After eight days of heat wave, the street scene was very different in many places in the Netherlands on Wednesday: no shorts but raincoats, no sunglasses but umbrellas. But despite that rain, the temperature will remain a few degrees above the long-term average, KNMI expects.
The question is whether it will be as warm again as last Sunday. Then it was the warmest August 14 since the measurements began in 1901. The average temperature in De Bilt was 24 degrees, calculated over 24 hours. Exactly 0.7 degrees warmer than in 2020, when the previous record was set.
It is one of five heat records broken this year. While without global warming, only three heat records would be broken over the whole of 2022, according to an analysis by NRC.
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It is no coincidence that more heat records will be broken in 2022 than expected. Since 1990, there have been more than 2.5 times as many ‘warmest days ever’ in De Bilt as would be expected without climate change. It was warmer than ever in three hundred days, while under normal circumstances ‘only’ would have broken a day record about 113 times.
In contrast to the many heat records, hardly any cold records have been broken. Since 1990, without climate change, it is also expected that some 113 cold-day records would have been broken, in reality 25 days were the coldest on record. It is the result of climate change, says Peter Siegmund, climate expert at KNMI.
NRC collected all temperature measurements from the weather station in De Bilt since 1901. These are average daytime temperatures over 24 hours. For the analysis, it was checked how often the average daytime temperature was higher or lower than measured up to then. For example, April 8, 1969 was the warmest with 14.2 degrees, the warmest April 8 measured so far – a heat record has been recorded. In 2018, it was 15.2 degrees on April 8, the warmest so far – that too has been registered as a record. In addition, it was calculated how many daily records per year could be expected if there had been no climate change. The number of records should then decrease the longer the measurements are taken. After all, in the first year of measurements, a daily record is set every day, because every measurement is the first. In the second year, in theory, half of the days should be warmer and half of the days colder. In the third year it is about a third of the days, and so on. This method was previously used by KNMI to analyze European temperature records.
Until the late 1980s, the number of broken heat records was roughly in line with the trend – without global warming. Fewer records every year. Since 1990, the number of weather records has been almost structurally above the number that could be expected. Every year, about nine daily records are broken, that number remains stable and is no longer decreasing.
This is remarkable because the number of records would have to decrease the longer the measurements are taken, if the climate had not changed. That looks like this:
Warmer ‘normal’ days
When you think of heat records, you quickly think of heat waves. In this case, it is much more often about days in the autumn or winter that were not so warm before. The differences per season are quite large. In principle, about 29 heat records should be broken every season since 1990. In reality, there were considerably more in autumn (78) and winter (87). Fewer heat records were broken in the summer (67) and spring (68), although these were still more than double what could be expected without warming. Cold records have not been broken in most years since the 1990s.
“This type of research shows that climate change doesn’t just affect more heat waves, or less ice skating,” says Siegmund. “The normal days are also getting warmer. Normally you don’t notice that so quickly.” Many heat records have been broken recently. Those are not always days that we remember as extremely warm: January 1, 2022 was the warmest on record at 12.3 degrees.
The cold records are much older: the cold record of March 26, 1901 was never broken again, when the average temperature over 24 hours was minus 1.4 degrees. You have to compare this with the ‘oldest’ heat record, otherwise it is a crazy comparison; maybe there is also a warm record that has been standing since 1901
Siegmund is not surprised that more heat records have been broken since 1990. “1988 is always mentioned as the year in which the temperature in the Netherlands jumped,” he says. The temperature had already risen gradually before that, in 1988 there was a clear break in the trend. “We don’t know exactly why. It may be because the wind comes from the west more often, bringing warmer air in.”
The weather records fit in with a trend in which the weather in the Netherlands is getting warmer. The average annual temperature in De Bilt has risen by 1.8 degrees since the early 1970s. Then the average temperature was 9.3 degrees Celsius, now it is 11.1 degrees.
A version of this article also appeared in the newspaper of August 18, 2022
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