## 4 Discussion  
### Climate Change and Its Impacts  
The results of this study indicate that precipitation (PRE), temperature (TEM), and ETa in the MRYR have fluctuating upward trends, and PET has experienced fluctuating downward trends since the beginning of the 21 st century. Overall, the degree of aridity in the MRYR has weakened, and the climate state has shifted in the direction of warming and humidification. The trend of humidification in Northwest China has expanded significantly eastward since the beginning of this century [58; 60]; the eastern part of Northwest China has also begun to enter a period of increased PRE [61], and its area of influence includes \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**LUCC (2001–2022)** & **Area/km\({}^{2}\)** & \begin{tabular}{c} **Percentage of** \\ **Change** \\ \end{tabular} & \
\begin{tabular}{c} **Cumulative** \\ **Percentage** \\ \end{tabular} &
\begin{tabular}{c} **Change in NEP/Tg** \\ **(1 Tg = 10\({}^{2}\) g)** \\ \end{tabular} \\ \hline Cropland–Grassland & 21,592.86 & 31.91\% & 31.91\% & 3.25 \\ Grassland–Cropland & 18,064.26 & 26.69\% & 58.60\% & 5.15 \\ Grassland–Forest & 10,166.11 & 15.02\% & 73.63\% & 4.54 \\ Cropland–Impervious & 5306.00 & 7.84\% & 81.47\% & –0.40 \\ Cropland–Forest & 3824.11 & 5.65\% & 87.12\% & 1.49 \\ Barren–Grassland & 3340.92 & 4.94\% & 92.05\% & 0.754 \\ Forest–Cropland & 1407.19 & 2.08\% & 94.13\% & 0.096 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Major land type transformations and their impact on NEP. Figure 12: (a) Land transfer distribution map, 2001–2022; (b) land transfer flow map, 2001–2022. most of the MRYR. There are two views on the change in PET in Northwest China: one is that evaporation is decreasing, i.e., the \"evaporation paradox\", which is contrary to the possibility that global warming may lead to an increase in evaporation [62; 63]; the other is that evaporation shifted from a decrease to an increase in the 1990s [64; 65]. The results of the PET study in this paper are similar to the first view. The increase in ETa is most likely caused by the greening of large areas of vegetation in the study area under ecological measures. In general, vegetation greening increases evaporation in an area, which enhances the water vapor cycle in that area and results in increased PRE in the downwind area [66]. In the context of warming, winters in the middle and high latitudes will be milder [67], and freeze-thaw phenomena will become more widespread and frequent [68]. The rate of soil CO2 emission increases during thawing [69], thus further exacerbating the greenhouse effect. The soil respiration in the study area was relatively strong during the initial phase of the freeze-thaw (fall-winter alternation, November), eventually declined to a more stable level (winter freeze), and then soil respiration increased again (winter-spring alternation, March) (Figure 13a). The R\({}_{\rm H}\) in winter, spring, summer, and fall accounted for 7.04%, 22.96%, 46.87%, and 23.13% of the total, respectively. Since the NPP is an annual dataset, we analyzed the monthly changes in the GPP from 2001 to 2022 (Figure 13a). There were many months when the GPP was lower than the R\({}_{\rm H}\) in winter, suggesting that the MRYR is dominated by carbon sources in winter. It can be seen that the changes in soil respiration and carbon sinks in the MRYR were mainly concentrated in summer, which had higher temperatures and more concentrated PRE (Figure 13b). The soil respiration fell sharply at temperatures below freezing and rose sharply at temperatures above freezing [70]. The same finding was observed in this study, where the R\({}_{\rm H}\) was significantly higher at temperatures \(>0\)\({}^{\circ}\)C than at temperatures \(\leq 0\)\({}^{\circ}\)C under very low PRE in winter (Figure 13c). Overwinter temperature changes (including the frequency of freeze-thaw and snowmelt processes) are more pronounced in open agricultural land compared to forest or grassland soils [71].