Changes in the thermal growing season in Nordic countries during the past century and prospects for the future Timothy R. Carter Agricultural Research Centre of Finland. Plant Production Research. Office address; Finnish Meteorological Institute, PO Box 503, FIN-00101 Helsinki, Finland, e-mail: tim.carter@fmi.fi The start, end, duration and intensity of the thermal growing season (the period with mean daily temperatures exceeding 5°C) during the past century (1890-1995) was analysed at nine sites in the Nordic region. Statistical comparisons were made between three adjacent 35-year periods. The re- sults indicate that the growing season lengthened considerably at all sites between 1891-1925 and 1926-1960. Lengthening has continued at a slower rate up to the present at the eight Fennoscandian sites but not at the Icelandic site. In contrast, the intensity of the growing season, expressed by effec- tive temperature sum above 5°C, which increased at all sites between the first two periods, has de- creased slightly at all locations except Turku in recent decades. Under three scenarios, representing the range of estimated greenhouse gas-induced warming by the 20505, the growing season is expected to lengthen at all sites. For a “Central” scenario, the great- est lengthening is computed for southern and western Scandinavia (7-8 weeks) with smaller changes in Finland (4 weeks) and Iceland (3 weeks). With a lengthening growing season during the past century in Fennoscandia, there are likely to have been impacts on natural and managed ecosystems. Some evidence of recent biotic and abiotic effects already exists, but other indicators of long-term change remain to be analysed. Key words: climate change, duration, effective temperature sum, impact, indicators ntroduction During the past century developments in agri- cultural management and technology have revo- lutionised production potential, with the Nordic countries among the world leaders in embracing these advances. However, in spite of this progress much of the region still finds itself at a disad- vantage compared with other areas ofcentral and southern Europe due to a limiting factor that currently defies technological ingenuity: the short growing season. In the Nordic region the growing season is © Agricultural and Food Science in Finland Manuscript received March 1998 161 Voi 7(1998):161-179. AGRICULTURAL AND FOOD SCIENCE IN FINLAND largely temperature-limited, and is convention- ally represented as the period during which the surface mean daily air temperature exceeds 5°C. The physiological significance of this period dif- fers among plant types. It is most relevant to perennial species that are exposed to the weath- er throughout the year, such as trees, shrubs and some grasses. It is less meaningful (though still relevant) in relation to annual crops, many of which are sown after the start and harvested be- fore the end of the period. For these species, es- pecially cereals, oilseed and legumes (and to a lesser extent tubers and root crops, which are harvested later), it is necessary to distinguish between the growing “season”, which is the en- tireperiod in which growth can theoretically take place, and the growing “period”, which is the actual period of growth. The length of the growing season is a natural indicator of the thermal climate, varying both spatially (from over 220 days in southern Den- mark to less than 100 days in the marginal range- lands of Iceland and northern and upland Fen- noscandia) and from year to year. However, it describes only one facet of thermal suitability for plant growth: duration. It does not indicate the intensity of the season, which can be de- scribed using mean temperature or a measure of accumulated temperature (effective temperature sum - ETS). In fact, as is shown below, the du- ration of the season may bear little relation to its intensity. This paper focuses on variations in growing season characteristics in the Nordic region dur- ing the past century and prospects for the future. The following section describes methods for deriving growing season duration and the data sets employed in this analysis. Subsequently, the results of an analysis of growing season dura- tion and ETS at nine sites in the Nordic region are presented for the period 1890-1995, along with some estimates of possible future changes in duration under scenarios of greenhouse gas- induced climate change. Finally, the implication of the results is discussed in relation to observed changes in plant behaviour and other indicators ofbiotic and abiotic response to climate. Material and methods The thermal growing season The active growing season is conventionally de- fined as that period during which the tempera- ture and soil moisture conditions are adequate for crop growth. In the Nordic region, the major control on the growing season is low tempera- tures during the winterpart of the year. This cold period is usually associated withprotracted snow cover and frozen soil conditions, especially away from the milder coastal regions. By convention, the season for active plant development and growth in the Nordic countries has long been calculated as the period during which mean dai- ly air temperatures remain above 5°C. Arguments for adopting this threshold in- clude: (i) its approximation to the mean temperature at which significant growth and development commences across a range of plant species including trees (Sarvas 1972), natural vege- tation (Heikinheimo and Lappalainen 1997) and agricultural crops (Lallukka et al. 1978); (ii) its widespread adoption as a base tempera- ture for computing ETS, a measure of ther- mal time that can be related to plant devel- opment (see references in (i)); and (iii) its approximation to the mean temperature at which continuous snowcover and soil frost disappear in many inland areas, marking the earliest opportunity for spring sowing of an- nual crop species. However, the threshold is arbitrary and highly generalised. Other thresholds have been applied for computing the growing season outside the Nordic region, including 5.6°C (42°F - Meteor- ological Office 1965) in the United Kingdom, 6°C in the UK (Taylor 1976) and France (AGPM 1987) and 4.4°C in various countries at northern latitudes (Nuttonson 1955). Moreover, some plants are known to commence development at temperatures lower than 5°C, such as winterand spring cereals (Gallagher 1979, Kleemola 1991, 162 Seminar in honour of the 100thanniversary ofMTT AGRICULTURAL AND FOOD SCIENCE IN FINLAND AGRICULTURAL AND FOOD SCIENCE IN FINLAND 163 Saarikko and Carter 1996) and potato (Kooman 1996). Other crops like maize (Hough 1975, Goudriaan 1988, AGPM 1987), sunflower (CE- TIOM 1986) or soya bean (CETIOM 1987) re- quire temperatures higher than 5°C. Finally, the measure is exclusively temperature-based and disregards other constraints on the growing sea- son, especially moisture availability, which can be important during regional droughts in some seasons, but is usually regarded as a secondary constraint in the Nordic countries, and the oc- currence of damaging frost, which severely cur- tails the effective growing period in some years and regions. Notwithstanding the above caveats, the 5°C threshold represents a convenient measure, wide- ly accepted and applied in the Nordic region. It is also advocated for defining the thermal com- ponent of a growing season measure adopted by the United Nations Food and Agriculture Organ- ization (FAO 1978) and has subsequently been applied in global studies to model regional agri- cultural productivity potential (Leemans and Solomon 1993). Method of computation Computation of the index is straightforward when using long-term means of daily tempera- ture, which generally display a smooth seasonal curve. An example is presented in Figure 1 for the 1961-1990 period at Helsinki. However, in any individual year the spring and autumn peri- ods are usually characterised by frequent depar- tures above and below the 5°C threshold. Thus, in order to compute the length of the growing season from daily temperatures, certain rules are commonly applied to avoid including warm spells in early spring or late autumn that are sep- arated from the main growing season by pro- longed colder conditions. For example, a meth- od applied in Finland specifies that the growing season starts when daily mean temperatures first Fig. 1.Air temperature at Helsinki, Kaisaniemi: 1961-1990 mean daily, 1988 daily mean, 1988 monthly mean (horizontal lines) and 1988 daily means interpolated from monthly means using the Brooks (1943) method. Also shown is the 5°C threshold temperature. Vol. 7(1998): 161-179. exceed 5°C for at least 5 consecutive days in the spring and there is less than 50% snow cover in open areas (Venäläinen and Nordlund 1988). The season ends when the 10-day running mean of mean daily temperature falls below 5°C. The method is illustrated in Figure 1 using the 1988 daily mean temperature curve for Hel- sinki, which exceeded 5°C on one occasion (17 April/day 107) before commencement of the growing season on 28 April (day 118) and fell below 5°C twice (11-12 and 18-20 October) before the end of the growing season proper on 24 October (day 297). Nonetheless, exceptions may occur even with these definitions, and the official duration of the growing season still re- quires expert verification before it can be used in other applications such as calculation ofETS. This study is concerned with century-scale variations in growing season duration. Unfortu- nately, there are few sites in the Nordic region for which daily mean temperature data have been computer coded over such long periods, so it was not possible to estimate the growing season di- rectly from daily data. Instead, an approximate method of computing the growing season was adopted employing monthly mean observations which are available over a network of sites in the region. The “monthly” method involves fitting a sine curve to the monthly mean temperatures for groups of 3 consecutive months in an approach proposed by Brooks (1943). This produces a smooth curve of daily temperatures through the monthly mean values, which can be used to estimate the start and end of the growing season. The procedure is illus- trated for 1988 monthly mean temperatures at Hel- sinki in Figure 1,where the interpolated daily tem- peratures are plotted alongside the original daily mean observations. In this example, the start of the growing season using the smoothed daily temper- atures occurs on 25 April or day 115 (compared with 28 April/day 118 using the original daily se- Fig. 2. Start and end dates of the thermal growing season at Helsin- ki (Kaisaniemi), 1961-1990, com- puted using the "daily" and "monthly" methods (see text for explanation). 164 Seminar in honour ofthe lOOth anniversary ofMTT AGRICULTURAL AND FOOD SCIENCE IN FINLAND Table 1.Meteorological stations supplying temperature data used in the analysis (locations are shown in Fig. 3). WMO no. is the official World Meteorological Organization code number for the station. Site WMO no. Country Location Alt.(m) Period Stykkishölmur 4013 Iceland 65°05’N, 22°44’W 8 1890-1996 Nordby - Denmark 55°26’N, B°24’E 5 1890-1995 Ferder Fyr 1482 Norway 59°02’N, 10°32’E 6 1890-1995 Falsterbo 2616 Sweden 55°23’N, 12°49’E 5 1890-1996 Uppsala 2462 Sweden 59°51’N, 17°37’E 13 1722-1997 Stockholm 2485 Sweden 59°20’N, 18°03’E 44 1756-1995 Turku 2972 Finland 60°31’N, 22°16’E 51 1890-1997 Helsinki 2978 Finland 60°10’N, 24°57’E 4 1829-1997 Kajaani 2897 Finland 64°17’N, 27°40’E 132 1890-1997 ries) and the end of the growing season falls on 18 October/day 291 (compared with 24 October/day 297), giving a growing season length of 176 days (179 days). A comparison of the “daily” and “monthly” methods of computing the growing season has been conducted for Helsinki over the 1961-1990 period (Fig. 2). On average, the monthly meth- od produced a slightly shorter season than the daily method (mean difference -3 days; stand- ard deviation 11.3 days), though in some years there can be differences of several weeks. These differences are primarily due to departures of daily temperatures above and below the temper- ature threshold during the transition seasons, which are smoothed out using the monthly meth- od. Arguably, because it employs smoothed tem- peratures, the monthly method provides a more consistent and reliable indicator of the general march of seasonal temperature than the daily method, enabling it to be applied in detecting general trends over the long term. The monthly method is used throughout the analysis present- ed in the remainder of this paper. It is also used to define the growing season for computing ef- fective temperature sum above 5°C: GSE ETS = X 5 I l where GSS and GSE are the start and the end of the thermal growing season, respectively, and T. is the mean temperature on day i. Fig. 3. Location of the meteorological stations used in this study. 165 Vol. 7(1998): 161-179. AGRICULTURAL AND FOOD SCIENCE IN FINLAND Climatological data The study employs century-long monthly mean temperature time series from nine sites across the Nordic region (Table 1 and Fig. 3), forming part of the North Atlantic Climatological Data- set (Frich et al. 1996). Two criteria were impor- tant in selecting the sites analysed here: (i) They should be relatively homogeneous over time, with few disturbances to therecords due to relocations, instrument changes, altered measurement times, and urbanisation or oth- er environmental effects. Six of the nine sites are reported as meeting this specification by Frich et al. (1996). Three others were select- ed for their long duration (see below). (ii)They should be fairly representative of the main agricultural regions of each country, to enable comparisons of growing season char- acteristics with long-term statistics of crop production. Stykkisholmur was selected to represent the agricultural climate of Iceland following Bergthörsson et al. (1988). The other sites were chosen by visual compari- son between the site location and a recent gridded 10’ x 10’ arable land use map of Eu- rope (de Smet and Heuvelmans 1997), though the Nordby site is more maritime than typi- cal agricultural sites in Denmark (J. Olesen, pers. comm. 1998) and Kajaani was selected to illustrate a marginal site north of the major arable zone in Finland. The analysis focuses on a common time pe- riod, 1890-1995, at all sites, though some have been updated to 1997 (H. Tuomenvirta, pers. comm. 1998). Earlier data are included from three sites to provide a longer term perspective: Uppsala from 1722 and Stockholm from 1756 (Moberg 1996, A. Moberg pers. comm. 1998) and Helsinki from 1829 (Heino 1994, H. Tuo- menvirta pers. comm. 1998). All three of these records have been corrected for urbanisation ef- fects. Climate change scenarios Aside from natural causes (e.g. volcanic erup- tions, changes in solar output, earth orbital var- iations, continental drift or factors internal to the climate system) there is growing evidence to suggest that anthropogenic activities are having a discernible effect on global climate (Santer et al. 1996). Rapid increases in concentration of greenhouse gases (GHGs), especially carbon di- oxide, methane, nitrous oxide, ozone and halo- carbons, have been observed in the lower atmos- phere, which are largely due to fossil fuel com- bustion, cement manufacture, deforestation and intensive agriculture. These gases are known to trap the sun’s energy, and are expected to cause a warming of the surface climate of the globe, though in some regions part of this warming may be offset by concentrations of aerosols in the atmosphere, another bi-product of industrialisa- tion. The magnitude and rate of the expected warming are not well known, especially at re- gional level, due to the complexity of the cli- mate system. However, estimates with general circulation models (GCMs) of the atmosphere and oceans suggest that northern latitude land areas may warm more rapidly than the global average, while locations in and around the North Atlantic ocean may warm more slowly (IPCC 1996). In order to summariserecent estimates of the future climate in Finland and surrounding re- gions, and to obtain a measure of uncertainty in these estimates, a set of temperature and precip- itation scenarios were developed as part of the Finnish Research Programme on Climate Change (SILMU) (Carter 1996). The temperature scenar- ios are depicted in Table 2. They show a gradi- ent of temperature change from the weakest in- creases expected over Iceland, in the North At- lanticregion, with little seasonal variation, to the strongest increases in the most continental part of the region, over Finland, with a marked win- ter maximum of warming. Uncertainties concerning future GHG emis- sions (which determine the radiative forcing of the climate) are accounted for in the scenarios 166 Seminar in honour of the 100th anniversary ofMTT AGRICULTURAL AND FOOD SCIENCE IN FINLAND Table 2. Scenarios of seasonal temperature changeby the year 2050 (°C) for each Nordic country prepared during the Finnish Research Programme on Climate Change (SILMU). Source: based on Carter (1996). Scenarios Iceland Denmark Norway Sweden Finland SILMU Winter 0.90 3.00 3.00 3.00 3.60 Central Spring 0.90 2.10 2.10 2.10 2.40 Summer 0.90 1.20 1.50 1.80 1.80 Autumn 0.90 2.10 2.10 2.10 2.40 SILMU Winter 0.30 0.86 0.86 0.86 0.90 Low Spring 0.30 0.60 0.60 0.60 0.60 Summer 0.30 0.34 0.43 0.51 0.45 Autumn 0.30 0.60 0.60 0.60 0.60 SILMU Winter 1.50 4.29 4.29 4.71 5.40 High Spring 1.50 3.00 3.00 3.30 3.60 Summer 1.50 1.71 2.14 2.83 2.70 Autumn 1.50 100 100 130 3.60 Winter (Dec-Feb), Spring (Mar-May), Summer (Jun-Aug), Autumn (Sep-Nov) by adopting the upper and lower projections de- fined by the Intergovernmental Panel on Climate Change (IPCC - Leggett et al. 1992). Uncertain- ties in the global mean temperature response to radiative forcing are also considered, by using the range assumed by the IPCC (1.5-4.5°C for an increase in GHG forcing equivalent to a dou- bling of atmospheric carbon dioxide - IPCC 1996). There are three scenarios: a central sce- nario (based on composite regional information from GCMs and mid-range assumptions about future atmospheric GHG concentrations and fu- ture gobal temperature response), a low scenar- io (assuming low GHG emissions and low tem- perature response) and a high scenario (assum- ing high emissions and high temperature re- sponse). Overall, these scenarios provide upper, mid-range and lower estimates of likely future temperature changes in the Nordic region (Cart- er 1996). Results The analysis consisted of three main steps. First, the GSS, GSE, GSL (growing season length) and ETS were plotted at all sites and statistical anal- ysis was applied to the common period 1890- 1995 to determine possible trends in the data. Second, to investigate how the growing season during the past century compares with earlier periods, the longer time series from Uppsala, Stockholm and Helsinki were plotted and ana- lysed. Finally, estimates were made of the fu- ture growing season under the scenarios of pos- sible temperature change. The growing season during the past century: 1890-1995 Plots of GSS, GSE, GSL and ETS during 1890- 1997 were constructed for all sites. The plots for Turku, Finland, are illustrated in Figure 4. In each plot a least squares linear regression line was fitted to provide a first impression of possi- ble trends in the data. The regression lines sug- gest that GSS has become progressively earlier at all Fennoscandian sites throughout this peri- od (by between 4 and 12 days), with the most marked change occurring between the turn of the century and the decades around the 1940s (Fig. 4a). There is a change towardsprecocity at 167 Vol. 7(1998): 161-179. AGRICULTURAL AND FOOD SCIENCE IN FINLAND Fig. 4. Computed growing season characteristics at Turku 1890-1997: (a) start, (b) end, (c) duration and (d) effective temperature sum above a base temperature of 5°C. Least-squares linear regression lines are fitted to each series. 168 Seminar in honour of the 100th anniversary ofMTT AGRICULTURAL AND FOOD SCIENCE IN FINLAND the Icelandic site too up to the 19505,but this is partially reversed in the decades up to the present. At the other end of the growing season there appears to be a concurrent trend towards a more delayed GSE throughout the period (by between 1 and 9 days), again stronger in the early dec- ades of the series (Fig. 4b). The exception is in Iceland, where the GSE has become slightly ear- lier. The net effect on GSL has been a strong ex- tension of the season of between 1 and 3 weeks throughout Fennoscandia (cf. Fig. 4c), with a more modest lengthening of 4 days at Stykkishöl- mur, which disguises a large gain peaking in the 19405, followed by a moderate shortening. Con- current with a lengthening of the growing sea- son, there has also been a general increase in thermal resources as expressed by the ETS. How- ever, this gain was already realised by the 1930s or 1940 s at all sites, and was followed by a de- cline, although at some sites recent values ofETS have begun to approach those high levels once again (Fig. 4d). On their own, the trend lines analysed above offer only partial information and can be mis- leading, as they: (i) are unable to provide a statistical measure of the strength and significance ofa given trend relative to the underlying variability of the data series; (ii) give undue weight to data points at the be- ginning and end of a series; (iii)can disguise possible temporal cycles and other non-linearities in the data series. Thus, in order to provide a more objective measure of the changes, each time series was divided into three non-overlapping 35-year pe- riods: 1891-1925, 1926-1960 and 1961-1995. To establish whether growing seasons during the most recent period display characteristics that are significantly different from previous periods, the two earlier periods were each compared with the most recent using a student’s t-test (2-tailed). The null hypothesis assumed no change in the mean between a given 35-year period and the 1961-1995 period, and the test was conducted at three levels ofstatistical significance: P