Impacts Of Last Millennium Land Use/Cover Change On The Climate Of China

Land use and cover change(LUCC)is one of the important external forcing for human to affect the climate system.To improve the recognition of the impacts of LUCC on regional climate in China,especially the impacts on decadal climate change over eastern China,the relative contribution of biogeophysical and biogeochemical processes of LUCC to climate change in China.It is of great significance for scientifically making policies to face the challenge of global climate change.The impacts of LUCC on precipitation and surface temperature over China are investigated based on ensemble simulations of LUCC sensitivity simulations(opening the carbonnitrogen cycle)with the CESM-LME model project and LUCC sensitivity simulations with and without the carbon-nitrogen cycle and the corresponding control experiments from the CESM.Based on the former,the spatial statistical analysis methods are used to explore the potential impacts of LUCC comprehensive climate process on decadal variability of precipitation and temperature over eastern China.And we study the biogeochemical and biogeophysical effects of LUCC on annual and seasonal precipitation and temperature change separately by using the latter one.The main conclusions of this study are summarized as follows:1.Compositive impacts of LUCC on summer climate change at decadal scale over eastern ChinaBased on the data derive from CESM-LME,the low-pass filter was used to extract the decadal signals,and the linear regression and spatial correlation analysis was used to explore the physical mechanism.It is found that LUCC causes the decadal increase of summer(June-July-August,JJA)precipitation over the southern part of eastern China and the decadal decrease of summer precipitation over the northern part of eastern China.From the perspective of spatial distributions,the LUCC plays an important role in modulating the JJA precipitation distributing pattern over eastern China at decadal scale.Before land use and land cover changed dramatically(850-1749 AD,hereafter “Before LUCC”),the first leading mode of decadal JJA precipitation over eastern China exhibits a meridional dipole pattern while the second leading mode exhibits a meridional tripole pattern.And after land use and cover changed dramatically(1750-2005 AD,hereafter “After LUCC”),the spatial pattern is reversed,i.e.,the tripole pattern becomes the first leading mode and the dipole pattern becomes the second leading mode.LUCC influence decadal low-level circulation over eastern part of East Asia,especially the "anticyclone-cyclone" / "anticyclone-cyclone-anticyclone" anomalous circulation over East Asia,and moisture transportation and vertical airflow through winter(December-January-February,DJF)SST in Pacific and Atlantic at decadal timescale,thus has impacts on the decadal JJA precipitation distribution over eastern China.The decadal variation of summer surface temperature over eastern China shows a consistent cooling in the whole region.As for the spatial distribution pattern,“Before LUCC”,the first leading mode shows that surface temperature is consistently increasing or decreasing over eastern China,while the second mode is a dipole distribution which shows reversed changes between the south and the north.“After LUCC”,the first leading mode exhibits the reversed distribution pattern between southwest part of eastern China and other regions over eastern China and the large value area appears in the north part.The second leading mode is same with that of “Before LUCC”.The physical mechanisms of temperature variation in different regions over eastern China are different.The decadal variations of leaf area index,surface latent heat flux and net longwave flux at surface contribute to the decadal variations of surface temperature in the region north of 35° N in eastern China,while the influence of surface albedo is relatively weak.And the decadal variations of surface albedo,surface sensible heat flux and net radiation flux in summer have great influences on the decadal variations of surface temperature in the regions south of 35° N in eastern China,although the decrease of LAI may increase the surface temperature,it is not enough to offset the cooling caused by surface albedo.2.Biogeophysical and biogeochemical impacts of LUCC on temperature and precipitation change over ChinaBased on the results derive from CESM,it is found that the increase of surface temperature in most regions over China is due to the effects of LUCC biogeophysical process,while the LUCC biogeochemical process results in the decreasing temperature over China.The effects of LUCC biogeophysical and biogeochemical processes on temperature vary from region to region and from season to season,but in general,LUCC biogeophysical process has a stronger effect on temperature changes than biogeochemical process.From the annual and seasonal average results,the biogeophysical warming in most areas over China is the joint influences of the net radiation flux,sensible heat flux and latent heat flux,the changes of net radiation flux and sensible heat flux are consistent with the change of surface temperature but opposite to the surface latent heat flux.But there are differences in different seasons and regions,that is,the relative contribution of the three flux are different.The relative contribution of LUCC biogeophysical and biogeochemical processes to precipitation over China varies from region to region.The precipitation change in the west of 100° E in China is mainly caused by the biogeochemical process of LUCC,but for most areas that are east of 100° E,the effects of LUCC biogeophysical process are stronger than that of biogeochemistry.In terms of physical mechanisms,500 h Pa specific humidity and vertical velocity have the consistent effects on annual average or seasonal average precipitation under the influence of LUCC,however,the effects of LUCC biogeophysical process are more complex.