Dutch Climate Skeptics Vindicated: Seven Pre-1950s Heatwaves Reinstated

A miracle just happened. KNMI, the national weather and climate institute in The Netherlands, admitted publicly that we, a group of four skeptical scientists, were right in our critique of their homogenization. This admission means the end of a ‘battle’ that has been dragging on for about seven years.

What was this whole discussion actually about? In 2016, KNMI homogenized their daily temperatures for the period 1901-1950 because of a change in measurement method in 1950 (Pagoda screen to Stevenson screen) and a displacement 300 meter towards open field in 1951. They had parallel measurement for the change in screens but not for the displacement and therefore they had decided to homogenize De Bilt statistically by comparing it with a station 150 kilometers northeast (a place called Eelde) from De Bilt. The homogenization had a negligible effect on the average temperature. However the hottest days of the year (In The Netherlands this means Tmax of around 30^oC) in the period 1901-1950 were corrected downwards by up to 1.9^oC. Because of this, 16 out of those 23 heatwaves vanished from the official records.

Hot Summer

In 2018, when The Netherlands experienced a hot summer, KNMI started claiming in the media that heatwaves nowadays are much more frequent than in the past. I, together with three others, decided to critically examine the KNMI corrections. In March 2019 we launched our first extensive report (in Dutch) about the matter, titled The Mystery of the vanished Heatwaves. The report showed that the KNMI had overcorrected far too much. An article was prepared for a major Dutch newspaper, but after interference by the director of KNMI, the editor in chief of the newspaper decided not to publish the article. A spokesman of KNMI used ad hominem arguments against us (or mainly me as I am the most visible of us four). After questioning this in an email I had a conversation with the director of KNMI and the spokesman. It was a shocking experience. They told me they wouldn’t response to our extensive report as they didn’t “trust me”. I replied science isn’t about trust. “Our report is either right or wrong and in both cases I would like to know”, I replied.

This was the end of it and in the years after they kept using their – in our opinion – fraudulent corrections to claim a strong increase in heatwaves.