# Remote Sensing of Environmental Changes in Cold Regions: Methods, Achievements and Challenges
## 3 Changes and Trends
### Northern High Latitudes  
The long-term (1979-2017) anomalies of the annual non-frozen season derived from the satellite microwave FT observational record show a lengthening non-frozen season trend over vegetated lands (excluding large water bodies and permanent ice/snow covered areas) in the high-northern latitudes (3.30 day decade\({}^{-1}\); \(p\)-value \(<\)0.001), with similar trends over the Northern Hemisphere (3.98 day decade\({}^{-1}\)), Southern Hemisphere (3.61 day decade\({}^{-1}\)), and global domains (3.93 day decade\({}^{-1}\)) as shown in Figure 4.  
The changes in the annual non-frozen season length reflect the overall warming trend in the climate system. The timing and duration of the non-frozen season is an important factor affecting water, carbon, and energy budgets in cold land areas. Recent trends toward earlier and longer non-frozen seasons coincide with global warming and have been shown to be a major driver of northern vegetation greening, active layer deepening and permafrost degradation, enhanced evapotranspiration, earlier snowmelt onset, and associated changes in terrestrial water and energy budgets [21; 34; 149]. For example, satellite observations indicate that the snow end date in spring advanced by 5.11 days from 2001 to 2014 in the high northern latitudes (52-75\({}^{\circ}\)N) [238], along with shorter lake ice cover duration at higher latitudes from 2002 to 2015 [17]. The melting of permafrost ice provides the water supply to thermokarst lakes [239] and leads to surface water expansion detected within continuous and discontinuous permafrost zones [29]. Satellite remote sensing observations have been used to detect changes in warming Arctic-boreal ecosystems. Increasing temperatures and atmospheric CO\({}_{2}\) concentrations can impact the production, dynamics, and composition of vegetation, as well as soil moisture and other soil properties [240]. One of the most pronounced ongoing changes is tundra shrub expansion [203]. Changes to vegetation are likely to occur around vegetation ecotones [203; 240]. Much effort has been given to identifying  
Figure 4: Non-frozen season trend (1979–2017) for the Northern Hemisphere (NH), Southern Hemisphere (SH), high-northern latitudes (HNL), and global domain. The anomalies were calculated as annual differences from the long-term mean. Grid cells dominated by permanent snow/ice cover and large water bodies (open water fraction \(\geq\) 20%) are excluded from the analysis. regions of vegetation production increase (greening) or decrease (browning). A 10-year time series of monthly average AVHRR NDVI indicated an increase in photosynthetic activity in the northern high latitudes from 1981 to 1991 driven by earlier growing season onset [241]. A subsequent study using a longer AVHRR time series indicated an increase in tundra vegetation growth in boreal North America, largely driven by longer growing seasons, but decreasing growth in interior forests [242]. Recent work used time series of Landsat vegetation indices in Arctic sites, with results indicating more areas with increasing growth than decreasing growth [243]. Analysis of a 28-year Landsat NDVI record indicated that greening and browning of Canadian boreal forests were largely driven by disturbances from wild fire and insect damages; and, in forests not affected by disturbance, climate changes were associated with both areas of greening and browning [240]. Regarding phenological changes, a 30-year Landsat record was used to detect an earlier/heterogeneous leaf emergence trend in temperate/boreal deciduous forests [197], while a 33-year AVHRR NDVI time series revealed regional trends toward earlier growing season onset, later growing season end, and longer growing season duration over the high northern latitudes [31].