Search

δ18O in foraminifera (δ18Oc) is a useful proxy for density, and the strength of the Atlantic Meridional Overturning Circulation (AMOC) can be reconstructed by the zonal density contrast in the Atlantic. However, whether the deglacial zonal δ18Occontrast can represent the AMOC change is still unclear. δ18Occontrast across the Florida Straits has been hypothesized as a proxy for the AMOC evolution during the last deglaciation, but the strength of Florida Current could also be influenced by wind forcing. Here we examine the ability of the zonal δ18Occontrast to reconstruct AMOC in a deglacial model simulation. The model simulation suggests that the deglacial variation of the Florida Current strength is dominated by AMOC, with the wind effect on the variation of the Florida Current being negligible. Furthermore, the δ18Occontrast across the western boundary along the entire Atlantic and the basin‐wide δ18Occontrast in the North Atlantic in the upper ocean can also be used to reconstruct AMOC. However, using basin‐wide δ18Occontrast to reconstruct AMOC in the South Atlantic is not possible at all water depths. In the subtropical South Atlantic, the basin‐wide δ18Occontrast is decoupled from the density contrast between 400 to 600 m through the deglaciation because of the deglacial change of the Antarctic Intermediate Water. Therefore, δ18Ocis a useful proxy to reconstruct past density and in turn past AMOC, but caution has to be used when using the basin‐wide δ18Occontrast to reconstruct the basin‐wide density contrast in the South Atlantic. The Florida Current strength and the δ18Occontrast in the Florida Straits are determined dominantly by AMOC during the last deglaciationδ18Occontrast across the western boundary currents can be used to reconstruct deglacial density contrast in the upper oceanThe basin‐wide δ18Occontrast can be used to reconstruct AMOC variation but depends on the latitude and depth

Understanding how anthropogenic CO2emissions will influence future precipitation is critical for sustainably managing ecosystems, particularly for drought-sensitive tropical forests. Although tropical precipitation change remains uncertain, nearly all models from the Coupled Model Intercomparison Project Phase 5 predict a strengthening zonal precipitation asymmetry by 2100, with relative increases over Asian and African tropical forests and decreases over South American forests. Here we show that the plant physiological response to increasing CO2is a primary mechanism responsible for this pattern. Applying a simulation design in the Community Earth System Model in which CO2increases are isolated over individual continents, we demonstrate that different circulation, moisture and stability changes arise over each continent due to declines in stomatal conductance and transpiration. The sum of local atmospheric responses over individual continents explains the pan-tropical precipitation asymmetry. Our analysis suggests that South American forests may be more vulnerable to rising CO2than Asian or African forests. Increasing zonal asymmetry in tropical precipitation is projected by 2100, with increases over Asian and African forests and decreases over South American forests. Plant physiological responses to increasing CO2are now identified as a primary driving mechanism.

The location of the Southern Ocean Silicate Front (SF) is a key indicator of physical circulation, biological productivity, and biogeography, but its variability in space and time is currently not well understood due to a lack of time‐varying nutrient observations. This study provides a first estimate of the spatiotemporal variability of the SF, defined using the silicate‐to‐nitrate (Si:N) ratio as simulated by the Community Earth System Model (CESM) Large Ensemble (1920–2100), and its response to a changing Southern Ocean. The latitude where Si:N = 1 largely coincides with regions of high gradients in silicate and the observed position of the Antarctic Polar Front (PF) and serves as an indicator of waters with adequate nutrients available for diatom growth. On seasonal to interdecadal time scales, variability in the location of the SF is largely determined by biological nutrient utilization and Southern Ocean bathymetry, respectively. From 1920 to 2100, under historical and RCP8.5 forcing, the zonally averaged SF shifts poleward by ∼3° latitude, with no discernible shift in the position of the simulated location of the PF or the core of the Antarctic Circumpolar Current. A more poleward SF is primarily driven by long‐term reductions in silicate and nitrate concentrations at the surface as a consequence of greater iron availability and a warmer, more stratified Southern Ocean. These results suggest a decoupling of the SF and PF by the end of the century, with implications for local biogeography, global thermocline nutrient cycling, and the interpretation of paleoclimate records from deep sea sediments. Silicate Front position is influenced by biology and bathymetry on seasonal to interdecadal time scalesTwenty‐first century climate warming drives a poleward shift in the Silicate Front independent of the Antarctic Polar FrontA more poleward Silicate Front is driven by a combination of increased stratification and silicate utilization

Dissolved oxygen (O2) is essential for the survival of marine animals. Climate change impacts on future oxygen distributions could modify species biogeography, trophic interactions, biodiversity, and biogeochemistry. The Coupled Model Intercomparison Project Phase 5 models predict a decreasing trend in marine O2over the 21st century. Here we show that this increasing hypoxia trend reverses in the tropics after 2100 in the Community Earth System Model forced by atmospheric CO2from the Representative Concentration Pathway 8.5 and Extended Concentration Pathway 8.5. In tropical intermediate waters between 200 and 1,000 m, the model predicts a steady decline of O2and an expansion of oxygen minimum zones (OMZs) during the 21st century. By 2150, however, the trend reverses with oxygen concentration increasing and OMZ volume shrinking through 2300. A novel five‐box model approach in conjunction with output from the full Earth system model is used to separate the contributions of biological and physical processes to the trends in tropical oxygen. The tropical O2recovery is caused mainly by reductions in tropical biological export, coupled with a modest increase in ventilation after 2200. The time‐evolving oxygen distribution impacts marine nitrogen cycling, with potentially important climate feedbacks. Ocean interior O2responds to changes in solubility, ventilation, and the strength of the biological pump. While climate‐driven solubility changes have received considerable attention, the influence of climate on ventilation and net primary production may yield a complex response that is difficult to predict, particularly over a period of centuries. The recovery of O2in the tropical intermediate waters after 2100 that we describe highlights a possible unanticipated long‐term implication of a coupling of Southern Ocean and tropical ocean net primary production and has important implication for biological feedbacks to climate warming. Specifically, denitrification levels in tropical intermediate waters decline below pre‐industrial levels, suggesting that the contracting OMZs may offset anthropogenic N2O emissions originating from agriculture and other land sources. We report a reversal of increasing hypoxia in the tropical ocean after 2100 in the Community Earth System Model with strong climate warmingA novel five‐box model approach is developed to quantify the contributions of biological and physical processes to the temporal evolution of tropical oxygenPartial recovery of O2in tropical intermediate waters is caused mainly by decreasing export production, along with a modest increase in ventilation

A coupled global numerical simulation (conducted with the Community Earth System Model) is used in conjunction with satellite remote sensing observations to examine the role of top-down (grazing pressure) and bottom-up (light, nutrients) controls on marine phytoplankton bloom dynamics in the Southern Ocean. Phytoplankton seasonal phenology is evaluated in the context of the recently proposed “disturbance-recovery” hypothesis relative to more traditional, exclusively “bottom-up” frameworks. All blooms occur when phytoplankton division rates exceed loss rates to permit sustained net population growth; however, the nature of this decoupling period varies regionally in Community Earth System Model. Regional case studies illustrate how unique pathways allow blooms to emerge despite very poor division rates or very strong grazing rates. In the Subantarctic, southeast Pacific small spring blooms initiate early cooccurring with deep mixing and low division rates, consistent with the disturbance-recovery hypothesis. Similar systematics are present in the Subantarctic, southwest Atlantic during the spring but are eclipsed by a subsequent, larger summer bloom that is coincident with shallow mixing and the annual maximum in division rates, consistent with a bottom-up, light limited framework. In the model simulation, increased iron stress prevents a similar summer bloom in the southeast Pacific. In the simulated Antarctic zone (70°S–65°S) seasonal sea ice acts as a dominant phytoplankton-zooplankton decoupling agent, triggering a delayed but substantial bloom as ice recedes. Satellite ocean color remote sensing and ocean physical reanalysis products do not precisely match model-predicted phenology, but observed patterns do indicate regional variability in mechanism across the Atlantic and Pacific. CESM explicitly simulates variable pathways for Southern Ocean bloom formation driven by both bottom-up and top-down controlsSimulated blooms can initiate amidst both strong or poor cell division rates depending on iron/light availability and trophic decouplingImplicit evidence in the remote sensing data supports these unique mechanistic pathways as well as a strong but variable grazing control

Using the Community Earth System Model, we explore the role of human land use and land cover change (LULCC) in modifying the terrestrial carbon budget in simulations forced by Representative Concentration Pathway 8.5, extended to year 2300. Overall, conversion of land (e.g., from forest to croplands via deforestation) results in a model-estimated, cumulative carbon loss of 490?Pg?C between 1850 and 2300, larger than the 230?Pg?C loss of carbon caused by climate change over this same interval. The LULCC carbon loss is a combination of a direct loss at the time of conversion and an indirect loss from the reduction of potential terrestrial carbon sinks. Approximately 40% of the carbon loss associated with LULCC in the simulations arises from direct human modification of the land surface; the remaining 60% is an indirect consequence of the loss of potential natural carbon sinks. Because of the multicentury carbon cycle legacy of current land use decisions, a globally averaged amplification factor of 2.6 must be applied to 2015 land use carbon losses to adjust for indirect effects. This estimate is 30% higher when considering the carbon cycle evolution after 2100. Most of the terrestrial uptake of anthropogenic carbon in the model occurs from the influence of rising atmospheric CO2on photosynthesis in trees, and thus, model-projected carbon feedbacks are especially sensitive to deforestation. Land use and land cover change are more important than climate in modifying land carbon uptakeWhen integrated to the year 2100 or 2300, indirect effects of land use on carbon are larger than direct effectsCurrent land use carbon fluxes should be multiplied by 2.6 to account for future loss of natural carbon cycle sinks

As anthropogenic emissions and warming continue to alter Earth's environment, it is essential to highlight future impacts that can be avoided through mitigation. Here we use two ensembles of the Community Earth System Model (CESM) run under the business-as-usual scenario, RCP 8.5, and the mitigation scenario, RCP 4.5, to identify avoidable impacts of anthropogenic warming on marine net primary production (NPP). We emphasize the use of ensembles so as to distinguish long-term, anthropogenic trends in marine productivity from internal variability. Twentieth century globally integrated marine NPP is 55.7 ± 1 Pg C, with much of the variability attributable to certain regions (e.g., the equatorial Pacific). CESM projections indicate that global marine NPP will drop by ~4% by 2080 if we follow RCP 8.5, but only by 2% under RCP 4.5. The response to warming on a global scale includes compensating regional effects; NPP increases in polar and eastern equatorial Pacific waters but decreases in the Atlantic, western Pacific, and Indian Oceans. The two main phytoplankton groups simulated in CESM show distinct responses: diatoms decrease their NPP, while small phytoplankton NPP increases over the mid-21st century. Trends in NPP from mid-21st century to 2080 are significantly different between the two emission scenarios mainly in the Atlantic Ocean basin and therefore impacts here are “avoidable” if we follow RCP 4.5, rather than RCP 8.5. In contrast, changes in NPP on a global scale and in most areas of the Pacific and Indian basins and the Southern Ocean are not distinguishable between forcing scenarios. Climate warming during the 21st century results in reduced marine net primary production on a global scaleEnsemble simulations allow for an investigation of ensemble mean trends across two emission scenarios in the presence of natural variabilitySignificant decreases in marine net primary production in the Atlantic basin may be avoided with a mitigation emission scenario

We quantify and isolate the sources of projection uncertainty in annual-mean sea-air CO2flux over the period 2006–2080 on global and regional scales using output from two sets of ensembles with the Community Earth System Model (CESM) and models participating in the 5th Coupled Model Intercomparison Project (CMIP5). For annual-mean, globally-integrated sea-air CO2flux, uncertainty grows with prediction lead time and is primarily attributed to uncertainty in emission scenario. At the regional scale of the California Current System, we observe relatively high uncertainty that is nearly constant for all prediction lead times, and is dominated by internal climate variability and model structure, respectively in the CESM and CMIP5 model suites. Analysis of CO2flux projections over 17 biogeographical biomes reveals a spatially heterogenous pattern of projection uncertainty. On the biome scale, uncertainty is driven by a combination of internal climate variability and model structure, with emission scenario emerging as the dominant source for long projection lead times in both modeling suites. Uncertainty in ocean carbon uptake projections is quantified and partitionedEmission scenario dominates uncertainty in globally integrated carbon uptake projectionsInternal variability and model structure dominate uncertainty on regional scales

The ocean has absorbed 41 per cent of all anthropogenic carbon emitted as a result of fossil fuel burning and cement manufacture. The magnitude and the large-scale distribution of the ocean carbon sink is well quantified for recent decades. In contrast, temporal changes in the oceanic carbon sink remain poorly understood. It has proved difficult to distinguish between air-to-sea carbon flux trends that are due to anthropogenic climate change and those due to internal climate variability. Here we use a modelling approach that allows for this separation, revealing how the ocean carbon sink may be expected to change throughout this century in different oceanic regions. Our findings suggest that, owing to large internal climate variability, it is unlikely that changes in the rate of anthropogenic carbon uptake can be directly observed in most oceanic regions at present, but that this may become possible between 2020 and 2050 in some regions.

Letters to the editor are presented in response to previous articles, including a report on functional magnetic resonance imaging in brain research from the October 19, 2013 issue, an article from the September 28, 2013 issue about the fate of Earth without life, and a letter by Lawrence D'Oliveiro in the October 12, 2013 issue about the role of disease prevention in evolution of manners.

A particle method is presented for computing vortex sheet motion in three-dimensional flow. The particles representing the sheet are advected by a regularized Biot–Savart integral in which the exact singular kernel is replaced by the Rosenhead–Moore kernel. New particles are inserted to maintain resolution as the sheet rolls up. The particle velocities are evaluated by an adaptive treecode algorithm based on Taylor approximation in Cartesian coordinates, and the necessary Taylor coefficients are computed by a recurrence relation. The adaptive features include a divide-and-conquer evaluation strategy, nonuniform rectangular clusters, variable-order approximation, and a run-time choice between Taylor approximation and direct summation. Tests are performed to document the treecode's accuracy and efficiency. The method is applied to simulate the roll-up of a circular-disk vortex sheet into a vortex ring. Two examples are presented, azimuthal waves on a vortex ring and the merger of two vortex rings.

Earth system models are intended to make long‐term projections, but they can be evaluated at interannual and seasonal time scales. Although the Community Earth System Model (CESM2) showed improvements in a number of terrestrial carbon cycle benchmarks, relative to its predecessor, our analysis suggests that the interannual variability (IAV) in net terrestrial carbon fluxes did not show similar improvements. The model simulated low IAV of net ecosystem production (NEP), resulting in a weaker than observed sensitivity of the carbon cycle to climate variability. Low IAV in net fluxes likely resulted from low variability in gross primary productivity (GPP)—especially in the tropics—and a high covariation between GPP and ecosystem respiration. Although lower than observed, the IAV of NEP had significant climate sensitivities, with positive NEP anomalies associated with warmer and drier conditions in high latitudes, and with wetter and cooler conditions in mid and low latitudes. We identified two dominant modes of seasonal variability in carbon cycle flux anomalies in our fully coupled CESM2 simulations that are characterized by seasonal amplification and redistribution of ecosystem fluxes. Seasonal amplification of net and gross carbon fluxes showed climate sensitivities mirroring those of annual fluxes. Seasonal redistribution of carbon fluxes is initiated by springtime temperature anomalies, but subsequently negative feedbacks in soil moisture during the summer and fall result in net annual carbon losses from land. These modes of variability are also seen in satellite proxies of GPP, suggesting that CESM2 appropriately represents regional sensitivities of photosynthesis to climate variability on seasonal time scales. Earth system models that are intended to make climate change projections also represent the global exchange of carbon dioxide (CO2) between the atmosphere, oceans, and land. As such, the growth rate and variability of CO2concentrations in the atmosphere provide a robust measurement to evaluate models. We looked at the interannual variability (IAV) of terrestrial carbon fluxes and their sensitivity to variations in temperature and water that were simulated by the Community Earth System Model and compared them to observations. We found that the model underestimates the IAV of net terrestrial carbon fluxes, especially in the tropics. We also identified two modes of variability that correspond to an increase in summer land carbon uptake (amplification) and a change in the seasonal timing (redistribution) of land carbon fluxes. Notably, seasonal redistribution was initialized by warmer springs that increased early‐season productivity, but subsequent water limitations in the summer and fall resulted in lower‐than‐average productivity over the growing season and net annual C losses to the atmosphere. Similar patterns of seasonal amplification and redistribution are seen in satellite observations of photosynthesis, suggesting that the model is realistically simulating characteristics of terrestrial ecosystems necessary for capturing carbon cycle‐climate feedbacks. Simulated net carbon fluxes show low interannual variability because of low tropical variability and high correlation in component fluxesSeasonal amplification and redistribution of fluxes characterize dominant modes of carbon cycle variability, consistent with observationsSeasonal redistribution is characterized by warm springs, dry summers, and can result in a net carbon loss from land Simulated net carbon fluxes show low interannual variability because of low tropical variability and high correlation in component fluxes Seasonal amplification and redistribution of fluxes characterize dominant modes of carbon cycle variability, consistent with observations Seasonal redistribution is characterized by warm springs, dry summers, and can result in a net carbon loss from land

One of the most robust changes in the hydrological cycle predicted by Earth System Models (ESMs) during the remainder of 21st century is an increase in the difference between precipitation and evapotranspiration (P‐E) in arctic and boreal regions. We explore the long‐term consequences of this change for marine ecosystems in the Arctic Ocean using the Community Earth System Model forced with a business as usual scenario of future greenhouse gas concentrations. We find that by the year 2300 increases in freshwater delivery considerably reduce Arctic Ocean surface salinity, creating a freshwater lens that has far‐reaching impacts on marine biogeochemistry. The expanding freshwater lens limits vertical nutrient supply into the euphotic zone by enhancing vertical stratification and accelerating surface lateral mixing with surface waters in the North Atlantic, which become increasingly nutrient depleted from weakening of the Atlantic Meridional Overturning Circulation (AMOC). The resulting increase in nutrient stress reduces marine export production in the Arctic Ocean by 53% in 2300 relative to the 1990s and triggers a shift in community composition with small phytoplankton replacing diatoms. At the same time, the seasonal timing of export production undergoes a 2‐month forward shift, with the peak advancing from July to May. This suggests that the threat to food webs and higher trophic levels may intensify after the year 2100 as gains in productivity from sea ice loss saturate and freshwater impacts on nutrient stress continue to strengthen. Our analysis highlights the critical importance of changing terrestrial hydrology and land‐ocean coupling as drivers of long‐term biogeochemical change in the Arctic Ocean and the necessity of multi‐century climate change projections. Climate warming will cause more rain and snow to fall in northern regions, increasing river runoff and causing the Arctic Ocean to freshen. Using a global climate model, here we explore the long‐term consequences of this freshening for the marine biosphere. As the Earth responds to a future scenario of “business‐as‐usual” fossil fuel emissions, an expanding lens of freshwater will form a cap on the surface of the Arctic Ocean, limiting the upward mixing of nutrient‐rich water from deeper ocean layers. This stunts the growth of phytoplankton, counteracting by 2100 gains in marine productivity caused by increasing light availability from sea ice loss. After 2100, as the freshwater cap strengthens, small phytoplankton mostly replace diatoms in arctic ecosystems as a consequence of increasing nutrient stress. This transition happens rather suddenly, suggesting that the marine biosphere may pass through an ecological tipping point. By 2300, export production, a measure of the flow of organic matter available to support zooplankton, fish, and marine mammals, declines by over 50%. At the same time, a shift in the timing of the peak phytoplankton bloom from July to May is likely to further disrupt arctic food webs. Our work shows that climate change impacts on the hydrological cycle in the far north will have long‐lasting and far‐reaching impacts on the marine biosphere in the Arctic and highlights the importance exploring the potential for ecological tipping points in deep future time. A growing lens of freshwater in the Arctic Ocean continues to expand after the year 2100 in CESM1(BGC) for RCP 8.5The expanding freshwater lens increases nutrient stress, overriding gains in marine productivity associated with sea ice lossBy 2300, export production declines by 53% and the peak bloom shifts from July to May, strongly influencing arctic marine food web dynamics

Iron can be a growth‐limiting nutrient for phytoplankton, modifying rates of net primary production, nitrogen fixation, and carbon export ‐ highlighting the importance of new iron inputs from the atmosphere. The bioavailable iron fraction depends on the emission source and the dissolution during transport. The impacts of anthropogenic combustion and land use change on emissions from industrial, domestic, shipping, desert, and wildfire sources suggest that Northern Hemisphere soluble iron deposition has likely been enhanced between 2% and 68% over the Industrial Era. If policy and climate follow the intermediate Representative Concentration Pathway 4.5 trajectory, then results suggest that Southern Ocean (>30°S) soluble iron deposition would be enhanced between 63% and 95% by 2100. Marine net primary productivity and carbon export within the open ocean are most sensitive to changes in soluble iron deposition in the Southern Hemisphere; this is predominantly driven by fire rather than dust iron sources. Changes in iron deposition cause large perturbations to the marine nitrogen cycle, up to 70% increase in denitrification and 15% increase in nitrogen fixation, but only modestly impacts the carbon cycle and atmospheric CO2concentrations (1–3 ppm). Regionally, primary productivity increases due to increased iron deposition are often compensated by offsetting decreases downstream corresponding to equivalent changes in the rate of phytoplankton macronutrient uptake, particularly in the equatorial Pacific. These effects are weaker in the Southern Ocean, suggesting that changes in iron deposition in this region dominates the global carbon cycle and climate response. Human activity significantly modifies the magnitude and location of atmospheric soluble iron deposition to the oceansMarine carbon cycle responses to Anthropocene iron flux changes are modest but more sensitive to varying fire than dust iron emissionsIncreasing the iron flux produces offsetting patterns in phytoplankton macronutrient uptake and productivity rates at the basin scale

Climate change is expected to increase the frequency of flooding events and, thus, the risks of flood‐related mortality and infrastructure damage. Global‐scale assessments of future flooding from Earth system models based only on precipitation changes neglect important processes that occur within the land surface, particularly plant physiological responses to rising CO2. Higher CO2can reduce stomatal conductance and transpiration, which may lead to increased soil moisture and runoff in some regions, promoting flooding even without changes in precipitation. Here we assess the relative impacts of plant physiological and radiative greenhouse effects on changes in daily runoff intensity over tropical continents using the Community Earth System Model. We find that extreme percentile rates increase significantly more than mean runoff in response to higher CO2. Plant physiological effects have a small impact on precipitation intensity but are a dominant driver of runoff intensification, contributing to one half of the 99th and one third of the 99.9th percentile runoff intensity changes. Floods are one of the most devastating natural disasters in the world, contributing to thousands of deaths and billions of dollars in damages annually. Climate change is expected to increase flood exposure considerably through the 21st century. However, recent studies assessing future flood risk on global scales by downscaling precipitation from Earth system models often neglect important plant physiological responses to rising CO2. In particular, higher CO2concentrations may lower stomatal conductance and, in the absence of significant plant growth, reduce water loss through transpiration, increasing soil moisture in many regions. For a given precipitation rate, higher soil moisture can decrease the amount of rain that infiltrates the soil and increase runoff. Here we apply a simulation design that isolates the independent effects of higher CO2on radiatively driven precipitation intensification from plant‐driven soil moisture changes. We show that plant‐physiological responses to increasing CO2are major drivers of the runoff intensity change in the tropics. Land surface changes contribute to one half of the 99th percentile runoff change and one third of the 99.9th percentile change. Our results suggest that comprehensive flood assessments should account for plant physiology as well as radiative impacts of higher CO2in order to better inform flood prediction and mitigation practice. Plant physiological responses drive a larger increase in the daily intensity of runoff than the annual mean in simulations with rising CO2Plant physiological responses to rising CO2contribute as much as radiative greenhouse effects to changes in the 99th percentile daily runoffAssessments of future flood risk on global scales should include both atmosphere and land‐driven impacts on rainfall and runoff intensity

The authors comment on the ineffectiveness of the consumptive utilization of wildlife, which refers to the harvesting wild animals for profit, and considers the three main enterprises of consumptive utilization that were tried in Kenya in the past half century. It includes commercial wildlife cropping, wildlife and livestock ranching, and commercial hunting.