The conclusion was that in the twentieth century, a winter as cold as the winter of 2010 or colder occurred once every six years. Due to the warming trend in the Netherlands the return time has now increased to approximately once every fifteen years. The large amount of snow, which caused most problems, was much more unusual. In this article we investigate the global return times of temperature and snow cover.
In Figure 1 the return times for the temperature of the winter of 2010 are plotted assuming a stationary climate. The pattern of temperature anomalies of this winters shows two areas with below-normal temperatures (Europe/Siberia/China and the United States) and two regions where the winter was milder than usual (Canada/Greenland and North Africa/Middle East). If the climate would not be changing we would not be able to call the winter very cold in many areas: return times are less than once every ten years in Europe, 10-50 years in the southern US and reach 100 years in Siberia. The warm extremes would be much more unusual: up to 200 year in northeastern Canada and southern Greenland, and much higher still in North Africa and the Middle East.
However, due to global warming the probabilities of cold extremes have decreased and the chances of warm extremes have increased. This has not been taken into account in Figure 1. In Figure 2 we attempt to estimate the return times in a changing climate. This has been done by subtracting the trend from the observations at each grid point before computing the return times. The trend is defined as a locally varying factor times the smoothed global mean temperature rise.
The winter of 2010 was more exceptional in the large quantities of snow than in the temperature, both in Europe and in the United States. The deviation of winter-averaged weekly NOAA snow cover from the 1971-2000 normals is shown in Figure 3. In Europe, the United States and China there was a lot more snow on the ground than usual this winter, in Central Asia there was less snow. The return times in the United States vary largely, in most places this much snow is not very uncommon, but there are a few exceptions. In Europe the return times reach once every 20-50 years in Germany, North Germany and Scotland. The snow in Spain was really exceptional.