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\"Readers will discover that the natural element carbon is found in all living things, including people. This fascinating book explains how the Earth's supply of carbon moves among Earth's oceans, atmosphere, ecosystem, and geosphere in a process called the carbon cycle. Accessible text and detailed images help explain such processes as photosynthesis, respiration, and decomposition. Feature boxes highlight examples of the ways in which human activity, such as burning fossil fuels, releases too much carbon dioxide into the air disrupting the balance of the carbon cycle. Readers are encouraged to find ways to take action and find solutions\"-- Provided by publisher.

Anthropogenically emitted CO.sub.2 from fossil fuel use and land use change is partly absorbed by terrestrial ecosystems and the ocean, while the remainder retained in the atmosphere adds to the ongoing increase in atmospheric CO.sub.2 concentration. Earth system models (ESMs) can simulate such dynamics of the global carbon cycle and consider its interaction with the physical climate system. The ESMs that participated in the Coupled Model Intercomparison Project phase 6 (CMIP6) performed historical simulations to reproduce past climate-carbon cycle dynamics. This study investigated the cause of CO.sub.2 concentration biases in ESMs and identified how they might be reduced. First, we compared simulated historical carbon budgets in two types of experiments: one with prescribed CO.sub.2 emissions (the emission-driven experiment, \"E-HIST\") and the other with a prescribed CO.sub.2 concentration (the concentration-driven experiment, \"C-HIST\"). Because the design of CMIP7 is being considered, it is important to explore any differences or implications associated with such variations. The findings of this confirmed that the multi-model means of the carbon budgets simulated by one type of experiment generally showed good agreement with those simulated by the other. However, the multi-model average of cumulative compatible fossil fuel emission diagnosed from the C-HIST experiment was lower by 35 PgC than that used as the prescribed input data to drive the E-HIST experiment; the multi-model average of the simulated CO.sub.2 concentration for 2014 in E-HIST was higher by 7 ppmv than that used to drive C-HIST. Regarding individual models, some showed a distinctly different magnitude of ocean carbon uptake from C-HIST because the E-HIST setting allows ocean carbon fluxes to be dependent on land carbon fluxes via CO.sub.2 concentration. Second, we investigated the potential linkages of two types of carbon cycle indices: simulated CO.sub.2 concentration in E-HIST and compatible fossil fuel emission in C-HIST. It was confirmed quantitatively that the two indices are reasonable indicators of overall model performance in the context of carbon cycle feedbacks, although most models cannot accurately reproduce the cumulative compatible fossil fuel emission and thus cannot reproduce the CO.sub.2 concentration precisely. Third, analysis of the atmospheric CO.sub.2 concentration in five historical eras enabled the identification of periods that caused the concentration bias in individual models. Fourth, it is suggested that this non-CO.sub.2 effect is likely to be the reason why the magnitude of the natural land carbon sink in historical simulations is difficult to explain based on analysis of idealized experiments. Finally, accurate reproduction of land use change emission is critical for better reproduction of the global carbon budget and CO.sub.2 concentration. The magnitude of simulated land use change emission not only affects the level of net land carbon uptake but also determines the magnitude of the ocean carbon sink in the emission-driven experiment. This study confirmed that E-HIST enables an evaluation of the full span of the uncertainty range covering the entire carbon-climate system and allows for an explicit simulation of the interlinking process of the carbon cycle between land and ocean. By isolating the forced responses and feedback processes of the carbon cycle processes, the usefulness of C-HIST in elucidating climate-carbon cycle systems and in identifying the cause of CO.sub.2 biases was confirmed.

\"This textbook for advanced undergraduate and graduate students presents a multidisciplinary approach to understanding ocean circulation and how it drives and controls marine biogeochemistry and biological productivity at a global scale. Background chapters on ocean physics, chemistry and biology provide students with the tools to examine the range of large-scale physical and dynamic phenomena that control the ocean carbon cycle and its interaction with the atmosphere. Throughout the text observational data is integrated with basic physical theory to address cutting-edge research questions in ocean biogeochemistry. Simple theoretical models, data plots and schematic illustrations summarise key results and connect the physical theory to real observations. Advanced mathematics is provided in boxes and appendices where it can be drawn on to assist with the worked examples and homework exercises available online. Further reading lists for each chapter and a comprehensive glossary provide students and instructors with a complete learning package\"-- Provided by publisher.

The ocean plays a crucial role in the global carbon cycle by absorbing and storing about one-third of anthropogenic carbon dioxide (CO.sub.2). It is estimated that the ocean has sequestered approximately 26 % of CO.sub.2 emissions over the last decade, resulting in significant changes in the marine carbon system and impacting the marine environment. The sea-surface microlayer (SML) plays a crucial role in these processes, facilitating the transfer of matter and energy between the ocean and the atmosphere. However, most studies on the carbon cycle in the SML have primarily addressed daily variability and overlooked nocturnal processes, which may lead to inaccurate global carbon estimates. We analysed temperature, salinity, pH.sub.T25, and pCO.sub.2 using data collected over three complete diel cycles during an oceanographic campaign along the Croatian coast near Sibenik in the Middle Adriatic. Our analysis revealed statistically significant differences (p<0.05) between daytime and nighttime measurements of temperature, salinity, and pH.sub.T25 . Diel differences in pCO.sub.2, were also observed, with patterns largely driven by temperature effects and short-term mixing. These differences may be related to the occurrence of buoyancy fluxes, which are typically more pronounced during the day and could enhance CO.sub.2 fluxes, as observed with values of 1.98 ± 2.52 mmol cm.sup.-2 h.sup.-1 during the day, while at night, they dropped to 0.01 ± 0.02 mmol cm.sup.-2 h.sup.-1 . These findings emphasise the importance of considering complete diurnal cycles to accurately capture the variability in thermohaline features and carbon exchange processes, thereby improving our understanding of the ocean's role in climate change.

Understanding the carbon cycle of the terrestrial critical zone, extending from the tree canopy to the aquifer, is crucial for accurate quantification of its total carbon storage and for modelling terrestrial carbon stock responses to climate change. Caves and their catchments offer a natural framework to sample and analyse carbon in unsaturated zone reservoirs across various spatial and temporal scales. In this study, we analyse the concentration, stable carbon isotopic ratio ([delta].sup.13 C), and radiocarbon (.sup.14 C) compositions of CO.sub.2 from the atmosphere, boreholes (0.5 to 5 m depth), and cave sampled every 2 months over 2 years at Milandre cave in northern Switzerland. High concentrations of up to 35 000 ppmV CO.sub.2 are measured in the boreholes. The [delta].sup.13 C values of CO.sub.2 in the boreholes reflect the [delta].sup.13 C of C.sub.3 plants (â¼ -26 0/00), which dominate the catchment ecosystem. Shallow meadow boreholes host older CO.sub.2 in winter and modern CO.sub.2 in summer, while forest ecosystems consistently export modern CO.sub.2 (F.sup.14 C = â¼ 1) to the unsaturated zone. Cave CO.sub.2 concentrations exceed atmospheric levels and are diluted by temperature-driven seasonal ventilation. Keeling plot intercepts indicate that the cave CO.sub.2, which mixes with atmospheric CO.sub.2, is younger in summer (F.sup.14 C = 0.94) and older in winter (F.sup.14 C = 0.88), with a [delta].sup.13 C consistent with the C.sub.3 -plant-dominated catchment. Mixing models utilizing drip water dissolved inorganic carbon .sup.14 C suggest that varying carbonate dissolution and degassing dynamics do not explain the F.sup.14 C variation and concurrent [delta].sup.13 C stability of the mixing endmember. Rather, contributions from aged carbon reservoirs in the deeper unsaturated zone are likely. This study provides valuable insights into CO.sub.2 source dynamics and cycling within the karstic critical zone, highlighting the impact of seasonal variations and ecological factors on downward carbon export from terrestrial ecosystems.

Feedbacks between the climate system and the carbon cycle represent a key source of uncertainty in model projections of Earth's climate, in part due to our inability to directly measure large-scale biosphere-atmosphere carbon fluxes. In situ measurements of the CO.sub.2 mole fraction from surface flasks, towers, and aircraft are used in inverse models to infer fluxes, but measurement networks remain sparse, with limited or no coverage over large parts of the planet. Satellite retrievals of total column CO.sub.2 (XCO2), such as those from NASA's Orbiting Carbon Observatory-2 (OCO-2), can potentially provide unprecedented global information about CO.sub.2 spatiotemporal variability. However, for use in inverse modeling, data need to be extremely stable, highly precise, and unbiased to distinguish abundance changes emanating from surface fluxes from those associated with variability in weather. Systematic errors in XCO2 have been identified and, while bias correction algorithms are applied globally, inconsistencies persist at regional and smaller scales that may complicate or confound flux estimation. To evaluate XCO2 retrievals and assess potential biases, we compare OCO-2 v10 retrievals with in situ data-constrained XCO2 simulations over North America estimated using surface fluxes and boundary conditions optimized with observations that are rigorously calibrated relative to the World Meteorological Organization X2007 CO.sub.2 scale. Systematic errors in simulated atmospheric transport are independently evaluated using unassimilated aircraft and AirCore profiles. We find that the global OCO-2 v10 bias correction shifts the distribution of retrievals closer to the simulated XCO2, as intended. Comparisons between bias-corrected and simulated XCO2 reveal differences that vary seasonally. Importantly, the difference between simulations and retrievals is of the same magnitude as the imprint of recent surface flux in the total column. This work demonstrates that systematic errors in OCO-2 v10 retrievals of XCO2 over land can be large enough to confound reliable surface flux estimation and that further improvements in retrieval and bias correction techniques are essential. Finally, we show that independent observations, especially vertical profile data, such as those from the National Oceanic and Atmospheric Administration aircraft and AirCore programs are critical for evaluating errors in both satellite retrievals and carbon cycle models.

Carbon stored in soils represents the largest terrestrial carbon pool and factors affecting this will be vital in the understanding of future atmospheric CO2 concentrations. This book provides an integrated view on measuring and modelling soil carbon dynamics.

Radiocarbon is a tracer that provides unique insights into the ocean's ability to sequester CO.sub.2 from the atmosphere. While spatial patterns of radiocarbon in the ocean interior can indicate the vectors and timescales for carbon transport through the ocean, estimates of the global average ocean-atmosphere radiocarbon age offset (B-Atm) place constraints on the closure of the global carbon cycle. Here, we apply a Bayesian interpolation method to compiled B-Atm data to generate global interpolated fields and mean ocean B-Atm estimates for a suite of time slices across the last deglaciation. The compiled data and interpolations confirm a stepwise and spatially heterogeneous \"rejuvenation\" of the ocean, suggesting that carbon was released to the atmosphere through two swings of a \"ventilation seesaw\" operating between the North Atlantic and both the Southern Ocean and the North Pacific. Sensitivity tests using the Bern3D model of intermediate complexity demonstrate that a portion of the reconstructed deglacial B-Atm changes may reflect \"phase-attenuation\" biases that are unrelated to ocean ventilation and that arise from independent atmospheric radiocarbon dynamics instead. A deglacial minimum in B-Atm offsets during the Bølling-Allerød could partly reflect such a bias. However, the sensitivity tests further demonstrate that when correcting for such biases, ocean \"ventilation\" could still account for at least one-third of deglacial atmospheric CO.sub.2 rise. This contribution to CO.sub.2 rise appears to have continued through the Younger Dryas, though much of the impact was likely achieved by the end of the Bølling-Allerød, indicating a key role for marine carbon cycle adjustment early in the deglacial process. Our global average B-Atm estimates place further new constraints on the long-standing mystery of global radiocarbon budget closure across the last deglaciation and suggest that glacial radiocarbon production levels are likely underestimated on average by existing reconstructions.