[1] The stable oxygen isotope ratio (δ 18 O) in precipitation is an integrated tracer of atmospheric processes worldwide. Since the 1990s, an intensive effort has been dedicated to studying precipitation isotopic composition at more than 20 stations in the Tibetan Plateau (TP) located at the convergence of air masses between the westerlies and Indian monsoon. In this paper, we establish a database of precipitation δ

No abstract

Abstract.Continental-scale estimations of terrestrial methane (CH 4 ) and nitrous oxide (N 2 O) fluxes over a long time period are crucial to accurately assess the global balance of greenhouse gases and enhance our understanding and prediction of global climate change and terrestrial ecosystem feedbacks. Using a process-based global biogeochemical model, the Dynamic Land Ecosystem Model (DLEM), we quantified simultaneously CH 4 and N 2 O fluxes in North America's terrestrial ecosystems from 1979 to 2008. During the past 30 years, approximately 14.69 ± 1.64 T g C a −1 (1 T g = 10 12 g) of CH 4 , and 1.94 ± 0.1 T g N a −1 of N 2 O were released from terrestrial ecosystems in North America. At the country level, both the US and Canada acted as CH 4 sources to the atmosphere, but Mexico mainly oxidized and consumed CH 4 from the atmosphere. Wetlands in North America contributed predominantly to the regional CH 4 source, while all other ecosystems acted as sinks for atmospheric CH 4 , of which forests accounted for 36.8%. Regarding N 2 O emission in North America, the US, Canada, and Mexico contributed 56.19%, 18.23%, and 25.58%, respectively, to the continental source over the past 30 years. Forests and croplands were the two ecosystems that contributed most to continental N 2 O emission. The inter-annual variations of CH 4 and N 2 O fluxes in North America were mainly attributed to year-to-year climatic variability. While only annual precipitation was found to have a significant effect on annual CH 4 flux, both mean annual temperature and annual precipitation were significantly correlated to annual Correspondence to: H. Tian (tianhan@auburn.edu) N 2 O flux. The regional estimates and spatiotemporal patterns of terrestrial ecosystem CH 4 and N 2 O fluxes in North America generated in this study provide useful information for global change research and policy making.

[1] The magnitude, spatial, and temporal patterns of the terrestrial carbon sink and the underlying mechanisms remain uncertain and need to be investigated. China is important in determining the global carbon balance in terms of both carbon emission and carbon uptake. Of particular importance to climate-change policy and carbon management is the ability to evaluate the relative contributions of multiple environmental factors to net carbon source and sink in China's terrestrial ecosystems. Here the effects of multiple environmental factors (climate, atmospheric CO 2 , ozone pollution, nitrogen deposition, nitrogen fertilizer application, and land cover/land use change) on net carbon balance in terrestrial ecosystems of China for the period 1961-2005 were modeled with newly developed, detailed historical information of these changes. For this period, results from two models indicated a mean land sink of 0.21 Pg C per year, with a multimodel range from 0.18 to 0.24 Pg C per year. The models' results are consistent with field observations and national inventory data and provide insights into the biogeochemical mechanisms responsible for the carbon sink in China's land ecosystems. In the simulations, nitrogen deposition and fertilizer applications together accounted for 61 percent of the net carbon storage in China's land ecosystems in recent decades, with atmospheric CO 2 increases and land use also functioning to stimulate carbon storage.

Soil is the largest organic carbon (C) pool of terrestrial ecosystems, and C loss from soil accounts for a large proportion of land‐atmosphere C exchange. Therefore, a small change in soil organic C (SOC) can affect atmospheric carbon dioxide (CO2) concentration and climate change. In the past decades, a wide variety of studies have been conducted to quantify global SOC stocks and soil C exchange with the atmosphere through site measurements, inventories, and empirical/process‐based modeling. However, these estimates are highly uncertain, and identifying major driving forces controlling soil C dynamics remains a key research challenge. This study has compiled century‐long (1901–2010) estimates of SOC storage and heterotrophic respiration (Rh) from 10 terrestrial biosphere models (TBMs) in the Multi‐scale Synthesis and Terrestrial Model Intercomparison Project and two observation‐based data sets. The 10 TBM ensemble shows that global SOC estimate ranges from 425 to 2111 Pg C (1 Pg = 1015 g) with a median value of 1158 Pg C in 2010. The models estimate a broad range of Rh from 35 to 69 Pg C yr−1 with a median value of 51 Pg C yr−1 during 2001–2010. The largest uncertainty in SOC stocks exists in the 40–65°N latitude whereas the largest cross‐model divergence in Rh are in the tropics. The modeled SOC change during 1901–2010 ranges from −70 Pg C to 86 Pg C, but in some models the SOC change has a different sign from the change of total C stock, implying very different contribution of vegetation and soil pools in determining the terrestrial C budget among models. The model ensemble‐estimated mean residence time of SOC shows a reduction of 3.4 years over the past century, which accelerate C cycling through the land biosphere. All the models agreed that climate and land use changes decreased SOC stocks, while elevated atmospheric CO2 and nitrogen deposition over intact ecosystems increased SOC stocks—even though the responses varied significantly among models. Model representations of temperature and moisture sensitivity, nutrient limitation, and land use partially explain the divergent estimates of global SOC stocks and soil C fluxes in this study. In addition, a major source of systematic error in model estimations relates to nonmodeled SOC storage in wetlands and peatlands, as well as to old C storage in deep soil layers.

[1] China's terrestrial ecosystems have been recognized as an atmospheric CO 2 sink ; however, it is uncertain whether this sink can alleviate global warming given the fluxes of CH 4 and N 2 O. In this study, we used a process-based ecosystem model driven by multiple environmental factors to examine the net warming potential resulting from net exchanges of CO 2 , CH 4 , and N 2 O between China's terrestrial ecosystems and the atmosphere during 1961-2005. In the past 45 years, China's terrestrial ecosystems were found to sequestrate CO 2 at a rate of 179.3 Tg C yr −1 with a 95% confidence range of (62.0 Tg C yr −1 , 264.9 Tg C yr) while emitting CH 4 and N 2 O at rates of 8.3 Tg C yrwith a 95% confidence range of (3.3 Tg C yr −1 , 12.4 Tg C yr −1 ) and 0.6 Tg N yr −1 with a 95% confidence range of (0.2 Tg N yr −1 , 1.1 Tg N yr −1 ), respectively. When translated into global warming potential, it is highly possible that China's terrestrial ecosystems mitigated global climate warming at a rate of 96.9 Tg CO 2 eq yr −1 (1 Tg = 10 12 g), substantially varying from a source of 766.8 Tg CO 2 eq yr −1 in 1997 to a sink of 705.2 Tg CO 2 eq yr −1 in 2002. The southeast and northeast of China slightly contributed to global climate warming; while the northwest, north, and southwest of China imposed cooling effects on the climate system. Paddy land, followed by natural wetland and dry cropland, was the largest contributor to national warming potential; forest, followed by woodland and grassland, played the most significant role in alleviating climate warming. Our simulated results indicate that CH 4 and N 2 O emissions offset approximately 84.8% of terrestrial CO 2 sink in China during . This study suggests that the relieving effects of China's terrestrial ecosystems on climate warming through sequestering CO 2 might be gradually offset by increasing N 2 O emission, in combination with CH 4 emission.

Greenhouse gas (GHG)‐induced climate change is among the most pressing sustainability challenges facing humanity today, posing serious risks for ecosystem health. Methane (CH4) and nitrous oxide (N2O) are the two most important GHGs after carbon dioxide (CO2), but their regional and global budgets are not well known. In this study, we applied a process‐based coupled biogeochemical model to concurrently estimate the magnitude and spatial and temporal patterns of CH4 and N2O fluxes as driven by multiple environmental changes, including climate variability, rising atmospheric CO2, increasing nitrogen deposition, tropospheric ozone pollution, land use change, and nitrogen fertilizer use. The estimated CH4 and N2O emissions from global land ecosystems during 1981–2010 were 144.39 ± 12.90 Tg C/yr (mean ± 2 SE; 1 Tg = 1012 g) and 12.52 ± 0.74 Tg N/yr, respectively. Our simulations indicated a significant (P < 0.01) annually increasing trend for CH4 (0.43 ± 0.06 Tg C/yr) and N2O (0.14 ± 0.02 Tg N/yr) in the study period. CH4 and N2O emissions increased significantly in most climatic zones and continents, especially in the tropical regions and Asia. The most rapid increase in CH4 emission was found in natural wetlands and rice fields due to increased rice cultivation area and climate warming. N2O emission increased substantially in all the biome types and the largest increase occurred in upland crops due to increasing air temperature and nitrogen fertilizer use. Clearly, the three major GHGs (CH4, N2O, and CO2) should be simultaneously considered when evaluating if a policy is effective to mitigate climate change.

The magnitude, spatiotemporal patterns, and controls of carbon flux from land to the ocean remain uncertain. Here we applied a process-based land model with explicit representation of carbon processes in streams and rivers to examine how changes in climate, land conversion, management practices, atmospheric CO 2 , and nitrogen deposition affected carbon fluxes from eastern North America to the Atlantic Ocean, specifically the Gulf of Maine (GOM), Middle Atlantic Bight (MAB), and South Atlantic Bight (SAB). Our simulation results indicate that the mean annual fluxes (±1 standard deviation) of dissolved organic carbon (DOC), particulate organic carbon (POC), and dissolved inorganic carbon (DIC) in the past three decades were 2.37 ± 0.60, 1.06 ± 0.20, and 3.57 ± 0.72 Tg C yr À1, respectively. Carbon export demonstrated substantial spatial and temporal variability. For the region as a whole, the model simulates a significant decrease in riverine DIC fluxes from 1901 to 2008, whereas there were no significant trends in DOC or POC fluxes. In the SAB, however, there were significant declines in the fluxes of all three forms of carbon, and in the MAB subregion, DIC and POC fluxes declined significantly. The only significant trend in the GOM subregion was an increase in DIC flux. Climate variability was the primary cause of interannual variability in carbon export. Land conversion from cropland to forest was the primary factor contributing to decreases in all forms of C export, while nitrogen deposition and fertilizer use, as well as atmospheric CO 2 increases, tended to increase DOC, POC, and DIC fluxes.