Warm Spells Research Articles

Warm spells in Northern Europe in relation to atmospheric circulation

This study describes warm spells in Northern Europe and determines the synoptic situations that cause their occurrence. In this article, a relatively warm day was defined as a day when the maximum temperature exceeded the 95th annual percentile, and a warm spell (WS) was considered to be a sequence of at least five relatively warm days. In the analysed multiannual period and within the investigated area, 24 (Kallax) to 53 (Oslo) WSs were observed. The occurrence of WSs was mainly connected with positive anomalies of sea level pressure and a 500-hPa isobaric surface, displaying the presence of high-pressure systems. This occurrence was also accompanied by positive T850 anomalies.

Calculating Lateral Frost Front Penetration in a Rapidly Retreating Cliff of Fine Sediments

An adapted version of the Stefan equation (CLIFFSE) was tested to predict lateral progression of the frost front into cohesive sediments that form coastal cliffs along the north shore of the maritime estuary and gulf of the St Lawrence River (Quebec, Canada). The equation was adapted to accommodate the influence of cliff erosion on lateral penetration of freezing and thawing into vertical cliff faces. As the cliff erodes, freezing and thawing are initiated from the newly revealed surface. Frost progression and erosion were measured with an automated thermal erosion pin system. Measured observations agreed with predictions from the adapted equation (78 to 99% of the variability explained). Erosion associated with thawing front progression during winter warm spells led to a relative reduction in the frost front depth. Subsequently, progression of the frost front into the cliff contributed to an additional 50 cm of sediment freezing and erosion by the end of the cold season, which was not predicted by the original Stefan equation. Our findings support the hypothesis that multiple warm spells influence the amount of lateral penetration of the frost front in vertical cliffs. Copyright © 2015 John Wiley & Sons, Ltd.

Cold Treatment Breaks Dormancy but Jeopardizes Flower Quality in Camellia japonica L.

Camellia japonica L. is an evergreen shrub whose cultivars are of great ornamental value. In autumn, after flower bud differentiation, dormancy is initiated. As in many other spring flowering woody ornamentals, winter low temperatures promote dormancy release of both flower and vegetative buds. However, warm spells during late autumn and winter can lead to unfulfilled chilling requirements leading to erratic and delayed flowering. We hypothesized that storing plants at no light and low temperature could favor dormancy breaking and lead to early and synchronized flowering in response to forcing conditions in C. japonica ‘Nuccio’s Pearl’. Plants with fully developed floral primordia were stored at dark, 7°C, and RH > 90% for up to 8 weeks. To monitor endodormancy release during the storage, we evaluated the content of abscisic acid (ABA) in flower buds and the expression profiles of five putative genes related to dormancy and cold acclimation metabolism in leaves and flower buds. In addition, the expression of four anthocyanin biosynthesis pathway genes was profiled in flower buds to assess the effect of the treatment on flower pigment biosynthesis. At 0, 4, 6, and 8 weeks of cold treatment, 10 plants were transferred to the greenhouse and forced to flower. Forced plant flower qualities and growth were observed. The ABA content and the expression profiles of two dormancy-related genes (CjARP and CjDEH) suggested that dormancy breaking occurred after 6–8 weeks of cold treatment. Overall, plants treated for 6–8 weeks showed earlier vegetative sprouting, enhanced, and homogeneous flowering with reduced forcing time. Prolonged cold treatments also reduced flower size and longevity, anthocyanin content, and pigment biosynthesis-related gene transcripts. In conclusion, the cold treatment had a promotive effect on dormancy breaking but caused severe drawbacks on flower quality.

Recent changes in Arctic temperature extremes: warm and cold spells during winter and summer

In the Arctic, climate change manifests with the strongest warming trends on the globe, especially in the cold season. It is under debate if climate extremes change similarly strong. Our study provides detailed regional information about two selected temperature extreme indices in the Arctic, namely warm and cold spells in winter and summer. We analyze their temporal evolution and variability from 1979–2013, based on daily station data and ERA-Interim reanalysis. Calculated trends from both datasets suggest a widespread decrease of cold spells in winter and summer of up to −4 days/decade, with regional patches where trends are statistically significant throughout the Arctic. Winter trends are spatially heterogeneous, the reanalysis also shows small areas with statistically significant increases of cold spells throughout Siberia. Calculated changes in warm spells from both datasets are mostly small throughout the Arctic (less than ±1 day/decade) and statistically not significant. Remarkable exceptions are the Lena river basin in winter with a statistically significant decrease of up to −1.5 days/decade and areas in Scandinavia with statistically significant increases of up to 2.5 days/decade in winter and summer (again from both datasets). From the analysis of spell lengths, we find that there are no shifts from longer to shorter spells or vice versa with time, but long cold spells (events lasting for more than 15 days) disappear almost completely after the year 2000. There is a distinct inter-annual and decadal variability in the spells, which hinders the detection of significant trends for all spell categories in all regions.

Climate change as a migration driver from rural and urban Mexico

Studies investigating migration as a response to climate variability have largely focused on rural locations to the exclusion of urban areas. This lack of urban focus is unfortunate given the sheer numbers of urban residents and continuing high levels of urbanization. To begin filling this empirical gap, this study investigates climate change impacts on US-bound migration from rural and urban Mexico, 1986–1999. We employ geostatistical interpolation methods to construct two climate change indices, capturing warm and wet spell duration, based on daily temperature and precipitation readings for 214 weather stations across Mexico. In combination with detailed migration histories obtained from the Mexican Migration Project, we model the influence of climate change on household-level migration from 68 rural and 49 urban municipalities. Results from multilevel event-history models reveal that a temperature warming and excessive precipitation significantly increased international migration during the study period. However, climate change impacts on international migration is only observed for rural areas. Interactions reveal a causal pathway in which temperature (but not precipitation) influences migration patterns through employment in the agricultural sector. As such, climate-related international migration may decline with continued urbanization and the resulting reductions in direct dependence of households on rural agriculture.

Time-dependent spatial distribution of thermal stresses in the ice cover of a small reservoir

Static ice loads (ice actions) are a key design parameter for dams in cold climates. However, their theoretical description is still elusive, introducing uncertainty in design and hindering development of remediation measures. We present and analyze measurements of stresses due to thermal loads in a small reservoir in northern Norway. Several weeks of observations, including both cold and warm spells, were well-described by a simple equation that accounts for thermal expansion and temperature-dependent creep. One model parameter was found to depend systematically on the location of measurements within the reservoir. Biaxial stress measurements showed that the stress field was not homogeneous. Results suggest that the stress field in reservoirs should be predictable from first principles with numerical methods and point toward a promising, simple parameterization.

Changes in extreme temperature and precipitation events in the Loess Plateau (China) during 1960–2013 under global warming

In recent decades, extreme climatic events have been a major issue worldwide. Regional assessments on various climates and geographic regions are needed for understanding uncertainties in extreme events' responses to global warming. The objective of this study was to assess the annual and decadal trends in 12 extreme temperature and 10 extreme precipitation indices in terms of intensity, frequency, and duration over the Loess Plateau during 1960–2013. The results indicated that the regionally averaged trends in temperature extremes were consistent with global warming. The occurrence of warm extremes, including summer days (SU), tropical nights (TR), warm days (TX90), and nights (TN90) and a warm spell duration indicator (WSDI), increased by 2.76 (P<0.01), 1.24 (P<0.01), 2.60 (P=0.0003), 3.41 (P<0.01), and 0.68 (P=0.0041) days/decade during the period of 1960–2013, particularly, sharp increases in these indices occurred in 1985–2000. Over the same period, the occurrence of cold extremes, including frost days (FD), ice days (ID), cold days (TX10) and nights (TN10), and a cold spell duration indicator (CSDI) exhibited decreases of −3.22 (P<0.01), −2.21 (P=0.0028), −2.71 (P=0.0028), −4.31 (P<0.01), and −0.69 (P=0.0951) days/decade, respectively. Moreover, extreme warm events in most regions tended to increase while cold indices tended to decrease in the Loess Plateau, but the trend magnitudes of cold extremes were greater than those of warm extremes. The growing season (GSL) in the Loess Plateau was lengthened at a rate of 3.16days/decade (P<0.01). Diurnal temperature range (DTR) declined at a rate of −0.06°C /decade (P=0.0931). Regarding the precipitation indices, the annual total precipitation (PRCPTOT) showed no obvious trends (P=0.7828). The regionally averaged daily rainfall intensity (SDII) exhibited significant decreases (−0.14mm/day/decade, P=0.0158), whereas consecutive dry days (CDD) significantly increased (1.96days/decade, P=0.0001) during 1960–2013. Most of stations with significant changes in SDII and CDD occurred in central and southeastern Loess Plateau. However, the changes in days of erosive rainfall, heavy rain, rainstorm, maximum 5-day precipitation, and very-wet-day and extremely wet-day precipitation were not significant. Large-scale atmospheric circulation indices, such as the Western Pacific Subtropical High Intensity Index (WPSHII) and Arctic Oscillation (AO), strongly influences warm/cold extremes and contributes significantly to climate changes in the Loess Plateau. The enhanced geopotential height over the Eurasian continent and increase in water vapor divergence in the rainy season have contributed to the changes of the rapid warming and consecutive drying in the Loess Plateau.

Undocumented migration in response to climate change.

In the face of climate change induced economic uncertainty, households may employ migration as an adaptation strategy to diversify their livelihood portfolio through remittances. However, it is unclear whether such climate migration will be documented or undocumented. In this study we combine detailed migration histories with daily temperature and precipitation information for 214 weather stations to investigate whether climate change more strongly impacts undocumented or documented migration from 68 rural Mexican municipalities to the U.S. during the years 1986-1999. We employ two measures of climate change, the warm spell duration index (WSDI) and the precipitation during extremely wet days (R99PTOT). Results from multi-level event-history models demonstrate that climate-related international migration from rural Mexico was predominantly undocumented. We conclude that programs to facilitate climate change adaptation in rural Mexico may be more effective in reducing undocumented border crossings than increased border fortification.

A statistical analysis of trends for warm and cold spells in uppsala by means of counts

Statistical studies of extremes are of interest in the climatic sciences, in particular trends of periods of unusually warm or cold weather, which could be labelled warm and cold spells, respectively. We study the yearly number of spells in Uppsala, Sweden which from a data‐analytic point of view truly are counts, and employ theory and methods from the field of regression models for counts. A possible trend for the period 1840–2012 was investigated. The trend for warm spells is positive and demonstrated to be larger in magnitude compared with the one for cold spells, and is found to be statistically significant. The methodology could be extended to analyse other climate indicators.

Nitrogen leaching is enhanced after a winter warm spell but mainly controlled by vegetation composition in temperate zone mesocosms

Leaching of nitrogen (N) from natural ecosystems poses serious environmental problems. Extreme events such as winter warm spells could exacerbate N leaching by disrupting biogeochemical cycles and will become more frequent with the projected climate change. We used lysimeters to investigate N leaching in response to a 12-day winter warm spell at two field sites with contrasting winter climate and in seven different vegetation covers in 140 well established mesocosms. Mean rates of N leaching reached 3.4 ± 0.4 mg N m−2 d−1 over the 49 days of observations in late winter/ early spring. The winter warm spell resulted in an 82 % increase of N leaching after the warm spell (up to 18 mg N l−1). Stronger leaching occurred at the colder site which can be explained by plants becoming photosynthetically active with subsequent frost damaging their above-ground tissue due to missing insulation by snow and/or dehardening, resulting in reduced N uptake. N leaching differed by >600 % among contrasting vegetation composition with almost no leaching from grassland types and strongest leaching from shrubland types that even surpassed leaching from bare ground controls. Winter warm spells can affect the biogeochemistry of temperate ecosystems with plant performance and vegetation composition controlling the amount of N leaching.

Analysis of extreme temperature events in the Qinling Mountains and surrounding area during 1960–2012

In this study, 16 indices of extreme temperature were calculated on the basis of daily maximum and minimum temperature data in the northern and southern regions of the Qinling Mountains (NSQ) using linear trend and correlation analysis and other methods to investigate the temporal variation trend, spatial distribution pattern and correlation of extreme temperature events. The results are as follows. (1) The extreme cold and warm indices exhibited contrasting variation trends over the entire region: the cold indices (TX10, TN10, ID, FD and CSDI) decreased; the warm indices (TX90, TN90, SU, TR and WSDI) increased; the extremal indices (TXn, TNn, TXx and TXn) increased; and the diurnal temperature range (DTR) decreased. (2) The absolute indices, extremal indices and other indices showed a certain latitudinal zonality: ice days (ID) and frost days (FD) decreased from north to south, while summer days (SU), tropical nights (TR), extremal indices (TXn, TNn, TXx and TXn), the cold and warm spell duration indicators (CSDI and WSDI, respectively) and growing season length (GSL) increased from north to south. (3) The correlation analysis results showed that, except for the extremal indices and individual indices, the extreme temperature indices correlated well. (4) The altitude has a large impact on spatial distribution of extreme temperature indices, and the ubiquity of the heat island effect in urban constructed regions also had an impact on amplitude of variation in extreme temperature.

Linkages between Quaternary climate change and sedimentary processes in Hala Lake, northern Tibetan Plateau, China

Profundal lake sediment cores are often interpreted in line with diverse and detailed sedimentological processes to infer paleoenvironmental conditions. The effects of frozen lake surfaces on terrigenous sediment deposition and how climate changes on the Tibetan Plateau are reflected in these lakes, however, is seldom discussed. A lake sediment core from Hala Lake (590km2), northeastern Tibetan Plateau spanning the time interval from the Last Glacial Maximum to the present was investigated using high-resolution grain-size composition of lacustrine deposits. Seismic analysis along a north–south profile across the lake was used to infer the sedimentary setting within the lake basin. Periods of freezing and melting processes on the lake surface were identified by MODIS (MOD10A1) satellite data. End-member modeling of the grain size distribution allowed the discrimination between lacustrine, eolian and fluvial sediments. The dominant clay sedimentation (slack water type) during the global Last Glacial Maximum (LGM) reflects ice interceptions in long cold periods, in contrast to abundant eolian input during abrupt cold events. Therefore, fluvial and slack water sedimentation processes can indicate changes in the local paleoclimate during periods of the lake being frozen, when eolian input was minor. Inferred warm (i.e., ∼22.7 and 19.5cal.ka BP) and cold (i.e., ∼11–9 and 3–1.5cal.ka BP) spells have significant environmental impacts, not only in the regional realm, but they are also coherent with global-scale climate events. The eolian input generally follows the trend of the mid-latitude westerly wind dynamics in winter, contributing medium-sized sand to the lake center, deposited within the ice cover during icing and melting phases. Enhanced input was dominant during the Younger Dryas, Heinrich Event 1 and at around 8.2ka, equivalent to the well-known events of the North Atlantic realm.

Recent Changes of Some Observed Climate Extreme Events in Kano

Observed rainfall and temperature data for the period 1960–2007 were used to examine recent changes of extreme climate over Kano, located in the Sahelian region of Nigeria. The RClimDex software package was employed to generate nine important climate indices as defined by the Expert Team on Climate Change Detection, Monitoring and Indices (ETCCDMI). For the entire period, the results show a warming trend, an increased number of cool nights, more warm days, and a strong increase in the number of warm spells. The rainfall indices show a slight increase in annual total rainfall, a decrease in the maximum number of consecutive wet days, and a significant increase in the number of extremely wet days. Such changes in climate may result in an increasing demand for domestic energy for cooling and a higher evaporation rate from water bodies and irrigated crop. These findings may give some guidance to politicians and planners in how to best cope with these extreme weather and climate events.

Recent changes in Georgia׳s temperature means and extremes: Annual and seasonal trends between 1961 and 2010

Sixteen temperature minimum and maximum series are used to quantify annual and seasonal changes in temperature means and extremes over Georgia (Southern Caucasus) during the period 1961 and 2010. Along with trends in mean minimum and maximum temperature, eight indices are selected from the list of climate extreme indices as defined by the Expert Team on Climate Change Detection and Indices (ETCCDI) of the Commission for Climatology of the World Meteorological Organization (WMO), for studying trends in temperature extremes. Between the analysis periods 1961–2010, 1971–2010 and 1981–2010 pronounced warming trends are determined for all Georgia-averaged trends in temperature means and extremes, while all magnitudes of trends increase towards the most recent period. During 1981 and 2010, significant warming trends for annual minimum and maximum temperature at a rate of 0.39°C (0.47°C) days/decade and particularly for the warm temperature extremes, summer days, warm days and nights and the warm spell duration index are evident, whereas warm extremes show larger trends than cold extremes. The most pronounced trends are determined for summer days 6.2 days/decade, while the warm spell duration index indicates an increase in the occurrence of warm spells by 5.4 days/decade during 1981 and 2010. In the comparison of seasonal changes in temperature means and extremes, the largest magnitudes of warming trends can be observed for temperature maximum in summer and temperature minimum in fall. Between 1981 and 2010, summer maximum temperature shows a significant warming at a rate of 0.84°C/decade, increasing almost twice as fast as its annual trend (0.47°C/decade). The Georgia-averaged trends for temperature minimum in fall increase by 0.59°C/decade. Strongest significant trends in temperature extremes are identified during 1981 and 2010 for warm nights (4.6 days/decade) in summer and fall as well as for warm days (5.6 days/decade) in summer. Analyses demonstrate that there have been increasing warming trends since the 1960s, particularly for warm extremes during summer and fall season, accompanied by a constant warming of temperature means in Georgia.

A boreal invasion in response to climate change? Range shifts and community effects in the borderland between forest and tundra

It has been hypothesized that climate warming will allow southern species to advance north and invade northern ecosystems. We review the changes in the Swedish mammal and bird community in boreal forest and alpine tundra since the nineteenth century, as well as suggested drivers of change. Observed changes include (1) range expansion and increased abundance in southern birds, ungulates, and carnivores; (2) range contraction and decline in northern birds and carnivores; and (3) abundance decline or periodically disrupted dynamics in cyclic populations of small and medium-sized mammals and birds. The first warm spell, 1930–1960, stands out as a period of substantial faunal change. However, in addition to climate warming, suggested drivers of change include land use and other anthropogenic factors. We hypothesize all these drivers interacted, primarily favoring southern generalists. Future research should aim to distinguish between effects of climate and land-use change in boreal and tundra ecosystems.Electronic supplementary materialThe online version of this article (doi:10.1007/s13280-014-0606-8) contains supplementary material, which is available to authorized users.

Cold De-acclimation Tolerance among Polish Accessions of Common Wheat and Triticale

Cold de-acclimation is a phenomenon the frequency of which is expected to increase due to unstable winter temperatures predicted in the future [1] The aim of the present work was to screen for cold-de-acclimation tolerance the breeding materials of common wheat and triticale, and to find out whether this trait may be, at least partially, responsible for their winter survival.The experiment was performed during winter 2013/14 with 44 accessions of triticale and 78 accessions of wheat. Winter hardiness was studied in seven locations. The studies of freezing tolerance/de-acclimation were conducted at two experimental points where the plants were tested for the freezing tolerance before and after a warm spell during winter. Freezing tolerance on the cellular level was additionally assessed by means of chlorophyll fluorescence measurements after freezing.Our results indicated that the studied accessions of both species were highly variable in their tolerance to de-acclimation. Although no correlation between the freezing tolerance and de-acclimation tolerance was observed, all de-acclimation tolerant accessions were also highly freezing-tolerant. The maximum level of de-acclimation tolerance observed in triticale was higher than in wheat. It was also found that de-acclimation tolerance was responsible for different winter survival of triticale plants in one out of seven field locations, while in five locations no differences in winter survival were observed. In the case of common wheat, de-acclimation tolerance was not related to winter survival at any of three locations in which it varied between the studied materials.Similar observations, together with the studies of other risk factors for winter survival during future winters, will be continued for the next four winters to create models of future winter hardiness risk for wheat and triticale and to select plants with higher resistance to these factors.Founding: the Ministry of Agriculture and Rural Development (Poland) grant HOR hn 801-10/14.

Dealing with Increased Variability of Spring Temperatures in Temperate Deciduous Orchard Crops

Productivity of temperate deciduous crops is potentially more vulnerable to climate change than many annual crops because temperate deciduous crops flower and set crops at the beginning of the growing season when temperatures vary greatly. The fruit development periods (time between bloom and harvest) of many fruit tree species (peach and nectarine (Prunus persica), plum (Prunus salicina), prune (Prunus domestica), sweet cherry (Prunus avium), almond (Prunus dulcis), pear (Pyrus communis)) are inversely related to accumulated heat during the first month after bloom – the higher the accumulated heat, the shorter the fruit development period (Fig. 1). The fruit development period of a cultivar can vary among years by as much as 30 days and most of the effects on development occur during the early stages of fruit development. Thus, spring temperatures can have significant effects on fruit sizes and/or tree yields because, during warm springs, rates of fruit growth cannot keep up with rates of fruit development when spring temperatures are excessively high. In recent years spring temperature variability appears to be increasing. While historic mean growing degree hour (GDH) accumulations in the month after peach tree bloom in California were around 6000 GDH, recently 3 years have registered accumulations over 8000 GDH and one year was above 9000 GDH. This caused early harvests and problems with fruit size. A harvest prediction model has been developed to help growers cope with these variable weather events (http://fruitsandnuts.ucdavis.edu/Weather_Services/Harvest_Prediction__About_Growing_Degree_Hours/). In addition, prolonged temperatures > 27o C occurring during bloom have also negatively affected fruit set of prunes (Prunus domestica) and sweet cherries (Prunus avium). California prune production is especially vulnerable because it is dependent a single cultivar. Breeding efforts are under way to spread the bloom period and decrease the risk of sporadic warm spells during spring (Table 1).

Metabolite changes in conifer buds and needles during forced bud break in Norway spruce (Picea abies) and European silver fir (Abies alba).

Environmental changes such as early spring and warm spells induce bud burst and photosynthetic processes in cold-acclimated coniferous trees and consequently, cellular metabolism in overwintering needles and buds. The purpose of the study was to examine metabolism in conifers under forced deacclimation (artificially induced spring) by exposing shoots of Picea abies (boreal species) and Abies alba (temperate species) to a greenhouse environment (22°C, 16/8 h D/N cycle) over a 9 weeks period. Each week, we scored bud opening and collected samples for GC/MS–based metabolite profiling. We detected a total of 169 assigned metabolites and 80 identified metabolites, comprising compounds such as mono- and disaccharides, Krebs cycle acids, amino acids, polyols, phenolics, and phosphorylated structures. Untargeted multivariate statistical analysis based on PCA and cluster analysis segregated samples by species, tissue type, and stage of tissue deacclimations. Similar patterns of metabolic regulation in both species were observed in buds (amino acids, Krebs cycle acids) and needles (hexoses, pentoses, and Krebs cycle acids). Based on correlation of bud opening score with compound levels, distinct metabolites could be associated with bud and shoot development, including amino acids, sugars, and acids with known osmolyte function, and secondary metabolites. This study has shed light on how elevated temperature affects metabolism in buds and needles of conifer species during the deacclimation phase, and contributes to the discussion about how phenological characters in conifers may respond to future global warming.

Trends and variability of daily temperature extremes during 1960–2012 in the Yangtze River Basin, China

The variability of surface air temperature extremes has been the focus of attention during the past several decades, and may exert a great influence on the global hydrologic cycle and energy balance through thermal forcing. Based on daily minimum (TN) and maximum temperature (TX) observed by the China Meteorological Administration at 143 meteorological stations in the Yangtze River Basin (YRB), a suite of temperature indices recommended by the Expert Team on Climate Change Detection and Indices, with a primary focus on extreme events, were computed and analyzed for the period of 1960–2012 for this area. The results show widespread significant changes in all temperature indices associated with warming in the YRB during 1960–2012. On the whole, cold-related indices, i.e., cold nights, cold days, frost days, icing days and cold spell duration index significantly decreased by −3.45, −1.03, −3.04, −0.42 and −1.6days/decade, respectively. In contrast, warm-related indices such as warm nights, warm days, summer days, tropical nights and warm spell duration index significantly increased by 2.95, 1.71, 2.16, 1.05 and 0.73days/decade. Minimum TN, maximum TN, minimum TX and maximum TX increased significantly by 0.42, 0.18, 0.19 and 0.14°C/decade. Because of a faster increase in minimum temperature than maximum temperature, the diurnal temperature range (DTR) exhibited a significant decreasing trend of −0.09°C/decade for the whole YRB during 1960–2012. However, the decreasing trends all occurred in 1960–1985, while increasing trends though insignificant were found in all sub-regions and the whole YRB during 1986–2012. Geographically, stations in the eastern Tibet Plateau and northeastern YRB showed stronger trends in almost all temperature indices. Time series analysis indicated that the YRB was dominated by a general cooling trend before the mid-1980s, but a warming trend afterwards. In general, the overall warming in the YRB was mainly due to the warming in 1986–2012. Strong relationships between temperature trends and elevation were detected in this study. The warming rates increased with elevation when elevation is above 350m, but decreased with elevation when elevation is below 350m.

Warmer and wetter winters: characteristics and implications of an extreme weather event in the High Arctic

One predicted consequence of global warming is an increased frequency of extreme weather events, such as heat waves, droughts, or heavy rainfalls. In parts of the Arctic, extreme warm spells and heavy rain-on-snow (ROS) events in winter are already more frequent. How these weather events impact snow-pack and permafrost characteristics is rarely documented empirically, and the implications for wildlife and society are hence far from understood. Here we characterize and document the effects of an extreme warm spell and ROS event that occurred in High Arctic Svalbard in January–February 2012, during the polar night. In this normally cold semi-desert environment, we recorded above-zero temperatures (up to 7 °C) across the entire archipelago and record-breaking precipitation, with up to 98 mm rainfall in one day (return period of >500 years prior to this event) and 272 mm over the two-week long warm spell. These precipitation amounts are equivalent to 25 and 70% respectively of the mean annual total precipitation. The extreme event caused significant increase in permafrost temperatures down to at least 5 m depth, induced slush avalanches with resultant damage to infrastructure, and left a significant ground-ice cover (∼5–20 cm thick basal ice). The ground-ice not only affected inhabitants by closing roads and airports as well as reducing mobility and thereby tourism income, but it also led to high starvation-induced mortality in all monitored populations of the wild reindeer by blocking access to the winter food source. Based on empirical-statistical downscaling of global climate models run under the moderate RCP4.5 emission scenario, we predict strong future warming with average mid-winter temperatures even approaching 0 °C, suggesting increased frequency of ROS. This will have far-reaching implications for Arctic ecosystems and societies through the changes in snow-pack and permafrost properties.