## 4 Discussion
However, they deduced that the wetting zone (high latitudes of Northern Hemisphere) is likely to disappear when the temporary global warming hiatus ended. In other words, they attributed this wetting trend to the slowdown of the increase in temperature. However, contrary to the prevailing view of a warming hiatus after 1998 that occurred in both the global and hemispheric scale [59], our results showed that increasing trends in the growing season temperatures of Northern Hemisphere during this time period were even more statistically significant than those before. This discrepancy indicated that the reversed trend (decreasing trend) in non-growing season temperature may have overweighed the increasing trend in growing season temperature and eventually resulted in slowing down of rising in annual mean temperature in the Northern Hemisphere. However, such assumption needs to be further validated by future work. In addition, the increase in growing season temperature may be related to the snow-albedo feedback because a rapid rise in temperatures will accelerate the melting of snow, which in turn will reduce the surface albedo in high-altitude areas of the northern hemisphere, resulting in enhanced absorption of solar radiation on the land surface and further increasing the temperature [60]. Beyond that, the accelerated melting of the snow and ice may be another important reason (besides the increase in precipitation) of why the areas with wetting trends are located mainly in the high latitudes. In addition, drought events primarily affect the productivity of the vegetation. However, an interesting phenomenon is that the significantly increasing trend in production before 1998 (referred to by the GPP and NDVI) was slowed or even reversed during the 1999-2015 period in a great part of the northern hemisphere, while the drought events obviously decreased. [PERSON] et al. [61] tried to explain the slowing down or even the decreased in vegetation production during this period in terms of an increased atmospheric vapor pressure deficit, which describes the difference between the water vapor pressure at saturation and the actual water vapor pressure for a given temperature, and plays a critical role in determining plant photosynthesis. Those authors concluded that the increased vapor pressure deficit had offset the positive CO\({}_{2}\) fertilization effect in 1999-2015 and eventually resulted in persistent and widespread decreases in vegetation productivity. However, our results revealed that the mismatch between the distribution of highly drought-vulnerable areas and significant decreases in the areas of drought may also be partly responsible for this discrepancy. Regarding the method, we suggest that the drought indices should preferentially include the dynamics of temperature in the context of global warming. Therefore, we selected SPEI, which combines the sensitivity of the PDSI with changes in the evaporation demand (mainly caused by temperature fluctuations and trends) and the multitemporal nature of the SPI, to define the drought events. In this context, our findings of a significant wetting trend within the growing season in the northern hemisphere should be more consistent with the real-life situation. Additionally, compared with the change in drought as characterized by area [62], our study used the drought duration, intensity, and frequency, which could comprehensively express the occurrence of drought within a time interval and had better applicability in the severity analysis method. Despite these achievements, this study still faced two limitations. First, because of the limited availability of data, we did not show the temporal and spatial variations in PET, which were also important to fully understand the climatic drivers in regional drought. Second, we used only CRU-TS temperature data to reveal changes in temperature within the growing season in vegetation areas of the northern hemisphere.