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Climate Change and Ocean Acidification Impacts on Lower EGU Journal Logos (RGB) Open Access Open Access Open Access Advances in Annales Nonlinear Processes Geosciences Geophysicae in Geophysics Open Access Open Access Natural Hazards Natural Hazards and Earth System and Earth System Sciences Sciences Discussions Open Access Open Access Atmospheric Atmospheric Chemistry Chemistry and Physics and Physics Discussions Open Access Open Access Atmospheric Atmospheric Measurement Measurement Techniques Techniques Discussions Open Access Biogeosciences, 10, 5831–5854, 2013 Open Access www.biogeosciences.net/10/5831/2013/ Biogeosciences doi:10.5194/bg-10-5831-2013 Biogeosciences Discussions © Author(s) 2013. CC Attribution 3.0 License. Open Access Open Access Climate Climate of the Past of the Past Discussions Climate change and ocean acidification impacts on lower trophic Open Access Open Access levels and the export of organic carbon to the deepEarth ocean System Earth System Dynamics 1 1 1 2 1 Dynamics A. Yool , E. E. Popova , A. C. Coward , D. Bernie , and T. R. Anderson Discussions 1National Oceanography Centre, University of Southampton Waterfront Campus, European Way, Southampton SO14 3ZH, UK Open Access Open Access 2Met Office Hadley Centre, FitzRoy Road, Exeter EX1 3PB, UK Geoscientific Geoscientific Instrumentation Instrumentation Correspondence to: A. Yool ([email protected]) Methods and Methods and Received: 29 January 2013 – Published in Biogeosciences Discuss.: 25 February 2013 Data Systems Data Systems Revised: 18 July 2013 – Accepted: 19 July 2013 – Published: 5 September 2013 Discussions Open Access Open Access Geoscientific Geoscientific Abstract. Most future projections forecast significant and casting of these ecosystems. For all processes, there is ge- Model Development ongoing climate change during the 21st century, but with the ographical variabilityModel in changeDevelopment – for instance, productivity Discussions severity of impacts dependent on efforts to restrain or reor- declines −21 % in the Atlantic and increases +59 % in the ganise human activity to limit carbon dioxide (CO2) emis- Arctic – and changes are much more pronounced under RCP Open Access Open Access sions. A major sink for atmospheric CO2, and a key source 8.5 than the RCP 2.6 scenario. of biological resources, the World Ocean is widely antici- Hydrology and Hydrology and pated to undergo profound physical and – via ocean acid- Earth System Earth System ification – chemical changes as direct and indirect results of these emissions. Given strong biophysical coupling, the 1 Introduction Sciences Sciences marine biota is also expected to experience strong changes Discussions Open Access in response to this anthropogenic forcing. Here we exam- Starting at the dawn of the industrial era, atmosphericOpen Access con- ine the large-scale response of ocean biogeochemistry to cli- centrations of carbon dioxide (CO2) have steadily increased Ocean Science mate and acidification impacts during the 21st century for from approximately 280Ocean ppm to almost Science 400 ppm, an absolute Discussions Representative Concentration Pathways (RCPs) 2.6 and 8.5 change that is greater than that between previous glacial and using an intermediate complexity global ecosystem model, interglacial periods. This change has been driven largely by MEDUSA-2.0. The primary impact of future change lies in the combustion of so-called fossil fuels, but also by concomi- Open Access Open Access stratification-led declines in the availability of key nutrients tant land-use changes and cement production. Many socio- in surface waters, which in turn leads to a global decrease economic analyses forecast it to continue into the future, with Solid Earth − some scenarios of future humanSolid impact Earth elevating CO2 to al- (1990s vs. 2090s) in ocean productivity ( 6.3 %). This im- Discussions pact has knock-on consequences for the abundance of the low most 1000 ppm by the close of the current century (Houghton trophic level biogeochemical actors modelled by MEDUSA- et al., 2001). − Nonetheless, rising atmospheric CO2 is already mitigated 2.0 ( 5.8 %), and these would be expected to similarly im- Open Access Open Access pact higher trophic level elements such as fisheries. Related by a suite of earth system processes, on both land and in impacts are found in the flux of organic material to seafloor the ocean, that act to remove anthropogenic CO2 to these The Cryosphere reservoirs. In the caseThe of theCryosphere land, this is largely believed communities (−40.7 % at 1000 m), and in the volume of Discussions ocean suboxic zones (+12.5 %). A sensitivity analysis re- to be driven by the “fertilisation effect” that elevated atmo- moving an acidification feedback on calcification finds that spheric pCO2 has on terrestrial plants, which are able to fix change in this process significantly impacts benthic com- CO2 at a lower cost in terms of water loss (Van Minnen et munities, suggesting that a better understanding of the OA- al., 2009). By contrast, uptake of anthropogenic CO2 by the sensitivity of calcifying organisms, and their role in ballast- ocean is believed to be driven by a series of physicochem- ing sinking organic carbon, may significantly improve fore- ical factors collectively referred to as the solubility pump. Here, elevated CO2 dissolution in cold waters combines with Published by Copernicus Publications on behalf of the European Geosciences Union. 5832 A. Yool et al.: Climate and ocean acidification impacts deep water formation in the same locations to draw CO2 into spectra, from primary producers, such as coccolithophorids, the ocean’s interior. It is estimated that this process has ac- through to heterotrophs, such as protistan foraminifera and counted for about 30 % of the total anthropogenic emissions metazoan pteropods. It is typically produced to form struc- of CO2 (Sabine et al., 2004), although it is suggested by some tures such as cell coverings, shells or skeletons, and occurs in workers that the strength of the ocean’s solubility pump is in two major crystal forms, calcite and aragonite, that differ in decline (Le Quer´ e´ et al., 2007). their solubility. As the ocean acidifies, carbonate chemistry Alongside the ocean’s solubility pump is the so-called bi- shifts such that the concentration of bicarbonate increases, ological pump, which represents the sum of a series of bi- but that of carbonate decreases. As a result, the solubility of ologically mediated processes that act to transport carbon CaCO3 will increase, with potentially serious consequences – as organic carbon – into the ocean’s interior. This pump for organisms using it for structural purposes (Orr et al., is ultimately driven by the production of organic carbon by 2005; Fabry et al., 2008; Gangstø et al., 2011). phytoplankton in the sunlit surface ocean and its subsequent A more indirect impact of increased CaCO3 solubility may aggregation into sinking particles (Volk and Hoffert, 1985; lie in its connection with the biological pump. Based on sed- Buesseler et al., 2007). As dissolved inorganic carbon (DIC) iment trap observations, it has been hypothesised that the is abundant in seawater, primary production in the ocean is sinking flux of organic material to the deep ocean is en- not limited by CO2, but may instead be limited by the avail- hanced in the presence of CaCO3, which acts to “protect” ability of nutrients such as nitrogen, silicon or iron (and by from remineralisation (Armstrong et al., 2002; Klaas and the activity of grazing zooplankton). As a result, the biologi- Archer, 2002). Should the abundance of calcifying organisms cal pump is not believed to have played any significant role in in the upper ocean decline in response to OA, this may shoal the ocean’s response to anthropogenic CO2 to date (Gruber the remineralisation of organic material and diminish its flux et al., 1996; but see also Boyce et al., 2010). into the abyssal ocean (Heinze, 2004). This may act to retain However, though the physical state of the ocean has nutrients at shallower depths, but it may also act to decrease changed only slightly since the start of the Industrial Rev- the food supply to seafloor communities reliant on sinking olution (cf. Levitus et al., 2005), future global warming is material. expected to cause significant changes to its stratification and Here we examine issues of future ocean productivity in re- circulation, with likely consequences for the biological pump sponse to climate change and OA using an intermediate com- (Van der Waal et al., 2010; Doney et al., 2012). An analy- plexity ecosystem model, MEDUSA-2.0. This model is used sis of phytoplankton abundance (based on in situ and satel- in simulations for the period 1860–2100 for two IPCC Repre- lite observations) has suggested that change is already de- sentative Concentration Pathways (RCPs) of future CO2 con- tectable and that they are declining in response to rising sea centrations (RCP 2.6 and RCP 8.5), and with sensitivity ex- surface temperatures (Boyce et al., 2010, 2012; but see also periments to explore the impact of OA on CaCO3 production. Mackas, 2011; McQuatters-Gollop et al., 2011; Rykaczewski The manuscript is structured as follows. First, MEDUSA-2.0 and Dunne, 2011; Boyce et al., 2011). And a number of mod- is briefly introduced, followed by a description of the simu- elling studies have found a similar relationship between in- lation protocol. This is followed by an overview of the simu- creasing ocean stratification and declining ocean productiv- lation’s performance, both physically and biogeochemically, ity and associated export (e.g. Bopp et al., 2001; Steinacher for the recent period (1980–2010). Next, results are presented et al., 2010; but see also Taucher and Oschlies, 2011). for the 21st century, with a particular focus on the changes Related to oceanic uptake of anthropogenic CO2 is ongo- found between the 1990s and the 2090s. Most of the analysis ing ocean acidification (OA). This occurs as the dissolved an- deals with the higher CO2 pathway, RCP 8.5, but RCP 2.6 thropogenic CO2 combines with H2O and speciates into bi- is also examined.
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