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Nearshore reefs, at the interface of land-sea interactions, provide essential ecosystem services, but are susceptible to multiple global and local stressors. These stressors can detrimentally impact coral growth and the continuity of the reef framework. Here, we analyse coral growth records (1998 – 2016) of massive Porites spp. colonies from nearshore reefs in Fiji. Our aim is to assess the role of thermal stress and turbidity on coral growth across a range of environments. Our findings reveal a negative linear relationship between linear extension and seawater turbidity across locations (GLM, R 2  = 0.42, p  < 0.001), indicating that average coral growth is significantly influenced by local environmental conditions. On interannual timescales, all locations experienced a 14% to 30% decrease in linear extension in response to acute thermal stress during the 2013 – 2016 period. This finding highlights the existence of compounding effects between water quality and thermal stress. We suggest that inshore, long-lived massive hard corals in areas of high turbidity are more vulnerable to increasing SSTs due to an already reduced mean growth. Integrated management strategies in these regions that considers managing for multiple, interacting local stressors are warranted to enhance resilience.

Aim Elucidating the environmental limits of coral reefs is central to projecting future impacts of climate change on these ecosystems and their global distribution. Recent developments in species distribution modelling (SDM) and the availability of comprehensive global environmental datasets have provided an opportunity to reassess the environmental factors that control the distribution of coral reefs at the global scale as well as to compare the performance of different SDM techniques. Location Shallow waters world‐wide. Methods The SDM methods used were maximum entropy (Maxent) and two presence/absence methods: classification and regression trees (CART) and boosted regression trees (BRT). The predictive variables considered included sea surface temperature (SST), salinity, aragonite saturation state (ΩArag), nutrients, irradiance, water transparency, dust, current speed and intensity of cyclone activity. For many variables both mean and SD were considered, and at weekly, monthly and annually averaged time‐scales. All were transformed to a global 1° × 1° grid to generate coral reef probability maps for comparison with known locations. Model performance was compared in terms of receiver operating characteristic (ROC) curves and area under the curve (AUC) scores. Potential geographical bias was explored via misclassification maps of false positive and negative errors on test data. Results Boosted regression trees consistently outperformed other methods, although Maxent also performed acceptably. The dominant environmental predictors were the temperature variables (annual mean SST, and monthly and weekly minimum SST), followed by, and with their relative importance differing between regions, nutrients, light availability and ΩArag. No systematic bias in SDM performance was found between major coral provinces, but false negatives were more likely for cells containing ‘marginal’ non‐reef‐forming coral communities, e.g. Bermuda. Main conclusions Agreement between BRT and Maxent models gives predictive confidence for exploring the environmental limits of coral reef ecosystems at a spatial scale relevant to global climate models (c. 1° × 1°). Although SST‐related variables dominate the coral reef distribution models, contributions from nutrients, ΩArag and light availability were critical in developing models of reef presence in regions such as the Bahamas, South Pacific and Coral Triangle. The steep response in SST‐driven probabilities at low temperatures indicates that latitudinal expansion of coral reef habitat is very sensitive to global warming.

Despite a growing literature on the climate response to solar geoengineering—proposals to cool the planet by increasing the planetary albedo—there has been little published on the impacts of solar geoengineering on natural and human systems such as agriculture, health, water resources, and ecosystems. An understanding of the impacts of different scenarios of solar geoengineering deployment will be crucial for informing decisions on whether and how to deploy it. Here we review the current state of knowledge about impacts of a solar‐geoengineered climate and identify the major research gaps. We suggest that a thorough assessment of the climate impacts of a range of scenarios of solar geoengineering deployment is needed and can be built upon existing frameworks. However, solar geoengineering poses a novel challenge for climate impacts research as the manner of deployment could be tailored to pursue different objectives making possible a wide range of climate outcomes. We present a number of ideas for approaches to extend the survey of climate impacts beyond standard scenarios of solar geoengineering deployment to address this challenge. Reducing the impacts of climate change is the fundamental motivator for emissions reductions and for considering whether and how to deploy solar geoengineering. This means that the active engagement of the climate impacts research community will be important for improving the overall understanding of the opportunities, challenges, and risks presented by solar geoengineering. Key Points The paucity of climate impacts studies on solar geoengineering is a key missing link in the interdisciplinary research on this topic The climate impacts community can use existing tools and datasets to assess many solar geoengineering effects on natural and human systems Solar geoengineering could be tailored to produce different climate outcomes demanding innovative approaches to impacts assessment Plain Language Summary Solar geoengineering, a set of ideas to cool the planet by reflecting away light, could offer a way to reduce the impacts of climate change as a complement to cutting emissions of greenhouse gases. Many studies in recent years have looked at how solar geoengineering could change the climate but little work has been done on how it would affect the impacts of climate change, e.g. to study its effects on water resources, crops, and ecosystems. We show how the tools and approaches used to understand the impacts of climate change could be directly applied to understand the impacts of solar geoengineering. However, solar geoengineering could be done in a wide range of different ways and the climate impacts of solar geoengineering would depend on how it was done which poses a novel challenge for climate impacts research. We therefore suggest some ways forward for climate impacts research that would help to address this challenge. The evidence is clear that solar geoengineering can't replace cutting emissions of greenhouse gases as it can't reverse all their harmful effects. However, future climate impacts research would help us understand whether it could help reduce the risks of climate change.

The effect of European settlement on water quality in the Great Barrier Reef of Australia is a long-standing and controversial issue 1 , 2 , 3 , 4 , 5 , 6 . Erosion and sediment transport in river catchments in this region have increased substantially since European settlement 6 , 7 , 8 , 9 , 10 , but the magnitude of these changes remains uncertain 1 , 2 , 3 , 4 , 5 , 6 , 7 , 8 , 9 , 10 . Here we report analyses of Ba/Ca ratios in long-lived Porites coral from Havannah Reef—a site on the inner Great Barrier Reef that is influenced by flood plumes from the Burdekin river—to establish a record of sediment fluxes from about 1750 to 1998. We find that, in the early part of the record, suspended sediment from river floods reached the inner reef area only occasionally, whereas after about 1870—following the beginning of European settlement—a five- to tenfold increase in the delivery of sediments is recorded with the highest fluxes occurring during the drought-breaking floods. We conclude that, since European settlement, land-use practices such as clearing and overstocking have led to major degradation of the semi-arid river catchments, resulting in substantially increased sediment loads entering the inner Great Barrier Reef.

The management of hydrological extremes and impacts on society is inadequately understood because of the combination of short-term hydrological records, an equally short-term assessment of societal responses and the complex multi-directional relationships between the two over longer timescales. Rainfall seasonality and inter-annual variability on the Pacific coast of Central America is high due to the passage of the Inter Tropical Convergence Zone (ITCZ) and the El Niño–Southern Oscillation (ENSO). Here we reconstruct hydrological variability and demonstrate the potential for assessing societal impacts by drawing on documentary sources from the cities of Santiago de Guatemala (now Antigua Guatemala) and Guatemala de la Asunción (now Guatemala City) over the period from 1640 to 1945. City and municipal council meetings provide a rich source of information dating back to the beginning of Spanish colonisation in the 16th century. We use almost continuous sources from 1640 AD onwards, including > 190 volumes of Actas de Cabildo and Actas Municipales (minutes of meetings of the city and municipal councils) held by the Archivo Histórico de la Municipalidad de Antigua Guatemala (AHMAG) and the Archivo General de Centro América (AGCA) in Guatemala City. For this 305-year period (with the exception of a total of 11 years during which the books were either missing or damaged), information relating to Catholic rogation ceremonies and reports of flooding events and crop shortages were used to classify the annual rainy season (May to October) on a five-point scale from very wet to very dry. In total, 12 years of very wet conditions, 25 years of wetter than usual conditions, 34 years of drier conditions and 21 years of very dry conditions were recorded. An extended drier period from the 1640s to the 1740s was identified and two shorter periods (the 1820s and the 1840s) were dominated by dry conditions. Wetter conditions dominated the 1760s–1810s and possibly record more persistent La Niña conditions that are typically associated with higher precipitation over the Pacific coast of Central America. The 1640s–1740s dry period coincides with the Little Ice Age and the associated southward displacement of the ITCZ.

X-ray micro–computed tomography (µCT) is increasingly used to record the skeletal growth banding of corals. However, the wealth of data generated is time consuming to analyse for growth rates and colony age. Here we test an artificial intelligence (AI) approach to assist the expert identification of annual density boundaries in small colonies of massive Porites spanning decades. A convolutional neural network (CNN) was trained with µCT images combined with manually labelled ground truths to learn banding-related features. The CNN successfully predicted the position of density boundaries in independent images not used in training. Linear extension rates derived from CNN-based outputs and the traditional method were consistent. In the future, well-resolved 2D density boundaries from AI can be used to reconstruct density surfaces and enable studies focused on variations in rugosity and growth gradients across colony 3D space. We recommend the development of a community platform to share annotated images for AI. Article Highlights AI can help facilitate the expert identification of coral density boundaries in well-resolved regions following minimal training. AI can be used to automate and upscale non-destructive coral density-banding analysis across museum collections. Holistic analyses of coral density banding are central to understand coral growth responses to changing environments.

A 420-year history of strontium/calcium, uranium/calcium, and oxygen isotope ratios in eight coral cores from the Great Barrier Reef, Australia, indicates that sea surface temperature and salinity were higher in the 18th century than in the 20th century. An abrupt freshening after 1870 occurred simultaneously throughout the southwestern Pacific, coinciding with cooling tropical temperatures. Higher salinities between 1565 and 1870 are best explained by a combination of advection and wind-induced evaporation resulting from a strong latitudinal temperature gradient and intensified circulation. The global Little Ice Age glacial expansion may have been driven, in part, by greater poleward transport of water vapor from the tropical Pacific.

X-ray micro-computed tomography (µCT) is widely used in environmental and ecological studies to visualise the complex skeletons of marine calcifying organisms. However, the potential to quantify skeletal density in marine calcifiers is complicated because µCT greyscale intensities are influenced by many factors. Here we constrain sources of error in coral skeletal densitometry and show how errors can be quantified, minimised and reported. We establish the accuracy and precision of density estimates by analysing a population of massive Porites spp. coral that vary in bulk average skeletal density (1.07–1.57 g cm−3), size (4–25 cm in diameter), mass (87.9–2176.5 g) and morphology (41 specimens scanned in 81 individual µCT acquisitions). The population mean offset between the µCT-derived density and actual values was 0.2 ± 4.6% (x- ± 1 SD, n = 41). Following acquisition-specific corrections based on bulk density offsets, equivalent internal regions of low and high density within repeatedly scanned corals under different settings varied by only ±0.8% (1 SD). We present a workflow for µCT laboratories to measure density, and minimise and report errors. Application of these recommendations will ultimately improve data comparison of coral skeletal growth assessments. The workflow recommendations are transferable to the µCT densitometry analysis of other marine calcifying organisms.

The Maritime Continent (MC) forms the western boundary of the tropical Pacific Ocean, and relatively small changes in this region can impact the climate locally and remotely. In the mid-Piacenzian warm period of the Pliocene (mPWP; 3.264 to 3.025 Ma) atmospheric CO2 concentrations were ∼ 400 ppm, and the subaerial Sunda and Sahul shelves made the land–sea distribution of the MC different to today. Topographic changes and elevated levels of CO2, combined with other forcings, are therefore expected to have driven a substantial climate signal in the MC region at this time. By using the results from the Pliocene Model Intercomparison Project Phase 2 (PlioMIP2), we study the mean climatic features of the MC in the mPWP and changes in Indonesian Throughflow (ITF) with respect to the preindustrial. Results show a warmer and wetter mPWP climate of the MC and lower sea surface salinity in the surrounding ocean compared with the preindustrial. Furthermore, we quantify the volume transfer through the ITF; although the ITF may be expected to be hindered by the subaerial shelves, 10 out of 15 models show an increased volume transport compared with the preindustrial. In order to avoid undue influence from closely related models that are present in the PlioMIP2 ensemble, we introduce a new metric, the multi-cluster mean (MCM), which is based on cluster analysis of the individual models. We study the effect that the choice of MCM versus the more traditional analysis of multi-model mean (MMM) and individual models has on the discrepancy between model results and data. We find that models, which reproduce modern MC climate well, are not always good at simulating the mPWP climate anomaly of the MC. By comparing with individual models, the MMM and MCM reproduce the preindustrial sea surface temperature (SST) of the reanalysis better than most individual models and produce less discrepancy with reconstructed sea surface temperature anomalies (SSTA) than most individual models in the MC. In addition, the clusters reveal spatial signals that are not captured by the MMM, so that the MCM provides us with a new way to explore the results from model ensembles that include similar models.

We experimentally examined the in situ, long-term effects of flow on the coral Pocillopora verrucosa over a 2-yr period. Eighteen coral nubbins were positioned on stages protected from the ambient flow with transparent walls. Six of the corals were exposed to “enhanced flow” with underwater pumps (15–20 cm s−1), while another other six were grown under “reduced-flow” conditions (∼ 1 cm s−1), and a further six nubbins were set up as a control treatment exposed to ambient natural flow conditions on stages without walls. Mortality was 50% in the reducedflow experiment. Compared with the reduced-flow and the control treatments, colonies growing in enhanced-flow conditions developed a more compact morphology and had significantly higher chlorophyll and protein concentrations, a higher density of zooxanthellae, and, most importantly, higher reproductive output. However, corals grown in reduced-flow conditions had lower skeleton density compared with those grown under ambient flow or enhanced flow. Flow rate had no measurable influence on average pH at the site of calcification as monitored by skeletal boron isotopic composition (δ 11B). Hence, the skeletal δ 11B–pH proxy in this coral appears to be independent of flow and the biological attributes that are modulated by flow, such as zooxanthellae density, chlorophyll concentrations, and rates of photosynthesis, respiration, and calcification. Calcification site pH in all colonies was raised by +0.3 relative to ambient seawater pH. Together with light, flow should be considered a key abiotic factor determining core biological characteristics of P. verrucosa.