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.

Spectral analyses of past relative sea-level oscillations as represented by the ages of 57 Phanerozoic (the last 545 Myr) stratigraphic sequence boundaries from the Canadian Arctic show a strong spectral peak at 32 Myr (>99.9% confidence). These findings concur with previous reports of significant cycles with periods of around 30 Myr in various records of fluctuations of sea level, and in potentially related episodes of tectonism, volcanism, climate, and biotic extinctions. Sequence boundaries commonly coincide with stage boundaries based on biostratigraphy, and are correlated with episodes of extinction and times of flood-basalt volcanism. The connection between tectonics and sea-level variations may come from changes in rates of ocean-floor spreading and subduction, intraplate stresses from plate-reorganizations, and pulsations of hotspot volcanism. These coordinated periodic fluctuations in tectonics, sea level and climate may be modulated by cyclical activity in the Earth's mantle, although some pacing by astronomical cycles is suspected.

To reduce atmospheric carbon dioxide emissions and mitigate impacts of climate change, countries across the world have mandated quotas for renewable electricity. But a question has remained largely unexplored: would low-cost, firm, zero-carbon electricity generation technologies enhance-or would they displace-deployment of variable renewable electricity generation technologies, i.e., wind and solar photovoltaics, in a least-cost, fully reliable, and deeply decarbonized electricity system? To address this question, we modeled idealized electricity systems based on historical weather data and considered only technoeconomic factors. We did not apply a predetermined use model. We found that cost reductions in firm generation technologies (starting at current costs, ramping down to nearly zero) uniformly resulted in increased penetration of the firm technologies and decreased penetration of variable renewable electricity generation, in electricity systems where technology deployment is primarily driven by relative costs, and across a wide array of future technology cost assumptions. Similarly, reduced costs of variable renewable electricity (starting at current costs, ramping down to nearly zero) drove out firm generation technologies. Yet relative to deployment of "must-run" firm generation technologies, and when the studied firm technologies have high fixed costs relative to variable costs, the addition of flexibility to firm generation technologies had only limited impacts on the system cost, less than a 9% system cost reduction in our idealized model. These results reveal that policies and funding that support particular technologies for lowor zero-carbon electricity generation can inhibit the development of other lowor zero-carbon alternatives.

The geophysical limit to maximum land-area power density of large wind farms is related to the rate of replenishment of kinetic energy removed from the atmosphere by wind turbines. Although observations and numerical simulations have indicated an upper bound to the power density in the order of 1 W/m(2), no theoretical foundation has yet been provided. Here, we study the role of atmospheric pressure gradients and the latitude-dependent Coriolis parameter in the power density of large-scale wind farms by means of both numerical atmospheric simulations and analytic expressions. We illustrate that energy transport to regional-scale wind farms is primarily governed by horizontal pressure gradients and their interaction with the Coriolis force and turbine-induced surface drag within the latitude-dependent Ekman layer. Higher pressure gradients and lower Coriolis parameters promote higher energy availability and, consequently, higher potential power density, suggesting that the power density of regional-scale wind farms is largely resource- and location-dependent.

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.

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.

As reliance on wind and solar power for electricity generation increases, so does the importance of understanding how variability in these resources affects the feasible, cost-effective ways of supplying energy services. We use hourly weather data over multiple decades and historical electricity demand data to analyze the gaps between wind and solar supply and electricity demand for California (CA) and the Western Interconnect (WECC). We quantify the occurrence of resource droughts when the daily power from each resource was less than half of the 39-year daily mean for that day of the year. Averaged over 39 years, CA experienced 6.6 days of solar and 48 days of wind drought per year, compared to 0.41 and 19 for WECC. Using a macro-scale electricity model, we evaluate the potential for both long-term storage and more geographically diverse generation resources to minimize system costs. For wind-solar-battery electricity systems, meeting California demand with WECC generation resources reduces the cost by 9% compared to constraining resources entirely to California. Adding long-duration storage lowers system costs by 21% when treating California as an island. This data-driven analysis quantifies rare weather-related events and provides an understanding that can be used to inform stakeholders in future electricity systems.

Electric sector capacity expansion models are widely used by academic, government, and industry researchers for policy analysis and planning. Many models overlap in their capabilities, spatial and temporal resolutions, and research purposes, but yield diverse results due to both parametric and structural differences. Previous work has attempted to identify some differences among commonly used capacity expansion models but has been unable to disentangle parametric from structural uncertainty. Here, we present a model benchmarking effort using highly simplified scenarios applied to four open-source models of the U.S. electric sector. We eliminate all parametric uncertainty through using a common dataset and leave only structural differences. We demonstrate how a systematic model comparison process allows us to pinpoint specific and important structural differences among our models, including specification of technologies as baseload or load following generation, battery state-ofcharge at the beginning and end of a modeled period, application of battery roundtrip efficiency, treatment of discount rates, formulation of model end effects, and digit precision of input parameters. Our results show that such a process can be effective for improving consistency across models and building model confidence, substantiating specific modeling choices, reporting uncertainties, and identifying areas for further research and development. We also introduce an open-source test dataset that the modeling community can use for unit testing and build on for benchmarking exercises of more complex models. A community benchmarking effort can increase collaboration among energy modelers and provide transparency regarding the energy transition and energy challenges, for other stakeholders such as policymakers.