Dutch climate data dispute reignites debate after historical heatwaves reinstated

A long-running dispute over temperature records in the Netherlands has resurfaced after the Dutch national weather agency, the Royal Netherlands Meteorological Institute (KNMI), revised part of its historical climate dataset and restored several pre-1950 heatwaves that had previously been removed from official records.

The controversy dates back to 2016, when the institute adjusted temperature data from its De Bilt weather station, lowering some daily maximum temperatures recorded between 1901 and 1950 by as much as 1.9C. Critics argued the changes significantly reduced the number of historic heatwaves on record, with 16 of 23 heatwaves effectively disappearing from the dataset.

Four independent researchers challenged the methodology, arguing that the adjustment process systematically reduced historical heat extremes. Their findings were later published in a peer-reviewed study that examined the homogenisation technique used by KNMI and concluded that the corrections had substantially altered the frequency of extreme heat events recorded at De Bilt.

The dispute centred on KNMI’s efforts to account for historical changes in measurement methods and station location. According to the institute, adjustments were made to improve consistency across long-term climate records. However, critics maintained the approach disproportionately reduced older temperature extremes and distorted comparisons between past and present heatwaves.

KNMI has since introduced an updated Version 2.0 homogenisation method. The agency now estimates that there were 14 heatwaves at De Bilt between 1901 and 1950, double the number identified under its previous methodology, effectively restoring several historical heatwave events, including the notable summer of 1947.

The revisions have been seized upon by climate policy critics, who argue the episode demonstrates the need for greater scrutiny and transparency in the handling of historical climate data.

“Here again, we have a government agency caught rewriting climate history. The Netherlands Meteorological Institute erased heatwaves of the past, ignored critics, and reinstated the truth only when the evidence became impossible to ignore. Policies were built on that manipulated record. Dutch farmers lost livelihoods. Industry and the wider economy paid the price. But accountability is coming”, wrote climate analysts Electroverse.

In 2016, the Netherlands Meteorological Institute adjusted temperatures at De Bilt, the country’s main climate station. Daily maximums from 1901 to 1950 were lowered by up to 1.9C, which removed 16 of 23 heatwaves from the record.

The altered data were then used to claim modern… pic.twitter.com/gkGOoGsxRB

— Electroverse (@Electroversenet) May 29, 2026