3.1 Vegetation response to global warming across Europe
NDVI trend analyses over the period 2001–2020 highlight regional vegetation and landcover change as response to climate variability in Europe (Fig. 1). Particularly strong vegetation greening and positive surface reflectance trends can be detected in central, southern, and towards the north-eastern parts of Europe. Western Europe and large parts of France, the Iberian Peninsula, Italy, and the British Islands show weaker but still significant greening. Eastern Europe shows a strongly negative vegetation response to climate variability and an increasing browning trend with significant peaks in south-eastern Ukraine and Russia. The strong greening trends of northern and eastern Europe match the temperature trends of the region during the past 20 years. Due to feedback mechanism of global warming and heat waves, the snow cover over northern Europe is decreasing, which influences the surface reflectance patterns indicating positive greening trends. These are, however, more likely to represent reduced snow cover in spring (Bormann et al. 2018; Dye and Tucker 2003; Zhang et al. 2020). Less and scattered as well as darkened snow cover resulting from particle deposition further amplifies solar heating of snowpacks and contributes to increased snow melt (Flanner et al. 2009). During spring, these interactions can lead to changes in surface reflectance that are not caused by an earlier onset of the phenological phase but by removed or scattered snow cover (Dye and Tucker 2003). Reverse browning trends, on the other hand, can be linked to enhanced fire activity (Eckert et al. 2015; Forkel et al. 2013).
In general, significant warming can be detected in most parts of continental Europe and particularly in the central and north-eastern parts. A strong increase in total precipitation further amplifies vegetation response in north-eastern Europe and large parts across Turkey, Greece, and toward the Black Sea. However, parts of central Europe show continuous greening signals despite increasing temperature and decreasing precipitation trends. Continental eastern Europe is more directly related to drying-up and increasing temperature trends.
Considering European vegetation response to climate change mechanisms on multiannual scales, a monthly, seasonal, and regional variability in trend behaviour can be detected. Particularly positive reflectance trends occur during November and December, which emphasize the strong temperature increase and a shift in snow cover duration in north-eastern continental climate zones (Fig. 2). A very strong warming signal during February further decreases snow cover and enhances the positive spectral reflectance (Fig. 3). The region shows constant greening throughout spring and summer, compared to a significant browning trend in eastern Europe and France and a less significant greening in central Europe. Browning trends increase significantly during September and October, affecting large parts of central-eastern and western Europe. Particularly the western part shows negative vegetation response, which can be traced back to strong negative precipitation trends and an increase in mean temperature from July onwards (Fig. 4). An increase in winter heavy precipitation has been predicted in western Europe for future climate models, particularly affecting the UK and parts of south-western Scandinavia (Chan et al. 2018; Christidis et al. 2021; Ketzler et al. 2021; Whan et al. 2020). The simulated increase in winter precipitation is predominantly visible during December, whereas spring and early summer rainfall decreases significantly. Despite the changes in mean precipitation trends, the annual variability strongly increases, confirming recent results (Chan et al. 2018; Zhang et al. 2021).
3.2 Correlation and attribution of climatic variables to spectral greening
Spatial correlation between temperature and landcover reflectance patterns shows strong significance with increasing temperatures and vegetation response across central and north-eastern Europe (Fig. 5). Precipitation trends are not significant in the north-eastern part and the increasing spectral greening signal is linked to the temperature trends and reduced snow coverage (Fig. 6). Soil moisture correlates positively with enhanced vegetation response but is spatially connected to a significant increase in precipitation. Eastern Europe and southern Russia and Ukraine show significantly positive correlation between precipitation and soil moisture decrease, and increased browning trends. The eastern parts of the Black Sea, however, reveal reverse correlation.
In central Europe, and particularly across Germany and central France, regional warming correlates strongly with increased greening trends. But vegetation response is negatively connected to the drying-up process and a very significant decrease in annual total precipitation stands against the positive spectral greening. Here, the monthly variability shows more positive feedbacks. In the southern parts of Europe, the correlation is regionally diverse. In central and southern Italy and Spain, total precipitation is strongly decreasing, which is not entirely mirrored in the greening trends – probably linked to growing irrigation measures (Cramer et al. 2018; Pool et al. 2021). Soil temperature and evapotranspiration are strongly increasing, and soil moisture and canopy surface water show negative trends, particularly affecting the infiltration rate and the deeper soil layers (see supplementary data to this article for trend analyses of GLDAS variables).
3.3 Trends in European climate variability and extremes
Recently, the discussion about cause and effect of the local and regional anthropogenic overprint on the ecosystem’s functionalities, fuelled by global climate change feedbacks, has been reinforced by the severe gradient of continental European flooding and drought spells (Ciais et al. 2005; Harris 2010; Herrmann and Hutchinson 2005; Lin et al. 2020; Zahradníček et al. 2015). Particularly the severe drought episodes of 2003, 2010, and between 2018 and 2020, followed by dramatic flooding events across Europe in summer 2021, have entered the political and economic debate (Brun et al. 2020; Büntgen et al. 2021; Cramer et al. 2018; Dirmeyer et al. 2021; Hari et al. 2020; Kahle et al. 2022; Kempf and Glaser 2020). Temperatures are constantly increasing across Europe and particularly over terrestrial surfaces. According to the 2021 IPCC report, the year 2020 has been marked the warmest on instrumental record (Masson-Delmotte et al. 2021), which contributes to the enhanced temperature trend of the observation period presented in this article. Compared to the IPCC, the past two decades show stronger regional trend variability, e.g. across the Iberian Peninsula, that demonstrates less positive temperature trends than over the reference period 1960–2020 used by the IPCC (Masson-Delmotte et al. 2021). Temperature anomalies, on the other hand, are less variable here with a tendency towards more hot year anomalies compared to colder events (Fig. 7). Precipitation anomalies, however, increase in both, positive and negative annual sums, and subsequent hot and dry year periods are expected to occur more frequently by the mid of the 21st century (Gampe et al. 2021; Vogel et al. 2021).
The strongest occurrence in positive temperature anomalies can be detected across Turkey, central France, south-western Germany, and extensive parts of Belarus, Ukraine, and Russia. In addition to this, negative temperature anomalies are lacking, which underlines the trend towards prolonged hot drought periods and strongly increasing annual average temperatures – affecting tree growth and enhance increased die-back (Del Martinez Castillo et al. 2022). Across northern Russia, both, positive and negative temperature anomalies are frequent and negative anomalies prevail in the Balkans, which strengthen the risk of crop failure during periods of either hot droughts or intensified cold waves (Piticar et al. 2018; Vogel et al. 2019). Regional temperature extremes are furthermore accompanied by growing negative precipitation anomalies, particularly across central Europe, the Adriatic, north-western parts of the Iberian Peninsula, and the Alpine region. Where no negative anomalies prevail, a trend towards lower precipitation rates poses a particular threat to regional croplands and the water balance of the forests and meadows. Across central Europe, NDVI anomalies show strongly negative and only partly positive occurrences, which stand against the rising spectral greening trend over the reference period. Accumulated anomalies are rather attributed to increasing numbers of drought periods and clearly indicate the rapidly rising vulnerability of vegetation response to an increase in extreme weather patterns.