Studying permafrost changes under different (e.g., glacial/interglacial) and changing (e.g., current various scenarios) climates can potentially advance our understanding of permafrost's responses to climate change and further enable informed policy making for mitigating impacts from permafrost changes. Despite existing studies generally focusing on permafrost change during certain periods, here, we have synthetically examined the changes of the Northern Hemisphere near-surface permafrost during the six periods (Last Glacial Maximum (LGM, similar to 21 ka), mid-Holocene (MH, similar to 6 ka), preindustrial (PI, ca 1850), future 1.5 degrees C and 2.0 degrees C global warming periods, and end of the 21st century), using the surface frost index (SFI) model and outputs of six climate models. Simulated climate anomalies plus present-day observed climatology are used to drive the SFI model in this study. This helps correct systematic biases in permafrost change simulations.The results show that multi-model ensemble extent of present-day near-surface permafrost in the Northern Hemisphere agree well with the observations, with an area bias of 0.27x106 km2 in area (1.8% of the total observed area). Minor deviations (1.55x106 km2) in the simulated present-day permafrost extents across the climate models indicate a low inter-model diversity. In response to changes in annual mean surface air temperature of -10.3 +/- 2.3 degrees C (LGM), +0.1 +/- 0.5 degrees C (MH), +2.6 +/- 0.7 degrees C (1.5 degrees C global warming, RCP4.5), +3.6 +/- 1.0 degrees C (2.0 degrees C global warming, RCP4.5), and +5.0 +/- 1.3 degrees C (end of the 21st century, RCP4.5) in present-day permafrost regions relative to the PI, the changes in near-surface permafrost area are +33%+/- 30% (LGM), -13%+/- 6% (MH), -25%+/- 8% (1.5 degrees C warming, RCP4.5), -35% +/- 10% (2.0 degrees C warming, RCP4.5), and -55%+/- 12% (end of the 21st century, RCP4.5), respectively. From the LGM to the future, near-surface permafrost extent substantially decreases, underlining its high sensitivity to climate change and implying its potentially profound impacts in a warming future.

In this study, the period that corresponds to the threshold of a 1.5 degrees C rise (relative to 1861-1880) in surface temperature is validated using a multi-model ensemble mean from 17 global climate models in the Coupled Model Intercomparison Project Phase 5 (CMIP5). On this basis, the changes in permafrost and snow cover in the Northern Hemisphere are investigated under a scenario in which the global surface temperature has risen by 1.5 degrees C, and the uncertainties of the results are further discussed. The results show that the threshold of 1.5 degrees C warming will be reached in 2027, 2026, and 2023 under RCP2.6, RCP4.5, RCP8.5, respectively. When the global average surface temperature rises by 1.5 degrees C, the southern boundary of the permafrost will move 1-3.5 degrees northward (relative to 1986-2005), particularly in the southern Central Siberian Plateau. The permafrost area will be reduced by 3.43 x 10(6) km(2) (21.12%), 3.91 x 10(6) km(2) (24.1%) and 4.15 x 10(6) km(2) (25.55%) relative to 1986-2005 in RCP2.6, RCP4.5 and RCP8.5, respectively. The snow water equivalent will decrease in over half of the regions in the Northern Hemisphere but increase only slightly in the Central Siberian Plateau. The snow water equivalent will decrease significantly (more than 40% relative to 1986-2005) in central North America, western Europe, and northwestern Russia. The permafrost area in the QinghaieTibet Plateau will decrease by 0.15 x 10(6) km(2) (7.28%), 0.18 x 10(6) km(2) (8.74%), and 0.17 x 10(6) km(2) (8.25%), respectively, in RCP2.6, RCP4.5, RCP8.5. The snow water equivalent in winter (DJF) and spring (MAM) over the QinghaieTibet Plateau will decrease by 14.9% and 13.8%, respectively.