Results are presented of export production, dissolved organic matter (DOM) and dissolved oxygen simulated by 12 global ocean models participating in the second phase of the Ocean Carbon‐cycle Model Intercomparison Project. A common, simple biogeochemical model is utilized in different coarse‐resolution ocean circulation models. The model mean (±1σ) downward flux of organic matter across 75 m depth is 17 ± 6 Pg C yr⁻¹. Model means of globally averaged particle export, the fraction of total export in dissolved form, surface semilabile dissolved organic carbon (DOC), and seasonal net outgassing (SNO) of oxygen are in good agreement with observation‐based estimates, but particle export and surface DOC are too high in the tropics. There is a high sensitivity of the results to circulation, as evidenced by (1) the correlation of surface DOC and export with circulation metrics, including chlorofluorocarbon inventory and deep‐ocean radiocarbon, (2) very large intermodel differences in Southern Ocean export, and (3) greater export production, fraction of export as DOM, and SNO in models with explicit mixed layer physics. However, deep‐ocean oxygen, which varies widely among the models, is poorly correlated with other model indices. Cross‐model means of several biogeochemical metrics show better agreement with observation‐based estimates when restricted to those models that best simulate deep‐ocean radiocarbon. Overall, the results emphasize the importance of physical processes in marine biogeochemical modeling and suggest that the development of circulation models can be accelerated by evaluating them with marine biogeochemical metrics.

Climate change is known to cause deoxygenation in the open ocean, but its effects on eutrophic and seasonally hypoxic estuaries and coastal oceans are less clear. Using Chesapeake Bay as a study site, we conducted climate downscaling projections for dissolved oxygen and found that the hypoxic and anoxic volumes would increase by 10‐30% between the late 20^th and mid‐21^st century. A budget analysis of dissolved oxygen in the bottom water revealed differing physical and biogeochemical responses to climate change. Sea level rise and larger winter‐spring runoff led to stronger stratification and large reductions in the vertical oxygen supply to the bottom water. On the other hand, warming led to earlier initiation of hypoxia, accompanied by weaker summer respiration and more rapid termination of hypoxia. Decreasing solubility due to warming accounted for about 50% of the reduction in the bottom water oxygen content.

Global carbon dioxide (CO₂) evasion from inland waters (rivers, lakes, and reservoirs) and carbon (C) export from land to oceans constitute critical terms in the global C budget. However, the magnitudes, spatiotemporal patterns, and underlying mechanisms of these fluxes are poorly constrained. Here, we used a coupled terrestrial–aquatic model to assess how multiple changes in climate, land use, atmospheric CO₂ concentration, nitrogen (N) deposition, N fertilizer and manure applications have affected global CO₂ evasion and riverine C export along the terrestrial‐aquatic continuum. We estimate that terrestrial C loadings, riverine C export, and CO₂ evasion in the preindustrial period (1800s) were 1,820 ± 507 (mean ± standard deviation), 765 ± 132, and 841 ± 190 Tg C yr⁻¹, respectively. During 1800–2019, multifactorial global changes caused an increase of 25% (461 Tg C yr⁻¹) in terrestrial C loadings, reaching 2,281 Tg C yr⁻¹ in the 2010s, with 23% (104 Tg C yr⁻¹) of this increase exported to the ocean and 59% (273 Tg C yr⁻¹) being emitted to the atmosphere. Our results showed that global inland water recycles and exports nearly half of the net land C sink into the atmosphere and oceans, highlighting the important role of inland waters in the global C balance, an amount that should be taken into account in future C budgets. Our analysis supports the view that a major feature of the global C cycle–the transfer from land to ocean–has undergone a dramatic change over the last two centuries as a result of human activities.

Secular tidal trends are present in many tide gauge records, but their causes are often unclear. This study examines trends in tides over the last century in the Chesapeake and Delaware Bays. Statistical models show negative M2 amplitude trends at the mouths of both bays, while some upstream locations have insignificant or positive trends. To determine whether sea level rise is responsible for these trends, we include a term for mean sea level in the statistical models and compare the results with predictions from numerical and analytical models. The observed and predicted sensitivities of M2 amplitude and phase to mean sea level are similar, although the numerical model amplitude is less sensitive to sea level. The sensitivity occurs as a result of strengthening and shifting of the amphidromic system in the Chesapeake Bay and decreasing frictional effects and increasing convergence in the Delaware Bay. After accounting for the effect of sea level, significant negative background M2 and S2 amplitude trends are present; these trends may be related to other factors such as dredging, tide gauge errors, or river discharge. Projected changes in tidal amplitudes due to sea level rise over the 21st century are substantial in some areas, but depend significantly on modeling assumptions.