Greater Toronto, Hamilton, Ottawa and Niagara region expected to have hottest Halloween on record

If you're trick-or-treating in or around Hamilton tonight, you probably won't need a winter coat under your costume.

Today is forecast to be the hottest Halloween on record in Ontario municipalities including Hamilton, Toronto, Burlington, St. Catharines, Lincoln, Niagara Falls, Toronto, Oakville, Mississauga and Brampton. Ottawa and a number of places in Quebec are also forecast to set heat records. 

According to CBC's Climate Data Dashboard, which sources data from the Meteorological Service of Canada, Oct. 31 will see 122 heat records set. 

In Hamilton, the forecast high is 24 C, which — according to the dashboard  — is 13 C above the historical average.  That average is calculated using data from 1981 to 2010.

It's "not actually out of the ordinary" to experience temperatures this hot in October, Peter Kimbell, a warning preparedness meteorologist with Environment and Climate Change Canada told CBC Hamilton. "We've seen it many times before."

For example, Kimbell said, the previous record for Oct. 31 in Hamilton is 22.8 C, which occurred in 1971 and 1876. He said the region experienced similarly warm days around this time in 1901, 1933, 1946 and 1950 too.

What's happening right now, Kimbell said, is that "a big gradient of pressure" from the eastern seaboard to the central part of the United States is causing winds to blow warm air from the southern U.S. and into southern Ontario.

"We have very warm air across southern Ontario and actually had warm air across northeastern Ontario yesterday," he said.

Climate modelling shows temperatures rising

In Hamilton, the average temperature for the last 30 days is 11.6 C. That's 2 C above the historic average.

While Hamilton's hottest Halloween is not in itself a "smoking gun," Ian Borsuk, of Environmental Hamilton, says it's indicative of a wider trend toward a warmer climate.

Last year was the hottest on record, according to the European Union's climate agency, and in recent weeks, we've seen episodes of severe weather such as deadly flash floods in Spain, which scientists say are increasing in frequency due to climate change.   

"These issues are only going to get worse," Borsuk said. "We've locked in this warming."

Borsuk told CBC Hamilton he hopes this record is a "wake-up call" that we need more action to reduce greenhouse gas emissions. 

"It's entirely possible that this is not going to be the hottest Halloween ever," he said, adding that while some people grew up having to alter their Halloween costumes for warmth, future kids may have to get used to dressing down due to heat. 

CBC's Climate Dashboard notes that Hamilton is expected to be hotter on average by the end of the century. How much depends on the scale of greenhouse gas emissions. In a low-emission scenario, the temperature will likely increase between 1.5 C and 3.8 C. In a high-emission scenario, the average annual temperature could increase as much as 8.1 C.

"The hottest Halloween on record might seem like a quaint impact of climate change in 10 to 20 years because the effect in the future will be much, much worse," Borsuk said. 

A good opportunity to speak to neighbours about climate change

If the topic of heat comes up while trick-or-treating, Borsuk recommends talking about it.

"The more we talk about this and the more we make individual commitments in our own lives, the easier it is for us to imagine and conceive of the collective action that we desperately need and require governments to follow through on."

You may also want to pack an umbrella.

Kimbell said the forecast calls for rain tonight.