3.3. The projection of MAGT and ALT
In view of a strong statistical rule of MAGT and ALT in climatic factors
(e.g., TDD and FDD) and topographic factors (e.g., Lon, Lat, and Ele),
most studies have begun to use similar statistical methods to
investigate the present and future development trends of the periglacial
climate realm (Koven et al., 2013; Aalto et al., 2017, 2018; Zhang et
al., 2019). In this study, the optimal fitting model for the present
state was employed to forecast MAGT and ALT under different future
climate scenarios. For ALT, the spatial domain was limited to the area
with simulated MAGT ≤ 0°C during each associated period and/or RCP
scenario.
Due to climate change, the permafrost temperature exhibits an obvious
rising trend on the QTP. We simulated the future change of permafrost on
the QTP after half a century. The results revealed that the future
changes of MAGT and ALT are predicted to be pronounced, but
region-specific (Figure 6). The forecasted average MAGT over the QTP
permafrost regions will increase from -1.35°C (present status) to
-0.66°C by 2061−2080 (RCP2.6) and to 0.25°C for RCP8.5 (Table 2). The
ALT, however, was only predicted to increase from 2.3 m (2000−2015) to
2.7 m (2061−2080) for RCP8.5. The
reason for the consistency or small change of the ALT is that, the
section of the permafrost with a MAGT near 0°C is forecasted to degrade
to seasonally frozen ground, and this part of the permafrost usually
corresponds to a thicker active layer. Additionally, the uncertainties
related to the forecasts of MAGT and ALT under different RCPs in the
future were given. And, the uncertainties are characterized by the range
of MAGT value and ALT value. As can be seen in Figure 7, even under the
different RCPs scenarios, the fluctuation range of MAGT and ALT is
basically the consistent.
Over the next half century, the near-surface permafrost areas are
predicted to continue to decrease by 0.13 × 106km2 (12%), 0.42 × 106km2 (40%) and 0.60 × 106km2 (58%) on the QTP by 2070 (2061−2080), under the
RCP2.6, RCP4.5 and RCP8.5 scenarios, respectively. The result is
basically consistent with the projected change by Chang et al. (2018)
(Figure 8). Permafrost is in non-equilibrium under the influence of
climate change, and there may be no permafrost that is driven by the
current climate. In fact, it may be that permafrost is degrading, so the
distribution range of the simulation results may be underestimated (Zhao
et al., 2019). The changes in MAGT and ALT are not only related to the
changes in temperature and precipitation but also closely related to
hydrothermal parameters and surface energy balance (Guo and Wang, 2016;
Hu et al., 2019). Based on the existing observation data and improved
soil physics, the estimated changes in previous studies are generally
larger than that of actual change (Lawrence et al., 2012; Cheng et al.,
2019; Wang et al., 2019b).
4. Discussion
In order to project the possible future changes of permafrost, we
simulated MAGT and ALT changes under the present state and future
scenarios based on statistical and ML methods. The results show that
under different RCPs, significant degradation of the QTP permafrost may
occur (e.g., MAGT rising and ALT thickening); in particular, under
RCP8.5, more than half of the near-surface permafrost will disappear,
and regional differences were observed. In this section, to further
verify the feasibility of our results, we compared our simulated MAGT
and ALT with those of previous studies and then analyzed the
vulnerability of permafrost to climate change under the present state.
Based on these findings, we proposed urgent action should be taken to
adapt climate change. Finally, the model performance and potential
sources of the uncertainty in this study were discussed.