Snow: Exploration And Evaluation Of The Normalized Difference Vegetation Index Changes Of Western Europe

In an attempt to determine terrestrial vegetation growth levels for the region of Western Europe between the years 1982-1999 a study was conducted. The study was intended to discern whether or not greening or browning trends had occurred throughout the region as well as potential causes of the vegetation growth or decline. Through monitoring and observations of the normalized difference vegetation index (NDVI) patterns of the region throughout the entirety of the time span, we were able to produce a series of figures and graphs that were conducive to evaluating vegetation trends. Subsequently, a comprehensive literature exploration was undertaken to identify causal factors of the particular vegetation trends for the region. It was discovered, through the implementation of the NDVI analysis of the region, that greening had taken place over the time period as the mean NDVI had increased. In addition, evidence was found that supported the scientific notion that increased temperature, snow cover extent, growing season length, and land-use changes were influential factors behind the observed greening trends through initiating climatic changes within the region. As a result, a direct correlation was established between increased the NDVI and greening trends originating from various drivers that altered climate conditions for Western Europe. Further research may be pursued in an attempt to determine the extent of global impacts that develop from various regional climate and vegetation changes.

Introduction

Many facets of modern society and natural ecosystems rely on vegetation as it is effectively the link between the soil, water, and atmosphere in addition to being a defining component of earth system energy and material exchanges (Ma and Frank, 2006). As such, a multitude of studies and research projects have been undertaken to identify vegetation trends in various regions across the world and were found to vary by year and location. Patterns in terrestrial vegetation activity were found to often be the result of climatic drivers that, once analyzed, aided researchers in determining discernible causal factors and evaluating their regional effects. However, after assessing the ecological impacts of human land-use change projects, human activities were also determined to have varying effects on terrestrial vegetation changes (Fonge et al., 2018). Positive and negative changes, respectively referred to as greening and browning, are of particular interest as they are often indicative of ecosystem changes (Pausas and Millán, 2019). Past vegetation growth trends act as indicators for future biospheric activity. As such, determining relationships between causes, rooted in climatic and anthropogenic influential elements, and effects on plant growth is a critical component of understanding long term environmental ramifications that will ultimately affect aspects of the natural world and human civilization. This study aimed to identify and evaluate factors of vegetation growth patterns through the NDVI analysis of Western Europe over an eighteen-year time span. In order to gain valuable insight regarding long term regional, and potentially global impacts, stemming from vegetation activity changes through a short time span the normalized difference vegetation index was monitored through a time series to recognize trends (Jong et al., 2011). Observing the NDVI and reviewing relevant scientific literature for the region through a specific time period allowed for vegetation changes to be detected and potential acting processes of the region to be assessed (Colombo, 2012).

Methodology

In an attempt to best identify potential greening and browning trends throughout the time period in Western Europe the mean NDVI was monitored. The objective was to observe terrestrial vegetation growth patterns. Through the implementation of Rscript and analysis of the data set produced from remote sensing, sources figures were created in order to determine net vegetation growth or decline. As such, the study averaged the NDVI over the time scale and generated visual representations that illustrated, through the incorporation of thematic color gradients to aid in pinpointing the particular NDVI levels of areas throughout the region, the net increase or decrease of the NDVI (Forkel et al., 2013). Furthermore, the study attempted a statistical methods approach as it produced a line graph aimed at establishing a correlation between overall increased mean NDVI and greening as well as to account for potential inter-annual variability (Forkel et al., 2013). The mean NDVI per year was calculated by adding up the NDVI values acquired for each day throughout each year and subsequently dividing them by the number of days in the year to attain an average value. Additionally, a pixelated regression of the NDVI slope was constructed for the entirety of the region. The slope was computed within the script through the utilization of the slope regression formula applied to satellite images of Western Europe in order to generate a per-pixel regression over multiple rasters of yearly NDVI data. This approach was undertaken in order to effectively illustrate the NDVI slope increase or decrease over the time series as well as to detect hotspots of greening or browning.

Results

After the completion of the scientific study dedicated to exploring the changes of the normalized difference vegetation index throughout Western Europe, it was confirmed that greening had occurred throughout the region between the years 1982-1999. Figures produced during the study allowed for visual representations of the region’s terrestrial greening patterns. The areas of land displaying a high mean NDVI initially in Figure 1, in comparison to the same areas towards the end of the time period in Figure 2, show a marked increase over time. As can best be expressed by Figure 3, throughout the time frame of the study the areas of land consisting of an increased mean NDVI spread throughout the region, implying that causal factors were introduced that led to favorable climates for vegetation growth within a multitude of countries and areas across Western Europe. In addition, Figure 3 illustrates that areas subject to the largest mean NDVI increases, while a majority were within central portions of the region, were primarily countries in the northeast areas of mainland Western Europe. Other countries, also shown by Figure 3, such as Sweden and Finland displayed perceivably no change in their NDVI suggesting that the factors influencing greening patterns were variable for different areas of land. Figure 4 allows for a graphical representation of the analyzed data. The line graph depicts a steady increase in the mean NDVI throughout the entirety of the time period in Western Europe. The increased mean NDVI can also be seen through a slope regression, expressed in Figure 5, as the computed slope indicates substantial greening within the region as well as identifies increased mean NDVI hotspots through a color gradient pixelated image. The results of the study, considered through various figures generated and analyzed from the data set, conclusively indicate that the region of Western Europe was subject to areas of notable greening and overall an increased mean NDVI.

Figure 1. Displays the mean NDVI over the first five years of the data set from 1982-1987 for Western Europe. The image represents the averaged NDVI from the data, allowing for a visual representation of vegetation levels in particular terrestrial areas in the early years of the time period in question.

Figure 2. Depicts the mean NDVI of the last five years of the data set from 1994-1999 for Western Europe. The average NDVI, through a visual portrayal given by the image, illustrates the significant increase in vegetation levels throughout the region towards the end of the time period.

Figure 3. Represents the mean change in NDVI between the years 1982-1987 and 1994-1999 for Western Europe. The image exhibits the difference in the normalized difference vegetation index between the beginning years and the final years so as to effectively provide a visual delineation of the substantial vegetation growth within the region, on average, within the established time period.

Figure 4. Is a line graph that shows the mean NDVI over Western Europe from 1982-1999. The blue line represents the seasonal variation present within the region whilst the red line represents the average NDVI over the years. The graph indicates that vegetation growth within Western Europe increased in a relatively consistent and steady manner throughout the determined time period.

Figure 5. Displays, through the implementation of a pixelated visual, the slope of the normalized difference vegetation index regression for Western Europe between the years 1982-1999. As can be seen by the image, the region of Western Europe has a high slope of NDVI regression implying the terrestrial areas were subject to notable increases in vegetation levels over the years.

Discussion

4.1 Increased Temperature

Western Europe, from the years 1982 through 1999, was shown by the results of this study to have experienced substantial greening across the region. As such, the mean normalized difference vegetation index for the specific region increased. Research has indicated that there was a multitude of factors that led to the greening seen throughout the time period. Climate variability, for instance, is a critical component of vegetation dynamics (Liu et al., 2015). Composed of both temperature and precipitation, climate variability within Western Europe significantly impacted the NDVI throughout the designated time period. Correlations between mean increases in the temperature and precipitation levels, varying in strength as temperature increases were greater (approximately 0.730 °C per year in central Western Europe) and therefore played a more substantial role in greening, had positive effects on NDVI and vegetation growth within the region (Liu et al., 2015). Increased temperatures, specifically occurring around the spring months during the typical vegetation growing season, led to a substantial increase in biospheric activity and biomass formation within central Western Europe. As can be seen in Figure 5, the most substantial increases in the slope of the mean NDVI occurred mainly throughout the central portion of the region. Warmer temperatures not only directly influenced high latitude greening trends in Western Europe, but also suggested that regional changes amplified global responses as positive feedbacks could have been established leading to increased photosynthetic processes, global warming trends, and reductions in permafrost and snow cover (Dye and Tucker, 2003).

4.2 Snow Cover Extent

Snow cover also plays a critical role in determining aspects of climatic variability as it impacts atmospheric temperatures, soil moisture, and surface albedo. In an attempt to identify and produce climatology of European snow cover extent for industry operations and tourism purposes, a study was undertaken by researchers which yielded results indicating a considerable reduction in snow cover in the spring months for the region of land stretching from westernmost mainland Europe to the Ural Mountains (Henderson and Leathers, 2009). The data revealed that snow cover increased until mid-February where it then consistently declined until June, however, a rapid ablation occurred from the end of March to the beginning of April (Henderson and Leathers, 2009). The reduction of snow cover during the spring season decreased surface albedo, initiating a positive feedback loop that resulted in the continued decline in snow cover extent, increased atmospheric temperatures, advancement of the natural tree line and vegetation, and increased mean NDVI. As shown in Figure 3, the region experiencing the greatest increases in mean NDVI (central Western Europe) aligns accordingly with the snow cover extent data within the appropriate time period implying that the significant reduction in snow cover positively influenced vegetation growth within the region. The decrease in snow cover across mainland Western Europe decreased the Earth’s overall surface reflectivity, contributing to significant disruptions in surface temperature regulation systems and hydrological cycles that have the potential for far-reaching global impacts (Trivedi et al., 2007).

4.3 Lengthening of Growing Season

The increase in terrestrial vegetation from 1982 through 1999 was also shown, through additional research aimed at discovering plant growth patterns during spring months within the high northern latitudes, to be influenced by the lengthening of the growing season. More specifically, areas of land such as central Western Europe that lie between 45 °N and 70 °N were subject to notable warming as the amplitude of active seasonal growth extended between a range of four to twelve days (Myneni et al., 1997). In addition, the decline of the growing season was delayed by approximately two to four days, further prolonging seasonal terrestrial vegetation growth (Myneni et al., 1997). The increase in the length of the growing season, and subsequent increase in photosynthetic activity in plants, lent itself to a mean increase in NDVI within the central Western European region. Results found through the completion of this study substantiate the findings as, can be witnessed through the NDVI changes for that particular area over both Figure 1 and Figure 2, prominent vegetation growth took place within the previously mentioned area. Phenological modeling, in conjunction with additional studies conducted, indicated temperature increases were the primary causal factor in seasonal growth length variations (Menzel and Fabian, 1999). Similarly, warmer temperatures leading to increased regional biomass generation through longer growing seasons are effects of induced global warming trends.

4.4 Anthropogenic Drivers

Vegetation changes within the time frame could also have been impacted by human activities. Anthropogenic influences of increased NDVI could have stemmed from land use changes within the region. Researchers, with the intent to track land use management intensity changes across Europe, identified hotspots of the ten percent largest change values that dominated processes of land use change through bouts of agricultural intensification, urbanization, and abandoned cropland (Kuemmerle, Estel and Müller, 2019). Upon completion of the study, it was determined that Poland and the Czech Republic were hotspots as they underwent considerable declines in cropland, causing forest regrowth in the areas and subsequent decrease in albedo (Kuemmerle, Estel and Müller, 2019). Additionally, hotspots of decrease in pasture extent were greatest along the northern boundary and Spain and across the United Kingdom and Ireland (Kuemmerle, Estel and Müller, 2019). Moreover, hotspots of a considerable increase in forestland extent were identified in the northern areas of Spain whilst hotspots of moderate increases in agricultural abandonment were noted across central Western Europe (Moretti et al., 2019). Figure 3 corresponds with the findings of the researchers as marked increases in the mean NDVI throughout the time range occurred within the very same countries and areas of land documented in the literature. The lower albedo associated with reforestation and reductions in pastures and agricultural land increases the surface temperature and is climatically conducive to vegetation growth. The continued feedback impacts of a low albedo do not perpetually maintain regionally exclusive effects as they influence global warming patterns.

Conclusion

In an attempt to explore and evaluate the normalized difference vegetation index changes of Western Europe a study was undertaken. The region was found, through the production and analysis of a series of images derived from running scripts of a given data set, to be subjected to vegetation growth levels over the time period in question. Further research provided insight as it was determined that causal factors such as increased temperatures, declining snow cover extent, the lengthening of growing seasons, and anthropogenic activities greatly influenced the increased NDVI. As a result, between the years of 1982-1999, the region of Western Europe experienced greening.