Carbon dynamics in coral reefs

Coral reefs show high organic and inorganic carbon production and create unique landforms on tropical coastlines. The balance between organic and inorganic carbon production is determined by benthic organisms such as corals, macroalgae, and seagrasses, and also by reef hydrodynamics, which in turn determine CO2 sinks and sources within the ecosystem. Many studies have shown that net organic carbon production in coral reef ecosystems is almost zero (balanced), with net positive calcification resulting in reefs acting as CO2 sources. However, the relationships among productivity, benthic organisms, and hydrodynamics have not been well documented; more detailed information is required from both field observations and coupled physical–biological models. Reef sediments have low organic carbon content (median, 0.35% dry weight), approximately 50% those of tropical and subtropical seagrass beds (median, 0.67%) and 5% those of mangrove forests (median, 6.3%). Sedimentation rates do not vary significantly between these three ecosystems, so organic carbon input and decomposition in the surface sediments are key factors controlling organic carbon burial rates. Coral reefs provide calm conditions that enhance sedimentation of fine sediments, which facilitates the development of seagrass beds and mangrove forests. Seagrass meadows and mangrove forests in turn trap fine sediments from terrestrial sources and prevent high-turbidity water from reaching coral reefs. Coral reefs, seagrass meadows, and mangrove forests are thus interdependent ecosystems; to effectively store and export blue carbon in tropical coastal areas, it is necessary to maintain the health of these ecosystems.

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Robust constraints on past CO2 climate forcing from the boron isotope proxy

The atmospheric concentration of the greenhouse gas carbon dioxide, CO2, is intimately coupled to the carbon chemistry of seawater, such that the radiative climate forcing from CO2 can be changed by an array of physical, geochemical and biological ocean processes. For instance, biological carbon sequestration, seawater cooling and net CaCO3 dissolution are commonly invoked as the primary drivers of CO2 change that amplify the orbitally‐paced ice age cycles of the late Pleistocene. Based on first‐principle arguments with regard to ocean chemistry we demonstrate that seawater pH change (∆pH) is the dominant control that effectively sets CO2 radiative forcing (∆F) on orbital timescales, as is evident from independent late Pleistocene reconstructions of pH and CO2. In short, all processes relevant for CO2 on orbital timescales, including temperature change, cause pH to change to bring about fractional CO2 change so as to yield a linear relationship of ∆pH to CO2 climate forcing. Further, we show that ∆pH and CO2 climate forcing can be reconstructed using the boron isotope pH‐proxy more accurately than absolute pH or CO2, even if seawater boron isotope composition is poorly constrained and without information on a second carbonate system parameter. Thus, our formalism relaxes otherwise necessary assumptions to allow the accurate determination of orbital timescale CO2 radiative forcing from boron isotope‐pH reconstructions alone, thereby eliminating a major limitation of current methods to estimate our planet’s climate sensitivity from the geologic record.

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Marine biogeochemical modelling  and data assimilation for operational forecasting, reanalysis, and climate research

Predictions of marine biogeochemistry are of importance for a range of applications, from operational forecasting of harmful algal blooms, to seasonal prediction of primary production, to understanding the influence of the marine carbon cycle on future climate change. Reanalyses, which include data assimilation in model hindcasts, are also required for the assessment of long-term environmental change. The inclusion of marine biogeochemistry in ocean forecasting and reanalysis systems is still in its early stages, but is already providing valuable insights. This chapter begins by giving an overview of biogeochemical modelling and data assimilation, and discussing challenges around physical-biogeochemical coupling and the use of observations. A summary of current applications to operational forecasting, reanalysis and climate studies is then given, before a vision is presented for a fully integrated prediction framework, linking five-day regional forecasting to global climate research.

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Seasonal asymmetry in the evolution of surface ocean pCO2 and pH thermodynamic drivers and the influence on sea‐air CO2 flux

It has become clear that anthropogenic carbon invasion into the surface ocean drives changes in the seasonal cycles of carbon dioxide partial pressure (pCO2) and pH. However, it is not yet known whether the resulting sea‐air CO2 fluxes are symmetric in their seasonal expression. Here we consider a novel application of observational constraints and modeling inferences to test the hypothesis that changes in the ocean’s Revelle Factor facilitate a seasonally asymmetric response in pCO2 and the sea‐air CO2 flux. We use an analytical framework that builds on observed sea surface pCO2 variability for the modern era and incorporates transient dissolved inorganic carbon (DIC) concentrations from an Earth system model. Our findings reveal asymmetric amplification of pCO2 and pH seasonal cycles by a factor of two (or more) above pre‐industrial levels under RCP8.5. These changes are significantly larger than observed modes of interannual variability and are relevant to climate feedbacks associated with Revelle Factor perturbations. Notably, this response occurs in the absence of changes to the seasonal cycle amplitudes of DIC, total alkalinity, salinity, and temperature, indicating that significant alteration of surface pCO2 can occur without modifying the physical or biological ocean state. This result challenges the historical paradigm that if the same amount of carbon and nutrients is entrained and subsequently exported, there is no impact on anthropogenic carbon uptake. Anticipation of seasonal asymmetries in the sea surface pCO2 and CO2 flux response to ocean carbon uptake over the 21st century may have important implications for carbon cycle feedbacks.

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Longstanding signals of marine community structuring by winter storm wave-base

Coastal marine communities face both physical oceanographic changes and altered ecological relationships due to indirect human activities, such as climate-related changes, and direct human activities, such as extraction of wave energy as a renewable resource. Often single physical oceanographic changes and altered ecological relationships are investigated, rather than multiple potential drivers. Here we investigated the links between the structure of offshore benthic bivalve communities to multiple physical drivers including wave-base, and more traditional drivers of marine soft sediment community structure (e.g. temperature, pH, dissolved oxygen, salinity, and nutrients). Our benthic bivalve community data (both modern and historical) were collected from bulk sediment box-core samples taken over a depth range of 20-70 m on the continental shelf of Newport, Oregon, USA. Environmental data were collected through CTD casts at sampling locations and through NOAA Buoy Station 46094. We used a non-linear hierarchical regression approach to look for a systematic response in the benthos. Subtidal bivalve communities structured themselves along a depth gradient with a distinct shift in species’ rank abundance at 50 m, and this shift was most strongly associated with storm wave-base. This distinct wave-driven community structure was present in both modern-day bivalve communities and century-scale historical communities, suggesting both the importance of waves and the long-standing nature of their impacts on biological communities in this system. These results emphasize potential consequences of changing wave-base on this shelf, which could occur indirectly through changing storm regimes due to anthropogenic climate change or directly through large-scale wave energy harvest.

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Tactile approaches to help learners visualize key processes in environmental health sciences

This chapter describes how hands-on models, or manipulatives, can be employed to improve the environmental health literacy of a variety of people, from science teachers and students in classrooms, to global audiences in large festival gatherings. Environmental health concepts can be quite abstract. For example, the effect of wood smoke on human lungs. People are concerned about the exposure to toxic molecules from the smoke, but find an explanation of the chemical process by which wood smoke harms human health too difficult to fully understand. Hands-on activities and models are a visual and tactile way of communicating essential molecular environmental health concepts in an inviting way without requiring a technical background.

The MIT Edgerton Center Molecule Set (hereafter referred to as the Molecule Set) is one example of an engaging model set that employs a simple design of differently colored LEGO® bricks to represent atoms. The set was designed to teach chemical principles to middle school students, and has evolved to include new topics with a more environmental health emphasis such as climate change and air pollution. The success of the Molecule Set and corresponding lessons stems from a unique collaboration between MIT scientists and the MIT Edgerton Center. This chapter highlights the Molecule Set and other relevant examples where hands-on models have been used to communicate abstract science concepts and improve environmental health literacy.

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Increase of dissolved inorganic carbon and decrease in pH in near-surface waters in the Mediterranean Sea during the past two decades (update)

Two 3-year time series of hourly measurements of the fugacity of CO2 (fCO2) in the upper 10m of the surface layer of the northwestern Mediterranean Sea have been recorded by CARIOCA sensors almost two decades apart, in 1995–1997 and 2013–2015. By combining them with the alkalinity derived from measured temperature and salinity, we calculate changes in pH and dissolved inorganic carbon (DIC). DIC increased in surface seawater by  ∼ 25µmolkg−1 and fCO2 by 40µatm, whereas seawater pH decreased by  ∼ 0.04 (0.0022yr−1). The DIC increase is about 15% larger than expected from the equilibrium with atmospheric CO2. This could result from natural variability, e.g. the increase between the two periods in the frequency and intensity of winter convection events. Likewise, it could be the signature of the contribution of the Atlantic Ocean as a source of anthropogenic carbon to the Mediterranean Sea through the Strait of Gibraltar. We then estimate that the part of DIC accumulated over the last 18 years represents  ∼ 30% of the total inventory of anthropogenic carbon in the Mediterranean Sea.

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Ocean acidification in the IPCC AR5 WG II

OUP book