Science - August 23, 2012

Weather station readings too high

The temperature measured by the Netherlands Meteorological Institute (KNMI) in De Bilt is too high. The cause: influence from city heat in Utrecht.

This is reported by Wageningen researchers (Meteorology and Air Quality) in an article in the Dutch journal Meteorologica. The recorded temperature trend in the past century is 0.3 C too high. This is caused by increased urbanization. The KNMI has maintained a correction for the proximity of the city which is three times smaller than this. It is becoming clearer that urbanization has a strong effect on the local climate. One good example is the heat island effect. The temperature difference between the city and its environs can easily be five degrees or even more during peak moments, both in summer and in winter.
The influence of the city also causes problems when determining climatic warming. Earlier estimates show that the effect on De Bilt is one-tenth of a degree. This figure was based on comparisons with other weather stations. The Wageningen researchers have calculated the deviation with an advanced atmospheric model. 'Inspired by what Belgium scientists have done for the weather station in Brussel (Ukkel),' researcher Gert-Jan Steeneveld points out. 'That reported an effect of 0.77-1.33 degrees. That is a lot.'  Steeneveld will investigate this with MSc student Sytse Koopmans.
The model used calculates the expected temperature for four different 7-day weather situations, based on land-use in 1900 and 2000. The differences between then and now are big. Utrecht is five times larger than a century ago when the size of its population was less than half. In particular, land-use plays a big role, explains Steeneveld. The contribution from mankind to the heat balance has hardly changed. It is true that we are now producing more heat due to electricity, heating and transportation, but the effect is cancelled out (for every square metre)  because people in the past lived much closer to one another.
The recorded climate series, according to the Wageningen study, has a trend which is 0.3 C per century too high. This is not considered high in a house-garden-kitchen context, but is considerably high in climate research. The IPCC has estimated global warming in the past century to be 0.74 degrees. But this global average does not suddenly have to be viewed in a new light. 'Research has shown that the average is not influenced by urbanization.'  The Bilt, on the other hand, cannot avoid having to make a new correction, he says. 'If the model is correct and the results are solid, KNMI would have to correct the temperature series.'
The former weather station of Meteorology at the Haarweg was also affected by the increase in built-up areas. That can be seen by comparing the measurements of the Haarweg with those from the new de Veenkampen station in the Binnenveld. The new station has been monitored for three years by taking double measurements. Whenever an east wind sweeps through Wageningen, Haarweg appears to be 0.6 to 0.8 degrees warmer than at de Veenkampen. According to Steenveld, the average difference for all the wind directions is 0.45 degrees.