A new estimation of carbon emissions from land use and land cover change in China over the past 300 years.

Scientific estimation of carbon emissions induced by historical land use and land cover change (LUCC) can improve the accuracy of terrestrial ecosystem carbon budget estimates and deepen understanding of the future carbon-sink potential of terrestrial ecosystems. The present study, using historical-document-based data for provincial cropland, forest, and grassland area in China, and experimental-data-based information for provincial vegetation and soil organic carbon density, re-estimates China's LUCC-induced carbon emissions for 1700-1980 using a bookkeeping model in which we updated tabulated functions for carbon losses and gains. The past 300 years have witnessed a dramatic LUCC in China. The cropland area has increased by 67.11 million ha, while the forest and grassland areas have decreased by 127.96 million ha and 16.72 million ha, respectively. Accordingly, the net carbon emissions for 1700-1980 are 6.17-12.35 Pg C, with 8.55 Pg C in the moderate scenario. Among the contributing factors, deforestation was the largest carbon source, accounting for over 90 % of the total carbon emissions. According to our estimates, over 70 % of carbon emissions were caused by harvesting wood, while <30 % were from converting forest and grassland to cropland. Spatially, for the whole period, carbon emissions in southwestern China (Chuan-Yu, Yunnan, and Guangxi), northeastern China (Liaoning, Jilin, and Heilongjiang), and parts of northwestern China (Gan-Ning, Qinghai, and Xinjiang) were as high as 6.03 Pg C, accounting for 70 % of the total carbon emissions. Extending previous studies, we updated the historical LUCC data, carbon density data, and tabulated functions for carbon losses and gains. The estimation results objectively reveal the historical spatiotemporal changes in LUCC-induced emissions.

Exploring Spatiotemporal Pattern of Grassland Cover in Western China from 1661 to 1996.

Historical grassland cover change is vital for global and regional environmental change modeling; however, in China, estimates of this are rare, and therefore, we propose a method to reconstruct grassland cover over the past 300 years. By synthesizing remote sensing-derived Chinese land use and land cover change (LULCC) data (1980-2015) and potential natural vegetation data simulated by the relationship between vegetation and environment, we first determined the potential extent of natural grassland vegetation (PENG) in the absence of human activities. Then we reconstructed grassland cover across western China between 1661 and 1996 at 10 km resolution by overlaying the Chinese historical cropland dataset (CHCD) over the PENG. As this land cover type has been significantly influenced by anthropogenic factors, the data show that the proportion of grassland in western China continuously decreased from 304.84 × 106 ha in 1661 to 277.69 × 106 ha in 1996. This reduction can be divided into four phases, comprising a rapid decrease between 1661 and 1724, a slow decrease between 1724 and 1873, a sharp decrease between 1873 and 1980, and a gradual increase since 1980. These reductions correspond to annual loss rates of 7.32 × 104 ha, 2.90 × 104 ha, 17.04 × 104 ha, and -2.37 × 104 ha, respectively. The data reconstructed here show that the decrease in grassland area between 1661 and 1724 was mainly limited to the Gan-Ning region (Gansu and Ningxia) and was driven by the early agricultural development policies of the Qing Dynasty. Grassland was extensively cultivated in northeastern China (Heilongjiang, Jilin, and Liaoning) and in the Xinjiang region between 1724 and 1980, a process which resulted from an exponential increase in immigrants to these provinces. The reconstruction results enable provide crucial data that can be used for modeling long-term climate change and carbon emissions.

Evaluation of global historical land use scenarios based on regional datasets on the Qinghai-Tibet Area.

Global historical land use scenarios are widely used to model human-induced climate change from the regional to global scales. It is necessary to conduct regional scale assessments of these global scenarios, identifying their uncertainties and pointing out directions for improvement. Based on the regional reconstruction Li-dataset, remotely sensed dataset, and grazing intensity dataset, the uncertainties of land use area and geographical distribution in HYDE3.1, HYDE3.2, and SAGE (a global land dataset from the Center for Sustainability and the Global Environment) scenarios for the Qinghai-Tibet Area (QTA) are evaluated. The comparisons show that the cropland areas on the QTA in HYDE3.2 for 1900-2000 are close to those of the Li-dataset, whereas HYDE3.1 underestimated and SAGE overestimated the cropland areas significantly. Spatially, HYDE3.1, HYDE3.2, and SAGE have large uncertainties, which cannot reflect the distribution of cropland on the QTA and its changes for 1900-2000 well, and too much cropland is allocated to southeastern Tibet. HYDE3.1 and HYDE3.2 overestimated the pasture area and its distribution on the QTA significantly. The distribution of pasture in SAGE showed overall an agreement with the spatial pattern for grazing intensity, but changes in grazing intensity for 2000-2010 was not reflected in SAGE. The FAO pasture definition and estimates and the method of using population as a proxy for pasture area are not appropriate for the QTA. Methodology which uses the pasture inventory data to calibrate satellite-based grassland maps to obtain the current pasture maps may also not be appropriate because of the lacking differentiation between natural and anthropogenic grasslands in remotely sensed data. More regional level land use estimates with concise definitions, define the land use more clearly, and stratification reconstruction based on differences in agro-climatic conditions and resource endowments may be used to improve global maps.

Reconstruction of the cropland cover changes in eastern China between the 10th century and 13th century using historical documents.

To evaluate and improve datasets of anthropogenic land cover change used in local and global climate models, great efforts were made to reconstruct historical land use, including the LandCover 6k project which dedicated to reconstructing human land use over the past 10,000 years. In this study, we utilized historical records, including taxed-cropland and cropland measurement areas, and data on the number of households in eastern China between the 10th century and 13th century in concert with coefficient calibration, model allocation, and per capita cropland estimation to reconstruct areas of provincial cropland for 22 provinces over five time periods. Our reconstructions indicate that total cropland areas of eastern China for AD 1000, 1066, 1078, 1162, and 1215 are 34.74 × 106 ha, 49.42 × 106 ha, 51.62 × 106 ha, 35.21 × 106 ha, and 51.21 × 106 ha, respectively. And the cropland area fluctuated because of dynasty shift and went through three phases. Cropland expansion and contraction mainly occurred in the middle and lower reaches of the Yangtze and Yellow Rivers as well as the Huaihe River Basin, while in some regions far away from battlefields, including northeastern and southern China, cropland area expanded continuously throughout the study period.