Weather is changeable but its caprices are well-examined. In Europe, we can access temperature data established through a dense network of measuring stations—a gigantic treasure trove of data that also reveals plenty of information about medium-term climate conditions.
One thing is for certain: Average temperatures are rising. But how exactly does this manifest itself locally? As a statistician at Universität Hamburg's Center for Earth System Research (CEN), I set out to approach measurement series from a different angle—producing astonishing results!
We normally derive averages from measured data. These provide us with a good overview. I can thus, for instance, compare the average temperature of a certain year to averages from other years. Alternatively, I can scrutinize all summers in a country or certain region since 1950, focusing on their development.
Yet, averages are only half the truth. If, for example, the nights would get colder while the days would become warmer in the same place and at the same time, the averages would not change at all—humans and nature, however, would be severely impacted. Therefore, I am particularly interested in finding out whether extreme temperatures are changing as well, for ecological systems tend to be most vulnerable under radical conditions.
But what values are extreme? In order to answer this question, I categorize ninety percent of all temperature values as “normal.” Merely the coldest and hottest five percent are considered extreme—a well-known statistics method not yet applied in climate research.
We would expect global warming to work as a simple addition to “normal temperatures” with hot days getting even hotter and colder days warmer.
Even so, the results clearly show: While average temperatures, particularly in France and Germany, have generally been increasing, extreme values have been developing very differently across Europe. As expected, hot days have become even hotter in Central and East Europe over the past sixty years. In Norway and Southeast Europe, by contrast, hot days have actually cooled down. Turkey is a special case: In the past decades, both hot and cold days have gotten colder.
How does this fit into patterns of warming due to climate change? Regionally, strong natural climate variations may mask the global temperature rise, so to speak. This knowledge is crucial to individual areas: Instead of assuming a general two-degree temperature increase, politicians can use our findings in adapting urban planning and agriculture to specific local requirements.
Hence, I have created additional profiles for individual cities. Trondheim in Norway, for instance, shows a diminishing temperature range since 1950. In the past, temperature lows averaged minus 18° Celsius; nowadays, they do not drop below about minus 14° anymore. Unaltered, temperatures on the warmest days do not exceed six degrees. Evidently, Trondheimers have “lost” several low temperature points in the past sixty years.
Quite differently, in Germany’s capital Berlin, colder days have stayed more or less the same, averaging about 7° Celsius. Hot days saw temperatures increasing by two degrees, roughly reaching 32° Celsius: Berlin’s temperature profile has broadened. I am currently analyzing data from the measuring station in Fuhlsbüttel, a district of Hamburg. We will know more about temperature trends in Hamburg shortly.
This content was first published as a guest article in the newspaper Hamburger Abendblatt on 12th June 2017
Christian Franzke is an expert on statistics in climate research at the CEN..
Franzke, L.E.: Local trend disparities of European minimum and maximum temperature extremes. Geophys. Res. Lett., 42, 6479-6484, doi: 10.1ßß2/2015GL065011. Go to original article