Hanqin Tian

Carbon balance of the terrestrial biosphere in the twentieth century: Analyses of CO2, climate and land use effects with four process-based ecosystem models

The concurrent effects of increasing atmospheric CO2 concentration, climate variability, and cropland establishment and abandonment on terrestrial carbon storage between 1920 and 1992 were assessed using a standard simulation protocol with four process-based terrestrial biosphere models. Over the long-term (1920-1992), the simulations yielded a time history of terrestrial uptake that is consistent (within the uncertainty) with a long-term analysis based on ice core and atmospheric CO2 data. Up to 1958, three of four analyses indicated a net release of carbon from terrestrial ecosystems to the atmosphere caused by cropland establishment. After 1958, all analyses indicate a net uptake of carbon by terrestrial ecosystems, primarily because of the physiological effects of rapidly rising atmospheric CO2. During the 1980s the simulations indicate that terrestrial ecosystems stored between 0.3 and 1.5 Pg C yr(-1), which is within the uncertainty of analysis based on CO2 and O-2 budgets. Three of the four models indicated tin accordance with O-2 evidence) that the tropics were approximately neutral while a net sink existed in ecosystems north of the tropics. Although all of the models agree that the long-term effect of climate on carbon storage has been small relative to the effects of increasing atmospheric CO2 and land use, the models disagree as to whether climate variability and change in the twentieth century has promoted carbon storage or release. Simulated interannual variability from 1958 generally reproduced the El Nino/Southern Oscillation (ENSO)-scale variability in the atmospheric CO2 increase, but there were substantial differences in the magnitude of interannual variability simulated by the models. The analysis of the ability of the models to simulate the changing amplitude of the seasonal cycle of atmospheric CO2 suggested that the observed trend may be a consequence of CO2 effects, climate variability, land use changes, or a combination of these effects. The next steps for improving the process-based simulation of historical terrestrial carbon include (1) the transfer of insight gained from stand-level process studies to improve the sensitivity of simulated carbon storage responses to changes in CO2 and climate, (2) improvements in the data sets used to drive the models so that they incorporate the timing, extent, and types of major disturbances, (3) the enhancement of the models so that they consider major crop types and management schemes, (4) development of data sets that identify the spatial extent of major crop types and management schemes through time, and (5) the consideration of the effects of anthropogenic nitrogen deposition. The evaluation of the performance of the models in the context of a more complete consideration of the factors influencing historical terrestrial carbon dynamics is important for reducing uncertainties in representing the role of terrestrial ecosystems in future projections of the Earth system.

Effect of interannual climate variability on carbon storage in Amazonian ecosystems

The Amazon Basin contains almost one-half of the worlds undisturbed tropical evergreen forest as well as large areas of tropical savanna,. The forests account for about 10 per cent of the worlds terrestrial primary productivity and for a similar fraction of the carbon stored in land ecosystems,, and short-term field measurements suggest that these ecosystems are globally important carbon sinks. But tropical land ecosystems have experienced substantial interannual climate variability owing to frequent El Niño episodes in recent decades. Of particular importance to climate change policy is how such climate variations, coupled with increases in atmospheric CO2 concentration, affect terrestrial carbon storage. Previous model analyses have demonstrated the importance of temperature in controlling carbon storage,. Here we use a transient process-based biogeochemical model of terrestrial ecosystems, to investigate interannual variations of carbon storage in undisturbed Amazonian ecosystems in response to climate variability and increasing atmospheric CO2 concentration during the period 1980 to 1994. In El Niño years, which bring hot, dry weather to much of the Amazon region, the ecosystems act as a source of carbon to the atmosphere (up to 0.2 petagrams of carbon in 1987 and 1992). In other years, these ecosystems act as a carbon sink (up to 0.7 Pg C in 1981 and 1993). These fluxes are large; they compare to a 0.3 Pg C per year source to the atmosphere associated with deforestation inthe Amazon Basin in the early 1990s. Soil moisture, which is affected by both precipitation and temperature, and which affects both plant and soil processes, appears to be an important control on carbon storage.

Spatial and temporal patterns of nitrogen deposition in China: Synthesis of observational data

[1] Anthropogenic nitrous pollutant emissions in China significantly increased during the last decades, which contributed to the accelerated nitrogen ( N) deposition. In order to characterize spatial pattern of nitrogen deposition, we employed the kriging technique to interpolate sampling data of precipitation chemistry and ambient air concentration from site-network observations over China. The estimation of wet deposition in China was limited to aqueous NO(3)(-) and NH(4)(+), while ambient NO(2) was the only species involved in the predicted dry deposition fluxes. To obtain wet deposition fluxes, precipitation concentration was multiplied by 20-year mean precipitation amounts with a resolution of 10 x 10 km. Dry deposition fluxes were products of the interpolated ambient NO(2) concentration and deposition velocities modeled for the main vegetation types in China. The total deposition rates of wet and dry deposition peaked over the central south China, with maximum values of 63.53 kg N ha(-1) yr(-1), and an average value of 12.89 kg N ha(-1) yr(-1). With ambient NO(2) concentration data spanning from the year 1990 through 2003, we detected and evaluated trends in the time series of the annual values of atmospheric NO(2) concentration. Significant upward trends at 21 of 102 sites were exhibited, with median percent change of 61.45% over the period 1990-2003. In addition, spatially continuous patterns of dry deposition fluxes based on ambient NO(2) measurements in two 5-year phases, 9 years apart, were carried out. On average, there was a rise of 7.66% in NO(2) dry deposition during 9 years throughout China.

China's changing landscape during the 1990s : large-scale land transformations estimated with satellite data

[1] Land-cover changes in China are being powered by demand for food for its growing population and by the nation’s transition from a largely rural society to one in which more than half of its people are expected to live in cities within two decades. Here we use an analysis of remotely sensed data gathered between 1990 and 2000, to map the magnitude and pattern of changes such as the conversion of grasslands and forests to croplands and the loss of croplands to urban expansion. With high-resolution (30 m) imagery from Landsat TM for the entire country, we show that between 1990 and 2000 the cropland area increased by 2.99 million hectares and urban areas increased by 0.82 million hectares. In northern China, large areas of woodlands, grasslands and wetlands were converted to croplands, while in southern China large areas of croplands were converted to urban areas. The land-cover products presented here give the Chinese government and international community, for the first time, an unambiguous understanding of the degree to which the nation’s landscape is being altered. Documentation of these changes in a reliable and spatially explicit way forms the foundation for management of China’s environment over the coming decades. Citation: Liu, J., H. Tian, M. Liu, D. Zhuang, J. M. Melillo, and Z. Zhang (2005), China’s changing landscape during the 1990s: Large-scale land transformations estimated with satellite data, Geophys. Res. Lett., 32, L02405, doi:10.1029/ 2004GL021649.

Regional carbon dynamics in monsoon Asia and its implications for the global carbon cycle

Data on three major determinants of the carbon storage in terrestrial ecosystems are used with the process-based Terrestrial Ecosystem Model (TEM) to simulate the combined effect of climate variability, increasing atmospheric CO2 concentration, and cropland establishment and abandonment on the exchange Of CO2 between the atmosphere and monsoon Asian ecosystems., During 1860-1990, modeled results suggest that monsoon Asia as a whole released 29.0 Pg C, which represents 50% of the global carbon release for this period. Carbon release varied across three subregions: East Asia (4.3 Pg C), South Asia (6.6 Pg C), and Southeast Asia (18.1 Pg C). For the entire region, the simulations indicate that land-use change alone has led to a loss of 42.6 Pg C. However, increasing CO2 and climate variability have added carbon to terrestrial ecosystems to compensate for 23% and 8% of the losses due to land-use change, respectively. During 1980-1989, monsoon Asia as a whole acted as a source of carbon to the atmosphere, releasing an average of 0.158 Pg C per year. Two of the subregions acted as net carbon source and one acted as a net carbon sink. Southeast Asia and South Asia were sources of 0.288 and 0.02 Pg C per year, respectively, while East Asia was a sink of 0.149 Pg C per year. Substantial interannual and decadal variations occur in the annual net carbon storage estimated by TEM due to comparable variations in summer precipitation and its effect on net primary production (NPP). At longer time scales, land-use change appears to be the important control on carbon dynamics in this region

Net exchanges of CO2, CH4, and N2O between China's terrestrial ecosystems and the atmosphere and their contributions to global climate warming

Chinas 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 Chinas terrestrial ecosystems and the atmosphere during 1961-2005. In the past 45 years, Chinas 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(-1)) while emitting CH(4) and N(2)O at rates of 8.3 Tg C yr(-1) with 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 Chinas 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 1961-2005. This study suggests that the relieving effects of Chinas 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.

The terrestrial biosphere as a net source of greenhouse gases to the atmosphere.

The terrestrial biosphere can release or absorb the greenhouse gases, carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O), and therefore has an important role in regulating atmospheric composition and climate. Anthropogenic activities such as land-use change, agriculture and waste management have altered terrestrial biogenic greenhouse gas fluxes, and the resulting increases in methane and nitrous oxide emissions in particular can contribute to climate change. The terrestrial biogenic fluxes of individual greenhouse gases have been studied extensively, but the net biogenic greenhouse gas balance resulting from anthropogenic activities and its effect on the climate system remains uncertain. Here we use bottom-up (inventory, statistical extrapolation of local flux measurements, and process-based modelling) and top-down (atmospheric inversions) approaches to quantify the global net biogenic greenhouse gas balance between 1981 and 2010 resulting from anthropogenic activities and its effect on the climate system. We find that the cumulative warming capacity of concurrent biogenic methane and nitrous oxide emissions is a factor of about two larger than the cooling effect resulting from the global land carbon dioxide uptake from 2001 to 2010. This results in a net positive cumulative impact of the three greenhouse gases on the planetary energy budget, with a best estimate (in petagrams of CO2 equivalent per year) of 3.9 ± 3.8 (top down) and 5.4 ± 4.8 (bottom up) based on the GWP100 metric (global warming potential on a 100-year time horizon). Our findings suggest that a reduction in agricultural methane and nitrous oxide emissions, particularly in Southern Asia, may help mitigate climate change.

Evaluating water stress controls on primary production in biogeochemical and remote sensing based models

[1] Water stress is one of the most important limiting factors controlling terrestrial primary production, and the performance of a primary production model is largely determined by its capacity to capture environmental water stress. The algorithm that generates the global near-real-time MODIS GPP/NPP products (MOD17) uses VPD (vapor pressure deficit) alone to estimate the environmental water stress. This paper compares the water stress calculation in the MOD17 algorithm with results simulated using a process-based biogeochemical model (Biome-BGC) to evaluate the performance of the water stress determined using the MOD17 algorithm. The investigation study areas include China and the conterminous United States because of the availability of daily meteorological observation data. Our study shows that VPD alone can capture interannual variability of the full water stress nearly over all the study areas. In wet regions, where annual precipitation is greater than 400 mm/yr, the VPD-based water stress estimate in MOD17 is adequate to explain the magnitude and variability of water stress determined from atmospheric VPD and soil water in Biome-BGC. In some dry regions, where soil water is severely limiting, MOD17 underestimates water stress, overestimates GPP, and fails to capture the intraannual variability of water stress. The MOD17 algorithm should add soil water stress to its calculations in these dry regions, thereby improving GPP estimates. Interannual variability in water stress is simpler to capture than the seasonality, but it is more difficult to capture this interannual variability in GPP. The MOD17 algorithm captures interannual and intraannual variability of both the Biome-BGC-calculated water stress and GPP better in the conterminous United States than in the strongly monsoon-controlled China.

Pattern and change of soil organic carbon storage in China: 1960s–1980s

Soils, an important component of the global carbon cycle, can be either net sources or net sinks of atmospheric carbon dioxide (CO2). In this study, we use the first and second national soil surveys of China to investigate patterns and changes in soil organic carbon storage (SOC) during the period from the 1960s to the 1980s. Our results show that there is a large amount of variability in SOC density among different soil types and land uses in the 1980s. The SOC density in the wetlands of Southwest China was the highest (45 kg m−2), followed by meadow soils in the South (26 kg m−2), forest and woodlands in the Northwest (19 kg m−2), steppe and grassland in the Northwest (15 kg m−2), shrubs in the Northwest (12 kg m−2), paddy lands in the Northwest (13 kg m−2), and drylands in the Northwest (11 kg m−1). The desert soils of the Western region ranked the lowest (1 kg m−2). The density of SOC was generally higher in the west than other regions. Eastern China had the lowest SOC density, which was associated with a long history of extensive land use in the region. The estimation of SOC storage for the entire nation was 93 Pg C in the 1960s and 92 Pg C in the 1980s. SOC storage decreased about 1 Pg C during the 1960s–1980s. This amount of decrease in SOC for the entire nation is small and statistically insignificant. To adequately characterize spatial variations in SOC, larger sampling sizes of soil profiles will be required in the future analyses.

Toward more realistic projections of soil carbon dynamics by Earth system models

Soil carbon (C) is a critical component of Earth system models (ESMs), and its diverse representations are a major source of the large spread across models in the terrestrial C sink from the third to fifth assessment reports of the Intergovernmental Panel on Climate Change (IPCC). Improving soil C projections is of a high priority for Earth system modeling in the future IPCC and other assessments. To achieve this goal, we suggest that (1) model structures should reflect real-world processes, (2) parameters should be calibrated to match model outputs with observations, and (3) external forcing variables should accurately prescribe the environmental conditions that soils experience. First, most soil C cycle models simulate C input from litter production and C release through decomposition. The latter process has traditionally been represented by first-order decay functions, regulated primarily by temperature, moisture, litter quality, and soil texture. While this formulation well captures macroscopic soil organic C (SOC) dynamics, better understanding is needed of their underlying mechanisms as related to microbial processes, depth-dependent environmental controls, and other processes that strongly affect soil C dynamics. Second, incomplete use of observations in model parameterization is a major cause of bias in soil C projections from ESMs. Optimal parameter calibration with both pool- and flux-based data sets through data assimilation is among the highest priorities for near-term research to reduce biases among ESMs. Third, external variables are represented inconsistently among ESMs, leading to differences in modeled soil C dynamics. We recommend the implementation of traceability analyses to identify how external variables and model parameterizations influence SOC dynamics in different ESMs. Overall, projections of the terrestrial C sink can be substantially improved when reliable data sets are available to select the most representative model structure, constrain parameters, and prescribe forcing fields.