Climate in Finland has become warmer


Climate is warming globally which is observable also in Finland. Long-term temperature time series reveal that Finland's climate has become warmer during all seasons. However, warming has been greatest in winter.

Finland's mean temperature has risen by about two degrees since the late-19th century

Based on the best estimate the annual Finnish mean temperature has risen approximately 2 °C since the 1880's (figure 1). The rise in temperature is statistically significant. [1]

Warming has been greatest in winter when the mean temperature has risen 2–3 degrees. Summer has warmed the least: a little over one degree. The measurement period also includes major variation, such as the cold winters in 1985 and 1987, and warm years in the 1930s. [1]

The warmer climate is also reflected on nature. In Finland, the average time of leaf bud burst in the spring has become earlier by some 12 days in 1846–2005. [2]

Figure 1. Finland's annual mean temperatures in 1847–2017 [1]. Before the 1880's temperature data is very uncertain due to the sparse density of observation stations. Yearly values showed as red dots are based on gridded monthly mean temperatures covering the whole Finland [3]. The mean temperature level given by a statistical model is shown as a red dotted line. The decadal average temperatures calculated based on these climatological annual mean temperatures are shown as mean (solid black line) and with 50 and 95 per cent probability limits (darker and lighter grey bars) [1].

The 1930s were warm particularly up north

The warm period of the 1930s extended over large areas of the northern hemisphere, the stronger the farther north you went. It was relatively warmest in the northern Arctic Ocean, Spitsbergen and northern parts of Greenland. The warmth would seem to be mostly due to natural variation in ocean currents, resulting in warm water from the Atlantic flowing into the Barents Sea.

In Finland, as well, the warmth of the 1930s is more distinct in Lapland than in southern Finland. As the time series for Helsinki and Sodankylä indicate, average temperatures in Sodankylä are just exceeding the level of the 1930s, whereas Helsinki has exceeded this level earlier (figure 2).

Figure 2. Annual mean temperatures for Kaisaniemi, Helsinki 1830–2017, Jyväskylä 1884–2017 and Sodankylä 1908–2017. The thin line indicates annual values, the thick one ten-year running mean. For Helsinki, an estimate of how much urbanisation has raised the temperature is included; the medium line describes estimated temperatures in case urbanisation had not affected temperatures. Click image to enlarge.

Probability of high temperatures multiplied already

Even though warming, measured by annual mean temperature, has for the time being been fairly meagre in comparison with the high year-to-year variation of temperatures in Finland, it has, however, already multiplied the probability of record-high monthly and seasonal mean temperatures. [4] For instance, the return period for the mean temperature of the record-warm July 2010 in Helsinki is circa 300 years, not taking into account warming that has already taken place. Considering global climate change, the return period is only around 60 years. [5]

Natural variation emphasised on a small scale and for short time series

It is important to realise that human-induced warming of the climate is a global phenomenon. Locality-specific temperature time series for one observation station primarily reveal high natural climatic variation. Hence, verification of human-induced climate change on the basis of temperature observation series is most successful when reviewing mean temperatures for large areas over a long period of time. In this case, random variation due to natural factors is evened out and the underlying systematic warming due to higher concentrations of greenhouse gases is better highlighted.

Temperature is only one, even if a very important, characteristic of climate. Precipitation, wind levels, cloud cover and amounts of snow are examples of other climate variables with a major impact on society and nature. [6]], [7] However, equally long, reliable and geographically comprehensive time series are not available on these as for temperature. Therefore, it is more difficult to observe consistent long-term change, a trend, in them.

No long-term change has been observed in other characteristics of climate – rather variation

Precipitation sum in May-September (whose reliability is not impeded by difficulties in measuring snowfall) vary a lot in Finland from year to year, and no clear trends have been observed for the time being. On the basis of observations from four observation stations for a period of circa 100 years, the number of dry spells is highest and they last longest on the average in coastal areas in the summer half of the year. The opposite is true for Lapland. Statistically significant trends have not occurred in the numbers of dry days and duration of dry spells, though there has been some decline. [8]

There has mainly been a decrease in the occurrence of high wind speeds, when observed on the basis of time series of atmospheric pressure observations of ten stations, spanning 120 years at most. In the past less than 50 years, wind speeds would seem to have increased slightly, but the change has not been statistically significant. [9]

Observations of the duration of sunshine reveal variation in cloud cover. When the number of sunshine hours is high, there are few clouds. Finland's longest time series of sunshine duration are from Helsinki and Sodankylä. [10], [11] Time series (figure 3) show that cloud cover varies from year to year. A relatively sunny period prevailed in the early 1970s both in Sodankylä and Helsinki, followed by more cloudy years in the 1980s and early 1990s. Early 2000s were relatively sunny, again, particularly in Helsinki. Exceptionally little sunshine was measured in Helsinki in a few years in early 20th century, with the daily mean of sunshine hours at less than three hours. This does not, however, directly indicate a lot of cloud, but is probably connected with the Novarupta vulcanic eruption in Alaska in 1912.

Figure 3. Mean of daily sunshine hours in 1906–2017 in Helsinki and in 1950–2017 in Sodankylä. The annual values covering the period from March to October are shown with a thin line, and ten-year running mean with a thick one.