Sensitivity of atmospheric CO2 and climate to explosive volcanic eruptions

Impacts of low-latitude, explosive volcanic eruptions on climate and the carbon cycle are quantified by forcing a comprehensive, fully coupled carbon cycle-climate model with pulse-like stratospheric aerosol optical depth changes. The model represents the radiative and dynamical response of the climate system to volcanic eruptions and simulates a decrease of global and regional atmospheric surface temperature, regionally distinct changes in precipitation, a positive phase of the North Atlantic Oscillation, and a decrease in atmospheric CO₂ after volcanic eruptions. The volcanic-induced cooling reduces overturning rates in tropical soils, which dominates over reduced litter input due to soil moisture decrease, resulting in higher land carbon inventories for several decades. The perturbation in the ocean carbon inventory changes sign from an initial weak carbon sink to a carbon source. Positive carbon and negative temperature anomalies in subsurface waters last up to several decades. The multi-decadal decrease in atmospheric CO₂ yields a small additional radiative forcing that amplifies the cooling and perturbs the Earth System on longer time scales than the atmospheric residence time of volcanic aerosols. In addition, century-scale global warming simulations with and without volcanic eruptions over the historical period show that the ocean integrates volcanic radiative cooling and responds for different physical and biogeochemical parameters such as steric sea level or dissolved oxygen. Results from a suite of sensitivity simulations with different magnitudes of stratospheric aerosol optical depth changes and from global warming simulations show that the carbon cycle-climate sensitivity γ, expressed as change in atmospheric CO₂ per unit change in global mean surface temperature, depends on the magnitude and temporal evolution of the perturbation, and time scale of interest. On decadal time scales, modeled γ is several times larger for a Pinatubo-like eruption than for the industrial period and for a high emission, 21st century scenario.

A gender bias in the calcification response to ocean acidification

The effects of nutrients and pCO₂ on zooxanthellate and azooxanthellate colonies of the temperate scleractinian coral Astrangia poculata (Ellis and Solander, 1786) were investigated at two different temperatures (16 °C and 24 °C). Corals exposed to elevated pCO₂ tended to have lower relative calcification rates, as estimated from changes in buoyant weights. No nutrient effect was observed. At 16 °C, gamete release was not observed, and no gender differences in calcification rate were observed. However, corals grown at 24 °C spawned repeatedly and male and female corals exhibited two different growth rate patterns. Female corals grown at 24 °C and exposed to CO₂ had calcification rates 39 % lower than females grown at ambient CO₂, while males showed only a 5 % decline in calcification under elevated CO₂. At 16 °C, female and male corals showed similar reductions in calcification rates in response to elevated CO₂ (15 % and 19 % respectively). At 24 °C, corals spawned repeatedly, while no spawning was observed at 16 °C. The increased sensitivity of females to elevated pCO₂ may reflect a greater investment of energy in reproduction (egg production) relative to males (sperm production). These results suggest that both gender and spawning are important factors in determining the sensitivity of corals to ocean acidification and their inclusion in future research may be critical to predicting how the population structures of marine calcifiers will change in response to ocean acidification.

Effect of salinity induced pH changes on benthic foraminifera: a laboratory culture experiment

The coastal water pH varies with salinity. Therefore, to study the effect of salinity induced pH variations on benthic foraminifera, live specimens of Rosalina globularis were subjected to different salinities (10, 15, 20, 25, 30, 35 and 40 ‰) with pH varying from 7.2 to 8.2. A total of 210 specimens were used and the experiment was conducted in replicates. It was observed that the salinity induced pH changes affect the calcification of foraminifera. However the response is not linear. The maximum growth is reported in the specimens kept at 35 ‰ salinity (pH 8.0) while the rest of the specimens maintained at salinity higher or lower than 35 ‰, showed comparatively lesser growth. A significant drop in pH severely hampers the calcification capability of benthic foraminifera. Specimens kept at 10 and 15 ‰ (pH 7.2 and 7.5, respectively) became opaque within two days of lowering the salinity and later on their tests dissolved within 24 and 43 days, respectively. Besides calcification capability, pH also affects reproduction. No specimen reproduced at 10 and 15 ‰ salinity while only a few specimens (3 %) reproduced at 20 ‰. As compared to 10–20 ‰ salinity, ∼60 % reproduction was observed in specimens subjected to 25–40 ‰ salinity. The drop in pH also decreased the calcification rate as specimens at 20 ‰ salinity took twice the time to reach maturity than normal range (25–40 ‰). We conclude that salinity induced drop in pH adversely affects the calcification capability and reproduction in benthic foraminifera. It is inferred that the time required to reach reproductive maturity increases at the extreme salinity tolerance limits. However, beyond a certain limit, a further increase in pH does not affect benthic foraminifera; rather they respond to salinity as per their salinity tolerance range.

Geographical variations in the effectiveness and side effects of deep ocean carbon sequestration

The capture and injection of carbon dioxide (CO2) into the deep ocean could provide a relatively long-term mitigation of climate change, but would come at the expense of enhancing acidification at the seafloor. We employ an Earth system model to survey the regional differences in the effectiveness and side effects of CO2 injection. Sequestration efficiency, as calculated relative to the ‘natural’ invasion from the atmosphere that would occur in the absence of mitigation, is highest for injection in the deep NW Pacific, but can be negative for shallow sites. For higher climate sensitivities and greater total emissions, sequestration efficiency is enhanced, decreasing the relative cost and increasing its potential value as a form of mitigation. However, CO2 injection increases the total area of seafloor bathed in under-saturated waters, with Atlantic sites inducing particularly large increases in seafloor undersaturation as well as having less favorable sequestration efficiency.

Editorial: Old carbon is killing oceans

“We’ve mailed a package to ourselves and it’s hard to call off delivery.” This pithy sound bite from Oregon State oceanographer Burke Hales refers to how the ocean absorbs human-related carbon dioxide and then cycles it back to the surface 30 to 50 years later.

Here on the Oregon Coast, this carbon cycle has been manifesting itself in the form of acidic seawater that rises from the depths and kills young oysters and clams. Especially right after hatching, their shells are tender enough to be dissolved by mild acid.