| Permafrost is the largest terrestrial ecosystem carbon pool in the world and plays an important role in global carbon cycle and climate change.The change of permafrost and its potential release of organic carbon under global warming have been a crucial scientific issue that is related to the trend of global climate change.In this study,17 CMIP5 models were used to estimate the time that correspond to 1.5? and 2? temperature rises.Based on observational data,the simulation performance of the spatial distribution and active layer depth of permafrost over Northern Hemisphere(NH)with CLM4.5 and semi-empirical method was examined.Furthermore,the change of permafrost and potential release of organic carbon emission under 1.5? and 2?warming within 3.8 m below groud level in permafrost region over NH were projected.The main conclusions of this study are as follows:(1)The time corresponding to 1.5? and 2? warming is different in different emission scenarios.The results of multi-model ensemble mean(MME)show that the global mean surface temperature will reach the threshold of 1.5? under three emissions scenarios(RCP2.6 RCP4.5,RCP8.5)in 2027,2026,and 2023,respectively.The global mean surface temperature will not reach to 2? temperature rise in RCP2.6.The global mean surface temperature would reach 2?temperature rise in 2046 and 2037 in RCP4.5 and RCP8.5 scenario,respectively.The climate change features represented by the MME are much larger than the noises among the models,implying that the projected results are reliable.(2)The estimation results of permafrost extent and active layer thickness by CLM4.5 and semi-empirical method are much different.CLM4.5 significantly underestimated the permafrost area over NH,only part of continuous and discontinuous permafrost can be simulated,with an area of 14.63×10~6 km~2.The permafrost extent over NH simulated by Kudryavtsev method and Nelson method is less than the results from International Permafrost Association(IPA)map,with an area of 18.28×10~6 km~2 and 18.48×10~6 km~2,respectively.The active layer thickness simulated by Stefan method,Kudryavtsev method,CLM4.5 are significantly positive related to observations,but different in magnitude.The mean biases of above three methods are 60.27 cm,23.55 cm and 5.49 cm,respectively.Due to the limited soil depth in surface data(3.8 m),the limitations in simulating the deep permafrost area and active layer thickness are still existed.(3)When the global mean surface temperature rises to 1.5? and 2?,the permafrost area over NH estimated by three methods are 13.41×10~6 km~2 and 11.73×10~6 km~2,which would reduce3.52×10~6 km~2(21.99%)and 5.4×10~6 km~2(31.98%)compared to historical period.The south boundary of permafrost would have a northward shift by 1~5 °.Permafrost extent over NH estimated by three methods has obvious uncertainties in the same emission pathway,which ranges from 4.42×10~6 km~2 to 5.85×10~6 km~2.However,the uncertainties of permafrost extent estimated by three methods are less in different emission pathways,which ranges from 0.08×10~6 km~2 to0.69×10~6 km~2.Active layer thickness simulated by three methods would deepen significantly in most regions over NH under global warming.(4)The organic carbon storage in permafrost region within 3.8 m below ground level over NH in historical period ranges from 287.63 to 346.22 Gt.When global mean surface temperature rises to 1.5 ? and 2 ?,the organic carbon released by permafrost would be 21.08~34.62 Gt and22.18~42.99 Gt,respectively.The uncertainties of organic carbon storage and emission in permafrost regions are influenced by the database of soil organic carbon and the simulation of active layer thickness and permafrost extent. |
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