Worldwide, coral reef ecosystems are experiencing increasing pressure from a variety of anthropogenic perturbations including ocean warming and acidification, increased sedimentation, eutrophication, and overfishing, which could shift reefs to a condition of net calcium carbonate (CaCO3) dissolution and erosion. Herein, we determine the net calcification potential and the relative balance of net organic carbon metabolism (net community production; NCP) and net inorganic carbon metabolism (net community calcification; NCC) within 23 coral reef locations across the globe. In light of these results, we consider the suitability of using these two metrics developed from total alkalinity (TA) and dissolved inorganic carbon (DIC) measurements collected on different spatiotemporal scales to monitor coral reef biogeochemistry under anthropogenic change. All reefs in this study were net calcifying for the majority of observations as inferred from alkalinity depletion relative to offshore, although occasional observations of net dissolution occurred at most locations. However, reefs with lower net calcification potential (i.e., lower TA depletion) could shift towards net dissolution sooner than reefs with a higher potential. The percent influence of organic carbon fluxes on total changes in dissolved inorganic carbon (DIC) (i.e., NCP compared to the sum of NCP and NCC) ranged from 32% to 88% and reflected inherent biogeochemical differences between reefs. Reefs with the largest relative percentage of NCP experienced the largest variability in seawater pH for a given change in DIC, which is directly related to the reefs ability to elevate or suppress local pH relative to the open ocean. This work highlights the value of measuring coral reef carbonate chemistry when evaluating their susceptibility to ongoing global environmental change and offers a baseline from which to guide future conservation efforts aimed at preserving these valuable ecosystems.

We analyze 36 years of global, hourly weather data (1980-2015) to quantify the covariability of solar and wind resources as a function of time and location, over multi-decadal time scales and up to continental length scales. Assuming minimal excess generation, lossless transmission, and no other generation sources, the analysis indicates that wind-heavy or solar-heavy U.S.-scale power generation portfolios could in principle provide similar to 80% of recent total annual U.S. electricity demand. However, to reliably meet 100% of total annual electricity demand, seasonal cycles and unpredictable weather events require several weeks' worth of energy storage and/or the installation of much more capacity of solar and wind power than is routinely necessary to meet peak demand. To obtain similar to 80% reliability, solar-heavy wind/solar generation mixes require sufficient energy storage to overcome the daily solar cycle, whereas wind-heavy wind/solar generation mixes require continental-scale transmission to exploit the geographic diversity of wind. Policy and planning aimed at providing a reliable electricity supply must therefore rigorously consider constraints associated with the geophysical variability of the solar and wind resourc-even over continental scales.

Some energy services and industrial processes-such as long-distance freight transport, air travel, highly reliable electricity, and steel and cement manufacturing-are particularly difficult to provide without adding carbon dioxide (CO2) to the atmosphere. Rapidly growing demand for these services, combined with long lead times for technology development and long lifetimes of energy infrastructure, make decarbonization of these services both essential and urgent. We examine barriers and opportunities associated with these difficult-to-decarbonize services and processes, including possible technological solutions and research and development priorities. A range of existing technologies could meet future demands for these services and processes without net addition of CO2 to the atmosphere, but their use may depend on a combination of cost reductions via research and innovation, as well as coordinated deployment and integration of operations across currently discrete energy industries.

Coral reefs feed millions of people worldwide, provide coastal protection and generate billions of dollars annually in tourism revenue(1). The underlying architecture of a reef is a biogenic carbonate structure that accretes over many years of active biomineralizat ion by calcifying organisms, including corals and algae(2). Ocean acidification poses a chronic threat to coral reefs by reducing the saturation state of the aragonite mineral of which coral skeletons are primarily composed, and lowering the concentration of carbonate ions required to maintain the carbonate reef. Reduced calcification, coupled with increased bioerosion and dissolution(3), may drive reefs into a state of net loss this century(4). Our ability to predict changes in ecosystem function and associated services ultimately hinges on our understanding of community- and ecosystem-scale responses. Past research has primarily focused on the responses of individual species rather than evaluating more complex, community-level responses. Here we use an in situ carbon dioxide enrichment experiment to quantify the net calcification response of a coral reef flat to acidification. We present an estimate of community-scale calcification sensitivity to ocean acidification that is, to our knowledge, the first to be based on a controlled experiment in the natural environment. This estimate provides evidence that near-future reductions in the aragonite saturation state will compromise the ecosystem function of coral reefs.

Aerosol-cloud radiative effects are determined and quantified in simulations of deep open-cell stratocumuli observed during the VAMOS Ocean-Cloud-Atmosphere-Land Study Regional Experiment (VOCALS-REx) campaign off the west coast of Chile. The cloud deck forms in a boundary layer 1.5 km deep, with cell sizes reaching 50 km in diameter. Global databases of ship tracks suggest that these linear structures are seldom found in boundary layers this deep. Here, we quantify the changes in cloud radiative properties to a continuous aerosol point source moving along a fixed emission line releasing 1017 particles per second. We show that a spatially coherent cloud perturbation is not evident along the emission line. Yet our model simulates an increase in domain-mean all-sky albedo of 0.05, corresponding to a diurnally averaged cloud radiative effect of 20 Wm(-2), given the annual mean solar insolation at the VOCALS-REx site. Therefore, marked changes in cloud radiative properties in precipitating deep open cells may be driven by anthropogenic near-surface aerosol perturbations, such as those generated by ships.

Many studies have linked mass extinction events with the catastrophic effects of large-body impacts and flood-basalt eruptions, sometimes as competing explanations. We find that the ages of at least 10 out of a total of 11 documented extinction events over the last 260 Myr (12 out of 13 if we include two lesser extinction events) coincide, within errors, with the best-known ages of either a large impact crater (>= 70 km diameter) or a continental flood-basalt eruption. The null hypothesis that this could occur by chance can be rejected with very high confidence (> 99.999%). The ages of large impact craters correlate with recognized extinction events at similar to 36 (two impacts), 66, 145 and 215 Myr ago (and possibly an event at similar to 168 Myr ago), and the ages of continental flood basalts correlate with extinctions at 66, similar to 94, similar to 116, 183, 201, 252 and 259 Myr ago (and possibly at similar to 133 Myr ago). Furthermore, at least 7 periods of widespread anoxia in the oceans of the last 260 Myr coincide with the ages of flood-basalt eruptions (with 99.999% confidence), and are coeval with extinctions, suggesting causal connections. These statistical relationships argue that most mass extinction events are related to climatic catastrophes produced by the largest impacts and large-volume continental flood-basalt eruptions.

What causes recurrent mass extinctions of life? We find that the ages of 10 of the 11 well-documented extinction episodes of the last 260 m.y. show correlations, at very high confidence (>99.99%), with the ages of the largest impact craters or the ages of massive continental flood-basalt eruptions. The four largest craters (>= 100 km diameter, impact energies >= 3 x 10(7) Mt trinitrotoluene [TNT]) can be linked with recognized extinction events at 36, 66, 145, and 215 Ma, and with stratigraphic distal impact debris correlative with the extinctions. The ages of 7 out of 11 major flood-basalt episodes can be correlated with extinction events at 66, 94, ca. 120, 183, 201, 252, and 260 Ma. All seven flood-basalt-extinction co-events have coincident volcanogenic mercury anomalies in the stratigraphic record, closely linking the extinctions to the volcanism. Furthermore, the seven major periods of widespread anoxia in the oceans of the last 260 m.y. are significantly correlated (>99.99%) with the ages of the flood-basalt-extinction events, supporting a causal connection through volcanism-induced climate warming. Over Phanerozoic time (the last 541 m.y.), the six "major" mass extinctions (>= 40% extinction of marine genera) are all correlated with the ages of flood-basalt episodes, and stratigraphically with related volcanogenic mercury anomalies. In only one case, the end of the Cretaceous (66 Ma), is there an apparent coincidence of a "major" mass-extinction event with both a very large crater (Chicxulub) and a continental flood-basalt eruption (the Deccan Traps).

Carbon fluxes on coral reefs (net community production and net community calcification) aggregate the collective activity of all coral reef community members. This integrated approach provides powerful community-level insights, but is unable to resolve the finer-scale contributions of different reef functional groups to the community-scale rates. Tools are required to disaggregate the community-scale approaches and evaluate the performance of co-existing reef functional groups. Such assessments are necessary to improve forecasts of coral reef responses to global and local environmental change. We present results from a coral reef field study on One Tree Island reef in the Great Barrier Reef, off northeastern Australia, in September-October 2016 where we combined observations of total alkalinity, dissolved inorganic carbon (DIC), and the stable isotopic composition of dissolved inorganic carbon (delta C-13(DIC)) to estimate carbon isotopic fractionation during organicmatter formation. Portions of the reef with greater abundance of non-calcifying algae fractionated DIC similar to 5 parts per thousand more (stronger preference for C-12) during organic metabolism than did portions of the reef with a greater abundance of calcifiers. These results were consistent across a wide range of assumed isotopic fractionation factors for net calcification. We attribute the observed differences in carbon isotopic fractionation to the metabolic activities of the ecological community underlying each section of the reef, rather than to environmental factors such as light availability or water temperature. The patterns in carbon isotopic fractionation were generally consistent with inferred ratios of calcification to primary production in each reef zone, giving further confidence to our inference that differences in carbon isotopic fractionation may be related to differences in the ecological community on small spatial scales.