Received: 2012-09-17 Accepted: 2013-04-29 Foundation: Academy of Finland, No.251441; The Project of Ministry of Finance, No.GYHY200706005; Kone Founda-
tion (Finland) Author: Samuli Helama (1974–), Ph.D and Adjuct Professor, specialized in time-series analysis of natural records.
Quantifying temporal changes in Tornionjoki river ice breakup dates and spring temperatures in Lapland since 1802
Samuli HELAMA1, Jianmin JIANG2, Johanna KORHONEN3, Jari HOLOPAINEN4, Mauri TIMONEN1
1. Finnish Forest Research Institute, Rovaniemi Northern Unit, 96300, Finland; 2. Training Centre of China Meteorological Administration, Beijing 100081, China; 3. Finnish Environment Institute, Helsinki P.O. Box 140, 00251, Finland; 4. Department of Geosciences and Geography, University of Helsinki, Helsinki P.O. Box 64, 00014, Finland
Abstract: Cryophenological records (i.e. observational series of freeze and breakup dates of ice) are of great importance when assessing the environmental variations in cold regions. Here we employed the extraordinarily long observational records of river ice breakup dates and air temperatures in northern Fennoscandia to examine their interrelations since 1802. Historical observations, along with modern data, comprise the informational setting for this analysis carried out using t-test. Temperature history of April-May season was used as cli-matic counterpart for the breakup timings. Both records (temperature and breakup) showed seven sub-periods during which their local means were distinctly different relative to preced-ing and subsequent sub-periods. The starting and ending years of these sub-periods oc-curred in temporal agreement. The main findings of this study are summarized as follows: (1) the synchrony between the temperature and river ice breakup records ruled out the possibility that the changes would have occurred due to quality of the historical series (i.e. inhomoge-neity problems often linked to historical time-series); (2) the studied records agreed to show lower spring temperatures and later river ice breakups during the 19th century, in comparison to the 20th century conditions, evidencing the prevalence of cooler spring temperatures in the study region, in agreement with the concept of the Little Ice Age (1570–1900) climate in North-West Europe; (3) the most recent sub-period demonstrate the highest spring tem-peratures with concomitantly earliest river ice breakups, showing the relative warmth of the current springtime climate in the study region in the context of the past two centuries; (4) the effects of anthropogenic changes in the river environment (e.g. construction and demolition of dams) during the 20th century should be considered for non-climatic variations in the breakup records; (5) this study emphasizes the importance of multi-centurial (i.e. historical) cryo-phenological information for highly interesting viewpoints of climate and environmental his-tory.
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Keywords: climatic change; historical climatology; Lapland; Little Ice Age; phenology
1 Introduction
River ice hydrology has experienced significant advances over the recent years with im-proved understanding of river ice processes (Morse and Hicks, 2005). Climate is an impor-tant factor influencing temporal variability of the river ice conditions. As a consequence, the recent concerns on the ongoing climatic variations have emphasized the value of cryo-phenology as a topical issue when delving into the natural and anthropogenic river ice proc-esses (Magnuson et al., 2000; Prowse and Beltaos, 2002; Beltaos and Burrell, 2003). In Lapland, Tornionjoki River runs as a border-river between northern Finland and northern Sweden with basin area and mean flow of 40,130 km2 and 378 m3/s, respectively (Kuusisto, 2006). The river ice breakup event has been recorded for Tornionjoki River extraordinarily since the late 17th century (Johansson, 1932). Kajander (1993, 1995) compiled the available documentary information to construct a continuous and homogenous time-series about the timing of the particular event in the river since 1693. Correspondingly, Zachrisson (1989), Kajander (1993) and Korhonen (2005) have shown that the breakup dates correlate nega-tively with the spring temperatures, especially with the monthly and bimonthly mean tem-peratures in April, May and April-May. Also Moberg et al. (2005) compared the river ice breakup series with the mean temperatures for bimonthly season of April-May in Haparanda (the local weather station) and found that the spring temperatures explained 67% of the variability in the river ice breakup record over the period 1860–1999. Moreover, Klingbjer and Moberg (2003) developed a continuous bicentennial air temperature series with monthly means for the same region since 1802. Using the combination of the data, Klingbjer and Moberg (2003) computed that the spring temperatures could explain as much as 58% of the cryophenological variability in Tornionjoki River over the longer period 1802–1999. Similar results were later derived by Korhonen (2005) using slightly extended the study period 1802–2002.
Another issue regarding the long-term changes in cryophenological records is related to their historical trends. Making use of historical freeze and breakup dates of ice on lakes and rivers, Magnuson et al. (2000) adopted 39 records over the Northern Hemisphere, one of them being the breakup record of Tornionjoki River, to conclude that the earliness of the breakup events has in general progressed especially since 1846. In addition, Moberg et al. (2006) used the cryophenological record from Tornionjoki River to conclude that the 20th century experienced warmer springs than the 18th to 19th centuries. However, already Zachrisson (1989) and Lammassaari (1990) speculated that some anthropogenic factors, in-cluding the intensified felling of timber, the drainage of wetlands and the removal of log-driving dams, could have contributed to the earlier start of the spring floods over the last half of the 20th century, with an influence to breakup dates in Tornionjoki River. The extent of the potentially non-climatic long-term influences on the river ice breakup data remains, however, still to be more rigorously evaluated.
The present study was initiated by the desire to detail the climatic component in the river ice variability of Tornionjoki River. Literature review showed that the analyses of tempera-ture and river ice breakup records has so far proceeded through linear correlations, that is, correlating the time-series of spring temperatures and Tornionjoki River ice breakup data
Samuli HELAMA et al.: Quantifying temporal changes in Tornionjoki river ice breakup dates 1071
using Pearson correlation and linear regression (Zachrisson, 1989; Kajander, 1993; Korho-nen, 2005; Klingbjer and Moberg, 2003; Moberg et al., 2005; Loader et al., 2011). Yet other studies have determined the monotonic trend of the Tornionjoki River ice breakup data (Magnuson et al., 2000; Korhonen, 2005). In this study, we aim to take a different viewpoint for detecting the historical changes in the river ice breakup changes and their connection to climate variability. Instead of linear correlations and monotonic trends, we made use of the scanning t-test and coherency detection (Jiang et al., 2002, 2007) to determine episodes of sub-periods that best characterize the records and their variations. The scientific research targets of our study can be classified as follows: (1) we aim to determine the sudden changes both in the temperature record and in the river ice breakup history; (2) taking the advantage of this method, we picture the temporal variations in spring temperatures and river ice breakup dates in more detail and for sudden temporal changes in the records; this approach will enhance the identification of the driving factors for the specified temporal changes, where simultaneous and non-simultaneous change could imply a change of (3) climatic and (4) non-climatic origin behind the described changes in the river ice breakup system, respec-tively; we hypothesize that such comparison could bear the potential to reveal possible arti-ficial inhomogeneities that may be present in the historical series (Klingbjer and Moberg, 2003) or anthropogenic changes in the river environment; (5) we aim to judge whether the ongoing warming of the springtime temperatures in the region (Tuomenvirta, 2004; Holo-painen, 2006; Holopainen et al., 2009) has resulted in an episode of unprecedentedly early breakups in the context of their multi-centennial history. In conclusion, the analyses were aimed to contribute to deeper understanding about the connections between the changes in the river ice breakup and spring temperatures, those of abrupt and longer term in character-istics.
2 Material and methods
2.1 Original time-series
Tornio (Torneå in Swedish) is a town in northern Fennoscandia, south-western Finnish Lap-land (65°51′N; 24°09′E), adjacent to Swedish border and situated in the lower reach of Tornionjoki River (both Torne älv and Torne River have also been used in the literature), the very vicinity of the Gulf of Bothnia (i.e. the northernmost part of the Baltic Sea). River ice breakup dates have been archived in Tornio since the late 17th century and the record is still updated every spring. Kajander (1993, 1995) constructed an extraordinary record of river ice breakup dates for the locality since 1693 by combining cryophenological observations made in Tornio and nearby villages into one series of breakup dates representative of the particular event in Tornio. Original documents came from archives of several historical and living persons, historical newspaper articles, institutional phenology diaries and data preserved due to local breakup time guessing competitions (Kajander, 1995). No attempt to estimate statis-tical reliability of the record was made (Kajander, 1993, 1995) but it has up to date been as-sumed that the veracity of this cryophenological record increases towards the late 20th cen-tury due to increasing number of independent sources of information. Here, we use the up-dated record of Kajander (1995) as continuous integer numbers where each year is associ-
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ated with the data that indicates how many days since April 1st the river ice breakup oc-curred that year (Table 1). Table 1 Descriptive statistics for cryophenology (number of days since April 1st) (Kajander 1993, 1995) and spring (April-May) temperature (℃) (Klingbjer and Moberg, 2003) series over different periods discussed in the text. Shown here are the number of observations (N), maximum (Max) and minimum (Min) values, the total range (R) of the observations, the mean (M) and its standard error (SE), standard deviation (SD) and variance (Var).
While the present work was greatly benefited by the extreme length of the cryo-
phenological observations, the comparisons were further complemented by the instrumental temperature observations made in parts of northern Fennoscandia already since 1802 (Holopainen, 2006). Using the available multi-station climatic dataset, Klingbjer and Mo-berg (2003) developed a continuous air temperature series with monthly means 1802–2002 for Tornedalen (approx. 66ºN; 24ºE). This historical data originated from four sites including Haapakylänsaari in Övertorneå (1802–1838), Rian in Kalix (1832–1856), Tureholm in Övertorneå (1856–1862) and Haparanda meteorological station (1859–2002). In their analyses, Klingbjer and Moberg (2003) first estimated the local monthly mean temperatures corresponding to 24 h averages at each site, translated the local mean temperatures from Kalix and Övertorneå to values representative for Haparanda, and, third, applied rigorous statistical homogeneity tests for their new composite series. According to Klingbjer and Moberg (2003), the extended temperature series they constructed is to be considered suffi-ciently homogenous. Accordingly, we adopted their record here as a highly reliable estimate of temperature variations in the region (Table 1).
2.2 Methodology
An algorithm to detect the significant changes in the records of climate and cryophenology was applied to data. This was the scanning t-test (Jiang et al., 2002, 2007) where the statistic (n, j) was defined to measure differences of sub-period averages between the adjoining sub-periods (i.e. time-window notably shorter than the full length of the record) with equal temporal length (n) as
1/2 2 2 1/22 1 2 1( , ) ( ) ( )j j j s jt n j x x n s s (1)
where
1
1
1( )
j
ji j n
x x in
, 1
2
1( )
j n
ji j
x x in
;
1
2 21 1
1( ( ) )
1
j
j ji j n
s x i xn
,
12 22 2
1( ( ) )
1
j n
j ji j
s x i xn
(2)
Samuli HELAMA et al.: Quantifying temporal changes in Tornionjoki river ice breakup dates 1073
where x is either the cryophenology (number of days since April 1st) or and spring (April-May) temperature (℃), and the sub-period size n may range as n=2, 3…<N/2, whereas the j=n+1, n+2…N–n+1 is a reference point at which an abrupt change is to be tested on the time-scale n in the series having total number of N observations. Thus the
statistic t(n, j) not only has the capacity to detect multi-scale abrupt changes but it also pro-vides a test of statistical significance for these changes. Natural time-series are however of-ten markedly autocorrelated. As a correction for temporal dependence in the series, we ap-plied the “Table-Look-Up Test” (von Storch and Zwiers, 1999). That is, the critical t-value was adjusted for every sub-period based on the pooled sample lag–1 autocorrelation coeffi-cient and sub-period size. The sub-period length was simply the length of the time window for comparisons between two episode means.
Then the ratio tr (n,j) = t (n, j) / t0.05 (3)
is taken as the index of the critical t-test for scanning detection. It is easier to understand that
an abrupt change is significant at = 0.05 when |tr(n, j)|>1.0, where tr(n, j)<1.0 denotes an abrupt decrease, and tr (n, j)>1.0 indicates an abrupt increasing change. Then the significant decrease and increase change points may be found at the local minimum and maximum cen-tres in |tr(n, j)|>1.0.
Finally, the statistic
1
2( , ) sign ( , ) ( , ) ( , ) ( , )rc ru rv ru rvt n j t n j t n j t n j t n j (4)
is defined as the index of coherency of significant changes between two series u and v. Usu-ally, when the statistic trc(n, j)>1.0 with both |tru(n, j)| and |trv(n, j|>1.0 the two series have
abrupt changes in the same direction, while if trc(n, j)>1.0 the two series have abrupt changes in opposite directions. Similarly, the local maximum and minimum centres denote the coherency points in same and opposite direction, respectively.
3 Results
The cryophenological variations were compared in the following sections to mean tempera-tures of April-May season. The series of two types were found to alter in synchrony regard-ing the determined sub-period starting and ending years (Figures 1a and 1b). The coherency between the series is markedly negative (Figure 1c). The indication of significant changes between the 1880s and 1920s on time scales longer than 64 years suggested significant long-term changes in both series between these dates: the spring temperatures warmed and the river ice breakups occurred earlier.
A closer look at the temporal points of significant changes in the river ice breakup record (Figure 2a and Table 2a) and spring temperature series (Figure 2b and Table 2b) specified that the temporal points of abrupt change occurring in 1821, 1845 and 1955 were exactly the same in both series. In addition, the series showed further synchrony as the years of abrupt change 1886, 1918 and 1980 in the river ice breakup variability corresponded to 1889, 1919 and 1974 in the spring temperature series.
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Figure 1 Contours of the scanning t-test: (a) for the river ice breakup dates; (b) spring temperatures; and (c) coherency between (a) and (b)
Figure 2 Observed variability in river ice breakup dates (number of days since April 1st) and spring tempera-tures (dashed gray line), their sub-period averages (black line) and the long-term mean (black horizontal dashed line)
Samuli HELAMA et al.: Quantifying temporal changes in Tornionjoki river ice breakup dates 1075
Table 2 Characterizing the transient changes in the cryophenology (a) (number of days since April 1st) and (b) spring (April-May) temperature (℃) series over different periods discussed in the text. Shown here are the first year of the sub-period, its length (in years) and the mean value over the sub-period.
(a) Year Length Mean (b) Year Length Mean
1802 19 49.8 1802 19 0.85
1821 24 42.8 1821 24 2.14
1845 41 47.1 1845 44 0.96
1886 31 41.4 1889 30 1.73
1918 37 38.7 1919 36 2.58
1955 25 41.4 1955 19 2.04
1980 22 37.0 1974 28 3.15
4 Discussion
4.1 Climatic connections
Climatic control of the river ice breakup history in Tornionjoki River has previously been determined by correlation analyses. Previous studies have demonstrated strong and negative correlations between the breakup dates and the mean temperatures of April-May season (Kajander, 1993; Klingbjer and Moberg, 2003; Korhonen, 2005; Moberg et al., 2005). This negative correlation, indicating that warmer (cooler) springs occurred in association with earlier (delayed) river ice breakup, has been calculated over several sub-periods and tem-perature data from different weather stations or their combinations. Notwithstanding these alterations in the frameworks of the previous analyses, the correlation has remained mark-edly negative and therefore meaningful regardless of the period. According to Klingbjer and Moberg (2003), who calculated the correlation between spring temperatures and breakup dates using 31-year moving window since 1802, the coefficient has varied between –0.4 and –0.9 depending on the interval. Later, Korhonen (2005) made use of the same dataset to calculate moving correlations with 20-year window and derived very similar results. These studies also concluded that the correlations appeared strongest during the first half of the 20th century and weakest during the earliest part of the record (Klingbjer and Moberg, 2003; Korhonen, 2005). Our calculations expanded these analyses using an analysis of transient changes.
In shorter term context (1964–2000), the river ice thickness is known to increase in Tornio every fall on an average from the beginning of November progressively until the end of March, after which the river ice thinning proceeds relatively rapidly towards the breakup (Korhonen, 2005). Apart from direct melting influence on the river ice, the breakup depends also indirectly on the snow-melt around the drainage basin in association with concomitant spring high flow that is suggested to contribute to breakup. Discharge has been measured for Tornionjoki River since 1911. It could be concluded that the annual low flow occurs during the winter months but the high flow typically during the last weeks of May or in the begin-ning of June, and it is noteworthy that the high flow dates correlate with the coefficient of 0.44 with the breakup dates since the beginning of the record (Korhonen, 2007).
4.2 The early 18th century changes
Historical climatic time-series are comprised of observations made by enthusiastic contem-
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poraries. Nevertheless, the historical series are often subject of criticisms due to potential inhomogeneities representing artificial signals (Datsenko et al., 2002; Moberg et al., 2003; Holopainen, 2006). Potential inhomogeneity problems associated with the both series have been discussed earlier by Zachrisson (1989), Kajander (1993, 1995) and Klingbjer and Mo-berg (2003). A certain feature that could be characterized as a series inhomogeneity (i.e. ar-tificial change due for example to change in observer) is an abrupt shift in one record not in correspondence with change in other record. Here, the abrupt changes were determined us-ing the statistical test for two independently observed series. However, no major discrepan-cies between the starting and ending years of the test based episodes could be described. Generally, it has been previously assumed that it is the early period that could most likely be associated with the inhomogeneity problems (Kajander, 1995; Klingbjer and Moberg, 2003). This could be implied as the early 19th century showed relatively low Pearson correlations between the temperature and breakup date series (Klingbjer and Moberg, 2003; Korhonen, 2005). However, we found that the early 19th century was characterized by significant changes that occurred exactly during the very same years, 1821 and 1845, in both series (Figure 2 and Table 2). This synchrony implied that the particular changes were not due to artificial problems in the series. Regarding even earlier river ice breakup data, our approach was unable to make any conclusion about the potential veracity problems prior to 1802 due to lack of overlapping temperature records.
4.3 Little Ice Age and modern warming
In this study, we have augmented the aforementioned correlation analyses and made use of a statistical test previously proven to provide a powerful tool in detecting the transient changes in the variations of different climatic and palaeoclimatic records (Jiang et al., 2002, 2007). Importantly, a pervasive character revealed by the present analyses was the synchrony of the climatic episodes, determined by the specified sub-period means, in the air temperature and cryophenological series. In particular, we were able to quantify the cryophenology based view that was set by Korhonen (2005) and Moberg et al. (2006), that a shift from generally colder to warmer conditions occurred in the late 19th century, in the context of our test that detected coherent and statistically significant trend-like changes in both series between the 1880s and the1920s. Interestingly, this interval and the direction of observed climatic change were both consistent with the potential transition from the climatic conditions of the Little Ice Age (Grove, 1988; Matthews and Briffa, 2005) to the 20th century warming. The cooling of the Little Ice Age seemed to culminate during the period 1570–1900 during which the Northern Hemisphere summer temperatures dropped significantly below the mean of period 1961–1990 (Matthews and Briffa, 2005). In this regard, Crowley (2000) has referred to the pre-1850 period as pre-anthropogenic era owing to the relative absence of greenhouse gas forcing and due to governance of natural factors affecting the climatic fluctuations. Previ-ously, Tuomenvirta (2004) analyzed a set of Finnish meteorological series and concluded that there has been a significant increase in the Finnish annual and spring (March through May) mean temperatures during the last 150 years. Similar conclusion was also drawn by Holopainen (2006) and Holopainen et al. (2009) who analyzed these variations in southern Finland since 1750 using historical and modern temperature observations along with tem-perature-sensitive geological and geophysical time-series. Interestingly, the spring tempera-
Samuli HELAMA et al.: Quantifying temporal changes in Tornionjoki river ice breakup dates 1077
ture increase was found being more pronounced in Finland than that of annual mean tem-peratures (Tuomenvirta, 2004). In our study, the most recent sub-period detected by our test was warmer than any of the antecedent sub-periods. Moreover, the most recent cryo-phenological sub-period showed earlier dates than any other sub-period since 1802 (Figure 2 and Table 2).
4.4 Progressive earliness of springs
Observed synchrony between the climatic and cryophenological changes appears a firm conclusion due to previously presented correlations and the presently shown test of transient changes. However, there may be more speculation about the long-term trends in the two se-ries. According to Hyvärinen (2003), a linear trend explained 22.5% of the total variability in the breakup dates of Tornionjoki River between the years 1693 and 2001. Similarly, Kor-honen (2005, 2006) calculated, using the cryophenological data from Tornionjoki River over the period 1693–2002, that the linear trend over this period would expect the breakup timing to become 4.3 days earlier per each century. Parallel results for Tornionjoki River were de-rived by Magnuson et al. (2002).
Although our analyses did not aim to quantify the long-term linear or monotone trends, the results could bear implications for these discussions, especially regarding the potential non-climatic factors influencing the trend. For example, a total of 162 log-driving dams is known to be constructed to the Tornionjoki river-system during the 20th century; however, after the cessation of the log-driving in the early 1970s majority of the dams were demol-ished and the watercourses have been renovated (Lammassaari, 1990). It has accordingly been suggested that the dams may have delayed the spring high flow (Zachrisson, 1989; Lammassaari, 1990). Interestingly, the results evidenced a largest temporal discrepancy be-tween the episode changes during the demolition of the dams as it was observed that the transient spring temperature change in 1974 was followed by the corresponding change in the breakups no earlier than in 1980 (Figure 2 and Table 2). The watercourses renovation could thus be suggested playing a role behind the temporal discrepancy. We note that such discrepancy could also bias the interpretation of cryophenological record as climate data (Loader et al., 2011). Our results thus follow the recommnedation that climate change and river ice regime research should consider anthropogenic impacts (Takács et al., 2013).
Albeit the recent cryophenological changes have likely been affected also by non-climatic factors, this would not change the general conclusion that the 20th and 21st century data for Tornionjoki River indeed indicates warmer spring temperatures in relation to the 19th cen-tury (Moberg et al., 2006). Moreover, the past decades have undergone markedly warmer springs than any other period during the instrumental era alongside of the unprecedentedly early breakups in the context of their multi-centennial history. Since particularly the spring temperatures have already risen pronouncedly (Tuomenvirta, 2004; Holopainen, 2006; Holopainen et al., 2009) and because they have been predicted to continue to rise during the 21st century (Jylhä et al., 2004), the progressive earliness of the breakups is clearly a serious feature in the annual rhythm of cold rivers, their ecology and the landscape.
5 Conclusions
Historical records of spring climate and cryophenology were analysed to quantify temporal
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changes in April-May temperatures and Tornionjoki river ice breakup dates in Lapland since 1802. Althought the historical series of climate and phenology are regularly subject of criti-cisms due to potential inhomogeneities in the data, we did not identify such bias in the stud-ied records. Instead, the temporal changes quantified here for the early 19th century oc-curred exactly during the very same years, 1821 and 1845, in both series, implying no artifi-cial signals in the records. Subsequently, we detected a shift from colder to warmer condi-tions through the late 19th century, with coherent and statistically significant trend-like changes in both series between the 1880s and the1920s. In our interpretation, this view was highly consistent with the climatic transition from the Little Ice Age to the 20th century warming. The most recent decades have undergone shift towards warmest sub-period mean temperature and earliest river ice breakup timing. In general, this finding agreed with the previously published results showing the pronounced warming of the spring temperatures in the region and adjacent areas. Over the same period, however, a markedly delayed cryo-phenological response to spring temperatures was observed. The shift towards warmer spring temperatures in 1974 was followed by a corresponding shirt towards earlier river ice breakups in 1980. We connected these findings to the increased anthropogenic activities within the Tornionjoki river-system, which may have caused the described asynchrony. This showed that the late 20th century (and not the early 19th century) part of the cryo-phenological record may actually contain more artificial non-climatic bias than previously appreciated.
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