On the fate of anthropogenic nitrogen

This article provides a synthesis of literature values to trace the fate of 150 Tg/yr anthropogenic nitrogen applied by humans to the Earth''s land surface. Approximately 9 TgN/yr may be accumulating in the terrestrial biosphere in pools with residence times of ten to several hundred years. Enhanced fluvial transport of nitrogen in rivers and percolation to groundwater accounts for ≈35 and 15 TgN/yr, respectively. Greater denitrification in terrestrial soils and wetlands may account for the loss of ≈17 TgN/yr from the land surface, calculated by a compilation of data on the fraction of N₂O emitted to the atmosphere and the current global rise of this gas in the atmosphere. A recent estimate of atmospheric transport of reactive nitrogen from land to sea (NO_x and NH_x) accounts for 48 TgN/yr. The total of these enhanced sinks, 124 TgN/yr, is less than the human-enhanced inputs to the land surface, indicating areas of needed additional attention to global nitrogen biogeochemistry. Policy makers should focus on increasing nitrogen-use efficiency in fertilization, reducing transport of reactive N to rivers and groundwater, and maximizing denitrification to its N₂ endproduct.     

Severity of ocean acidification following the end-Cretaceous asteroid impact

Most paleo-episodes of ocean acidification (OA) were either too slow or too small to be instructive in predicting near-future impacts. The end-Cretaceous event (66 Mya) is intriguing in this regard, both because of its rapid onset and also because many pelagic calcifying species (including 100% of ammonites and more than 90% of calcareous nannoplankton and foraminifera) went extinct at this time. Here we evaluate whether extinction-level OA could feasibly have been produced by the asteroid impact. Carbon cycle box models were used to estimate OA consequences of (i) vaporization of up to 60 × 10¹⁵ mol of sulfur from gypsum rocks at the point of impact; (ii) generation of up to 5 × 10¹⁵ mol of NO_x by the impact pressure wave and other sources; (iii) release of up to 6,500 Pg C as CO₂ from vaporization of carbonate rocks, wildfires, and soil carbon decay; and (iv) ocean overturn bringing high-CO₂ water to the surface. We find that the acidification produced by most processes is too weak to explain calcifier extinctions. Sulfuric acid additions could have made the surface ocean extremely undersaturated (Ω_calcite <0.5), but only if they reached the ocean very rapidly (over a few days) and if the quantity added was at the top end of literature estimates. We therefore conclude that severe ocean acidification might have been, but most likely was not, responsible for the great extinctions of planktonic calcifiers and ammonites at the end of the Cretaceous.From preindustrial times up to 2008, ca. 530 Pg of carbon were added to the atmosphere through burning of fossil fuels and deforestation (1). This has led to an increase in atmospheric CO₂ of 40% (from 280 in 1750 to 400 ppm today). Simultaneously, about 160 Pg C has been taken up by the ocean, causing ocean acidification (OA) (2).OA is of particular concern for calcifying organisms, because it leads to lower CO₃²⁻ concentrations and hence lower seawater saturation states with respect to CaCO₃ (Ω). In theory, lower Ω should make it energetically more costly for organisms to synthesize CaCO₃ shells and skeletons and, subsequently, if Ω falls below 1.0, to maintain them against dissolution. A large variety of short-term experiments have been carried out to test for such consequences (2). It is widely recognized, however, that one aspect that these experiments generally do not address is the degree to which organisms can evolve in response to the changing carbonate chemistry and thereby become more tolerant of the new conditions. As a result, there is a need for approaches that reveal the long-term response to OA with evolutionary adaptation factored in.     

Experimental ocean acidification alters the allocation of metabolic energy

Energy is required to maintain physiological homeostasis in response to environmental change. Although responses to environmental stressors frequently are assumed to involve high metabolic costs, the biochemical bases of actual energy demands are rarely quantified. We studied the impact of a near-future scenario of ocean acidification [800 µatm partial pressure of CO₂ (pCO₂)] during the development and growth of an important model organism in developmental and environmental biology, the sea urchin Strongylocentrotus purpuratus. Size, metabolic rate, biochemical content, and gene expression were not different in larvae growing under control and seawater acidification treatments. Measurements limited to those levels of biological analysis did not reveal the biochemical mechanisms of response to ocean acidification that occurred at the cellular level. In vivo rates of protein synthesis and ion transport increased ∼50% under acidification. Importantly, the in vivo physiological increases in ion transport were not predicted from total enzyme activity or gene expression. Under acidification, the increased rates of protein synthesis and ion transport that were sustained in growing larvae collectively accounted for the majority of available ATP (84%). In contrast, embryos and prefeeding and unfed larvae in control treatments allocated on average only 40% of ATP to these same two processes. Understanding the biochemical strategies for accommodating increases in metabolic energy demand and their biological limitations can serve as a quantitative basis for assessing sublethal effects of global change. Variation in the ability to allocate ATP differentially among essential functions may be a key basis of resilience to ocean acidification and other compounding environmental stressors.Studies of biological responses to future scenarios of global change are of significant interest, given the most recent projections of future environmental conditions (1). In addition to important impacts in the atmosphere and on terrestrial systems, anthropogenic CO₂ emission is causing acidification of the world’s oceans (2, 3). Determining the biological responses to ocean acidification is a critical component of the study of how marine ecosystems may be altered under future scenarios of anthropogenic global environmental change. Predicting the potential for evolutionary adaptation to global change requires an understanding of the biochemical mechanisms that maintain homeostasis of physiological systems (4, 5).The developmental stages of many marine organisms have evolved cellular defenses to mitigate the impact of current environmental stressors (6). Whether these protective mechanisms can respond to future, rapid anthropogenic changes is still an open question. Marine invertebrate larvae, and particularly those with calcareous structures, have been used in numerous investigations of the biological impact of ocean acidification (2, 7–10). Although the magnitude of a response appears to be species specific, acidification can, to varying degrees, impact a wide range of biological processes in developmental forms (7–14). For instance, under near-future global mean CO₂ conditions [720–1,000 µatm partial pressure of CO₂ (pCO₂)] (1), species of larval sea urchins generally are reduced in size by 10% or less (7, 9, 15–17), but studies of metabolic rate and ion regulation suggest that acidification may result in increased metabolic costs to maintain homeostasis (11, 12). By studying responses to seawater acidification at several levels of biological organization during the development of the sea urchin, Strongylocentrotus purpuratus—from whole-organism growth, to macromolecular synthesis rates, enzyme activities, and gene expressions—we show that, although the impact of acidification at the organismal level is minimal, dramatic compensation occurs at the cellular level. Specifically, growth is maintained by changes in energy allocation to accommodate the costs required to sustain increases in protein synthesis and ion transport. We conclude that measurements limited to morphological characteristics, metabolic rate, biochemical content, and gene expression do not reveal the major biochemical response mechanisms underlying the apparent resilience to acidification in developing sea urchins.     

Consumers mediate the effects of experimental ocean acidification and warming on primary producers

It is well known that ocean acidification can have profound impacts on marine organisms. However, we know little about the direct and indirect effects of ocean acidification and also how these effects interact with other features of environmental change such as warming and declining consumer pressure. In this study, we tested whether the presence of consumers (invertebrate mesograzers) influenced the interactive effects of ocean acidification and warming on benthic microalgae in a seagrass community mesocosm experiment. Net effects of acidification and warming on benthic microalgal biomass and production, as assessed by analysis of variance, were relatively weak regardless of grazer presence. However, partitioning these net effects into direct and indirect effects using structural equation modeling revealed several strong relationships. In the absence of grazers, benthic microalgae were negatively and indirectly affected by sediment-associated microalgal grazers and macroalgal shading, but directly and positively affected by acidification and warming. Combining indirect and direct effects yielded no or weak net effects. In the presence of grazers, almost all direct and indirect climate effects were nonsignificant. Our analyses highlight that (i) indirect effects of climate change may be at least as strong as direct effects, (ii) grazers are crucial in mediating these effects, and (iii) effects of ocean acidification may be apparent only through indirect effects and in combination with other variables (e.g., warming). These findings highlight the importance of experimental designs and statistical analyses that allow us to separate and quantify the direct and indirect effects of multiple climate variables on natural communities.     

On the effects of the ocean on atmospheric CFC-11 lifetimes and emissions

The ocean is a reservoir for CFC-11, a major ozone-depleting chemical. Anthropogenic production of CFC-11 dramatically decreased in the 1990s under the Montreal Protocol, which stipulated a global phase out of production by 2010. However, studies raise questions about current overall emission levels and indicate unexpected increases of CFC-11 emissions of about 10 Gg ⋅ yr⁻¹ after 2013 (based upon measured atmospheric concentrations and an assumed atmospheric lifetime). These findings heighten the need to understand processes that could affect the CFC-11 lifetime, including ocean fluxes. We evaluate how ocean uptake and release through 2300 affects CFC-11 lifetimes, emission estimates, and the long-term return of CFC-11 from the ocean reservoir. We show that ocean uptake yields a shorter total lifetime and larger inferred emission of atmospheric CFC-11 from 1930 to 2075 compared to estimates using only atmospheric processes. Ocean flux changes over time result in small but not completely negligible effects on the calculated unexpected emissions change (decreasing it by 0.4 ± 0.3 Gg ⋅ yr⁻¹). Moreover, it is expected that the ocean will eventually become a source of CFC-11, increasing its total lifetime thereafter. Ocean outgassing should produce detectable increases in global atmospheric CFC-11 abundances by the mid-2100s, with emission of around 0.5 Gg ⋅ yr⁻¹; this should not be confused with illicit production at that time. An illustrative model projection suggests that climate change is expected to make the ocean a weaker reservoir for CFC-11, advancing the detectable change in the global atmospheric mixing ratio by about 5 yr.

Man-made chlorofluorocarbons (CFCs) are the primary cause of the Antarctic ozone hole (1). The atmospheric lifetimes of these chemicals range from about 50 to 500 yr. The Montreal Protocol agreed to a complete phase out of worldwide CFC production and consumption by 2010. Evidence for healing of the Antarctic ozone layer has indeed emerged (2, 3), indicating the overall success of the Montreal Protocol. Atmospheric loss processes of CFC-11, the most abundant ozone-destroying CFC, are due to photolysis and reaction with excited oxygen (O¹D) once the gas reaches the stratosphere. The atmospheric lifetime of CFC-11 is assumed to be inversely related to the atmospheric abundance of the molecule, with due consideration of the lag times between tropospheric and stratospheric burdens (4). Given its lifetime of about 50 to 60 yr and continued emissions from storage banks such as chillers and building insulation foams (5), the CFC-11 inventory in the atmosphere is decreasing slowly. However, the rate of decrease in atmospheric concentrations has been slowing down since about 2012, suggesting higher overall emission and an unexpected additional post-2013 emission increase of CFC-11 of about 7 to 13 Gg ⋅ yr⁻¹ [10 to 20% of the total global emission during that time (6, 7)]. The latter is clearly inconsistent with the global zero new production that has been agreed to by the Montreal Protocol.CFC-11 is soluble in water, and therefore the ocean has absorbed some CFC-11 from the atmosphere. CFC-11 ocean uptake is greatest in high latitudes where cold sea surface temperatures (SSTs) enhance CFC-11 solubility (8), and mixing and transport from the surface into the deep ocean is enhanced. By 1994, the ocean had stored up to 1% of the total anthropogenic emissions of CFC-11 (9), and by 2014, the ocean held roughly 110 Gg of CFC-11 (10), or about 5 to 10% of the CFC-11 inventory in the various anthropogenic storage banks (5). While some CFC-11 is removed in sulfidic anoxic waters (11), this effect is small for the current climate, and CFC-11 has long been employed as a useful passive tracer to study ocean circulation (e.g., refs. 12 and 13). Early studies using a global model incorporating CFC-11 air–sea fluxes suggested that the ocean’s effects on atmospheric CFC-11 lifetimes and concentrations were negligible in the 1980s, when anthropogenic emissions were high (14). However, now that anthropogenic emissions have dramatically decreased and attention is focused on unexpected emissions of 10 Gg ⋅ yr⁻¹ or even less, changes in ocean uptake of CFC-11 could be affecting the atmospheric CFC-11 inventory enough to influence emission estimates and could introduce a time-dependent effect on its total lifetime. Further, as anthropogenic emissions continue to decrease in the future, the ocean must eventually become supersaturated with respect to atmospheric CFC-11 and turn into a source instead of a sink. No study has yet estimated when that should be expected to occur and what its magnitude will be.Here, we address the following questions: 1) How is the ocean affecting the atmospheric CFC-11 inventory, the lifetime of CFC-11 in the atmosphere and its time dependence, and how does this in turn influence emission estimates based on observed concentrations? 2) When will the ocean become a source of CFC-11 to the atmosphere, and how much will ocean outgassing affect the apparent emission and atmospheric mixing ratio in the future? 3) How will climate change affect ocean CFC-11 uptake in the future?For a conceptual understanding, we use a hierarchy of models starting with a simple six-box model that simulates the CFC-11 inventory in the atmosphere, ocean mixed layer, and deep ocean layers (each layer has two boxes representing the two hemispheres, see the schematic in Fig. 1A). CFC-11 in each box is assumed to be well mixed in this illustrative model. The atmospheric CFC-11 lifetime is kept constant at 55 yr and estimated emissions are taken from published work (15). We assume constant interhemispheric exchange timescales for each layer and constant cross-layer timescales for mixed layer to deep ocean exchange (SI Appendix, Table S1). Atmospheric CFC-11’s vertical distribution does affect its lifetime and surface concentration. Here, we subsume stratosphere–troposphere exchange into our adopted atmospheric lifetime estimates assuming a well-mixed atmosphere and focus on the ocean’s effect on atmospheric CFC-11. We then replace the four ocean boxes with a more sophisticated albeit low-resolution representation of the ocean (2.8° × 2.8° horizontal resolution and 15 vertical layers down to 5,000 m), the Massachusetts Institute of Technology general circulation model (MITgcm; 16, 17), which includes a physics-based CFC-11 air–sea flux and transport into the interior ocean and treats CFC-11 as a conservative tracer in the ocean (depicted in Fig. 1B). The MITgcm (for brevity, we refer to the combined coupled box model atmosphere–ocean model simply as the MITgcm) is run in two modes. First, we use the model forced with climatological average wind stress and buoyancy fluxes (Hist run) to assess the influence of parameters (i.e., SST, wind stress, etc.,) on air–sea CFC-11 fluxes. Second, we force the MITgcm using global monthly representative concentration pathway 8.5 condition (RCP8.5) output from the Max Planck Institute Earth System Model low-resolution version (MPI-ESM-LR) fully coupled global climate model (RCP8.5 run; 18, 19). This model has been shown to provide a realistic response of the Southern Ocean (55 to 70 °S), the region that stores the most CFC-11, to the southern annular mode (20). In the RCP8.5 run, interannual variability within the MPI-ESM-LR output provides changes in the forcing of the ocean applied after 1930, but variability in the atmospheric circulation is not explicitly incorporated into the box model atmosphere. We compare these runs to a “no-ocean” run in which the CFC-11 air–sea flux is turned off. Both the box model and MITgcm runs extend from 1930 (essentially the start of emission of this anthropogenic gas) to 2300.Open in a separate windowFig. 1.Schematic diagrams showing the box model (A) and the MITgcm setup (B). The box model has three layers that represent the atmosphere, ocean mixed layer, and deep ocean. Each layer has two boxes that indicate the NH and the SH. The MITgcm setup replaces the four ocean boxes with the MITgcm ocean but keeps the atmospheric boxes unchanged. One-way arrows indicate CFC-11 atmospheric loss; two-way arrows indicate CFC-11 transport into/out of the box.     

Outdoor air pollution: nitrogen dioxide, sulfur dioxide, and carbon monoxide health effects

Nitrogen dioxide (NO2), sulfur dioxide (SO2), and carbon monoxide are important ambient air pollutants. High-intensity, confined space exposure to NO2 has caused catastrophic injury to humans, including death. Ambient NO2 exposure may increase the risk of respiratory tract infections through the pollutant's interaction with the immune system. Sulfur dioxide (SO2) contributes to respiratory symptoms in both healthy patients and those with underlying pulmonary disease. Controlled human exposure studies have demonstrated that experimental SO2 exposure causes changes in airway physiology, including increased airways resistance. Both acute and chronic exposure to carbon monoxide are associated with increased risk for adverse cardiopulmonary events, including death. However, studies have not demonstrated a clear dose-dependent health risk response to increasing amounts of these pollutants except at high concentrations. In addition, a number of studies examining the effects of ambient level exposure to NO2, SO2, and CO have failed to find associations with adverse health outcomes.     

An effect of dietary sulfur on liver inorganic sulfate in the rat

The effect of dietary sulfur on liver inorganic sulfate concentration was determined by feeding rats diets containing 0.0002, 0.02, 0.1 and 0.42% of inorganic sulfate for a period of 17 days. Each diet contained 15% of casein supplemented with decreasing levels of methionine as the dietary inorganic sulfate was increased, to keep the sulfur as sulfate concentration constant at 0.67%. The molarity of the liver sulfate calculated on the basis of moles of sulfate per 1,000 g of wet tissue was 0.98, 1.3, 2.2 and 1.5 mM for rats fed the diets containing 0.0002, 0.02, 0.1 and 0.42% of inorganic sulfate, respectively. Thus it appears that the liver sulfate pool is limited by a combination of the rate of oxidation of the sulfur-containing amino acids and the extraction of sulfate from the portal system.     

From the Cover: Physical and biogeochemical modulation of ocean acidification in the central North Pacific

Atmospheric carbon dioxide (CO₂) is increasing at an accelerating rate, primarily due to fossil fuel combustion and land use change. A substantial fraction of anthropogenic CO₂ emissions is absorbed by the oceans, resulting in a reduction of seawater pH. Continued acidification may over time have profound effects on marine biota and biogeochemical cycles. Although the physical and chemical basis for ocean acidification is well understood, there exist few field data of sufficient duration, resolution, and accuracy to document the acidification rate and to elucidate the factors governing its variability. Here we report the results of nearly 20 years of time-series measurements of seawater pH and associated parameters at Station ALOHA in the central North Pacific Ocean near Hawaii. We document a significant long-term decreasing trend of −0.0019 ± 0.0002 y⁻¹ in surface pH, which is indistinguishable from the rate of acidification expected from equilibration with the atmosphere. Superimposed upon this trend is a strong seasonal pH cycle driven by temperature, mixing, and net photosynthetic CO₂ assimilation. We also observe substantial interannual variability in surface pH, influenced by climate-induced fluctuations in upper ocean stability. Below the mixed layer, we find that the change in acidification is enhanced within distinct subsurface strata. These zones are influenced by remote water mass formation and intrusion, biological carbon remineralization, or both. We suggest that physical and biogeochemical processes alter the acidification rate with depth and time and must therefore be given due consideration when designing and interpreting ocean pH monitoring efforts and predictive models.     

Phanerozoic cycles of sedimentary carbon and sulfur

A reservoir model of a Recent steady-state sedimentary system in which the reduced sulfur and oxidized sulfur reservoirs were coupled with the oxidized carbon and reduced carbon reservoirs was constructed. The time curve of the sulfur isotope ratios of the sedimentary sulfate reservoir was used to drive the model back to the beginning of Cambrian time (600 million years ago), producing the reservoir sizes and isotope values and material fluxes of the carbon-sulfur system. The predicted values of carbon isotope ratios of the carbonate reservoir agree well with observed values, showing that the model is basically sound. Some general conclusions from this success are (i) material flux rates in the carbon-oxygen-sulfur system of the geologic past (averaged over tens of millions of years) lie within about a factor of 2 of Recent rates. (ii) The oxidation-reduction balances of Phanerozoic time were dominated by reciprocal relationships between carbon and sulfur compounds. (iii) The rate of production of atmospheric oxygen by storage in sediments of organic carbon of photosynthetic origin increased from the Cambrian Period to the Permian Period and declined somewhat from the Permian Period to the Present. (iv) The storage of oxygen in oxidized sulfur compounds kept pace (within the limits of the data) with oxygen production. (v) Transfer of oxygen from CO₂ to SO₄ from the Cambrian to the Permian Period was several times the Recent free oxygen content of the atmosphere.     

From the Cover: Global declines in coral reef calcium carbonate production under ocean acidification and warming

Ocean warming and acidification threaten the future growth of coral reefs. This is because the calcifying coral reef taxa that construct the calcium carbonate frameworks and cement the reef together are highly sensitive to ocean warming and acidification. However, the global-scale effects of ocean warming and acidification on rates of coral reef net carbonate production remain poorly constrained despite a wealth of studies assessing their effects on the calcification of individual organisms. Here, we present global estimates of projected future changes in coral reef net carbonate production under ocean warming and acidification. We apply a meta-analysis of responses of coral reef taxa calcification and bioerosion rates to predicted changes in coral cover driven by climate change to estimate the net carbonate production rates of 183 reefs worldwide by 2050 and 2100. We forecast mean global reef net carbonate production under representative concentration pathways (RCP) 2.6, 4.5, and 8.5 will decline by 76, 149, and 156%, respectively, by 2100. While 63% of reefs are projected to continue to accrete by 2100 under RCP2.6, 94% will be eroding by 2050 under RCP8.5, and no reefs will continue to accrete at rates matching projected sea level rise under RCP4.5 or 8.5 by 2100. Projected reduced coral cover due to bleaching events predominately drives these declines rather than the direct physiological impacts of ocean warming and acidification on calcification or bioerosion. Presently degraded reefs were also more sensitive in our analysis. These findings highlight the low likelihood that the world’s coral reefs will maintain their functional roles without near-term stabilization of atmospheric CO₂ emissions.

Coral reef ecosystems provide a habitat for a vast array of biodiversity (1, 2), yield billions of dollars of global revenue from fisheries and tourism (3, 4), and protect tropical shorelines from hazards such as storms (5). These functions are dependent on the maintenance of the framework structure of the reefs, the accumulation of which requires the net production of calcium carbonate by resident taxa. This net calcium carbonate production is a balance between gross production minus the loss due to physical, chemical, and biological erosion. However, the net calcium carbonate production and related potential vertical accretion of reefs is increasingly threatened by anthropogenic climate change (5). Vertical reef accretion is the product of a number of processes that include 1) biological net calcium carbonate production (gross production by calcifying taxa minus bioerosion), 2) net sediment production (gross production minus endolithic and pore water–driven dissolution), 3) sediment transport (import and export), 4) physical erosion, and 5) cementation rates (Fig. 1). We refer to this potential vertical accretion of reefs simply as “accretion” hereafter, and note that we focus on sediment dissolution and biological net carbonate production (hereafter referred to as “net carbonate production,” the measurement of which is referred to as “carbonate budgets”), perhaps the best quantified and largest contributors to accretion rates on reefs on short timescales.Open in a separate windowFig. 1.Processes involved in net carbonate production and accretion on reefs as well as the associated methods typically employed to measure this. +ve = positive contribution to accretion with solid lines; −ve = negative contribution with dashed lines. Processes in gray are not included in most carbonate budgets or here. Here, we project the effects of ocean acidification and warming on CCA and coral calcification, chemical components of bioerosion, and sediment dissolution. Only chemical components of bioerosion are included in hydrochemical measurements, while direct sediment production by bioeroders is also included here.Climate change will impact both the abundance and calcification rates of reef taxa responsible for producing calcium carbonate, such as corals and coralline algae (2, 6, 7), while simultaneously altering the bioerosion and recycling of this calcium carbonate by resident bioeroders, such as sponges and cyanobacteria (8, 9). Both net carbonate production and accretion are already declining regionally in response to fishing pressure, disease, and marine heatwaves (10–13). Such changes have profound implications for societally relevant ecosystem service provisioning (11), and rapid climate change impacts are projected to further exacerbate these negative trajectories. Specifically, ocean warming and associated marine heatwaves will reduce gross carbonate production rates on coral reefs, as coral cover is reduced by more frequent and severe mass bleaching events (14–16) and as elevated temperatures decrease the calcification rates of coral and coralline algae under more severe warming scenarios (6, 17). Ocean acidification is also projected to reduce the calcification rates of key taxa such as corals and coralline algae that form reef structures and associated sediments (6, 7, 18, 19) while further reducing accretion by increasing the dissolution of carbonate sediments (20) and enhancing rates of bioerosion (8, 9). Furthermore, the combined impacts of ocean warming and acidification are predicted to be amplified under higher CO₂ emission scenarios (6, 19).While the responses of reef-forming taxa to ocean warming and acidification have been the focus of considerable scientific effort in recent decades (2, 6, 7), quantitative predictions of the impacts of climate change on global coral reef net carbonate production and reef accretion are limited. Specifically, existing projections are largely theoretical, limited to specific locations, only include sea level rise and not ocean acidification or warming, or do not include some of the major processes controlling coral reef net carbonate production (5, 10, 20–23). For example, one prominent model provided important data on lagoon sediment dissolution rates (20), although the link between changes in these rates and forereef accretion is unclear. Other global-scale projections do not include the impacts of ocean warming or acidification (5). How the combined effects of changes in the mortality, calcification, and bioerosion rates of individual reef taxa will manifest spatially across different ocean basins due to ocean warming and acidification remains unresolved.Predicting the trajectories of future net carbonate production is complicated by uncertainties around the magnitude of future declines in coral cover, which is likely to be one of the major drivers of future carbonate budgets of coral reefs; yet, estimating future coral cover is difficult. While coral cover is declining globally due to repeated mass coral bleaching (hereafter referred to as “bleaching”) and other local stressors, there is clear temporal and spatial variability of local anthropogenic impacts (16, 24–26). This makes estimating future coral cover a complex and heavily debated process, even on local scales (27–29). The impacts of marine heatwaves on coral mortality, recovery, and subsequent recruitment cannot be captured accurately in short-term laboratory experiments in the way that changes to calcification can, and currently available projections of coral cover into the future are encumbered with uncertainties that do not allow us to predict exact future cover for any specific region (16, 30, 31).Here, we resolve these challenges of applying laboratory results to real coral reef locations by assessing changes in future carbonate budgets of reefs as a function of integrated robust estimates of the responses of major components of the carbonate budget to climate change as well as including estimates of changes in future coral cover. We collate or measure data from 233 locations on 183 distinct reefs globally (49% Atlantic, 39% in the Indian, and 11% in the Pacific Ocean) to quantify the impacts of ocean warming and acidification on coral reef net carbonate production and then use these data to estimate the impacts on net carbonate production and accretion by 2050 and 2100. We incorporate more than 800 empirically measured changes in net calcification rates of the main producers of calcium carbonate on coral reefs (corals and coralline algae), bioerosion rates, and sediment dissolution in response to ocean warming, acidification, and their interaction from 98 studies. We model the size of the effects of ocean acidification, ocean warming, and their interaction under contrasting Intergovernmental Panel on Climate Change emissions scenarios for representative concentration pathways (RCP) 2.6, 4.5, and 8.5 for the year 2050 and 2100. We then apply these estimated effects to reefs with previously measured rates of net carbonate production, where the cover of corals and coralline algae is well defined (SI Appendix, Table S1). Importantly, we account for the impact of reduced coral cover, which, in most locations, will be further diminished by more severe and frequent bleaching events (16, 25), including estimates of its impacts based on currently available information. We calculate region-specific projections of degree heating weeks (DHW), a commonly used metric that accounts for the severity and duration of marine heatwaves on corals (32) and combine them with reductions in coral cover that were measured after exposure to differing DHWs during the 2016 El Niño event (25). These models (Materials and Methods) are then used to explore the effects of ocean warming and acidification independently, and in interaction with each other, under each climatic scenario on rates of reef net carbonate production and accretion.     

Impact of urban atmospheric pollution on coronary disease.

Recent epidemiological findings have suggested that urban atmospheric pollution may have adverse effects on the cardiovascular system as well as on the respiratory system. We carried out an exhaustive search of published studies investigating links between coronary heart disease and urban atmospheric pollution. The review was conducted on cited articles published between 1994 and 2005 and whose main objective was to measure the risk of ischaemic heart diseases related to urban pollution. Of the 236 references identified, 46 epidemiological studies were selected for analysis on the basis of pre-defined criteria. The studies were analysed according to short-term effects (time series and case-crossover designs) and long-term effects (case-control and cohort studies). A link between coronary heart disease and at least one of the pollutants studied (PM10, O3, NOx, CO, SO2) emerged in 40 publications. Particulate matter, nitrogen oxides, and carbon monoxide were the pollutants most often linked with coronary heart disease. The association was inconstant for O3. Although the mean mortality or morbidity risk related to urban atmospheric pollution is low compared with that associated with other better-known risk factors, its impact on health is nevertheless major because of the large number of people who are exposed. This exhaustive review supports the possibility that urban pollution is indeed an environmental cardiovascular risk factor and should be considered as such by the cardiologists.     

Impact of atmospheric dispersion and transport of viral aerosols on the epidemiology of influenza

Current theories of influenza viral epidemiology have not explained the persistence, seasonality, and explosive outbreaks of virus over large geographic areas. It is postulated in this paper that atmospheric dispersion and intercontinental scale transport of airborne aerosolized influenza virus may contribute to the spread, persistence, and ubiquity of the disease, the explosiveness of epidemics, and the prompt region-wide occurrence of outbreaks and that seasonal changes in circulation patterns and the dispersive character of the atmosphere may help to explain the regular annual cycle of influenza activity.     

Biodiversity of coral reef cryptobiota shuffles but does not decline under the combined stressors of ocean warming and acidification

Ocean-warming and acidification are predicted to reduce coral reef biodiversity, but the combined effects of these stressors on overall biodiversity are largely unmeasured. Here, we examined the individual and combined effects of elevated temperature (+2 °C) and reduced pH (−0.2 units) on the biodiversity of coral reef communities that developed on standardized sampling units over a 2-y mesocosm experiment. Biodiversity and species composition were measured using amplicon sequencing libraries targeting the cytochrome oxidase I (COI) barcoding gene. Ocean-warming significantly increased species richness relative to present-day control conditions, whereas acidification significantly reduced richness. Contrary to expectations, species richness in the combined future ocean treatment with both warming and acidification was not significantly different from the present-day control treatment. Rather than the predicted collapse of biodiversity under the dual stressors, we find significant changes in the relative abundance but not in the occurrence of species, resulting in a shuffling of coral reef community structure among the highly species-rich cryptobenthic community. The ultimate outcome of altered community structure for coral reef ecosystems will depend on species-specific ecological functions and community interactions. Given that most species on coral reefs are members of the understudied cryptobenthos, holistic research on reef communities is needed to accurately predict diversity–function relationships and ecosystem responses to future climate conditions.

As the concentration of atmospheric carbon dioxide (pCO₂) continues to rise, marine biodiversity is predicted to decline due to ocean-warming and acidification (1). Warming seas and increased acidity are expected to disproportionately affect marine ecosystems built by calcifying biota (2–4). Coral reefs are among the most sensitive marine ecosystems affected by global stressors, because the primary ecosystem engineers, calcifying scleractinian corals and coralline algae, show direct physiological responses to both elevated temperature and acidification, resulting in strong indirect effects on habitat structure and community composition (5, 6). In this century alone, record-breaking sea surface temperature anomalies have resulted in widespread coral mortality (7, 8), leading to a reduction in topographic complexity (9) and a shift in community composition (10, 11). Likewise, in situ observations of coral reefs along naturally occurring gradients of acidification have shown declines in habitat complexity (5, 6) and diversity (12, 13), as well as changes in community structure (14, 15). The combination of both thermal stress and acidification stress over the coming decades is predicted to have synergistic negative effects on reef resilience (2, 3, 16) by eroding the reef framework (17), shifting the structural dominance away from calcifiers and severely diminishing the biodiversity of this iconic ecosystem (2, 4). Coral reefs occupy less than 1% of the seafloor but house over 25% of all marine species; the loss of biodiversity due to anthropogenic stressors is predicted to lead to the functional collapse of these ecosystems later this century (2, 4, 18, 19). However, future projections of the combined effects of increased temperature and acidity on biodiversity have typically been derived from reviews and meta-analyses based on short-term, single-species experimental manipulations (20–23) or from in situ observations of a handful of taxa along natural gradients of seawater chemistry or temperature (5, 7, 12, 13, 24).Although such studies have informed our understanding of how some reef communities may change in the future, tradeoffs also exist for each approach in understanding climate impacts on biodiversity. Natural gradient studies do not simultaneously incorporate end-of-the-century levels of both acidification and warming, and short-term perturbation experiments are typically performed over days to weeks on single focal species. While short-term perturbation experiments across life stages have been instrumental in understanding how changes in ocean temperature, chemistry, and their combined effects influence organismal physiology (25, 26), they do not include diurnal or seasonal environmental changes (27–29) or realistic multispecies communities, which inherently excludes the roles of environmental variation and ecological interactions from contributing to the measured responses. Species interactions could be critical to experimental outcomes because they can modify population growth rates, behavior, consumption, reproduction, production, the efficacy of defensive structures, and resource availability, thereby influencing species densities, composition, and richness through competition, facilitation, or predation (30–34). Ultimately, species interactions determine whether ecosystem functions are maintained or diminished under altered environmental conditions (35–37). Thus, there is a pressing need for long-term, multispecies experimental work to understand the responses of complex communities to future climate change scenarios.Here, we examined the independent and combined effects of ocean-warming and acidification on the biodiversity of coral reef communities in long-term (2-y) mesocosms. In experimental flow-through mesocosms that received unfiltered seawater drawn from an adjacent reef slope, we examined the cryptobiota communities that developed on standardized habitats (two-tiered Autonomous Reef Monitoring Structures, or ARMS) (38) in each of four treatments: present-day pH and temperature (Control treatment), ocean acidification (−0.2 pH units—Acidified treatment), ocean-warming (+2 °C—Heated treatment), and future ocean combined stressors (−0.2 pH units and +2 °C—Acidified-Heated treatment) (SI Appendix, Fig. S1). These experimental ocean-warming and acidification conditions reflect those predicted for the late 21st century given current commitments under the Paris Climate Accord (roughly intermediate between Representative Concentration Pathways RCP 6.0 and RCP 8.5) (39).Each mesocosm was initially established with a 2-cm layer of carbonate reef sand and gravel as well as pieces of reef rubble (three replicate 10- to 20-cm pieces randomly divided among mesocosms) collected from the adjacent reef, thereby including natural infaunal and surface-attached communities. A juvenile (3- to 8-cm) Convict surgeonfish (Acanthurus triostegus), a generalist grazer on benthic algae, a Threadfin butterflyfish (Chaetodon auriga), a generalist grazer on noncoral invertebrates, and five herbivorous reef snails (Trochus sp.) were added to each tank to provide the essential ecological functions of herbivory and predation in the mesocosms at biomass values approximating Hawaiian reefs (40). Finally, the eight regionally most common reef-building coral species (Montipora capitata, Montipora flabellata, Montipora patula, Pocillopora acuta, Pocillopora meandrina, Porites compressa, Porites evermanni, and Porites lobata) were added as small fragments to each the mesocosms for an initial coral cover of ∼10% to begin the experiment. The corals and rubble were placed on a plastic grate 6 cm above the sediments to simulate their attachment to hard substrate in nature, and the ARMS were placed underneath the grate to simulate the location of the cryptobenthic habitat (SI Appendix, Fig. S2). Among the added species, only one species of coral was extirpated from a single treatment. Thus, we target the cryptobenthic community here, because they comprise the vast majority of biodiversity on coral reefs (41) and show significant community responses to our experimental treatments. Furthermore, due to the challenges associated with surveying the cryptobiota using visual census, these organisms are often overlooked in coral reef climate change research despite their essential roles in nutrient cycling, cementation, trophodynamics, and other ecological processes (42–45). As studies are increasingly pointing toward the critical functional importance of this community in food webs and the maintenance of biodiversity on coral reefs (43, 45, 46), there is a need to diminish the existing knowledge gap on both taxonomic composition and ecosystem function of this community in response to climate change.After two years of exposure, we examined the coral reef community that had developed on each ARMS unit. We generated amplicon sequence libraries targeting cytochrome oxidase I (COI) (the most extensive barcode database currently available) from each unit to test whether species richness, community composition (occurrence), or community structure (relative abundance) of the cryptobenthic community changed with treatment. This experimental study evaluates the richness and composition of an entire coral reef community which developed over a multiyear time frame under predicted future ocean conditions.     

Carbohydrate-like composition of submicron atmospheric particles and their production from ocean bubble bursting

Oceans cover over two-thirds of the Earth’s surface, and the particles emitted to the atmosphere by waves breaking on sea surfaces provide an important contribution to the planetary albedo. During the International Chemistry Experiment in the Arctic LOwer Troposphere (ICEALOT) cruise on the R/V Knorr in March and April of 2008, organic mass accounted for 15–47% of the submicron particle mass in the air masses sampled over the North Atlantic and Arctic Oceans. A majority of this organic component (0.1 - 0.4 μ m^-3) consisted of organic hydroxyl (including polyol and other alcohol) groups characteristic of saccharides, similar to biogenic carbohydrates found in seawater. The large fraction of organic hydroxyl groups measured during ICEALOT in submicron atmospheric aerosol exceeded those measured in most previous campaigns but were similar to particles in marine air masses in the open ocean (Southeast Pacific Ocean) and coastal sites at northern Alaska (Barrow) and northeastern North America (Appledore Island and Chebogue Point). The ocean-derived organic hydroxyl mass concentration during ICEALOT correlated strongly to submicron Na concentration and wind speed. The observed submicron particle ratios of marine organic mass to Na were enriched by factors of ∼10²–∼10³ over reported sea surface organic to Na ratios, suggesting that the surface-controlled process of film bursting is influenced by the dissolved organic components present in the sea surface microlayer. Both marine organic components and Na increased with increasing number mean diameter of the accumulation mode, suggesting a possible link between organic components in the ocean surface and aerosol–cloud interactions.     

Ocean acidification slows nitrogen fixation and growth in the dominant diazotroph Trichodesmium under low-iron conditions

Dissolution of anthropogenic CO₂ increases the partial pressure of CO₂ (pCO₂) and decreases the pH of seawater. The rate of Fe uptake by the dominant N₂-fixing cyanobacterium Trichodesmium declines as pH decreases in metal-buffered medium. The slower Fe-uptake rate at low pH results from changes in Fe chemistry and not from a physiological response of the organism. Contrary to previous observations in nutrient-replete media, increasing pCO₂/decreasing pH causes a decrease in the rates of N₂ fixation and growth in Trichodesmium under low-Fe conditions. This result was obtained even though the bioavailability of Fe was maintained at a constant level by increasing the total Fe concentration at low pH. Short-term experiments in which pCO₂ and pH were varied independently showed that the decrease in N₂ fixation is caused by decreasing pH rather than by increasing pCO₂ and corresponds to a lower efficiency of the nitrogenase enzyme. To compensate partially for the loss of N₂ fixation efficiency at low pH, Trichodesmium synthesizes additional nitrogenase. This increase comes partly at the cost of down-regulation of Fe-containing photosynthetic proteins. Our results show that although increasing pCO₂ often is beneficial to photosynthetic marine organisms, the concurrent decreasing pH can affect primary producers negatively. Such negative effects can occur both through chemical mechanisms, such as the bioavailability of key nutrients like Fe, and through biological mechanisms, as shown by the decrease in N₂ fixation in Fe-limited Trichodesmium.     

The role of nutricline depth in regulating the ocean carbon cycle

Carbon uptake by marine phytoplankton, and its export as organic matter to the ocean interior (i.e., the “biological pump”), lowers the partial pressure of carbon dioxide (pCO₂) in the upper ocean and facilitates the diffusive drawdown of atmospheric CO₂. Conversely, precipitation of calcium carbonate by marine planktonic calcifiers such as coccolithophorids increases pCO₂ and promotes its outgassing (i.e., the “alkalinity pump”). Over the past ≈100 million years, these two carbon fluxes have been modulated by the relative abundance of diatoms and coccolithophores, resulting in biological feedback on atmospheric CO₂ and Earth's climate; yet, the processes determining the relative distribution of these two phytoplankton taxa remain poorly understood. We analyzed phytoplankton community composition in the Atlantic Ocean and show that the distribution of diatoms and coccolithophorids is correlated with the nutricline depth, a proxy of nutrient supply to the upper mixed layer of the ocean. Using this analysis in conjunction with a coupled atmosphere–ocean intermediate complexity model, we predict a dramatic reduction in the nutrient supply to the euphotic layer in the coming century as a result of increased thermal stratification. Our findings indicate that, by altering phytoplankton community composition, this causal relationship may lead to a decreased efficiency of the biological pump in sequestering atmospheric CO₂, implying a positive feedback in the climate system. These results provide a mechanistic basis for understanding the connection between upper ocean dynamics, the calcium carbonate-to-organic C production ratio and atmospheric pCO₂ variations on time scales ranging from seasonal cycles to geological transitions.     

Attenuation of sinking particulate organic carbon flux through the mesopelagic ocean

The biological carbon pump, which transports particulate organic carbon (POC) from the surface to the deep ocean, plays an important role in regulating atmospheric carbon dioxide (CO₂) concentrations. We know very little about geographical variability in the remineralization depth of this sinking material and less about what controls such variability. Here we present previously unpublished profiles of mesopelagic POC flux derived from neutrally buoyant sediment traps deployed in the North Atlantic, from which we calculate the remineralization length scale for each site. Combining these results with corresponding data from the North Pacific, we show that the observed variability in attenuation of vertical POC flux can largely be explained by temperature, with shallower remineralization occurring in warmer waters. This is seemingly inconsistent with conclusions drawn from earlier analyses of deep-sea sediment trap and export flux data, which suggest lowest transfer efficiency at high latitudes. However, the two patterns can be reconciled by considering relatively intense remineralization of a labile fraction of material in warm waters, followed by efficient downward transfer of the remaining refractory fraction, while in cold environments, a larger labile fraction undergoes slower remineralization that continues over a longer length scale. Based on the observed relationship, future increases in ocean temperature will likely lead to shallower remineralization of POC and hence reduced storage of CO₂ by the ocean.Atmospheric carbon dioxide (CO₂) levels are strongly influenced by the production, sinking, and subsequent remineralization of particulate organic carbon (POC) in the ocean (1), with the atmospheric concentration partially set by the depth at which regeneration occurs (2). Numerous studies have endeavored to describe the complex interactions that produce the typically observed depth profile of sinking POC flux attenuation as relatively simple mathematical forms (3–6), with perhaps the most commonly used being a power law equation:f_z = f_z0(z/z₀)^?b[1]where f_z is the flux at depth z, normalized to flux at some reference depth, z₀, and b is the coefficient of flux attenuation (7). This relationship was originally derived from POC flux measurements from several eastern North Pacific locations, and an open ocean composite b value of 0.86 was calculated (7), a value which has since been used extensively in biogeochemical models (8) and to normalize fluxes measured in different regions and at different depths (9, 10). Regional variations in b, from 0.6 to 2.0, have since been demonstrated by deep-sea (>2,000 m) sediment trap studies (11, 12). More recently, mesopelagic POC flux attenuation between 150 m and 500 m depth was measured in the Vertical Transport in the Global Ocean (VERTIGO) project at two contrasting sites in the North Pacific (13), using neutrally buoyant sediment traps (NBSTs) developed to improve the reliability of upper ocean flux measurements (14). During VERTIGO, two deployments of multiple NBSTs at station ALOHA [“A Long-term Oligotrophic Habitat Assessment”; a tropical oligotrophic site characterized by low surface chlorophyll and warm temperatures (15)] and two at station K2 [a seasonally variable mesotrophic site in the northwest Pacific subarctic gyre with relatively cold waters (15)] yielded b values of 1.25 and 1.36 and of 0.57 and 0.49, respectively (16). Community structure (17), mineral ballasting (4, 12, 18), temperature (19), and oxygen concentration (20) have all been proposed as factors important in explaining these variations in the vertical profile of organic carbon remineralization, through their influence on particle sinking speed, POC degradation rate, or both.An alternative approach proposed to describe POC flux attenuation in the upper ocean is the use of an exponential equation (Eq. 2) that relates the flux at any depth to the flux measured at a reference depth by the remineralization length scale, z*, defined as the depth interval over which the flux decreases by a factor of 1/e (5, 6, 21),f_z = f_z0exp(?(z ? z₀)/z^?).[2]This approach is advocated by some because, unlike the power law equation (Eq. 1), which is sensitive to the reference depth used, the resulting length scale is not affected by use of either the absolute depth or the depth relative to the base of the mixed layer or euphotic zone (21).In this study, multiple neutrally buoyant PELAGRA (particle export measurement using a Lagrangian trap) sediment traps (22) (SI Text) were deployed at each of four locations in the North Atlantic: the Porcupine Abyssal Plain (PAP) time-series site, the Irminger and Iceland Basins, and within the North Atlantic subtropical gyre (NASG; Fig. 1A and Table S1). Fluxes of POC, lithogenic material, and the biominerals opal and calcium carbonate (CaCO₃) were measured at each site (Fig. 1 B and C), thereby increasing the range of locations for which these measurements have been made using NBSTs at multiple (≥3) depths.Open in a separate windowFig. 1.(A) Bathymetry map of the North Atlantic showing the locations of four multiple PELAGRA trap deployments; (B) measured fluxes of POM, opal, CaCO₃, and lithogenic material at each site; (C) corresponding measured POC fluxes at each site, with profiles fitted using Eq. 1 and extrapolated between 50 m and 1000 m depth. Error bars represent relative SD from replicate measurements and are generally smaller than symbols. POC flux plots show the calculated b value along with SE, r² value, and P value.