## 3 Results
### Changes in TDD, TDI, and DF at the Biome Level  
The mean TDD, TDI, and drought frequency at each biome during the 1982-1998 and 1999-2015 periods were presented in Figure 9. Results showed that the largest decrease in the mean TDD occurred in the tundra (TUN; decreased from 44.7 to 33.1 months) biome, followed by the boreal forests or taiga (BF; 39.3 to 31.2 months), temperate coniferous forests (TCF; 37.9 to 31.9 months), temperate grasslands, savannas, and shrulands (TGSS; 37.6 to 33.9 months), and montane grasslands and shrulands (MGS; 39.8 to 36.6 months) biomes, whereas the deserts and xeric shrulands (DXS), temperate broadleaf and mixed forests (TBMF), and Mediterranean forests, woodlands, and scrub (MFWS) biomes showed no obvious change (Figure 9a). At the same time, except for MFWS, all the biomes presented increased in TDI (Figure 9b). The increased amplitudes of each biome were roughly consistent with the changes in TDD, whereas the largest increase in TDI also occurred in the TUN (\(-\)35.5 to \(-\)25.8) biome, followed by the BF (\(-\)32.0 to \(-\)23.1), TCF (\(-\)29.2 to \(-\)22.8), TGSS (\(-\)27.2 to \(-\)23.5), and MGS (\(-\)29.8 to \(-\)26.3) biomes. As for the changes in mean drought frequency (DF), only the TUN, BF, and TCF biomes exhibited obvious decreases, from 9.9 to 7.9, 9.3 to 7.6, and 8.7 to 7.4, respectively (Figure 9c).  
Figure 7: The spatial distribution of total drought intensity (TDI) of each pixel in Northern Hemisphere during 1982–1998 and 1999–2015. (**a,b**) exhibited the spatial distribution of TDI during 1982–1998 and 1999–2015, respectively; (**c**) showed the spatial patterns of the TDI different between 1982–1998 and 1999–2015. (**d**) represented the frequency distribution of TDI in 1982–1998 and 1999–2015. (**e**) represented the frequency distribution of the TDI different between 1982–1998 and 1999–2015.  
As regard to the interannual variations in mean drought duration and drought intensity during the two study periods among different biomes (Table 1), we found that the annual trend for drought duration had transformed from increasing to decreasing trend in the TBMF, TCF, and TGSS biomes. Among them, the trend in the TCF biome decreased significantly (\(p<0.05\)) by 0.06 months per year during the 1999-2015 period. In the TUN biome, the drought duration exhibited a slight decrease  
Figure 8: The spatial distribution of drought frequency (DF) of each pixel in Northern Hemisphere during 1982–1998 and 1999–2015. (**a,b**) exhibited the spatial distribution of DF during 1982–1998 and 1999–2015, respectively; (**c**) showed the spatial patterns of the DF different between 1982–1998 and 1999–2015. (**d**) represented the frequency distribution of DF in 1982–1998 and 1999–2015. (**e**) represented the frequency distribution of the DF different between 1982–1998 and 1999–2015.  
Figure 9: The average total drought duration (TDD) (**a**), total drought intensity (TDI) (**b**), and drought frequency (DF) (**c**) at each biome during 1982–1998 and 1999–2015.  
before 1998, but it increased more strongly afterward, with an 18 times larger trend. The drought intensity showed nonsignificant trends in the TBMF, TCF, TGSS, MGS, MFWS, DXS, and TUN biomes in 1982-1998, but a significant increase afterward in the TCF (\(p<0.05\)) and TUN (\(p<0.01\)) biomes. In addition, the drought intensity increased significantly (\(p<0.05\)) in the BF biome before 1998, but this trend was subsequently stalled.