Archive for February, 2016

Apparent increase in coccolithophore abundance in the subtropical North Atlantic from 1990 to 2014 (update)

Abstract. As environmental conditions evolve with rapidly increasing atmospheric CO2, biological communities will change as species reorient their distributions, adapt, or alter their abundance. In the surface ocean, dissolved inorganic carbon (DIC) has been increasing over the past several decades as anthropogenic CO2 dissolves into seawater, causing acidification (decreases in pH and carbonate ion concentration). Calcifying phytoplankton, such as coccolithophores, are thought to be especially vulnerable to ocean acidification. How coccolithophores will respond to increasing carbon input has been a subject of much speculation and inspired numerous laboratory and mesocosm experiments, but how they are currently responding in situ is less well documented. In this study, we use coccolithophore (haptophyte) pigment data collected at the Bermuda Atlantic Time-series Study (BATS) site together with satellite estimates (1998–2014) of surface chlorophyll and particulate inorganic carbon (PIC) as a proxy for coccolithophore abundance to show that coccolithophore populations in the North Atlantic subtropical gyre have been increasing significantly over the past 2 decades. Over 1990–2012, we observe a 37 % increase in euphotic zone-integrated coccolithophore pigment abundance at BATS, though we note that this is sensitive to the period being analyzed. We further demonstrate that variability in coccolithophore chlorophyll a here is positively correlated with variability in nitrate and DIC (and especially the bicarbonate ion) in the upper 30 m of the water column. Previous studies have suggested that coccolithophore photosynthesis may benefit from increasing CO2, but calcification may eventually be hindered by low pHT (< 7.7). Given that DIC has been increasing at BATS by  ∼ 1.4 µmol kg−1 yr−1 over the period of 1991–2012, we speculate that coccolithophore photosynthesis and perhaps calcification may have increased in response to anthropogenic CO2 input.

Continue reading ‘Apparent increase in coccolithophore abundance in the subtropical North Atlantic from 1990 to 2014 (update)’

Péril acide sur les coraux (in French)

C’est l’autre conséquence, moins connue, de nos trop importantes émissions de gaz à effet de serre dans l’atmosphère. En plus d’être responsables de changements climatiques, ces gaz en excès perturbent la chimie des océans, les rendant plus acides. Cette acidification de l’eau menace particulièrement les écosystèmes fragiles que sont les récifs coralliens, comme le soulignent deux nouvelles études, publiées cette semaine dans des revues du groupe Nature. La Grande Barrière de corail australienne, en particulier, pourrait disparaître plus rapidement qu’escompté. Les scientifiques appellent à des actions urgentes pour protéger ces milieux uniques.

Continue reading ‘Péril acide sur les coraux (in French)’

Coastal-ocean uptake of anthropogenic carbon

Anthropogenic changes in atmosphere-ocean and atmosphere-land CO2 fluxes have been quantified extensively, but few studies have addressed the connection between land and ocean. In this transition zone, the coastal ocean, spatial and temporal data coverage is inadequate to assess its global budget. Thus we use a global ocean biogeochemical model to assess the coastal ocean’s global inventory of anthropogenic CO2 and its spatial variability. We used an intermediate resolution, eddying version of the NEMO-PISCES model (ORCA05), varying from 20 to 50 km horizontally, i.e., coarse enough to allow multiple century-scale simulations but finer than coarse resolution models (~ 200 km), to begin to better resolve coastal bathymetry. Simulated results indicated that the global ocean absorbed 2.3 Pg C yr−1 of anthropogenic carbon during 1993–2012, consistent with previous estimates. Yet only 4.5 % of that (0.10 Pg C yr−1) is absorbed by the global coastal ocean, i.e., less than its 7.5 % proportion of the global ocean surface area. Coastal uptake is weakened due to a bottleneck in offshore transport, which is inadequate to reduce the mean anthropogenic carbon concentration of coastal waters to the mean level found in the open-ocean mixed layer. Continue reading ‘Coastal-ocean uptake of anthropogenic carbon’

Transient Earth system responses to cumulative carbon dioxide emissions: linearities, uncertainties, and probabilities in an observation-constrained model ensemble

Information on the relationship between cumulative fossil CO2 emissions and multiple climate targets is essential to design emission mitigation and climate adaptation strategies. In this study, the transient response of a climate or environmental variable per trillion tonnes of CO2 emissions, termed TRE, is quantified for a set of impact-relevant climate variables and from a large set of multi-forcing scenarios extended to year 2300 towards stabilization. An  ∼ 1000-member ensemble of the Bern3D-LPJ carbon–climate model is applied and model outcomes are constrained by 26 physical and biogeochemical observational data sets in a Bayesian, Monte Carlo-type framework. Uncertainties in TRE estimates include both scenario uncertainty and model response uncertainty. Cumulative fossil emissions of 1000 Gt C result in a global mean surface air temperature change of 1.9 °C (68 % confidence interval (c.i.): 1.3 to 2.7 °C), a decrease in surface ocean pH of 0.19 (0.18 to 0.22), and a steric sea level rise of 20 cm (13 to 27 cm until 2300). Linearity between cumulative emissions and transient response is high for pH and reasonably high for surface air and sea surface temperatures, but less pronounced for changes in Atlantic meridional overturning, Southern Ocean and tropical surface water saturation with respect to biogenic structures of calcium carbonate, and carbon stocks in soils. The constrained model ensemble is also applied to determine the response to a pulse-like emission and in idealized CO2-only simulations. The transient climate response is constrained, primarily by long-term ocean heat observations, to 1.7 °C (68 % c.i.: 1.3 to 2.2 °C) and the equilibrium climate sensitivity to 2.9 °C (2.0 to 4.2 °C). This is consistent with results by CMIP5 models but inconsistent with recent studies that relied on short-term air temperature data affected by natural climate variability.

Continue reading ‘Transient Earth system responses to cumulative carbon dioxide emissions: linearities, uncertainties, and probabilities in an observation-constrained model ensemble’

Negligible effects of ocean acidification on Eurytemora affinis (Copepoda) offspring production

Ocean acidification is caused by increasing amounts of carbon dioxide dissolving in the oceans leading to lower seawater pH. We studied the effects of lowered pH on the calanoid copepod Eurytemora affinis during a mesocosm experiment conducted in a coastal area of the Baltic Sea. We measured copepod reproductive success as a function of pH, chlorophyll a concentration, diatom and dinoflagellate biomass, carbon to nitrogen (C : N) ratio of suspended particulate organic matter, as well as copepod fatty acid composition. The laboratory-based experiment was repeated four times during 4 consecutive weeks, with water and copepods sampled from pelagic mesocosms enriched with different CO2 concentrations. In addition, oxygen radical absorbance capacity (ORAC) of animals from the mesocosms was measured weekly to test whether the copepod’s defence against oxidative stress was affected by pH. We found no effect of pH on offspring production. Phytoplankton biomass, as indicated by chlorophyll a concentration and dinoflagellate biomass, had a positive effect. The concentration of polyunsaturated fatty acids in the females was reflected in the eggs and had a positive effect on offspring production, whereas monounsaturated fatty acids of the females were reflected in their eggs but had no significant effect. ORAC was not affected by pH. From these experiments we conclude that E. affinis seems robust against direct exposure to ocean acidification on a physiological level, for the variables covered in the study. E. affinis may not have faced acute pH stress in the treatments as the species naturally face large pH fluctuations.

Continue reading ‘Negligible effects of ocean acidification on Eurytemora affinis (Copepoda) offspring production’

The exposure of the Great Barrier Reef to ocean acidification

The Great Barrier Reef (GBR) is founded on reef-building corals. Corals build their exoskeleton with aragonite, but ocean acidification is lowering the aragonite saturation state of seawater (Ωa). The downscaling of ocean acidification projections from global to GBR scales requires the set of regional drivers controlling Ωa to be resolved. Here we use a regional coupled circulation–biogeochemical model and observations to estimate the Ωa experienced by the 3,581 reefs of the GBR, and to apportion the contributions of the hydrological cycle, regional hydrodynamics and metabolism on Ωa variability. We find more detail, and a greater range (1.43), than previously compiled coarse maps of Ωa of the region (0.4), or in observations (1.0). Most of the variability in Ωa is due to processes upstream of the reef in question. As a result, future decline in Ωa is likely to be steeper on the GBR than currently projected by the IPCC assessment report. Continue reading ‘The exposure of the Great Barrier Reef to ocean acidification’

Ocean acidification: decline of Great Barrier Reef likely to be worse than feared

Parts of the Great Barrier Reef are more vulnerable to ocean acidification than previously thought, according to new research published today in the journal Nature Communications.

The first-of-a-kind study of more than 3,000 coral reefs off the northeast coast of Australia reveals how some parts are already experiencing the kind of conditions scientists were predicting many decades from now.

This means the decline of the iconic ecosystem is likely to be steeper than scientists feared, say the team from the Commonwealth Scientific and Industrial Research Organisation (CSIRO) in Australia and the King Abdullah University of Science and Technology in Saudi Arabia.

Continue reading ‘Ocean acidification: decline of Great Barrier Reef likely to be worse than feared’

Mussel power: How ocean acidification is changing shells

One of the big problems with the world’s heavy carbon emissions is that they are driving up the levels of carbon dioxide in our oceans, which is making them more acidic. The surface pH of the oceans has already dropped from 8.1 to 8.0 over the past couple of decades, and is projected to reach 7.7 by 2100 – a huge change in biological terms.

This is reducing the carbonate in the water that marine organisms including shellfish, corals and sea urchins depend on to make their shells and exoskeletons. I co-published a study two years ago into how this would affect mussels. By simulating the ocean conditions of 2100, we found that their shells did not grow as large and were harder and more brittle. Now, in a new study, we have seen fascinating signs of them adapting to these changes.

When we looked at the mussel shells of the future in our first study, we found they fractured considerably more easily. This made them more vulnerable to predators such as birds and crabs – and also to stormy conditions, since the stronger waves can bang them against rocks and other mussels. As an economically important food source across the world, it has worrying implications for those who depend on them to make their living – indeed, mussel farmers tell me they are noticing these changes even now. It also raises the prospect of similar problems for other shellfish such as oysters and cockles, not to mention sea urchins and corals.

Continue reading ‘Mussel power: How ocean acidification is changing shells’

Locally driven interannual variability of near-surface pH and ΩA in the strait of Georgia

Declines in mean ocean pH and aragonite saturation state (ΩA) driven by anthropogenic CO2 emissions have raised concerns regarding the trends of pH and ΩA in estuaries. Low pH and ΩA can be harmful to a variety of marine organisms, especially those with calcium carbonate (CaCO3) shells, and so may threaten the productive ecosystems and commercial fisheries found in many estuarine environments. The Strait of Georgia is a large, temperate, productive estuarine system with numerous wild and aquaculture shellfish and finfish populations. We determine the seasonality and variability of near-surface pH and ΩA in the Strait using a one-dimensional, biophysical, mixing layer model. We further evaluate the sensitivity of these quantities to local wind, freshwater, and cloud forcing by running the model over a wide range of scenarios using 12 years of observations. Near-surface pH and ΩA demonstrate strong seasonal cycles characterized by low pH, aragonite-undersaturated waters in winter and high pH, aragonite-supersaturated waters in summer. The aragonite saturation horizon generally lies at ∼20 m depth except in winter and during strong Fraser River freshets when it shoals to the surface. Periods of strong interannual variability in pH and aragonite saturation horizon depth arise in spring and summer. We determine that at different times of year, each of wind speed, freshwater flux, and cloud fraction are the dominant driver of this variability. These results establish the mechanisms behind the emerging observations of highly-variable near-surface carbonate chemistry in the Strait. This article is protected by copyright. All rights reserved.

Continue reading ‘Locally driven interannual variability of near-surface pH and ΩA in the strait of Georgia’

Deep-Sea DuraFET: A pressure tolerant pH sensor designed for global sensor networks

Increasing atmospheric carbon dioxide is driving a long-term decrease in ocean pH which is superimposed on daily to seasonal variability. These changes impact ecosystem processes and they serve as a record of ecosystem metabolism. However, the temporal variability in pH is observed at only a few locations in the ocean because a ship is required to support pH observations of sufficient precision and accuracy. This paper describes a pressure tolerant Ion Sensitive Field Effect Transistor pH sensor that is based on the Honeywell Durafet® ISFET die. When combined with a AgCl pseudo-reference sensor that is immersed directly in seawater, the system is capable of operating for years at a time on platforms that cycle from depths of several km to the surface. The paper also describes the calibration scheme developed to allow calibrated pH measurements to be derived from the activity of HCl reported by the sensor system over the range of ocean pressure and temperature. Deployments on vertical profiling platforms enable self-calibration in deep waters where pH values are stable. Measurements with the sensor indicate that it is capable of reporting pH with an accuracy of 0.01 or better on the total proton scale and a precision over multi-year periods of 0.005. This system enables a global ocean observing system for ocean pH.

Continue reading ‘Deep-Sea DuraFET: A pressure tolerant pH sensor designed for global sensor networks’

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Ocean acidification in the IPCC AR5 WG II

OUP book