Numerical Simulation Of Key Parameters Of The Future Global Carbon And Water Cycle Based On FLUS And CoLM Coupling

The carbon and water cycles in terrestrial ecosystems are primarily driven by climate change,CO₂ fertilization,and land use/land cover（LULC）changes.However,predicting the effects of these factors on future terrestrial ecosystem carbon-water cycles is challenging due to uncertainties in socioeconomic development and global climate change.To address this,process-based ecosystem models are used to simulate the effects of climate change,CO₂concentration changes,and LULC changes on the carbon and water cycles.Unfortunately,the lack of high-resolution LULC datasets that can be effectively applied to land surface models（LSMs）has led to significant uncertainty in assessing future carbon and water cycles.To address this issue,this study used a coupling scenario framework of the new generation of Shared Socioeconomic Pathways（SSP）and Representative Concentration Pathways（RCP）to establish the relationship between future human activities and surface biogeochemical cycles.The study used the Future Land Use Simulation（FLUS）model and the Common Land Model（CoLM）to evaluate the impact of climate change,CO₂ concentration changes,and LULC changes on three key indicators of future global carbon-water cycles:gross primary productivity（GPP）,evapotranspiration（ET）,and water use efficiency（WUE）.This study accomplished three major objectives.First,it used the FLUS model to simulate global land use and land cover（LULC）products（1 km resolution）under eight SSP-RCP scenarios（2020-2100,5-year intervals）by matching the Land Use Harmonization version 2（LUH2）data and MODIS land cover data（MCD12Q1）.This step provided input data for land surface models（LSMs）and solved the problem of coupling between land use simulation models and LSMs.Second,the CoLM was used to evaluate the impact of climate change,CO₂ concentration,and LULC changes on GPP and ET,which are the largest carbon and second-largest water fluxes between land and air,respectively.The study also quantified the relative contributions of these three factors to changes in GPP and ET.Finally,the study analyzed the response of the key indicators of carbon-water coupling cycles,such as WUE,to climate change,CO₂ concentration changes,and LULC changes.The study highlights the critical importance of high-resolution LULC datasets to accurately model the carbon and water cycles and the pivotal role that LULC plays in the carbon and water cycling in terrestrial ecosystems.Overall,this study shed light on the uncertainty of carbon-water coupling cycles and provides valuable insights into the complex and diverse carbon-water cycles and their coupling processes in terrestrial ecosystems,and the findings have important implications for our understanding of future global changes.The major conclusions are as follows:（1）This study has addressed the issue of the discrepancy between the LUH2 dataset and the actual land cover（MODIS）at a regional scale and the coarse resolution of the LUH2 dataset by simulating future LULC data.The simulated LULC data has a higher resolution of 1 km and a spatial pattern and area that is similar to the MODIS land cover product.The results of this study showed that the global land use changes under various future scenarios have a wide range of possibilities.While sustainable development and low-carbon emission paths result in moderate land use changes,extreme scenarios,such as SSP4-RCP3.4,may lead to a 50%to80%expansion of cropland.The simulated LULC data under multiple scenarios can provide reliable data support for ecosystem change analysis and can serve as shared input data for LSMs to effectively model the carbon and water cycles in terrestrial ecosystems.（2）The response of global GPP to climate change,CO₂ concentration rise,and LULC changes was analyzed under different combinations of scenarios.The results show that global land GPP is expected to decrease in the 21st century due to urbanization,agricultural expansion,and deforestation.The contribution of LULC changes to the dynamic of global GPP ranges from 3.43%to 10.78%during the 21st century,with 7%to 9%of global land area dominated by LULC changes.However,during 2000-2050,the contribution of LULC changes to GPP changes is higher,ranging from 10.92%to 16.16%,and 1.41%to 14.57%for 2050-2100.In Southeast Asia,LULC changes can cause up to 56.08%of GPP changes in 2050-2100 due to deforestation and agricultural expansion.While LULC changes play an important role in GPP changes,the impact of CO₂ fertilization effect is greater on a global scale,and the relative contribution of LULC changes to GPP dynamics during 2000-2100 is estimated to be between1.24%and 2.51%.Therefore,the CO₂ fertilization effect is expected to be the primary influence on future terrestrial ecosystems GPP,with the influence of LULC changes on GPP mainly at regional scales.（3）The coupling between ET and GPP in terrestrial ecosystems is strong,and land-use change affects these two parameters in distinct ways.This study investigated the impact of eight LULC scenarios on global terrestrial ET and found that,overall,the total ET is expected to increase in the future,with an increase ranging from 0.55 to 2.38 Tg H₂O yr^-2.However,the effects of land-use change on ET vary across regions.For instance,western North America,Western Europe,tropical savanna in the south of Sahara,southern China,northern Asia,and Australia are expected to experience a decrease in ET due to LULC change,whereas eastern North America,Eastern Europe,South Asia and Southeast Asia,Northeast Asia,the Amazon basin,and tropical forests in Central Africa are expected to see an increase.Notably,the Amazon basin and tropical forests in Central Africa are projected to experience a relatively larger increase in ET.While climate change is the dominant factor driving ET changes in most regions of the world（91%～95%of the terrestrial area）,the impact of LULC change and increased CO₂ concentration cannot be ignored at the regional scale.These findings highlight the distinct responses of ET and GPP to LULC change,climate change,and CO₂ fertilization.（4）Changes in WUE are closely related to changes in GPP and ET,as WUE is coupled with both.Future LULC change is expected to have a negative impact on WUE,with an annual decreasing trend of 0.00017 to 0.17707 mg C Kg^-1 H₂O yr^-1 under the eight scenarios considered in this study,with the maximum trend observed in the SSP4-RCP3.4 scenario.The changes in WUE vary greatly across different regions of the world.For example,in northern China,WUE is expected to decrease under all scenarios,especially in the SSP4-RCP3.4scenario,whereas southern China is expected to show an increase in WUE under most scenarios except for the SSP4-RCP3.4 scenario.Climate warming in the future is expected to decrease global GPP and increase ET,leading to a decrease in WUE,while the effect of CO₂fertilization is expected to have the opposite effect.Overall,the CO₂ change contributes the most to the change in WUE,accounting for 75.43%to 78.25%of the total,followed by climate change,which accounts for 20.54%to 24.13%,while the contribution of LULC change is the smallest,accounting for 0.39%to 1.21%.Additionally,this study shows that the response of WUE to LULC,climate change,and CO₂ fertilization effects is dominated by GPP,and thus the trend of WUE is similar to that of GPP.In summary,this study produced a high-resolution future LULC dataset that can be used as input for LSMs.Based on this dataset,future climate change and CO₂ concentration data,this study elucidated the response patterns of GPP,ET,and WUE to the three influencing factors.The dominant role of climate or CO₂ concentration change at the global scale and the important impact of LULC change at the regional scale were revealed.This study improves the accuracy and scientific basis of future carbon-water cycle prediction,and provides effective scientific evidence for evaluating global carbon sequestration,water resources management,and ecosystem health in the future.