Posts Tagged 'globalmodeling'

Amplification of global warming through pH-dependence of DMS-production simulated with a fully coupled Earth system model

We estimate the additional transient surface warming ΔTs caused by a potential reduction of marine dimethyl sulfide (DMS) production due to ocean-acidification under the high emission scenario RCP8.5 until the year 2200. Since we use a fully coupled Earth system model, our results include a range of feedbacks, such as the response of marine DMS-production to the additional changes in temperature and sea-ice cover. Our results are broadly consistent with the findings of a previous study that employed an off-line model set-up. Assuming a medium (strong) sensitivity of DMS-production to pH, we find an additional transient global warming of 0.30 K (0.47 K) towards the end of the 22nd century when DMS-emission are reduced by 7.3 Tg S yr−1 or 31 % (11.5 Tg S yr−1 or 48 %). The main mechanism behind the additional warming is a reduction of cloud albedo, but a change in short-wave radiative fluxes under clear-sky conditions due to reduced sulfate aerosol load also contributes significantly. We find an approximately linear relationship between reduction of DMS-emissions and changes in top of the atmosphere radiative fluxes as well as changes in surface temperature for the range of DMS-emissions considered here. For example, global average Ts changes by −0.041 K per 1 Tg S yr−1 change in sea-air DMS-fluxes. The additional warming in our model has a pronounced asymmetry between northern and southern high latitudes. It is largest over the Antarctic continent, where the additional temperature increase of 0.56 K (0.89 K) is almost twice the global average. We find that feedbacks are small on the global scale due to opposing regional contributions. The most pronounced feedback is found for the Southern Ocean, where we estimate that the additional climate change enhances sea-air DMS-fluxes by about 9 % (15 %), which counteracts the reduction due to ocean acidification.

Continue reading ‘Amplification of global warming through pH-dependence of DMS-production simulated with a fully coupled Earth system model’

Risk and resilience: variations in magnesium in echinoid skeletal calcite

Echinoids have high-magnesium (Mg) calcite endoskeletons that may be vulnerable to CO2-driven ocean acidification. Amalgamated data for echinoid species from a range of environments and life-history stages allowed characterization of the factors controlling Mg content in their skeletons. Published measurements of Mg in calcite (N = 261), supplemented by new X-ray diffractometry data (N = 382), produced a database including 8 orders, 23 families and 73 species (~7% of the ~1000 known extant species), spanning latitudes 77°S to 72°N, and including 9 skeletal elements or life stages. Mean (± SD) skeletal carbonate mineralogy in the Echinoidea is 7.5 ± 3.23 wt% MgCO3 in calcite (range: 1.5-16.4 wt%, N = 643). Variation in Mg within individuals was small (SD = 0.4-0.9 wt% MgCO3). We found significant differences among skeletal elements: jaw demi-pyramids were the highest in Mg, whereas tests, teeth and spines were intermediate in Mg, but generally higher than larvae. Higher taxa have consistent mineralogical patterns, with orders in particular showing Mg related to first appearance in the fossil record. Latitude was a good proxy for sea-surface temperature (SST), although incorporating SST where available produced a slightly better model. Mg content varied with latitude; higher Mg content in warmer waters may reflect increased metabolic and growth rates. Although the skeletons of some adult urchins may be partially resistant to ocean acidification, larvae and some species may prove to be vulnerable to lowered pH, resulting in ecosystem changes in coastal marine environments.

Continue reading ‘Risk and resilience: variations in magnesium in echinoid skeletal calcite’

Earth 2075—CO2 II. Targeting 0°C Global Warming, Ocean pH 8.2, and an Early Return to 280 ppm

In this second paper of CRT’s EARTH 2075—CO2 series, revised emissions targets take into account fossil fuel combustion and cement production trends, global change inertia, and developing countries’ future energy needs. Global CO2 emissions are currently ~10 GtC/yr. A 12 GtC/yr cap is recommended by 2023. A realistic schedule for subsequently reducing emissions is recommended—returning to 10.5 GtC/yr by 2030 and cutting to 6 GtC/yr by 2050, 3 GtC/yr by 2062, and 1 GtC/yr by 2078. Our forecasting model assumes developing countries (DC) would moderately burn fossil fuels through 2062 while today’s industrial nations compensate with high-impact atmospheric carbon capture, plus offsetting emissions reductions of their own—sufficient to cap global emissions by 2023 and enable the above-targeted reductions through 2062. Developing countries would begin or accelerate their emissions cuts in 2063. Our forecasting model projects emissions cap and reduction impact on the accumulated mixing ratio (ppm) for atmospheric CO2. Emissions cuts alone are no longer likely to prevent CO2 from reaching 450 ppm tipping levels. More drastic emergency intervention is required to forestall tipping level crossings and prevent disastrous future consequences, including ≥ 2°C warming accompanied by mega-drought, superstorms, partial polar ice collapse, and abrupt catastrophic sea rise. A new approach involving massively amplified safe capture of atmospheric CO2 at sea is proposed. This paper establishes open-ocean amplified capture targets and forecasts the beneficial impacts of meeting them. Recommended high impact targets for mid-ocean capture and sequestration of atmospheric CO2 include contingency for delays and energy to drive multi-stage (land/sea) amplified capture plus extra contingency to offset feedbacks, outgassing, and permafrost thaw-release, which the model didn’t anticipate. CRT recommends starting multi-stage short-cycle ocean-amplified carbon capture (OACC) in 2019 and ramping it up to net 10 GtC/yr average CO2 capture by 2025 across vast mid-latitude, mid-ocean expanses—far out at sea and well away from coastal waters—plus simultaneously compounding benefits of the 12 GtC/yr 2023 emissions cap and above-targeted post-2023 emissions reductions, culminating in 92 percent reduction by 2078. With the sum of OACC plus natural sinks matching capped emissions by 2023 and substantially exceeding reduced post-2023 emissions, accumulated atmospheric CO2 may be capped at ≤ 425 ppm by 2023 and reduced to 350 ppm by 2050, with an option to restore 280 ppm by 2075 and reduce twenty-first century warming to 0°C. High-impact ocean-amplified carbon capture (OACC) at the rate of 10 GtC/yr could enable DC emissions leniency and still turn 280 ppm atmospheric CO2, ∆T = 0°C, and ocean pH 8.2 into viable twenty-first century targets that can be met approximately 250 years earlier than with emissions reduction alone—if tropospheric aerosol pollution is concurrently reduced.

Continue reading ‘Earth 2075—CO2 II. Targeting 0°C Global Warming, Ocean pH 8.2, and an Early Return to 280 ppm’

An abyssal carbonate compensation depth overshoot in the aftermath of the Palaeocene–Eocene Thermal Maximum

During the Palaeocene–Eocene Thermal Maximum (PETM) about 56 million years ago, thousands of petagrams of carbon were released into the atmosphere and ocean in just a few thousand years, followed by gradual sequestration over approximately 200,000 years. If silicate weathering is one of the key negative feedbacks that removed this carbon, a period of seawater calcium carbonate saturation greater than pre-event levels would be expected during the event’s recovery phase. In marine sediments, this should be recorded as a temporary deepening of the depth below which no calcite is preserved — the calcite compensation depth (CCD). Previous and new sedimentary records from sites that were above the pre-PETM CCD show enhanced carbonate accumulation following the PETM. A new record from an abyssal site in the North Atlantic that lay below the pre-PETM CCD shows a period of carbonate preservation beginning about 70,000 years after the onset of the PETM, providing the first direct evidence for an over-deepening of the CCD. This record confirms an overshoot in ocean carbonate saturation during the PETM recovery. Simulations with two earth system models support scenarios for the PETM that involve a large initial carbon release followed by prolonged low-level emissions, consistent with the timing of CCD deepening in our record. Our findings indicate that sequestration of these carbon emissions was most likely the result of both globally enhanced calcite burial above the CCD and, at least in the North Atlantic, an over-deepening of the CCD.

Continue reading ‘An abyssal carbonate compensation depth overshoot in the aftermath of the Palaeocene–Eocene Thermal Maximum’

Quantifying the volcanic emissions which triggered Oceanic Anoxic Event 1a and their effect on ocean acidification

The Cretaceous Oceanic Anoxic Event 1a (Early Aptian) is thought to be causally related to the eruption of the Ontong–Java Plateau large igneous province. This study uses osmium isotope records to quantify the magnitude of the respective CO2 emissions up to the onset of Ocean Anoxic Event 1a, and model the associated changes in carbonate saturation state (omega), atmospheric pCO2, carbon isotope ratios and the carbonate compensation depth with a carbon cycle model. These model results suggest that volcanism associated with the rapid negative 187/188 osmium ratios observed during the onset of Oceanic Anoxic Event 1a (Selli Event) increased the planetary CO2 degassing flux at least six-fold, causing a negative δ13C excursion of ca 1.5‰ in the dissolved surface ocean inorganic carbon pool. This is consistent with previously published δ13C data. Volcanic degassing of this magnitude would also suppress the aragonite saturation state of surface water to near under saturated values (Omega ca 1.1 to 0.9), shoal the carbonate compensation depth by 1500 m, and increase the atmospheric pCO2 by 3000 ppm, before increased weathering and anoxia would counter the pCO2 increase.

Continue reading ‘Quantifying the volcanic emissions which triggered Oceanic Anoxic Event 1a and their effect on ocean acidification’

Impacts of artificial ocean alkalinization on the carbon cycle and climate in Earth system simulations

Using the state-of-the-art emissions-driven Max Planck Institute Earth system model, we explore the impacts of artificial ocean alkalinization (AOA) with a scenario based on the Representative Concentration Pathway (RCP) framework. Addition of 114 Pmol of alkalinity to the surface ocean stabilizes atmospheric CO2 concentration to RCP4.5 levels under RCP8.5 emissions. This scenario removes 940 GtC from the atmosphere and mitigates 1.5 K of global warming within this century. The climate adjusts to the lower CO2 concentration preventing the loss of sea ice and high sea level rise. Seawater pH and the carbonate saturation state (Ω) rise substantially above levels of the current decade. Pronounced differences in regional sensitivities to AOA are projected, with the Arctic Ocean and tropical oceans emerging as hot spots for biogeochemical changes induced by AOA. Thus, the CO2 mitigation potential of AOA comes at a price of an unprecedented ocean biogeochemistry perturbation with unknown ecological consequences.

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An alternative model for CaCO3 over-shooting during the PETM: Biological carbonate compensation

Decreased CaCO3 content of deep-sea sediments argues for rapid and massive acidification of the oceans during the Paleocene–Eocene Thermal Maximum (PETM, ∼56 Ma BP). In the course of the subsequent recovery from this acidification, sediment CaCO3 content came to exceed pre-PETM levels, known as over-shooting. Past studies have largely attributed the latter to increased alkalinity input to the oceans via enhanced weathering, but this ignores potentially important biological factors. We successfully reproduce the CaCO3 records from Walvis Ridge in the Atlantic Ocean, including over-shooting, using a biogeochemical box model. Replication of the CaCO3 records required: 1) introduction of a maximum of ∼6500 GtC of CO2 directly into deep-ocean waters or ∼8000 GtC into the atmosphere, 2) limited deep-water exchange between the Indo-Atlantic and Pacific oceans, 3) the disappearance of sediment bioturbation during a portion of the PETM, and 4) most central to this study, a ∼50% reduction in net CaCO3 production, during acidification. In our simulations, over-shooting is an emergent property, generated at constant alkalinity input (no weathering feedback) as a consequence of attenuated CaCO3 productivity. This occurs because lower net CaCO3 production from surface waters allows alkalinity to build-up in the deep oceans (alkalinization), thus promoting deep-water super-saturation. Restoration of CaCO3 productivity later in the PETM, particularly in the Indo-Atlantic Ocean, leads to greater accumulation of CaCO3, ergo over-shooting, which returns the ocean to pre-PETM conditions over a time scale greater than 200 ka.

Continue reading ‘An alternative model for CaCO3 over-shooting during the PETM: Biological carbonate compensation’

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Ocean acidification in the IPCC AR5 WG II

OUP book