part of every government’s agenda" (The Globe and Mail. September 25, 2018., or as in this op-ed in the Financial Post by Craig Stewart "Counterpoint: Insurance claim costs are rising because severe weather is making flooding worse" (Financial Post. February 7, 2019.

Unfortunately, the insurance industry has not ever offered any data on increasing extreme weather trends to counter Minister McKenna's recent statement or the CBC Ombudsman findings since 2015. The insurance industry has in the past mixed up future predicted extreme rain trends with past observations as in the "Telling the Weather Story" report:

As noted in our Financial Post op-ed, the insurance industry has claimed a correlation and causation between extreme weather and flood damages. Unfortunately, there is no rain trend to correlate to making any causation discussion moot. As Dr. Dickinson explains in the video, warmer winters mean lower spring flood potential, and urbanization drives urban flood stresses, not changes in rainfall. More on this University of Guelph analysis is here:

Our rebuttal to the insurance industry's suggested correlation / causation was in the Financial Post:

Well done John Robson and Climate Discussion Nexus for sharing information on this topic.


More reading? - what do engineering studies in southern Ontario say about extreme rainfall trends? typically no past change - see compiled reports / analysis here:

The Ombudsman has also recently noted in a new review that "It would have been wrong to state categorically that Canada has already seen an increase in extreme rainfall events" and that its reporting should be unambiguous about the past, present and future, with "distinctions made between observed phenomena and predicted phenomena" (  The Ombudsman notes that CBC reporting describes future changes in extreme weather saying:
  • Canada is warming at twice the global rate, and our north is warming at three times that rate.
  • We can expect more extreme heat, warmer winters, earlier springs and rising sea levels.
  • Precipitation will increase in much of the country.
  • Weather extremes will intensify.

  • The last two bullet points are careful to use the future tense.