The growing seasons in the past two years were long and warm enough to allow growth of trees both on Svalbard’s high-Arctic and on barren mountaintops in East Finnmark. High temperatures combined with low precipitation also led to drought with implications for reindeer herding in East Finnmark.
By: Rolf A Ims, Leif Einar Støvern and Nigel G Yoccoz // UiT – The Arctic University of Norway, Ole Einar Tveito and Ketil Isaksen // Norwegian Meteorological Institute, Jane Uhd Jepsen // Norwegian Institute of Nature Research and Jan Ivvár Smuk // Várjjat Siida
It is now well established that land temperatures in the Arctic are rising faster than in many other land areas, a phenomenon called Arctic amplification. This warming is expected to be stronger in the winter season than in the summer season. However, the summers of 2023 and 2024 saw exceptional warmth in the two Arctic regions monitored by the Climate-Ecological Observatory for Arctic Tundra (COAT): East Finnmark and West Spitsbergen. In 2023, it garnered significant media attention that Longyearbyen recorded an average July temperature above 10°C for the first time, and thus exceeded the classic threshold for what is considered a polar climate. The summer of 2024 was even warmer. In statistical terms, the August 2024 mean temperatures at Svalbard Airport and in Vardø were respectively 5.1 and 3.6 standard deviations from the 1991–2020 mean (the most recent climate normal). This implies that temperatures as high as those recorded were only expected about once every five million years at Svalbard Airport and once in 500 years in Vardø. So, there is no doubt that we are seeing extreme temperatures.
Growth seasons for trees
A polar climate means that it is too cold for trees to grow. Recent studies have refined temperature boundaries for tree growth. One of the best global descriptions of alpine and Arctic treelines considers both the length of the growing season and the average temperature of the season. Specifically, the growing season must consist of at least 94 snow-free days with average temperatures above 0.9°C, and the average temperature over the entire season must be at least 6.4°C.
In both 2023 and 2024 these criteria for tree growth were met at all COAT weather stations in East Finnmark, as well as some stations on West Spitsbergen. At Reinhaugen/Boazoaivi, located at 470 metres above sea level and deep within the low-Arctic tundra of the Varanger Peninsula, the growing season lasted 104 days with an average temperature of 12.0°C. The Nedre Sassendalen station on the high-Arctic tundra in Spitsbergen had a 102-day growing season with an average temperature of 7.3°C.
It is important to remember that these criteria represent a physiological limit for tree growth that needs to be maintained over many years. Moreover, alpine and Arctic treelines are often heavily influenced by other factors such as physical disturbances (e.g. avalanches), biotic interactions (e.g. herbivores and pests), and human activities (logging).
The physiological limit also differs between tree species. Mountain birch is able to grow at shorter and colder growth seasons than Scots pine. Therefore, mountain birch forms the treeline and the northernmost forest stands in northern Norway. Note that neither birch nor pine are constrained by low winter temperatures in the Norwegian Arctic. Birch can survive temperatures of -65°C.
Drought
As summer temperatures get higher, evapotranspiration increases and if precipitation does not increase, this may lead to drought. Drought is often not precisely defined, as it can relate to hydrological, ecological and human impacts. Various indices have been used, the most common being the SPEI (Standardised Precipitation Evapotranspiration Index), which considers the difference between precipitation and potential evapotranspiration. The validity of such indices in polar regions is poorly understood, as drought has not been considered as a potential problem until very recently. The recent extreme summers with severe drought—such as 2018 in Fennoscandia and 2020 in Siberia which led to extensive fires—have shown that drought should also be investigated. Indeed, drought can limit tree recruitment and growth.
As drought affects vegetation productivity, it may also affect the condition of herbivores like reindeer. Reindeer herders in East Finnmark experienced that the last part of the summer 2024 was exceptionally dry. Many smaller tundra lakes dried up. The reindeer’s weight was affected to the extent that the seasonal slaughter, which normally takes place at the end of August, was postponed by one month in Várjjat Siida.
Drought represents a novel concern for reindeer herding. How the consequences can be mitigated is not easy to decide. Changed use of pastures can be an option. In any case, there is a need for increased preparedness for such events that may become more frequent and severe.
On warm summer days, reindeer herds often seek out patches of snow where they can cool off and escape insects. Last year, all the snow patches on the Varanger Peninsula melted away early in the summer. Photo: Geir Vie
Broader significance
The record-breaking warmth of the two last summers aligns with projections about changes in the plant growing season in northern regions. Such projections, published twelve years ago, gained much attention as they suggested that southern growing seasons could shift up to 20 degrees latitude northward during this century. The exceptional temperatures observed in the past two years fit with a pattern observed elsewhere in Europe, that summer temperatures have increased faster than climate models simulate. The reason appears to be changes in atmospheric circulation patterns.
Although Arctic warming is fastest in the winter season, the climate of the plant growing season is most decisive for the state of ecosystems. Cold and short growing seasons result in a polar desert with only patchy vegetation of mosses and lichens, while growing seasons that meet criteria for tree growth may eventually yield continuous forest, provided that the climate does not become too dry. Note that treelines and forest lines are different—there are often isolated trees or small groups of trees growing beyond forests.
However, establishing a forest on the tundra can take a long time. Studies of past climate changes have shown that it can take many centuries from when the climate becomes warm enough for tree growth to the establishment of fully developed forest ecosystems. With today’s extremely rapid climate change in the Arctic, this type of mismatch between climate and vegetation can result in unstable ecosystems with unknown characteristics and functions. It is therefore crucial to closely monitor developments using “ecosystem-wide observation systems” like COAT. In addition to weather stations, COAT has recently established an extensive network of automated sensors and manual field measurements that continuously document how biodiversity and critical ecosystem functions respond to climate warming both in East Finnmark and West Spitsbergen. The long-term operation of this observation system will provide a knowledge base for managing the impending climate and nature crisis in our northern regions.
