From the Cliff Mass Weather Blog
During the last few weeks, large fires have initiated and grown over northern Alberta, resulting in massive smoke plumes (see the image below from one week ago). Here in Washington State, we experienced a few days of smoke aloft from these fires last week.
I have received a number of emails asking whether such Alberta fires are unusual for this time of the year and whether global warming (climate change) could be the cause.
In addition, several media outlets have published headline articles about the topic, suggesting that human emissions of greenhouse gases were the main cause.
It turns out that reality is more complicated.
May is typically the biggest month for Alberta wildfires, there is little upward trend in Canadian wildfires, and anthropogenic greenhouse gas emissions are a very small part of the story.
Canadian Wildfire Statistics
Based on an official Canadian government wildfire database, here is a map of Canadian wildfires from 1980-2020. Alberta wildfires are generally found over the northern half of the province, which includes large areas of boreal forest and grassland.
What is the long-term trend of Canadian wildfires? Increasing due to global warming?
Perhaps surprising to some, the answer is no (see below).
For all of Canada, the number of fires is DECREASING, and there is no obvious trend in the area burned.
What about Alberta, where the current batch of wildfires are burning?
Lots of variability but little upward trend (see below). The biggest fire was in the early 1980s.
The lack of a long-term trend in wildfires is important: one WOULD expect an upward trend if global warming/climate change was a significant contributor.
Some folks have argued that having big Alberta fires in May points to global warming. They suggest that it is warming so fast that wildfires are occurring early!
But folks making such claims need to look at the data. Historically, May is the month of the most frequent wildfires in Alberta. (a graphic from a paper on trends in Alberta wildfire below).
Furthermore, many of Alberta’s great wildfires occurred in May, such as the huge Fort McMurray fire in early May 2016 and 2011 Great Slave Lake fire of 2011.
But why is May such a big month for northern Alberta wildfires?
It has to do with surface fuels. After a long, cool/wet winter, there are a lot of dead fuels (e.g., dried grass, annuals) from the previous year that are on the ground after the snow has melted. Such light fuels dry very quickly during the first warm weather and are ready to burn in early May.
But the optimal burn season is limited in time. Only a few weeks later, there is a greening of the surface vegetation (grasses start to grow, annuals sprout leave) and such greening REDUCES the flammability of the surface fuels. Which side would you expect to burn in the picture below?
There is a short favorable May window for large Alberta fires before the greening. But to take “advantage” of it you need favorable drying conditions, which are associated with a strong, upper-level ridge of high pressure over the region. And strong winds and lightning are favorable as well.
Furthermore, rainfall peaks during the summer (see monthly precipitation in Edmonton below).
The setup for the fires earlier this month was nearly perfect. In early May, an intense ridge of high pressure developed over southern Canada (see a plot of the difference from normal heights at 500 hPa pressure below for 1-15 May). Red and orange indicate MUCH higher than normal pressure in the lower atmosphere (around 18,000 ft).
Such high pressure resulted in intense drying and warming that helped dry the surface vegetation before greening occurred. There is no evidence that such a pattern is the result of climate change.
There were periods of strong, dry winds in early May due to the large pressure gradient at the edge of the high pressure at lower elevations (see example below on May 6). Strong wind is a primo accelerator of fire.
You can understand the situation by looking at the observed weather at Edmonton Airport from late April to early May (below). Rainy and around 60F in late April, then rapid warming and wind on April 30th with a jump to 85F on May 1, followed by 86 and 88 on May 3-4.
Perfect weather to dry out the surface fuels, followed by lightning at the high pressure shifted eastward. Human ignitions are also distinct possibilities.
In summary, there is little evidence that the Alberta wildfires represent a climate event.
Intense drying weather occurred exactly when the surface fuels were most vulnerable before greening. May is typically the month of the biggest Alberta fires for a reason and the smoke that reached the Northwest aloft from the fires was simply the result of a favorable wind pattern aloft (easterly flow), not the results of a slowly warming planet.
Note: I will do a special online Zoom session this Saturday (May 27th for my Patreon supporters). Will talk about wildfire meteorology and answer your questions.
via Watts Up With That?
May 23, 2023 at 08:55PM