Changes in weather patterns
Since 1900, the average temperature in and around Zagreb increased from an average of 10.5°C between 1900 and 1999 to an average of 11.7°C between 2000 and 2018. The warmest years in and around Zagreb were 2000, 2014, 2018, 2007, and 2012.
In the 20th century, the average number of hot days (days for which the 24-hour average temperature is above 27°C) per year was 0.3. Between 2000 and 2018, the average number of hot days were 3.5 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 33.6 days per year in the 20th century, on average. Between 2000 and 2018, the number of freezing days were 26 per year.
What does it mean for Zagreb?
Health and heat waves
Higher temperatures lead to excess mortality. The heatwave of July and August 2003, for instance, killed over 52,000 people in Europe, according to the Earth Policy Institute (Larsen, 2006), a think-tank. The elderly and infants are most at risk.
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).
Researchers showed that when the daily average temperature increases above 22°C (Graff Zivin et al., 2018), cognitive abilities of school children decrease, especially in mathematics.
In Zagreb, the number of school days above 22°C went from 2.3 per school year in the 20th century to 8.2 since 2000. It might not seem much but if exams took place on these days, pupils of Zagreb were at a disadvantage.
Zagreb and its environs in context
The most rapidly warming locations in Europe
Among the 58 biggest cities in the EU, Copenhagen and its environs is the fastest warming, and Zagreb ranks as number eight. We have defined “big cities” as cities with more than 500,000 people in the city proper.
|1 / 58||Copenhagen||Denmark||+1.5|
|2 / 58||Genoa||Italy||+1.4|
|3 / 58||Bucharest||Romania||+1.4|
|4 / 58||Milan||Italy||+1.4|
|5 / 58||Vilnius||Lithuania||+1.4|
|6 / 58||Helsinki||Finland||+1.3|
|7 / 58||Gothenburg||Sweden||+1.3|
|8 / 58||Zagreb||Croatia||+1.2|
|9 / 58||Dresden||Germany||+1.2|
|10 / 58||Łódź||Poland||+1.2|
|11 / 58||Warsaw||Poland||+1.2|
|12 / 58||Turin||Italy||+1.2|
|13 / 58||Madrid||Spain||+1.2|
|14 / 58||Kraków||Poland||+1.2|
|15 / 58||Wrocław||Poland||+1.2|
|16 / 58||Zaragoza||Spain||+1.2|
|17 / 58||Hamburg||Germany||+1.2|
|18 / 58||Barcelona||Spain||+1.2|
|19 / 58||Riga||Latvia||+1.1|
|20 / 58||Poznań||Poland||+1.1|
Zagreb and nearby cities
Here are the five locations closest to Zagreb, among the 558 we analyzed:
Cities of Croatia
Zagreb is one of seven locations in Croatia 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 Zagreb 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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Graff Zivin, Joshua, Solomon M. Hsiang, and Matthew Neidell. "Temperature and Human Capital in the Short and Long Run." Journal of the Association of Environmental and Resource Economists 5.1 (2018): 77-105.
Gray, J. S., et al. "Effects of climate change on ticks and tick-borne diseases in Europe." Interdisciplinary perspectives on infectious diseases (2009).
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