Signs of climate change in Finland

Temperature has increased by 0.76°C in the 20th century

A comprehensive assessment of observed changes in Finland's temperature and precipitation was made by Tuomenvirta (2004). The assessment was based on about 300 temperature and 700 precipitation data series, which were thoroughly homogenised. The longest series exceeded 150 years but the systematic analysis was mainly focused on the 20th century.

According to linear trend tests, the mean temperature in Finland increased by 0.76°C in the 20th century. The warming took place during the first two and last three decades of the century, while a slight but statistically insignificant cooling occurred in the time period between them. There was also some evidence of warming in the late 19th century, but the number of observation stations was too small for a reliable analysis.

The warmest year on record was 1938, when the average over the whole country was 2.4°C higher than the mean for the reference period of 1961-1990. The second warmest year was 1989, and the third warmest 2000. By far the coldest was 1867, the year of the great famine, with the nationwide average 3.4°C below the reference period.

Most of the warming occurred in spring. The mean temperature in March-May over the whole country was 1.8°C higher in 1963-2002 than in 1847-1876. The diurnal temperature variation had become smaller, again mainly in spring. A similar trend has been observed widely on the land areas of the Northern Hemisphere, together with an increase of cloudiness.

No significant, nation-wide precipitation trends were found. This is in contradiction with a 15-20% increase of precipitation in Sweden in the 20th century. Both countries have had changes in instrumentation and observation practices. The wettest year in Finland was 1974, with a nationwide mean of 740 mm, while the driest was 1941, with only 394 mm. In addition to significant year-to-year variation, the precipitation climate of Finland is also characterised by notable inter-decadal variability, which partly offsets the statistical detection of trends.

The warming can be observed in ice break-up series

Ice conditions in lakes and rivers have been observed in Finland for a long time. The longest continuous ice break-up series from River Tornionjoki dates back to the late 17th century. The break-up date has become earlier and the freezing date later at many observation sites during the last few decades. The trends are statistically significant mainly in those sites which have records at least since the late 19th century.

Trends of series that started later were in most cases not statistically significant. In the longest series that started in the late 19th century, the ice break-up has moved 6-9 days earlier in a hundred years. The freezing has been delayed since the late 19th century, in most cases by 0-8 days per century. Also the duration of ice cover has significantly shortened at sites with the longest records. Not all of the longest freezing and duration of ice cover trends are statistically significant. The trends of break-up are stronger, because the variation of break-up date is smaller than that of the freezing date. The large variation of freezing dates hides the trends of freezing. There were no statistically significant trends found in northern Lapland but the time series from this part of the country are still relatively short.

The Tornionjoki River ice break-up dates in 1693-2002

Sources

Tuomenvirta, H. 2004. Reliable estimation of climatic variations in Finland.Finnish Meteorological Institute, Contributions 43, 82 p.