Temporal And Spatial Variations Characteristics And Future Changes Of CO₂ Exchange Across The Arctic Permafrost Regions

With global warming,temperature increasing not only accelerates ecosystem carbon emission caused by permafrost degradation but also promotes carbon uptake by vegetation growth.However,it is largely unknown about the carbon sink or source of Arctic permafrost regions.Based on the in situ observations from the global carbon flux network and published literatures,the spatial and temporal distribution characteristics of Net Ecosystem Exchange(NEE)and correlation between NEE and the influenced climate variables was systematically analyzed by the curve fitting and the lineal regression.The Random Forest model was used to describe the interaction between NEE and influencing variables,then make the correspond geographic spatial dataset as input data to estimate the situation of NEE in the Arctic region during the latest two decades.The future change of NEE until 2100 was projected by using the different SSPs(Shared Social-economic Pathways)data from CMIP6(the sixth Coupled Model Intercomparison Project).The main study results were as follows:1)During 2002?2017,it showed that the annual NEE is-8.2±4.1 for forest,-4.8±4.1 for grassland,-3.6±3.0 for wetland,-3.3±2.6,0.02 for shrub,0.62 g CO₂ m^-2d^-1 for tundra.In the entire Arctic permafrost region,the interannual slope of NEE(p<0.05)is-66.9,-55.8,4.82 g CO₂ m^-2 yr^-2 in the spring,summer,and winter,respectively.Simultaneously,it's-100 for shrub,-70.2 for grassland,-47 for wetland,17 for forest,-16 g CO₂ m^-2 yr^-2 for tundra,which demonstrates the strength of carbon sink is gradually increasing year by year.2)The relationship between NEE and the soil temperature(0?20 cm)in the NGS(the nongrowing season,October to April)and GS(the growing season,May to September)conforms an exponential equation and a negative exponential equation,respectively.There is the significant seasonal difference in the sensitivity of ecosystem respiration to soil temperature(Q₁₀),of which the maximum in the winter(4.9±1.9)and the minimum in the summer(2.2±0.5).The relationship between NEE and soil water content(0?20 cm)can be described by the general linear equation.When soil water content per rising 1%,the change of NEE increased by 3.5 for wetland,8.9 for forest,20 for shrub,24 g C m^–2d^–1for grassland,respectively.3)According to the RF model,the total amount of carbon emission in the NGS and carbon uptake in the GS were estimated to be 1706?–2645 Tg C yr^–1 during the2002?2017,respectively,of which uncertainty(RMSE)severally was approximately1407.0?1362.7 Tg C yr^–1.In general,the total annual carbon exchange was–939 Tg C throughout the Arctic permafrost region,indicating that the region is currently in a carbon sink.In the NGS,soil temperature plays a key role in NEE,while leaf area index had the second impact on it besides soil temperature in the GS.4)During the NGS,the interannual rate of carbon loss gross around the Arctic permafrost reguons is 6.2,12.2,25.9 Tg C yr^–2 in SSP126,SSP245,SSP585,while the rate of carbon uptake gross during the GS is-2.0,-3.7,-6.1 Tg C yr^–2,respectively.It reveals that the increasing rate of carbon emission in the NGS is higher than the rate of carbon uptake in the GS,which means the strength of carbon sink is gradually declining in the Arctic permafrost region.It is estimated that the total annual carbon exchange throughout the entire Arctic permafrost was-545.2,-413.6,2.9 Tg C yr^–1 in SSP126,SSP245,SSP585 in 2057,respectively,which implies the entire permafrost across Arctic will shift from carbon sink to carbon origin in SSP585.In the study,the temporal and spatial change characteristics of NEE were analyzed and the relationship between NEE and influencing variables was explored.Then,based on the RF model,we calculated the total amount of NEE in the entire Arctic permafrost region during the 2002?2017,and the interannual changes of NEE was projected in the different SSPs until 2100.The study can provide the important data for the ecological protection in the Arctic permafrost region even the global,promote knowledge of response of carbon exchange to climate change and the further exploration,and reduce the uncertainty of climate projecting.