Climate projections for Toronto according to three CMIP5 scenarios

2019-08-28 In June 2019, Environment and Climate Change Canada (ECCC) launched a new Canadian climate data portal.

Through this portal, decision makers in the private sector, municipalities, provincial and federal departments are now better equipped to make informed decisions about future development options across Canada, taking into account projections of future climate change.

In this post, my goal is simply to illustrate the types of climate data that are available for thousands of municipalities by taking the example of Canada’s largest city: Toronto. A second example is provided for the city that I live in: Rimouski (in French). I encourage decision makers to explore what information ClimateData.ca has in store for the communities they live in.

For the City of Toronto (43.7417°N, 79.3733°W), I present plots of historical (1950-2005) and plausible futures (2006-2100) in response to three greenhouse gas (GHG) emissions scenarios that are referred to as Representative Concentration Pathways (RCP) in the scientific literature. These RCPs were included in the fifth phase of the Coupled Model Intercomparison Project (CMIP5).

• RCP 2.6 has a net GHG radiative forcing of 2.6 Watts/m2 in 2100 that would allow to limit global mean warming below 2°C. This scenario can be achieved provided that 1) quick and very substantial actions to reduce GHG emissions are taken worldwide; 2) CO2 capture technologies become sufficiently efficient and affordable to remove large amounts of CO2 from the atmosphere.
• RCP 4.5 has a net GHG radiative forcing of 4.5 Watts/m2 in 2100 and would cause a global mean warming of about 2.5°C to 3°C. In comparison with RCP2.6, this involves less rapid and less substantial actions to reduce GHG emissions. International commitments to reduce GHG emissions made in the Paris climate agreement in 2015 would imply an average global climate warming of around 3 to 3.5ºC. More substantial reductions in GHG emissions therefore need to be planned and implemented to first put us on the path of RCP4.5, and then continue our efforts to do better, en route towards RCP2.6 or even lower.
• RCP 8.5 has a net GHG radiative forcing of 8.5 Watts/m2 in 2100 and would cause a global mean warming of about 5°C in 2100. This corresponds to a high-end CO2 emissions scenario, often referred to as the worst-case scenario. Fortunately, efforts to mitigate GHG emissions are starting to bear fruit and it seems less and less likely that human activities can emit as large amounts of GHGs as RCP8.5 in the coming decades (Hausfather & Peters 2020).

All results displayed below come from an ensemble of twenty-four climate models. 

Summer temperature indices

How hot will it be in Toronto during the hottest day of the year? If we want the answer to be something less than 40°C, we will need to steer away from the worst-case scenario RCP8.5 pathway. 

OK, looking at the hottest day of the year is a bit extreme, isn’t it? How many days will have maximum temperatures above a given threshold? Let’s pick a threshold of 30°C for example. We’ve historically had 20 to 30 days per year with maximum temperatures above 30°C. But if we do not address global warming seriously, this number might go up to 80 days by 2100 if we take the high-end CO2 emissions RCP 8.5 pathway.

The increased number of warm summer days will require more frequent use of air conditioning in businesses, homes, and apartments. This will imply more expensive power bills in the summer months in comparison with the historical period. 

For people without access to air conditioning, the greatest amount of discomfort and health risk associated with summer heat waves has to do with warm tropical nights during which temperature never drops below a certain threshold (say 20°C). To avoid sleepless nights, more people will need air conditioning in the next decades. 

The increased warmth may present opportunities to grow new types of crops in the greater Toronto metropolitan area. But adequate precipitation will be needed if these new crops are to succeed. 

Winter temperature indices

Winter conditions in Toronto are projected to get milder. Whereas the coldest day used to be around -23°C, under the RCP 8.5 (RCP 4.5) pathway, the coldest day of the year would be around -10°C (-15°C) by the end of the century. 

Moreover, the number of days with minimum temperature below the -15°C threshold, which was historically about 15 days, will dwindle to close to zero.

Consequently, the total number of frost days will also be decreasing. 

And winter power bills are expected to get cheaper due to a decrease in the number of heating degree days. However, the decreased cost of heating in the winter will be at least partially offset by the increased cost of air conditioning in the summer. 

Precipitation indices

Under global warming, the water cycle should intensify, leading to increased total precipitation.

Some of this extra rainfall will be delivered in the form of extreme rainfall events. 

Toronto’s future climate depends on global GHG emissions

Climate models are mathematical representations of the physical climate system. These models have intrinsic uncertainties built into them. But it turns out those model uncertainties are smaller than uncertainties about what will be humanity’s future greenhouse gas emissions (RCP 2.6, RCP 4.5, or RCP 8.5). Individual persons, businesses, municipalities, and Canadian provinces can help move the needle in the right direction (RCP 2.6), towards much smaller CO2 emissions. 

Other resources

• Canada’s changing climate report (CCCR, 2019)
•  The Climate Atlas of Canada contains 17 city reports, including one for Toronto.
•  Search for the city/town you live in by clicking on “Explore by location” at ClimateData.ca
• The representative concentration pathways: an overview. This scientific paper describes in detail the RCP 2.6, RCP 4.5, and RCP 8.5 GHG emissions pathways.