• Energy Research

AbstractMost climate mitigation scenarios involve negative emissions, especially those that aim to limit global temperature increase to 2°C or less. However, the carbon uptake potential in land‐based climate change mitigation efforts is highly uncertain. Here, we address this uncertainty by using two land‐based mitigation scenarios from two land‐use models (IMAGE and MAgPIE) as input to four dynamic global vegetation models (DGVMs; LPJ‐GUESS, ORCHIDEE, JULES, LPJmL). Each of the four combinations of land‐use models and mitigation scenarios aimed for a cumulative carbon uptake of ~130 GtC by the end of the century, achieved either via the cultivation of bioenergy crops combined with carbon capture and storage (BECCS) or avoided deforestation and afforestation (ADAFF). Results suggest large uncertainty in simulated future land demand and carbon uptake rates, depending on the assumptions related to land use and land management in the models. Total cumulative carbon uptake in the DGVMs is highly variable across mitigation scenarios, ranging between 19 and 130 GtC by year 2099. Only one out of the 16 combinations of mitigation scenarios and DGVMs achieves an equivalent or higher carbon uptake than achieved in the land‐use models. The large differences in carbon uptake between the DGVMs and their discrepancy against the carbon uptake in IMAGE and MAgPIE are mainly due to different model assumptions regarding bioenergy crop yields and due to the simulation of soil carbon response to land‐use change. Differences between land‐use models and DGVMs regarding forest biomass and the rate of forest regrowth also have an impact, albeit smaller, on the results. Given the low confidence in simulated carbon uptake for a given land‐based mitigation scenario, and that negative emissions simulated by the DGVMs are typically lower than assumed in scenarios consistent with the 2°C target, relying on negative emissions to mitigate climate change is a highly uncertain strategy.

Global terrestrial models currently predict that the Amazon rainforest will continue to act as a carbon sink in the future, primarily owing to the rising atmospheric carbon dioxide (CO2) concentration. Soil phosphorus impoverishment in parts of the Amazon basin largely controls its functioning, but the role of phosphorus availability has not been considered in global model ensembles—for example, during the Fifth Climate Model Intercomparison Project. Here we simulate the planned free-air CO2 enrichment experiment AmazonFACE with an ensemble of 14 terrestrial ecosystem models. We show that phosphorus availability reduces the projected CO2-induced biomass carbon growth by about 50% to 79 ± 63 g C m−2 yr−1 over 15 years compared to estimates from carbon and carbon–nitrogen models. Our results suggest that the resilience of the region to climate change may be much less than previously assumed. Variation in the biomass carbon response among the phosphorus-enabled models is considerable, ranging from 5 to 140 g C m−2 yr−1, owing to the contrasting plant phosphorus use and acquisition strategies considered among the models. The Amazon forest response thus depends on the interactions and relative contributions of the phosphorus acquisition and use strategies across individuals, and to what extent these processes can be upregulated under elevated CO2.

Regional increases in atmospheric O3, mainly produced photochemically from anthropogenic precursor gases, have phytotoxicity due to its strong oxidizing properties. To determine the response of bamboo physiology to elevated O3 levels, three-year-old dwarf bamboo (Indocalamus decorus) clones were exposed to three O3 concentrations (Ambient-AA, 21.3 to 80.9 ppb in the daytime; -AA+70, 70 ppb O3 above ambient; -AA+140, 140 ppb O3 above ambient) in open-top chambers for one growing season in Beijing, China. Gas exchange, biomass, growth, soluble sugar, and starch contents were examined at the end of the experiment. Our findings indicated that: (1) elevated O3 treatments decreased the photosynthesis rate, total biomass, and bud numbers but increased individual bud biomass and rhizome bud to rhizome biomass ratio. The most severe reduction was observed in new rhizome biomass (35.9% reduction in AA+70 and 57.2% reduction in AA+140), whereas individual bud biomass increased by 50% and 75% in the AA+70 and AA+140 groups compared with AA, respectively; (2) the starch contents in the rhizome decreased by 28.4%, whereas soluble sugar increased by 38.1% in the AA+140 rhizome buds compared to AA; (3) only the culm numbers of pachymorph rhizomes (clumped) decreased, whereas no changes in leptomorph rhizomes were observed. However, the mean distance between two ramets was lengthened by 49.4% and 86.5% in AA+70 and AA+140, respectively. In conclusion, Indocalamus decorus allocated more nonstructural carbohydrates (NSCs) from the rhizome to the buds to form stronger buds and ensure the survival of newer generations as a high priority in response to O3 exposure. Indocalamus decorus may be conducive to escaping from disadvantaged habitats and decreasing resource competition by lengthening the distance between two ramets.

Abstract Tropospheric ozone (O3) is harmful to plant productivity and negatively impacts crop yields. O3 concentrations are projected to decrease globally in the optimistic Representative Concentration Pathway of 2.6 W m–2 (RCP2.6) but increase globally following the high-emission scenario under the RCP8.5, with substantial implications for global food security. The damaging effect of O3 on future crop yield is affected by CO2 fertilization and climate change, and their interactions for RCP scenarios have yet to be quantified. In this study, we used the Joint UK Land Environment Simulator modified to include crops (JULES-crop) to quantify the impacts, and relative importance of present-day and future O3, CO2 concentration and meteorology on crop production at the regional scale until 2100 following RCP2.6 and RCP8.5 scenarios. We focus on eight major crop-producing regions that cover the production of wheat, soybean, maize, and rice. Our results show that CO2 alone has the largest effect on regional yields, followed by climate and O3. However, the CO2 fertilization effect is offset by the negative impact of tropospheric O3 in regions with high O3 concentrations, such as South Asia and China. Simulated crop yields in 2050 were compared with Food and Agriculture Organisation (FAO) statistics to investigate the differences between a socioeconomic and a biophysical process-based approach. Results showed that FAO estimates are closer to our JULES-crop RCP8.5 scenario. This study demonstrates that air pollution could be the biggest threat to future food production and highlights an urgent policy need to mitigate the threat of climate change and O3 pollution on food security.