Has there been a decline in global and regional snow cover?

Snow cover affects the global surface energy balance and, with its high albedo, exerts a cooling effect on the Earth’s climate. Decreases in snow cover alter the flow of solar energy from being reflected away from Earth to being absorbed, increasing the Earth’s surface temperature. To gain a global understanding of snow cover change, in situ measurements are too few and far between, so remotely sensed data are needed. This research used the medium-resolution sensor MODIS on the Terra satellite, which has been observing global snow cover almost daily since the year 2000. Here, the MOD10C2 eight-day maximum value composite time series data from February 2000 to March 2023 were analyzed to detect global and regional trends in snow cover extent for the first 23 years of the 21st century. Trends in snow cover change during different time periods (seasons and snow-year) were examined using the Mann—Kendall test and the univariate differencing analysis. Both methods produced similar results. Globally, snow cover declined two to ten times as much as it increased, depending on the season of analysis, and annually, global snow cover decreased 5.12% (not including Antarctica or Greenland) based on the Mann—Kendall test at the 95th percentile (p < 0.05). Regionally, Asia had the greatest net area decline in snow cover, followed by Europe. Although North America has the second-largest extent of snow cover, it had the least amount of net decreasing snow cover relative to its size. South America had the greatest local decline in snow cover, decreasing 20.60% of its annual (snow-year) snow cover area. The Australia–New Zealand region, with just 0.34% of the global snow cover, was the only region to have a net increase in snow cover, increasing 3.61% of its annual snow cover area.Snow and ice influences the global surface energy balance and hydrologic cycle as well as modifying feedbacks that control these aspects of the world’s climate [1]. When snow cover melts, less solar radiation reflects to space and more of this energy is absorbed by the Earth, and this snow cover feedback is a probable contributor to the polar amplification, which is further warming Arctic regions [2–5]. Various studies have explored the relationship between snow and ice cover, Earth’s albedo, and surface energy [6,7], showing that when snow cover declines, albedo decreases and the Earth’s surface warms [8–10]. Snow also provides a critical short-term water storage mechanism for many people around the world [11]. Snow cover extent (SCE) has been declining in many parts of the world [8,12,13]. These observed changes in SCE are a direct response to climate change, mainly from warming temperatures and changes in precipitation [14–17]. Climate projections indicate that in the future, SCE will continue shrinking [14], enhancing the snow cover feedback and leading to warmer temperatures [12,15]. Even though SCE globally has decreased in the last four decades, there has been considerable inter-annual variability [18]. Anomalously cold periods and large snowfalls in recent winters have been experienced in North America, Asia, and Europe [19], leading to increasing SCE for some areas [20]. Indications are that the quick warming of the Arctic is associated with changes in atmospheric circulation [21,22] and may be responsible for these anomalous events and areas of increasing SCE.Monitoring SCE change in many parts of the world is difficult due to the lack of local observing networks, snow cover’s spatial variability due to local conditions and other physiographic characteristics, frequent cloud cover, and confusion between lake ice and snow cover during the melt season [12]. In situ data provides detailed local data but remains extremely sparse across the globe. With satellite imaging, snow cover recognition is becoming more precise. There are now multiple satellite-based sensors, from thermal and microwave sensors to visible light sensors, that can capture snow cover. This research uses data from the Moderate Resolution Imaging Spectroradiometer (MODIS), which observes snow surface properties using solar illumination (visible and infrared wavelengths) in cloud-free periods and has been shown to be excellent for mapping SCE and duration [23,24]. The MODIS snow products have been validated by many studies [25–29], such as Hall and Riggs (2007), who found an overall absolute accuracy of the base 500 m resolution data to be about 93% [24]. This study uses MODIS data because it now has a greater than 23-year time span of consistent global-scale snow data, which have been used in numerous snow cover studies [8,10,16,20,25,30]. The MOD10C2 data set was used because it is a consistent data set, which minimizes cloud cover contamination, has over 23 years of reliable data, has been used in many other SCE studies [8,31–33] and has been validated, such as by Lei et al. (2011) who evaluated the snow identification accuracies of the MOD10C2 data to station data in northeast China and found the accuracy to be greater than 88%, with cloud cover being the main problem [34]. Different remote sensing studies have shown that SCE is broadly declining in different regions across the globe. Hammond et al. (2018) mapped global snow zones with MODIS data and found that between 2001 and 2016 for areas of snow cover, 5.8% declined in snow persistence while 1.0% increased in snow persistence. They also found that declining trends were greatest in the winter months [30]. Notarnicola (2020), using MODIS snow products, studied hot spots of snow cover change in mountain regions across the globe between 2000 and 2018 and found that 78% of observed areas were affected by snow decline and a snow cover area decrease of up to 13%, while above 4000 m only negative changes were observed [35]. In another publication, Notarnicola (2022) used a combination of snow cover data sets including MODIS and found that between 1982 and 2020 over global mountain areas an overall negative trend of −3.6% ± 2.7% for yearly SCE [36]. The season most affected by negative trends was winter. While most mountain ranges had negative trends in SCE, like the Alps, some mountain ranges, such as those in the northern highmountain Asia region (Karakorum, Kunlun Shan, Pamir Mountains) had positive trends Because the Northern Hemisphere has the highest percent (98%) of the world’s snow cover between the Arctic and Antarctic circles [30], most snow cover studies have focused on the Northern Hemisphere. Kunkel et al. (2016) studied trends in extreme SCE in the Northern Hemisphere based on satellite observations for 1967 to 2015 and found an overall negative trend in SCE [37]. Using a multi-source remote sensing data set, Wang et al. (2018) found that between 2000 and 2015, the maximum, minimum, and annual average SCE in the Northern Hemisphere exhibited a fluctuating downward trend [38]. Using MODIS and AVHRR data, Hori et al. (2017) found an average decrease in SCE of 10 days/decade in the Northern Hemisphere since 1978 [39]. Using MODIS data, Eythorsson et al. (2019) estimated a decrease in Arctic snow cover frequency of 9.1 days/decade since 2001 [40]. Hernández-Henríquez et al. (2015) examined different latitudes and elevations of SCE declines in the Northern Hemisphere between 1971 and 2014 based on the NOAA snow chart climate data record and found the majority of statistically significant negative trends in the mid- to high latitudes [41]. Brown et al. (2021) analyzed snow cover trends for Canada (1955–2017) using the daily snow-depth-observing network of Environment and Climate Change Canada (ECCC) where results are broadly similar to previously published assessments showing long-term decreases in annual snow cover duration and snow depth over most of Canada [42]. Some large regions of snow cover have stayed relatively stable. Wang et al. (2017) used MODIS data and found no widespread decline in snow cover over on the Tibetan Plateau from 2000 to 2015 [43].Many SCE studies have focused on snow onset dates and snow end dates [15] as well as the Northern Hemisphere’s spring season [12,14,18,44] because with a high sun angle, spring snow in northern Canada, Alaska, and Siberia reflects extensive energy back to space that would otherwise potentially be absorbed and heat the planet further [2]. Shi et al. (2013), using the NOAA weekly snow cover maps, found that late spring/early summer SCE significantly decreased over the Arctic between 1972 and 2006 [45]. Derksen and Brown (2012), using the NOAA snow chart climate data records from April to June, found the Eurasia region set successive records for the lowest June SCE every year from 2008 to 2012 while North America set the June record 3 out of the 5 years (2008-2012) [46]. They also found the rate of loss of June snow cover extent between 1979 and 2011 (−17.8% decade−1 ) is greater than the loss of September sea ice extent (−10.6% decade−1 ) over the same period [8]. Brown and Robinson (2011), using the NOAA weekly SCE dataset, found that Northern Hemisphere spring SCE has undergone significant reductions over the past 90 years and that the rate of decrease has accelerated over the past 40 years. They also found that Eurasia had a significant earlier spring snow melt (March) than North America [14]. Musselman et al. (2021), using station data, found that in western North America (30 years+) snowmelt is increasing during the snow accumulation season [11]. This research differs from previous studies in that it analyzes changes in persistent SCE decline and increase globally and regionally. Although snow cover regions are quickly warming, changes in snow cover vary by region [14,18,21,47] and it is important to study the regional variation in a global context. There are few global scale SCE studies which focus on annual and seasonal areas increasing and decreasing the fastest. This study looks at areas of persistent decline and persistent increases in SCE globally and regionally from 2000 through 2022, and 2023 for the winter season. Most SCE change studies have generally focused on the spring season, when higher snow albedo feedbacks occur [14,46,48], but this study analyzes all four seasons and the snow-year (Northern Hemisphere: September to August of the following year, Southern Hemisphere: March to February of the following year). This study uses the Mann—Kendall test to analyze persistent changes in SCE globally and regionally as well as applying the univariate differencing analysis to determine SCE change between the beginning and end of the study period. The Mann—Kendall test is a analytical tool commonly used to analyze changes in snow cover [8,11,35,40,41] while the univariate differencing analysis for change analysis is also used, but not as much as the Mann—Kendall test [8,49,50]. This research differs from previous studies in that it analyzes changes in persistent SCE decline and increase globally and regionally. Although snow cover regions are quickly warming, changes in snow cover vary by region [14,18,21,47] and it is important to study the regional variation in a global context. There are few global scale SCE studies which focus on annual and seasonal areas increasing and decreasing the fastest. This study looks at areas of persistent decline and persistent increases in SCE globally and regionally from 2000 through 2022, and 2023 for the winter season. Most SCE change studies have generally focused on the spring season, when higher snow albedo feedbacks occur [14,46,48], but this study analyzes all four seasons and the snow-year (Northern Hemisphere: September to August of the following year, Southern Hemisphere: March to February of the following year). This study uses the Mann—Kendall test to analyze persistent changes in SCE globally and regionally as well as applying the univariate differencing analysis to determine SCE change between the beginning and end of the study period. The Mann—Kendall test is a analytical tool commonly used to analyze changes in snow cover [8,11,35,40,41] while the univariate differencing analysis for change analysis is also used, but not as much as the Mann—Kendall test [8,49,50]. The objectives of the study are: (1) to analyze persistent trends in SCE throughout the four different seasons and annually (snow-year) at both the global and regional scales; and (2) to analyze changes in SCE through univariate differencing between the beginning (average of 2000 to 2004) and end (average of 2018 to 2022) of the period. The novelty of this research, in addition to studying the four seasons and snow-year, is that it uses the Z-values of the Mann—Kendall test to show the intensity of significant changes (p < 0.05, 0.01) as well as using the Mann—Kendall test significant values to filter the results of the univariate differencing analysis.Discussion Both the Mann—Kendall test and the univariate differencing analysis showed that the snow-covered portions of the world decreased during the first 23 years of the 21st century. Although some regions of the world have seen increases in SCE over the past 23 years, globally, declines were much greater than increases in all seasons and annually,with declines being more than twice to more than ten times the area of increases for the different seasons. The Mann—Kendall test, with a significance level of 95% (p < 0.05) and a Z-value of greater than 1 (less than −1), showed that the snow-covered portion of the Earth had a net loss of 5,275,456 km2 , or 5.12% of its snow area size, or an area more than nine times the size of France, during the period from 2000 to 2022. Although the time series used for this study is only 23 years long and is not at the length (30 years+) often used to analyze climate influences [70,71], Roessler and Dietz (2022) found that the 23 years of MODIS snow data (2000–2022) are already sufficient to determine significant trends for a considerable part of the observed areas globally [20]. The results of this study clearly show that changes in the 21st century are an extension of the declines in SCE seen in the 20th century [72]. There is documentation that the Northern Hemisphere has experienced SCE declines since the early 1920s [13]. Most of the world’s snow cover exists in the Northern Hemisphere. Large-scale analyses of Northern Hemisphere SCE change have found a decrease in SCE of 1 million km2 (or 4%) for the entire Northern Hemisphere compared to the long-term mean since 1966–2020 [73]. Another study found a global change in snow cover duration for the full hydrologic year (Northern Hemisphere: 1 September to 31 August of the following year; Southern Hemisphere: 1 March 2000 to 28 February of the following year) of −0.44 days/year between 2000 and 2022 [20]. Research into changes in SCE for global mountain ranges between 1982 and 2020 found an overall negative trend of −3.6% ± 2.7% for yearly SCE [36]. Using MODIS snow data from 2000 to 2018, it was found that around 78% of the global mountain areas are undergoing a SCE decline, with some areas declining up to 43 days and a SCE decrease of up to 13% [35]. Although changes in precipitation are noted as a cause of changing SCE, most studies point to warming surface temperatures driving the substantial reduction in the extent and duration of Northern Hemisphere snow cover [74]. These studies reflect the SCE changes found in this research. This study’s regional analysis shows similar comparisons with other SCE change analyses. As noted above, the Asian region had the greatest overall decline of SCE in the world. One of the major global areas in which a decrease in SCE occurred was Siberia, especially during the 03-04-05 (spring) and 06-07-08 (summer) seasons. Notarnicola (2020)noted a strong decline in snow cover mainly located in the southern and central parts of the region in the mountains [35], and Wu et al. (2023) has noted a decline in Siberian snow cover in the spring [75]. In the winter and annually, this research noted that central and eastern Japan has seen a decrease in SCE. Since the late 1980s, snow cover has been decreasing over the Japanese archipelago [76]. Broad areas in Iran saw a decline in SCE, and other research has noted that the Zagros Mountains experience a significant SCE decrease of −20.7% [35]. The Tianshan mountains of western China showed a decline in the central and eastern portions during the 12-01-02 season, and other research has shown that a warming trend in the region has led to declines in SCE in the central and eastern Tianshan mountains [77]. This research also showed an increase in the northern and western Tianshan mountains in the 09-10-11 season, and others have also noted this as an area of increasing SCE [75]. Concerning other areas of Asia increasing in SCE, Smith and Bookhagen (2020) analyzed a time series of high-resolution snow water equivalent data over the Tibetan Plateau from 1986 to 2016 and found positive trends in Karakorum, Hindu Kush, and Kunlun Shan during the winter and summer periods [78]. The research presented here also found winter and summer increases in SCE on the Tibetan Plateau. Multiple studies have shown that there appears to be no major decline in SCE on the Tibetan Plateau [43,79]. In the far northeast of Russia, in the Chukotka region, increases in SCE occurred in the 06-07-08 season, which other researchers have also noted [35]. Between 2000 and 2015, Liu et al. (2017) found an increasing trend in SCE in central Kazakhstan in areas similar to the increasing trends found in this research in the 09-10-11 season and annually [80].The second-largest area of SCE is North America. Unlike Asia, North America did not experience a decline in SCE as intensively as Asia or any of the other regions, except for the Australia–New Zealand region, which saw some net increases. Annually and for every season, except the 03-04-05 season, North America experienced the least local change of any region, and for the 03-04-05 season, only one other region experienced less change (Table 3). At the annual period regions in northern and northwestern Canada, along with multiple regions in Alaska, experienced declines. Research in Alaska has presented similar trends, with a strong decrease in SCE [81]. When averaged across the state, the disappearance of snow in the spring has occurred from 4 to 6 days earlier per decade, and snow return in fall has occurred approximately 2 days later per decade [81]. This change appears to be driven by climate warming rather than a decrease in winter precipitation, with average winter temperatures also increasing by about 2.5 ◦F [81]. Northern Alaska has been experiencing a decline in SCE over the first 17 years of the 21st century [82]. Brown et al. (2021) analyzed snow cover in Canada from 185 stations and found an SCD decrease of −1.68 days/decade in the boreal autumn and an increase of +0.28 days/decade in the boreal spring [42]. This research also noted increases in Canada during the 03-04-05 season. Increased snow north of Lake Superior might be due to lake-effect snowfall, which is increasing around the regions of Lake Michigan and Lake Superior [83]. One of the few contiguous areas of concentrated declines is in northeastern North America. Decreasing snow in the Northeast between 2000 and 2017 has been reported by others [8]. Across New England, there once were numerous local downhill skiing and sledding areas, but without reliable snow cover, many have closed, especially in southern New England [84,85], an area of snow cover that this research shows is quickly disappearing. The third major area of snow cover is Europe, which for every season experienced a greater percent decline of SCE than its percent of global SCE and a greater increase for two seasons (12-01-02, 03-04-05) (Table 3). Europe also had the greatest interannual fluctuation of change (Figure 2), where the predominant winter teleconnection pattern over the Euro-Atlantic region, i.e., North Atlantic Oscillation (NAO) is a driving factor for the interannual variability of European snow cover [86], and the inter-annual variability means some years can deviate tremendously from average snow cover patterns in Great Britain [87]. This research shows that much of continental Europe, especially the central and eastern areas, have lost SCE, particularly during the winter season. Tomczyk, (2021) has noted that lowland areas of central and northern Europe have experienced a reduction in snow cover duration, and central Europe has experienced several mild winters since the 1990s, with the winter of 2019/2020 being extremely warm and snowless [88]. The duration of the snow season in Europe has decreased by up to 25 days in western, northern, and eastern Europe due to earlier spring melt [89]. Significant negative trends are found in the Alps and the Carpathians [36]. For the European Alps, a recent study found that over all stations and all months, 87% of the trends were negative and 13% positive [90]. Concerning areas of increase, there is a prominent increase in SCE in Norway and Sweden during 03-04-05 season, which other research has documented [20]. During the winter season, much of Ireland and western Scotland also indicate an increase in SCE. Occasionally, cold air from Siberia brings unusual winter snow to the British Isles [91]. Excluding Antarctica, most of the SCE in the Southern Hemisphere is restricted to high altitude areas in the Andean region [92], though in the southern region of Patagonia there is also some SCE. The Andes stretch north and south along western South America, and its snowpack is the primary source of water for many communities. Like Europe, the change in SCE in South America experienced some interannual variability, though it was not as extensive as in Europe (Figure 2). The interannual variation in South America is caused by El Niño Southern Oscillation (ENSO) events [92]. Using Landsat images across a north–south transect of approximately 2500 km (18–40◦ S) between 1986 and 2018, it was found that SCE declined across the entire study area at an average rate of about −12% per decade [92]. This research discovered intense areas of decline (2000–2022) in the Andes and Patagonia and, at the annual snow-year level, SCE declined by 20.6%; a similar decadal rate of decline was found between 1986 and 2018 [92]. The greatest seasonal decline occurred in the 09-10-11 (spring) season, with a net decline of 25.08% (Table 3, Figure 6). According to MODIS estimates over the period 2000–2016, the annual SCE shrunk in the Andes by about 13% around latitude 34◦ S [93]. Decreasing snow trends may be partially attributed to increases in surface air temperature [94]; however, precipitation seems to be a main driver [95]. Precipitation changes are being driven by ENSO events [93]. The poleward migration of the westerly winds has led to precipitation drops at Andean mid-latitudes, leading likely to decreasing SCE [96]. Declines in SCE in Patagonia have also been documented [96,97], including a significant decreasing trend of SCE of 19% for Patagonia’s Brunswick Peninsula for a 45-year period (1972–2016), which is attributed to a significant long-term warming of 0.71 ◦C at Punta Arenas during the extended winter (April–September) [98]. The last region of declining SCE was in Africa, which has the lowest percent of global SCE at 0.27% annually and varies between seasons from 0.05% (03-04-05) to 0.32% (12-01-02). As noted above, snow cover in Africa is scattered in many places, with only two areas of relatively continuous snow cover, North Africa and Central Africa. Both regions were found to have a declining SCE (Table 3), though areas of decline were spread out and not strongly spatially clustered (Figure 7). Past research in the Atlas Mountains found no evidence of a significant long-term trend; however, it was discovered that snow cover increased in February–March and decreased in April–May for their research period of 2000–2013 period [99]. There is a dearth of published research into SCE change in snow areas of Africa. Much of the published research is on famous individual mountains like Mt. Kenya and Mt. Kilimanjaro, such as the snow cover area of Mt. Kilimanjaro largely decreased from 10.1 km2 to 2.3 km2 between 1984 and 2011, which corresponds to a 77.2% reduction [100]. The only region to exhibit more increasing SCE than decreasing for a season, or annually, was the Australia–New Zealand region. The Australia–New Zealand region makes up a very small portion of the global SCE, having just 0.34% of the global SCE at the annual level (Table 3). At the annual level, this region increased its SCE by 3.61% (7099 km2 ) and spatially these increases are seen across Tasmania along with the North and South Islands of New Zealand, while prominent declines are found in the mountainous South Island, especially in the central to southwestern regions. Other researchers have found that Australia and New Zealand show an overall increasing SCE, though there were no large areas with significant trends [35]. New Zealand has considerable interannual variability [101] and the increases found here may be due to fluctuations in SCE. According to Planet Ski news [102], both Australia and New Zealand had heavy winter snows in 2022 [102]. A 16-year (2000–2016) time series of daily snow-covered area, derived from MODIS imagery, was used to analyze SCE change for New Zealand’s largest catchment, the Clutha Catchment. In contrast to other regions globally, no significant decrease in snow cover was observed, but substantial spatial and temporal variability was present [101]. In Tasmania, the incidence of snow fluctuation between 1983 and 2013 at Mt Field was shown to have no overall trend [103]. 5. Conclusions This study used global scale snow cover data (MOD10C2) from February 2000 to March of 2023 to analyze how snow cover has changed over the first 23 years of the 21st century. Two methods were used to analyze the data, univariate differencing and the more commonly used Mann—Kendall test. Novel to this research is the use of the Mann—Kendall Z-value (standard deviations of change from the norm) to determine intensity of snow cover change and the use of the Mann—Kendall p-value (p < 0.05) to filter the results of the univariate differencing From this research and that of others, it is clear that the world is quickly losing its snow cover. The first 23 years of the 21st century show that changes in snow cover continue along a path of decline, which has been happening for the past 100 years. During the past 23 years, snow cover has been broadly disappearing around the world, especially in Asia, Europe, and South America, along with areas in North America and Africa. Snow cover in Asia is declining faster than the global average, and snow cover loss is extensive throughout much of the region. Europe and South America are also losing snow cover faster than the global average. North America, the world’s second-largest area of snow cover, is experiencing a slower decline with multiple areas of increase. Despite its slower pace, snow cover is also declining in North America and quickly in the New England region. Snow cover in Africa is not as concentrated as in other parts of the world, but the snow cover here is also declining. The only region with more increasing snow cover than decreasing was the Australia–New Zealand region, but the increases were slight and with the regional variation the future direction of change can go in either direction. With global greenhouse gas emissions continuing at a record pace [104], the trend of decreasing SCE will continue. The next step in this research will be to explore the relationship between land surface temperature and changes in SCE, using the MOD11C3 land surface temperature data with the MOD10C2 snow cover data used in this paper, and potentially also exploring the use of the MOD10A1 data.