This city report was originally published with errors in the data. An algorithm meant to reconciliate two ECMWF datasets to make them comparable was never applied for this city, and its temperature increase was therefore exaggarated. The affected numbers have now been corrected. For an in-depth explaination of this incident, see: www.onedegreewarmer.eu/correction.
Changes in weather patterns
Since 1900, the average temperature in and around Visby increased from an average of 7.3°C between 1900 and 1999 to an average of 8.3°C between 2000 and 2018. The warmest years in and around Visby were 2015, 1990, 2018, 2014, and 1989.
In the 20th century, the average number of hot days (days for which the 24-hour average temperature is above 20°C) per year was 1.8. Between 2000 and 2018, the average number of hot days were 10.5 per year.
A day is considered hot when its average temperature is over two standard deviations of the normal average.
Temperature averaged −1°C or less for 32.2 days per year in the 20th century, on average. Between 2000 and 2018, the number of freezing days were 25.9 per year.
What does it mean for Visby?
Health and heat waves
Higher temperatures lead to excess mortality, even in relatively cold countries. A study of nine European cities, including Stockholm and Helsinki, showed that heat-related deaths increase as soon as average daily temperatures reach 20°C (de’Donato, 2005).
Rising temperatures may also cause the number of deaths related to extremely cold weather to drop.
Rail buckling and tarmac softening
In high temperatures, asphalt exposed to the sun starts to soften. This causes delays and some roads have to be closed to traffic.
When temperatures rise above 30°C, rails exposed to the sun can move or buckle. This can cause trains to derail, as happened many times in Europe already, and forces them to run more slowly, causing major delays.
Tick and mosquito-borne diseases
Tick-borne encephalitis, and more recently ehrlichiosis have been spreading in the past decades, probably due to higher temperatures (Gray et al., 2009).
Visby and its environs in context
Visby and nearby cities
Here are the five locations closest to Visby, among the 558 we analyzed:
Cities of Sweden
Visby is one of 29 locations in Sweden we have analyzed. This is how temperature has changed in the rest of them.
We analyzed two data sets from the European Centre for Medium-Range Weather Forecasts (ECMWF), ERA-20C for the period 1900–1979 and ERA-interim for the period 1979–2018.
Both data sets are re-analysis, which means that ECMWF scientists used observations from a variety of sources (satellite, weather stations, buoys, weather balloons) to estimate a series of variables for squares of about 80 kilometers in side width (125 kilometers for ERA-20C). While weather stations offer a much better record for immediate daily observations, using the ECMWF re-analyses is much more adequate for the study of long-term trends. Weather stations might move, or the city might expand around them, making their data unreliable when looking at centennial trends. However, the ECMWF data does not take into account micro-climates or “heat island” effects, so that the actual weather in the streets of Visby was probably one or two degrees warmer than the values reported here (the trend, however, is the same).
Since the start of this project, ECMWF has adjusted the way historical temperatures are calculated, to give better estimates for e.g. coastal cities. Because of this, some figures published here in 2019 may differ slightly from corresponding figures published in 2018.
This report was produced by the European Data Journalism Network. Partners include OBC Transeuropa (Italy), J++ (Sweden), Spiegel Online (Germany), Vox Europe (France), Pod Crto (Slovenia), Mobile Reporter (Belgium), Rue89 (France), Alternatives Economiques (France), and El Confidencial (Spain).
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