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When assessing the socio‐economic impacts of climate change, it is sensible to make targeted climate projections for regions of high population density and economy activity. Much of human activity is concentrated at river basins, yet it has been difficult to resolve the complex boundaries of these basins in coarse resolution global climate models. The latest high‐resolution observation and climate projection datasets enable such basin‐based evaluations now, and this study assesses the historical and projected climate changes over three major river basins in China—the Yellow, Yangtze and Pearl River basins. Based on CN05.1 dataset, the Yellow River basin has significantly warmed by about 1.8°C over the past five decades, far more than the other two basins. The change in temperature extremes has been as severe, with the annual maxima of daily maximum temperatures (TXx) increasing by 1.5°C, and the annual minima of daily minimum temperatures (TNn) increasing by 2.5°C. Precipitation over the Yangtze River has significantly increased by about 0.2 mm·day−1, while changes over the other two basins were not statistically significant. The uncertainty in the change of precipitation was greater than that of temperature. A selection of simulations from the Fifth and Sixth Coupled Model Intercomparison Projects (CMIP5 and CMIP6) were validated against the CN05.1 dataset for the historical period of 1961–2018. Changes in temperature indices were well‐reproduced, but changes in precipitation indices poorly so. CMIP6 models performed better than the CMIP5 models. Both CMIP5 and CMIP6 multi‐model ensembles (MMEs) projected about 1.0–2.0°C warming over China and the three river basins by 2015–2050. Both MMEs projected wetting trends over most parts of China and the three river basins. Both warming and wetting were projected to accelerate with time, particularly warming over the Yellow River basin, and wetting over the Pearl River basin. [ABSTRACT FROM AUTHOR]

Global warming as quantified by surface air temperature has been shown to be approximately linearly related to cumulative emissions of CO2. Here, a coupled state-of-the-art Earth system model with an interactive carbon cycle (BNU-ESM) was used to investigate whether this proportionality extends to the complex Earth system model and to examine the climate system responses to different emission pathways with a common emission budget of man-made CO2. These new simulations show that, relative to the lower emissions earlier and higher emissions later (LH) scenario, the amount of carbon sequestration by the land and the ocean will be larger and Earth will experience earlier warming of climate under the higher emissions earlier and lower emissions later (HL) scenario. The processes within the atmosphere, land, and cryosphere, which are highly sensitive to climate, show a relatively linear relationship to cumulative CO2 emissions and will attain similar states under both scenarios, mainly because of the negative feedback between the radiative forcing and ocean heat uptake. However, the processes with larger internal inertias depend on both the CO2 emissions scenarios and the emission budget, such as ocean warming and sea level rise. [ABSTRACT FROM AUTHOR]

The default fractional vegetation cover and terrain height were replaced by the estimated fractional vegetation cover, which was calculated by the Normalized Difference Vegetation Index (NDVI) of Earth Observing System Moderate-Resolution Imaging Spectroradiometer (EOS-MODIS) and the Digital Elevation Model of the Shuttle Radar Topography Mission (SRTM) system. The near-surface meteorological elements over northeastern China were assimilated into the three-dimensional variational data assimilation system (3DVar) module in the Weather Research and Forecasting (WRF) model. The structure and daily variations of air temperature, humidity, wind and energy fields over northeastern China were simulated using the WRF model. Four groups of numerical experiments were performed, and the simulation results were analyzed of latent heat flux, sensible heat flux, and their relationships with changes in the surface energy flux due to soil moisture and precipitation over different surfaces. The simulations were compared with observations of the stations Tongyu, Naiman, Jinzhou, and Miyun from June to August, 2009. The results showed that the WRF model achieves high-quality simulations of the diurnal characteristics of the surface layer temperature, wind direction, net radiation, sensible heat flux, and latent heat flux over semiarid northeastern China in the summer. The simulated near-surface temperature, relative humidity, and wind speed were improved in the data assimilation case (Case 2) compared with control case (Case 1). The simulated sensible heat fluxes and surface heat fluxes were improved by the land surface parameterization case (Case 3) and the combined case (Case 4). The simulated temporal variations in soil moisture over the northeastern arid areas agree well with observations in Case 4, but the simulated precipitation should be improved in the WRF model. This study could improve the land surface parameters by utilizing remote sensing data and could further improve atmospheric elements with a data assimilation system. This work provides an effective attempt at combining multi-source data with different spatial and temporal scales into numerical simulations. The assimilation datasets generated by this work can be applied to research on climate change and environmental monitoring of arid lands, as well as research on the formation and stability of climate over semiarid areas. [ABSTRACT FROM AUTHOR]