Heatwave in May 2018: “It was unusually warm; exceptional, even”

How unusual is it that the May weather is as hot as it was this year in Sweden? Will this become commonplace in a changed climate? At SMHI, researchers and climatologists have analysed weather statistics and concluded that it was uncommonly warm and that it represented a record high which could remain unchallenged for a long time to come.

We can now look back at a heatwave which made the month of May 2018 unusually hot, all across the country. SMHI's monitoring stations have recorded many new record high temperatures for May, some more than 2 degrees higher than the previous highest average monthly temperature.

Sunny weather in Stockholm
In May 2018, the average monthly temperature in Stockholm was 16.1; considerably higher than the 10.5 degrees which is the normal average monthly temperature in Stockholm in May, according to the meteorological normal period 1961-1990 which is used in comparisons.

Exceptionally hot compared with the norm

“That we should experience such a high average temperature in Stockholm in May – 16.1 degrees – is an occurrence which, statistically speaking, happens just three times in a million years. So it was unusually warm; exceptionally, even, compared with what we tend to experience in May,” says Gustav Strandberg, climate researcher at SMHI's Rossby Centre. He has made a statistical analysis of one of the longest series of temperature measurements in Sweden – the “Stockholm temperature series” – with temperatures recorded from 1756.

The next highest average monthly temperate for Stockholm is from 1993, when the May average was 13.9 degrees, 2.2 degrees below this year’s average. This is also rare, statistically speaking only occurring once in over six hundred years. The average monthly temperature in Stockholm for May is 10.5 degrees, based on the meteorological normal period 1961-1990, which is used in today’s weather comparisons.

“The fact that the record has been beaten by such a wide margin is remarkable, and this new May record may stand for a long time to come.”

What impact will a new climate have?

At the same time, we have ongoing climate change and global warming. How does this affect the weather we will see in the future?

“It’s important to remember that the weather will continue to vary, even if we are experiencing ongoing climate change, so we will have periods which are both colder and hotter than usual in the future as well,” Gustav Strandberg explains.

“If we look at the weather statistics we have today, the likelihood of having such an exceptionally hot May again is very low, despite global warming,” says Gustav Strandberg.

The likelihood of extreme temperatures does however increase in a warmer climate. Heatwaves will be more common in the future.

“We also shouldn't forget that the climate is already hotter. The past 30 years have been clearly warmer than the average for the period as a whole, but it’s impossible to reliably calculate probabilities for extremes based on a period of just 30 years,” Gustav Strandberg tells us.

Climate change continues

The rapid climate change that we are seeing is largely caused by human emissions of carbon dioxide into the atmosphere. The countries of the world can work together to reduce climate change by limiting emissions. The more emissions are limited, the less global warming the climate change will entail, meaning the effects can also be limited. At the same time, societies the world over will need to adapt to both the current and the future climate through what is known as climate change adaptation.