Search

Global and local anthropogenic stressors such as climate change, acidification, overfishing, and pollution are expected to shift the benthic community composition of coral reefs from dominance by calcifying organisms to dominance by non‐calcifying algae. These changes could reduce the ability of coral reef ecosystems to maintain positive net calcium carbonate accretion. However, relationships between community composition and calcification rates remain unclear. We performed field experiments to quantify the metabolic rates of the two most dominant coral reef substrate types, live coral and dead coral substrate colonized by a mixed algal assemblage, using a novel underwater respirometer. Our results revealed that calcification rates in the daytime were similar for the live coral and dead coral substrate communities. However, in the dark, while live corals continued to calcify at slower rates, the dead coral substrate communities exhibited carbonate dissolution. Daytime net photosynthesis of the dead coral substrate communities was up to five times as much as for live corals, which we hypothesize may have created favorable conditions for the precipitation of carbonate minerals. We conclude that: (1) calcification from dead coral substrate communities can contribute to coral reef community calcification during the day, and (2) dead coral substrate communities can also contribute to carbonate mineral dissolution at night, decreasing ecosystem calcification over a diel cycle. This provides evidence that reefs could shift from slow, long‐term accretion of calcium carbonate to a state where large daily cycling of calcium carbonate occurs, but with little or no long‐term accumulation of the carbonate minerals needed to sustain the reef against erosional forces. [ABSTRACT FROM AUTHOR]

Solar radiation modification has been suggested as a backup option to reduce anthropogenic warming. Marine cloud brightening (MCB) and ocean albedo modification (OAM) are two proposed approaches to intentionally reflect sunlight back to space over oceanic regions. Using the NCAR Community Earth System Model, we compare climate response to MCB and OAM under the framework of fast adjustment and slow feedback. We implement MCB and OAM uniformly over the global ocean to offset CO2‐induced warming. We find that to offset 3.3 K global mean warming from a doubling of CO2, diagnosed effective radiative forcing is −4.8 and −3.6 W m−2 for OAM and MCB, respectively. Correspondingly, radiative forcing efficacy of OAM is about 70% of MCB. Fast climate adjustment differs in response to MCB and OAM forcing. MCB cools the lower atmosphere by reflecting sunlight from cloud, causing a reduction in sunlight absorption in the atmosphere. In contrast, OAM, by reflecting more sunlight from surface, increases shortwave heating of the lower atmosphere, leading to a decrease in low marine clouds and hence a positive cloudy‐sky shortwave forcing that partly compensates the negative clear‐sky shortwave forcing. The slow climate response and pattern of equilibrium climate change are similar between MCB and OAM. As for hydrological cycle, relative to the climate under a doubling of CO2, both MCB and OAM produce an increase in precipitation and runoff over tropical land. Plain Language Summary: To counteract anthropogenic CO2‐induced global warming, various climate intervention approaches have been proposed. Among them, two approaches intend to reflect more sunlight back to space over ocean. One is marine cloud brightening that involves enhancement of marine cloud albedo. The other is ocean albedo modification that involves enhancement of ocean surface albedo. This study uses an Earth system model to compare climate response to these two climate intervention approaches. In our simulations, both approaches are designed to offset global mean surface warming caused by a doubling of atmospheric CO2. For the same amount of global cooling, ocean albedo modification produces a much stronger negative radiative forcing, implying that the forcing efficacy is smaller than that of marine cloud brightening. Under fixed sea surface temperature, fast climate adjustments, including changes in cloud and precipitation, differ substantially between these two approaches. This is mainly because ocean albedo modification reflects more sunlight from ocean surface, heating the lower atmosphere, but marine cloud brightening reflects more sunlight from cloud, cooling the lower atmosphere. When the climate system reaches a new equilibrium state after the adjustment of the atmosphere and upper ocean, these two approaches produce similar pattern of climate change. Key Points: We use NCAR CESM to analyze the fast and slow climate response to ocean albedo modification and marine cloud brighteningDiagnosed forcing efficacy of global implementation for marine cloud brightening is 40% greater than for ocean albedo modificationOcean albedo modification and marine cloud brightening produce similar pattern of equilibrium climate change [ABSTRACT FROM AUTHOR]

Global mean surface air temperature (Tglobal) variability on subdecadal timescales can be of substantial magnitude relative to the long‐term global warming signal, and such variability has been associated with considerable environmental and societal impacts. Therefore, probabilistic foreknowledge of short‐term Tglobal evolution may be of value for anticipating and mitigating some course‐resolution climate‐related risks. Here we present a simple, empirically based methodology that utilizes only global spatial patterns of annual mean surface air temperature anomalies to predict subsequent annual Tglobal anomalies via partial least squares regression. The method's skill is primarily achieved via information on the state of long‐term global warming as well as the state and recent evolution of the El Niño–Southern Oscillation and the Interdecadal Pacific Oscillation. We test the out‐of‐sample skill of the methodology using cross validation and in a forecast mode where statistical predictions are made precisely as they would have been if the procedure had been operationalized starting in the year 2000. The average forecast errors for lead times of 1 to 4 years are smaller than naïve benchmarks on average, and they perform favorably relative to most dynamical Global Climate Models retrospectively initialized to the observed state of the climate system. Thus, this method can be used as a computationally efficient benchmark for dynamical model forecast systems. Plain Language Summary: Year‐to‐year global temperature variability can be large compared to the long‐term progression of global warming, and such year‐to‐year variability has been shown to have considerable environmental and societal effects. Thus, approximate foreknowledge of yearly global temperature deviations should be of value for anticipating some climate impacts. This study presents a relatively simple, empirical method for predicting year‐to‐year global temperature. We show that information on the global spatial patterns of surface air temperature alone can be used to skillfully predict global average temperature 1 to 4 years ahead of time. We find that the method performs favorably compared to predictions from much more computationally expensive Global Climate Models. Key Points: Global patterns of annual surface air temperature can be used to predict subsequent annual global mean temperature deviationsThe state of Global Warming, El Niño, and the IPO constrains subsequent annual global mean temperature to within ~0.45°C (95% confidence)The method is skillful relative to naïve benchmarks, and it is competitive with hindcasts from the best initialized Global Climate Models [ABSTRACT FROM AUTHOR]

A number of radiation modification approaches have been proposed to counteract anthropogenic warming by intentionally altering Earth's shortwave or longwave fluxes. While several previous studies have examined the climate effect of different radiation modification approaches, only a few have investigated the carbon cycle response. Our study examines the response of plant carbon uptake to four radiation modification approaches that are used to offset the global mean warming caused by a doubling of atmospheric CO2. Using the National Center for Atmospheric Research Community Earth System Model, we performed simulations that represent four idealized radiation modification options: solar constant reduction, sulfate aerosol increase (SAI), marine cloud brightening, and cirrus cloud thinning (CCT). Relative to the high CO2 state, all these approaches reduce gross primary production (GPP) and net primary production (NPP). In high latitudes, decrease in GPP is mainly due to the reduced plant growing season length, and in low latitudes, decrease in GPP is mainly caused by the enhanced nitrogen limitation due to surface cooling. The simulated GPP for sunlit leaves decreases for all approaches. Decrease in sunlit GPP is the largest for SAI which substantially decreases direct sunlight, and the smallest for CCT, which increases direct sunlight that reaches the land surface. GPP for shaded leaves increases in SAI associated with a substantial increase in surface diffuse sunlight, and decreases in all other cases. The combined effects of CO2 increase and radiation modification result in increases in primary production, indicating the dominant role of the CO2 fertilization effect on plant carbon uptake. Plain Language Summary: A number of radiation modification approaches have been proposed to intentionally alter Earth's radiation balance to counteract anthropogenic warming. However, only a few studies have analyzed the potential impact of these approaches on the terrestrial plant carbon cycle. Here, we simulate four idealized radiation modification approaches, which include direct reduction of incoming solar radiation, increase in stratospheric sulfate aerosols concentration, enhancement of marine low cloud albedo, and decrease in high‐level cirrus cloud cover, and analyze changes in plant photosynthesis and respiration. The first three approaches cool the earth by reducing incoming solar radiation, and the last approach allows more outgoing thermal radiation. These approaches are designed to offset the global mean warming caused by doubled atmospheric CO2. Compared to the high CO2 world, all approaches will limit plant growth due to induced surface cooling in high latitudes and will lead to reduced nitrogen supply in low latitudes, leading to an overall reduction in the plant carbon uptake over land. Different approaches also produce different changes in surface direct and diffuse sunlight, which has important implications for plant photosynthesis. Relative to the unperturbed climate, the combined effects of enhanced CO2 and radiation modifications leads to an increase in plants' primary production. Key Points: We use NCAR CESM to investigate the plant carbon uptake response to four radiation modification approachesAll radiation modification approaches reduce primary production over land relative to climate with high CO2 and no radiation modificationThe impact of radiation modification on primary production is sensitive to the treatment of nitrogen limitations [ABSTRACT FROM AUTHOR]

Solar geoengineering by deliberate injection of sulfate aerosols in the stratosphere is one of the proposed options to counter anthropogenic climate warming. In this study, we focus on the effect of a specific microphysical property of sulfate aerosols in the stratosphere: hygroscopic growth—the tendency of particles to grow by accumulating water. We show that stratospheric sulfate aerosols, for a given mass of sulfates, cause more cooling when prescribed at the lower levels of the stratosphere because of hygroscopic growth. The larger relative humidity in the lower stratosphere causes an increase in the aerosol size through hygroscopic growth that leads to a larger scattering efficiency. In our study, hygroscopic growth provides an additional cooling of 23% (0.7 K) when 20 Mt‐SO4 of sulfate aerosols, an amount that approximately offsets the warming due to a doubling of CO2, are prescribed at 100 hPa. The hygroscopic effect becomes weaker at higher levels as relative humidity decreases with height. Hygroscopic growth also leads to secondary effects such as an increase in near‐infrared shortwave absorption by the aerosols that causes a decrease in high clouds and an increase in stratospheric water vapor. The altitude dependence of the effects of hygroscopic growth is opposite to that of sedimentation effects or the fast adjustment effects due to aerosol‐induced warming identified in a recent study. Plain Language Summary: The injection of sulfate aerosols into the stratosphere is one of the proposed solar geoengineering methods to reduce anthropogenic warming. There are several uncertainties associated with the effects of stratospheric aerosol injection such as aerosol microphysics, transport, removal, and other physical and chemical changes of the aerosols. In this study, we have investigated the effects of a single source of uncertainty—the hygroscopic growth which depends on the ambient relative humidity in the stratosphere. Hygroscopic growth could lead to an increase in the size of the aerosols and consequently an increase in scattering efficiency. As the relative humidity is larger in the lower stratosphere, we find that the cooling efficiency of a fixed mass of sulfates is larger when they are prescribed in the lower levels of the stratosphere. This effect of hygroscopic growth can be substantial: for 20 Mt‐SO4 that offsets the warming from a doubling of CO2, we find an additional cooling of about 20%. Our results indicate the need for an accurate parametrization of aerosol microphysical and radiative processes in climate models for an improved understanding of the effects of stratospheric aerosol injection. Key Points: The effects of hygroscopic growth of sulfate aerosols in stratospheric aerosol geoengineering are investigatedA specified mass of sulfates is more efficient in cooling the climate when they reside in the lower stratosphere due to hygroscopic growthHygroscopic growth can provide ~20% additional cooling when sulfates are used to offset warming from a doubling of CO2 [ABSTRACT FROM AUTHOR]

Solar geoengineering has been suggested as a potential means to counteract anthropogenic warming. Major volcanic eruptions have been used as natural analogues to large‐scale deployments of stratospheric aerosol geoengineering, yet difference in climate responses to these forcings remains unclear. Using the National Center for Atmospheric Research Community Earth System Model, we compare climate responses to two highly idealized stratospheric aerosol forcings that have different durations: a short‐term pulse representative of volcanic eruptions and a long‐term sustained forcing representative of geoengineering. For the same amount of global mean cooling, decreases in land temperature, precipitation, and runoff in the pulse case are much larger than that in the sustained case. The spatial pattern changes differ substantially between these two cases. Thus, direct extrapolations from volcanic eruption observations provide limited insight into impacts of potential stratospheric aerosol geoengineering. However, simulations of volcanic eruptions can be useful to test process representations in models that are used to simulate geoengineering deployments. Plain Language Summary: Major volcanic eruptions are considered as natural analogues for stratospheric sulfate aerosol geoengineering that aims to cool the climate by increasing the burden of stratospheric sulfate aerosols. Volcanic eruptions produce a layer of sulfate aerosols that stays in the stratosphere for a couple of years, whereas geoengineering efforts would need to sustain the aerosol layer persistently to counteract CO2‐induced warming. Here we use a climate model to compare climate changes in response to a volcano‐like pulse aerosol forcing and a geoengineering‐like sustained aerosol forcing. When producing similar amount of global mean cooling, the pulse aerosol forcing results in a much larger reduction in land temperature and land minus ocean temperature when compared to that induced by a sustained aerosol forcing. Also, both land precipitation and runoff decrease more in response to the pulse aerosol forcing. Spatial patterns of temperature and the hydrological cycle change also differ substantially between these two types of forcings. These differences in the climate response between the pulse forcing and sustained forcing clearly show that caution should be taken when using climate consequences of volcanic eruptions to directly infer climate responses to stratospheric aerosol geoengineering. Key Points: We simulate the climate impact of a volcanic‐like pulse and a geoengineering‐like sustained stratospheric sulfate aerosol forcingFor the same global mean cooling, decrease in land temperature, precipitation, and runoff is much larger under the pulse forcingDifferent timescales of aerosol forcing related to volcanic eruptions and geoengineering would produce quite different climate outcomes [ABSTRACT FROM AUTHOR]

In this study, we use the National Center for Atmospheric Research Community Earth System Model to investigate the contribution of sea ice and land snow to the climate sensitivity in response to increased atmospheric carbon dioxide content. We focus on the overall effect arising from the presence or absence of sea ice and/or land snow. We analyze our results in terms of the radiative forcing and climate feedback parameter. We find that the presence of sea ice and land snow decreases the climate feedback parameter (and thus increases climate sensitivity). Adjusted radiative forcing from added carbon dioxide is comparatively less sensitive to the presence of sea ice or land snow. The effect of sea ice on the climate feedback parameter decreases as sea ice cover diminishes at higher CO2 concentration. However, the influence of both sea ice and land snow on the climate feedback parameter remains substantial under the CO2 concentration range considered here (to eight times preindustrial CO2 content). Approximately, one quarter of the effect of sea ice and land snow on the climate feedback parameter is a consequence of the effect of these components on longwave feedback that is mainly associated with cloud change. Polar warming in response to added CO2 is approximately doubled by the presence of sea ice and land snow. Relative to the case in which sea ice and land snow are absent in the model, in response to increased CO2 concentrations, the presence of sea ice and land snow results in an increase in global mean warming by over 40%. Plain Language Summary: Sea ice and land snow are two crucial components that affect the climate response to external forcings. Feedbacks between ice/snow and climate change cause amplified surface warming in high latitudes. In this study, we use a climate model to estimate the contribution of sea ice and land snow to climate change in response to increased CO2 concentrations. We compare the climate response to increased CO2 between the simulations with sea ice and/or land snow and the simulations without them. We show that the existence of sea ice and land snow substantially amplifies the global temperature response to increased CO2 with sea ice having a stronger effect than land snow. Under higher CO2 levels, the effect of sea ice diminishes more rapidly than does the effect of land snow. About one quarter of the total climate feedback from sea ice and land snow is associated with the change in longwave radiation. Also we show that the effect of sea ice and land snow on the sensitivity of top‐of‐atmosphere net energy flux to the global mean temperature change is approximately additive. Key Points: We use NCAR CESM to investigate the overall contribution of sea ice and land snow to climate feedback to increased carbon dioxide contentSea ice and land snow lead to substantial increases in polar warming to increased CO2 content with sea ice playing a more important roleThe presence of sea ice and land snow decreases the climate feedback parameter with about one quarter of the effect from longwave feedback [ABSTRACT FROM AUTHOR]

Abstract: Ocean acidification threatens many marine organisms, especially marine calcifiers. The only global‐scale solution to ocean acidification remains rapid reduction in CO2 emissions. Nevertheless, interest in localized mitigation strategies has grown rapidly because of the recognized threat ocean acidification imposes on natural communities, including ones important to humans. Protection of seagrass meadows has been considered as a possible approach for localized mitigation of ocean acidification due to their large standing stocks of organic carbon and high productivity. Yet much work remains to constrain the magnitudes and timescales of potential buffering effects from seagrasses. We developed a biogeochemical box model to better understand the potential for a temperate seagrass meadow to locally mitigate the effects of ocean acidification. Then we parameterized the model using data from Tomales Bay, an inlet on the coast of California, USA which supports a major oyster farming industry. We conducted a series of month‐long model simulations to characterize processes that occur during summer and winter. We found that average pH in the seagrass meadows was typically within 0.04 units of the pH of the primary source waters into the meadow, although we did find occasional periods (hours) when seagrass metabolism may modify the pH by up to ±0.2 units. Tidal phasing relative to the diel cycle modulates localized pH buffering within the seagrass meadow such that maximum buffering occurs during periods of the year with midday low tides. Our model results suggest that seagrass metabolism in Tomales Bay would not provide long‐term ocean acidification mitigation. However, we emphasize that our model results may not hold in meadows where assumptions about depth‐averaged net production and seawater residence time within the seagrass meadow differ from our model assumptions. Our modeling approach provides a framework that is easily adaptable to other seagrass meadows in order to evaluate the extent of their individual buffering capacities. Regardless of their ability to buffer ocean acidification, seagrass meadows maintain many critically important ecosystem goods and services that will be increasingly important as humans increasingly affect coastal ecosystems. [ABSTRACT FROM AUTHOR]

Earth's Future invited 'leading experts in the field of geoengineering research to contribute brief reflections (2-5 pages) on the development of the discussion over the past decade and to consider where it may be going in the next 10 years.' Responding to this request, we offer the following text in the spirit of reflections that emphasize our personal roles and viewpoints. The primary focus of many of our comments is solar geoengineering and not carbon dioxide removal (CDR). Thus, this text is not intended to comprise a comprehensive review or set of carefully documented analyses. Our primary conclusion is that sustained progress in 'geoengineering' research will depend on social and material support for experimental work that can provide the observational basis for improved modeling and analysis, and, potentially, development and deployment of systems that may help protect the environment and improve human well-being. Relevant issues, and potential future trajectories, for CDR technologies may differ dramatically from those for solar geoengineering technologies. [ABSTRACT FROM AUTHOR]

Forestry - including afforestation (the planting of trees on land where they have not recently existed), reforestation, avoided deforestation, and forest management - can lead to increased sequestration of atmospheric carbon dioxide and has therefore been proposed as a strategy to mitigate climate change. However, forestry also influences land-surface properties, including albedo (the fraction of incident sunlight reflected back to space), surface roughness, and evapotranspiration, all of which affect the amount and forms of energy transfer to the atmosphere. In some circumstances, these biophysical feedbacks can result in local climate warming, thereby counteracting the effects of carbon sequestration on global mean temperature and reducing or eliminating the net value of climate-change mitigation projects. Here, we review published and emerging research that suggests ways in which forestry projects can counteract the consequences associated with biophysical interactions, and highlight knowledge gaps in managing forests for climate protection. We also outline several ways in which biophysical effects can be incorporated into frameworks that use the maintenance of forests as a climate protection strategy. [ABSTRACT FROM AUTHOR]

The severe mass extinction of marine and terrestrial organisms at the end of the Permian Period ( c. 251 Ma) was accompanied by a rapid (<100 000 years and possibly <10 000 years) negative excursion of c. 3‰ in the δ13C of the global surface oceans and atmosphere that persisted for some 500 000 years into the Early Triassic. Simulations with an ocean–atmosphere/carbon-cycle model suggest that the isotope excursion can be explained by collapse of ocean primary productivity, and changes in the delivery and cycling of carbon in the oceans and on land. Model results suggest that severe reduction of marine productivity led to an increase in surface-ocean dissolved inorganic carbon and a rapid, short-term increase in atmospheric pCO2 (from a Late Permian base of 850 ppm to c. 2500 ppm). Increase in surface ocean alkalinity may have stimulated the widespread microbial and abiotic shallow-water carbonate deposition seen in the earliest Triassic. The model is also consistent with a long-term (>1 Ma) decrease in sedimentary burial of organic carbon in the early Triassic. [ABSTRACT FROM AUTHOR]

Oceanic uptake and transport of bomb-produced radiocarbon is used as a diagnostic in global ocean models to test parameterizations of mixing and air-sea gas exchange between the ocean and atmosphere. A model's ability to simulate bomb-produced 4C is also a good indicator of its ability to predict uptake of anthropogenic CO2. We have conducted a model-data comparison of surface radiocarbon time series from coral records from the coasts of Kenya and Sumatra and a suite of dynamical three-dimensional ocean models that were included in the second phase of the Ocean Carbon-Cycle Model Intercomlarson Project. The coral records comprise the first intrabasin record of surface water Δ14C variability in the equatorial Indian Ocean and provide an independent evaluation of model performance. Differing treatments of lateral subgrid scale mixing in different models appear to be less important than other, unknown factors in explaining differences in results between the models. Those models that include a dynamic vertical mixing scheme appear to be more capable of matching observed coral radiocarbon time series. However, among models with the same parameterization of lateral subgrid scale mixing, there is a large degree of variation, suggesting that at both sites, factors such as resolution, topography, physical forcing and horizontal advection are more important than mixing parameterization in explaininq intermodel differences. None of the models reproduce the time lag in the rate of bomb 14C response between the Kenya and Sumatra coral sites. Future efforts are needed to improve model simulation of radiocarbon in surface waters in the equatorial Indian Ocean. [ABSTRACT FROM AUTHOR]