Anyone who starts a conversation about the weather in these warm November days, once the refuge for those in need of a carefree chat, quickly comes across the worrying speed of climate change. Is that actually correct?
First of all: November has barely started yet. The image that the late autumn month conjures up for most people, of drizzly weather, rain, hail and perhaps even wet snow, is often only visible later in the month. the current temperatures are also exceptionally high for these days, according to figures from the KNMI NRC collected.
During the first six days of November, the average maximum temperature in De Bilt was 14.8 degrees. That was only higher in 1994, 2005, 2011 and 2015. The average of the first six days of November, taking into account all years since measurements began 125 years ago, is 11 degrees.
also sees Peter Siegmund, climate expert at KNMI. “And that heat is not over yet for the time being. According to the forecast, next Friday it will be even warmer than today, although there is of course a considerable margin of uncertainty. That is extra striking because we are still further into November, normally it is colder than today.”
‘Normal’ moves up
But the current temperature is also exceptional. “Today it is already three degrees warmer than normal, so a lot warmer, and it will be the same tomorrow,” says Siegmund. By ‘normal’ he does not mean the average of the past hundred years. “I then compare it with the average from 1991 to 2020, a period of thirty years. We always look at the last period of thirty years and calculate in whole tens.” Due to the changing climate, the “normal” shifts every ten years.
Last Wednesday was even an absolute record: with 18 degrees in De Bilt, it was the warmest November 5 ever. Such records automatically bring to mind climate change, but they are not the best evidence for that, says Siegmund. “We don’t look at daily records ourselves. Every year there are some records, on the warm or cold side, so you can keep busy.”
The KNMI does look at “record statistics”, for example the number of daily records that are broken in a year. “By chance you expect records anyway, although that chance becomes smaller as the series gets longer. In the first year you measure, every day is a record. In a series of a hundred years, you expect about one record every hundred days, so about three per year.”
It only becomes meteorologically interesting if the number of records deviates from that expectation. “And that turns out to be the case. In practice, the number of heat records is higher than you would expect by chance, but lower on the cold side,” says Siegmund. “And that is clearly due to climate change.”
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