These days, it seems like hardly a summer week goes by without news of extremely hot weather happening somewhere or other. As I write this, social media is alight with reports on a staggering heatwave affecting western Canada and the northwest corner of the USA.
Lytton, in British Columbia, set high temperature records three days in a row 27th-29th, with 46.6°C, then 47.9°C, followed by a gargantuan 49.6°C. That last figure breaks the old national record for any time of year by a whopping 4.6°C! It’s an eye-watering 25.3°C above the average June daytime maximum.
This is embedded within a far-reaching hot spell lasting a week or so – more than a flash in the pan.
Heatwaves are also roasting both the northern and southern reaches of Africa. South Africa hit 36.4°C on 29th June… in the midst of their winter season. It’s also been unusually hot for June in the Middle East, even by their standards, with temperatures close to the 50°C at times.
I expect many readers are of the impression that hot days are becoming more common here, there, and everywhere as part of climate change. But the truth is, it depends on where you are.
To demonstrate this, here’s a look at the frequency of hot days in weather observations spanning 1973-2020, in four key regions.
The USA: All Over the Place
To keep things relatively simple, let’s focus on a single threshold temperature for each region. For the USA, I’m going to use 90°F (32.2°C); any day reaching this or higher is considered a hot one.
Let’s begin with a by-decade look at the whole region divided across the summer months:
A word that jumps out to me here is ‘flat’. There’s little trend over time, with similar numbers across all decades. In fact, if you were to fit trendlines, only August would show an increasing trend over time.
So, what’s going on? To understand that, we need to consider the spatial distribution of trend:
Now we can see that the lack of overall direction is a result of conflicting regional trends. There’s a right mix of reductions and increases but with a slight focus of reductions in the Midwest (north-central USA).
This pattern may have ties to the Arctic. Here, huge losses of summertime sea ice coverage have changed the typical temperature and moisture profile of the atmosphere above. In turn, this appears to have caused the typical path of low pressure systems to shift southward, so that they affect north-eastern North America more often. These bring cloudy, rainy weather – not conducive to hot temperatures.
In parts of the West and Southeast, each new decade has seen at least 4 more summer days reaching 90°F. In the West, this has been accompanied by an increase in the duration of droughts. Right now, the region is suffering one of the worst on record.
Europe: Universal Heating
Over the other side of the pond, the situation could hardly be more straightforward.
Using 30°C as a benchmark, the by-decade breakdown shows an increasing trend for all months:
Meanwhile, the spatial map is dominated by increasing trends:
Broadly, those trends are largest where days reaching 30°C or more are most frequent already. That’s mainly in places out of reach of cooling North Atlantic winds, in the south and east of the continent. Here, with each passing decade, the typical number of hot summer days has been increasing by 5 or more.
Some places are now typically seeing several weeks’ worth of additional hot days compared to the 1970s!
The pattern fits a general warming trend across the continent. In all but a few places, the typical spread of temperatures is edging upward, meaning fewer very cool days and more hot ones. This hasn’t yet gone far enough to have much effect on 30°C+ days in northern parts yet, but with further climate warming in the coming decades, that’s likely to change.
Northeast Asia: Cooling Pockets
Here, we see patterns that sit between those of the USA and Europe. The by-decade breakdown shows slight but still appreciable increasing trends, but only since the 1980s. It’s interesting that the 1970s saw considerably more 30°C+ days than the two following decades.
It may represent a ‘natural cycle’ overlaid onto the multidecadal warming trend, driven by very slow changes in the oceans. Or it could be related to changes in atmospheric aerosol levels, which peaked in the 1980s to early 1990s. Whether it’s either, both, or something else entirely, is a subject of ongoing debate!
Mapping out the 1973-2020 trend, we see a mixed picture, but with more places increasing than reducing. Again, it sits between what we’ve seen for the USA and Europe.
The increasing trends are focused on a belt aligned from the northwest to the southeast of the region. Intriguingly, this and the two main reducing areas show no clear correspondence to the layout of current population density or pollution levels. However, if we look at population change in the past decade, we can see that the reducing areas are the ones that have seen the greatest population increase during that time.
This is contradictory to all the studies I can find on population change and temperature. There’s a strong consensus that higher population leads to higher air temperatures, via increased building density among other factors. The ‘urban heat island’ effect.
Clearly, the surprising observations here merit further study – a subject for a future blog!
Australia: Hot Under the Collar
Down under, the situation is not so easy to monitor. Reason being the population is heavily concentrated within a few busy stretches along the edges of the country.
The outback is so sparsely populated that there has been little drive to place weather stations out there. At least, not back in the 20th century, for which we need data to perform good quality trend analysis.
What we do have tells us that the number of hot days is shows no clear trend in Dec but a clear positive trend in Jan-Feb.
Studying the mapped trends, the only exceptions are found to be located right on the eastern coastline. Yet just inland of there, the largest positive trends to 30°C+ days are found.
To explain this, we must take to the water. Adjacent to that coastline from about Taree northward, is the only area of ocean next to Australia to have been slightly cooler in the past few decades when compared to the 1970s. This has in turn reduced moisture evaporation into the atmosphere, leading to clearer skies. The additional sunshine has increased the number of hot days inland, but not been able to outweigh the direct cooling effect of the ocean right on the coastline.
In this blog, we’ve seen a few such examples of inconsistent climate trends in hot days. It goes to show how dangerous it can be to assume that everywhere is getting hotter, all the time – despite the rising global average.
Case in point: late June into July has been record-cold in parts of Brazil, with some areas seeing the first snowfall in decades!
James Peacock MSc
Head Meteorologist at MetSwift
Cover Photo by Unknown Author is licensed under CC BY-ND