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Terrestrial and oceanic carbon sinks together sequester >50% of the anthropogenic emissions, and the major uncertainty in the global carbon budget is related to the terrestrial carbon cycle. Hence, it is important to understand the major drivers of the land carbon uptake to make informed decisions on climate change mitigation policies. In this paper, we assess the major drivers of the land carbon uptake-CO2 fertilization, nitrogen deposition, climate change, and land use/land cover changes (LULCC)-from existing literature for the historical period and future scenarios, focusing on the results from fifth Coupled Models Intercomparison Project (CMIP5). The existing literature shows that the LULCC fluxes have led to a decline in the terrestrial carbon stocks during the historical period, despite positive contributions from CO2 fertilization and nitrogen deposition. However, several studies find increases in the land carbon sink in recent decades and suggest that CO2 fertilization is the primary driver (up to 85%) of this increase followed by nitrogen deposition (∼10%-20%). For the 21st century, terrestrial carbon stocks are projected to increase in the majority of CMIP5 simulations under the representative concentration pathway 2.6 (RCP2.6), RCP4.5, and RCP8.5 scenarios, mainly due to CO2 fertilization. These projections indicate that the effects of nitrogen deposition in future scenarios are small (∼2%-10%), and climate warming would lead to a loss of land carbon. The vast majority of the studies consider the effects of only one or two of the drivers, impairing comprehensive assessments of the relative contributions of the drivers. Further, the broad range in magnitudes and scenario/model dependence of the sensitivity factors pose challenges in unambiguous projections of land carbon uptake. Improved representation of processes such as LULCC, fires, nutrient limitation and permafrost thawing in the models are necessary to constrain the present-day carbon cycle and for more accurate future projections.

Recent papers on planetary and safe Earth System boundaries have proposed a limit of 0.25 for regional aerosol optical depth (AOD) in South Asia (SA) with a zone of uncertainty of 0.25–0.50 to avoid major disruptions to regional hydrology. These values are based on expert judgment, and a rigorous model-based confirmation of these boundary values is lacking in literature. In this paper, we address this important research gap using idealized climate model simulations. We analyze the response of the South Asian summer monsoon precipitation when the regional mean AOD is increased from the modeled present-day value of 0.14 to the proposed planetary boundary values of 0.25 and 0.5. Our simulations confirm that a regional AOD of 0.25 could indeed lead to drought conditions (>10% mean precipitation reduction) in India, while an AOD of 0.5 reduces Indian summer monsoon precipitation by about 19%. The reduction in the summer monsoon precipitation is driven by both fast adjustments (rapid adjustment of the atmosphere to aerosol radiative forcing) and slow responses (responses to changes in sea surface temperature). The rapid adjustment to anthropogenic aerosols, predominantly sulfates, involves enhanced atmospheric stability, subsidence, and suppressed cloud formation and precipitation. The slow response involves zonal surface temperature gradients between the North Indian Ocean and Western Pacific Ocean leading to changes in the Walker circulation, anomalous subsidence over SA, and a decrease in monsoon precipitation. Enhanced aerosol loading over SA reduces monsoon precipitation, regardless of aerosol composition, though the magnitude of reduction depends on whether the aerosols are primarily reflective or absorbing. Our findings confirm the risk of major disruptions to regional hydrology in SA when regional aerosol loading exceeds the boundary values. Future studies should assess the robustness of our results using other climate models and for other monsoon regions.

Climate records of ratios of stable water isotopes of oxygen (δ 18 O) are used to reconstruct the past Indian monsoon precipitation. Identifying the sources of water vapor is important in understanding the role of monsoonal circulation in the δ 18 O values, to aid in monsoon reconstructions. Here, using an isotope-enabled Earth system model, we estimate the contributions of oceanic and terrestrial water vapor sources to two major precipitation seasons in India—the Southwest monsoon and the Northeast monsoon, and their effects on the δ 18 O in precipitation (δ 18 O p ). We find that the two monsoon seasons have different dominant sources of water vapor because of the reversal in atmospheric circulation. While Indian Ocean regions, Arabian Sea, and recycling are the major sources of the Southwest monsoon precipitation, North Pacific Ocean and recycling are two crucial sources of Northeast monsoon precipitation. The δ 18 O p of the Southwest monsoon precipitation is determined by contributions from the Indian Ocean sources and recycling. Despite reduced precipitation, more negative δ 18 O p values are simulated in the Northeast monsoon season due to larger negative δ 18 O p contributions from the North Pacific. Our results imply that changes in atmospheric circulation and water vapor sources in past climates can influence climate reconstructions using δ 18 O.

In this study, we investigate the changes in water isotope ratios in the Indian summer monsoon precipitation ([delta].sup.18 O.sub.precip) during the Last Glacial Maximum (LGM, â¼ 21 ka Before Present) compared to the pre-industrial (PI) period, and the mechanisms driving these changes, using a general circulation model with water isotope and novel water vapor source-tagging capabilities.

In this study, we investigate the changes in water isotope ratios in the Indian summer monsoon precipitation (δ18Oprecip) during the Last Glacial Maximum (LGM, ∼ 21 ka Before Present) compared to the pre-industrial (PI) period, and the mechanisms driving these changes, using a general circulation model with water isotope and novel water vapor source-tagging capabilities. During the LGM, the model simulates a substantial reduction (15 %) in monsoon precipitation over the Indian subcontinent, consistent with proxy records. This drying in LGM is associated with reduced atmospheric water vapor, a thermodynamic response to cooling, while the westerly circulation, a dynamics response, is strengthened over parts of the subcontinent. Additionally, zonal temperature gradients between a relatively less-cooled tropical Western Pacific Ocean and Indian subcontinent lead to anomalous subsidence over the Indian region, enhancing the drying. Water vapor source tagging shows that while the four dominant moisture sources for the monsoon (South Indian Ocean, Arabian Sea, Indian land recycling, and Central Indian Ocean) remained the same, their contributions were reduced during the LGM. The δ18Oprecip values over the Indian monsoon region are enriched by approximately 1 ‰ in the LGM simulation, and we find that this enrichment was not driven by the local amount effect. A decomposition analysis shows that the enrichment was primarily caused by reduced contributions from distant, isotopically depleted water vapor sources and secondarily by reduced rainout during moisture transport from the Indian Ocean. These findings have important implications for paleoclimate reconstructions, suggesting that δ18O records from the Indian region could be indicators of broad-scale atmospheric circulation rather than being direct proxies for local precipitation amount.

Anthropogenic influences and global climate change are expected to alter the land carbon stocks in the future. In this modeling study, using the NCAR Community Earth System Model (CESM), we assess the relative importance of CO 2 fertilization, nitrogen deposition, climate change, and land use and land cover changes (LULCC) on the land carbon uptake in three future scenarios used in phase 5 of the Coupled Model Intercomparison Project (CMIP5). Our simulations show that CO 2 fertilization is the primary driver of the increase in net primary production (NPP) and total ecosystem carbon (TEC) in the representative concentrations pathway 2.6 (RCP2.6), RCP4.5, and RCP8.5 scenarios. The effect of nitrogen deposition on NPP and TEC in the future scenarios is small. Climate warming causes increases in NPP in the RCP4.5 and RCP8.5 scenarios, but it leads to loss of TEC in the future scenarios because of increased heterotrophic respiration. LULCC leads to an enhancement of NPP in the future scenarios due to post-harvest regrowth in the RCP2.6 and RCP8.5 scenarios, and due to afforestation in the RCP4.5 scenario. We find that land is a source of carbon in the RC8.5 and RCP2.6 scenarios mainly because of LULCC and climate change, but afforestation and CO 2 fertilization in the RCP4.5 scenario facilitate the land to be a sink. Our findings, albeit from a single model, are in broader agreement with other studies that highlight the need for better land management practices and moderation in climate change for a continued land carbon sink.

The stability and resilience of the Earth system and human well-being are inseparably linked 1 – 3 , yet their interdependencies are generally under-recognized; consequently, they are often treated independently 4 , 5 . Here, we use modelling and literature assessment to quantify safe and just Earth system boundaries (ESBs) for climate, the biosphere, water and nutrient cycles, and aerosols at global and subglobal scales. We propose ESBs for maintaining the resilience and stability of the Earth system (safe ESBs) and minimizing exposure to significant harm to humans from Earth system change (a necessary but not sufficient condition for justice) 4 . The stricter of the safe or just boundaries sets the integrated safe and just ESB. Our findings show that justice considerations constrain the integrated ESBs more than safety considerations for climate and atmospheric aerosol loading. Seven of eight globally quantified safe and just ESBs and at least two regional safe and just ESBs in over half of global land area are already exceeded. We propose that our assessment provides a quantitative foundation for safeguarding the global commons for all people now and into the future. We find that justice considerations constrain the integrated Earth system boundaries more than safety considerations for climate and atmospheric aerosol loading, and our assessment provides a foundation for safeguarding the global commons for all people.

An analysis of warm periods in Earth’s history helps to clarify an apparent anomaly in projections of the future behaviour of the South Asian summer monsoon. An analysis of warm periods in Earth’s history helps to clarify an apparent anomaly in projections of the future behaviour of the South Asian summer monsoon.