## 4 Discussion  
Temperature, precipitation, and PET are three key variables of the SPEI that can greatly influence the variations in drought. Temperatures have increased significantly around the world in recent decades, and the predominant explanations given have been the increasing use of fossil fuels and the increase in greenhouse gas emissions caused by anthropogenic activities [(43; 44)]. In this context, recent work suggests that global aridity has increased in step with the observed warming trends and that this drying will aggravate for many regions as the continuous rise in temperature further increases evapotranspiration [(7; 18)]. However, in contrast to the general expectation that a warmer climate will bring about an increase in PET, several studies have demonstrated that the PET has shown a steady downward trend over the last 18-20 years, both globally [(45)] and regionally in many areas of the northern hemisphere, such as in northern China [(46)], India [(47)], Italy [(48)], and Canada [(49)]. This is known as the \"evaporation paradox\" phenomenon [(50)]. Recently, several studies have attempted to explain what causes this paradoxical phenomenon. Some authors have concluded that under the backdrop of climate change, a weakening of PET resulted from the combined effects of sunshine hours, wind speed, solar radiation, and relative humidity overcomes the increase in PET induced by the temperature elevation and eventually results in increased PET in some regions [(51)]. It is important to note that a regional study in China emphasized that the rising temperatures contribute to a very limited extent to the variation in PET compared with other climate factors [(52)]. Our study results supported the view that using only temperature as input data to estimate the variation in PET, such as by using the Thorthwaite method [(53)], will overestimate the effects of global warming and eventually result in producing incorrect drought trends [(1; 5)]. This may partially explain why recently published studies have generally produced apparently conflicting results regarding to drought trends in the context of climate change. In addition, a great deal of research has indicated that human-influenced global warming not only affects evapotranspiration but may also be partly responsible for increases in heavy precipitation [(54; 55)]. Our results revealed a significant increase in precipitation in the vegetation zone of the northern hemisphere (Figure 3a) and showed that the areas with a wetting trend in 1999-2015 were located mainly in the high and mid-latitudes (Figure 4), which is consistent with the result by [PERSON] et al. [(56)]. That study also attributed the significant increases in precipitation in the mid-latitudes of the northern hemisphere to anthropogenic forcing [(56)]. The wetting trend in high latitudes was also identified by [PERSON] et al. [(57)], but they contributed this wetting trend (especially for Eurasian Arctic regions) to the enhancement of poleward atmospheric moisture transport. In parallel, [PERSON] et al. [(58)] investigated the drought characteristics variations globally based on SPI and also revealed that the drought frequency decreased significantly in Northern Hemisphere in recent decades. Contrary to our results, however, they concluded that the drought duration and intensity have changed insignificantly. The main reason for this discrepancy was that the SPI calculation was based only on precipitation data, and the effects of temperature and PET on drought were overlooked. [PERSON] et al. [(59)] used AI and  
Figure 10: Spatial patterns of drought sensitivity of vegetation during (**a**) 1982–1999 and (**b**) 2000–2015. Significance of correlations was assessed at \(p<0.05\). (**c**) showed the proportion of high drought sensitivity areas (significantly positive correlation) at each biome. Palmer Drought Severity Index (PDSI) to study the response of drought to the recent global warming hiatus in Northern Hemisphere and also revealed a wetting trend over the mid-to-high latitudes.