Ocean acidification over the next three centuries using a simple global climate carbon-cycle model: projections and sensitivities

Abstract

Abstract. Continued oceanic uptake of anthropogenic CO 2 is projected to significantly alter the chemistry of the upper oceans over the next three centuries, with potentially serious consequences for marine ecosystems. Relatively few models have the capability to make projections of ocean acidification, limiting our ability to assess the impacts and probabilities of ocean changes. In this study we examine the ability of Hector v1.1, a reduced-form global model, to project changes in the upper ocean carbonate system over the next three centuries, and quantify the model's sensitivity to parametric inputs. Hector is run under prescribed emission pathways from the Representative Concentration Pathways (RCPs) and compared to both observations and a suite of Coupled Model Intercomparison (CMIP5) model outputs. Current observations confirm that ocean acidification is already taking place, and CMIP5 models project significant changes occurring to 2300. Hector is consistent with the observational record within both the high-(> 55 • ) and low-latitude oceans (< 55 • ). The model projects low-latitude surface ocean pH to decrease from preindustrial levels of 8.17 to 7.77 in 2100, and to 7.50 in 2300; aragonite saturation levels ( Ar ) decrease from 4.1 units to 2.2 in 2100 and 1.4 in 2300 under RCP 8.5. These magnitudes and trends of ocean acidification within Hector are largely consistent with the CMIP5 model outputs, although we identify some small biases within Hector's carbonate system. Of the parameters tested, changes in [H + ] are most sensitive to parameters that directly affect atmospheric CO 2 concentrations -Q 10 (terrestrial respiration temperature response) as well as changes in ocean circulation, while changes in Ar saturation levels are sensitive to changes in ocean salinity and Q 10 . We conclude that Hector is a robust tool well suited for rapid ocean acidification projections and sensitivity analyses, and it is capable of emulating both current observations and large-scale climate models under multiple emission pathways.

Effects of ocean acidification on Antarctic marine organisms: A meta‐analysis

“…The pH of Southern Ocean surface waters has already decreased by ≈0.1 pH units and is predicted to decrease by a further ≈0.3 units by 2100 under the IPCC RCP8.5 “business as usual” scenario, equating to surface water CO 2 levels >1,000 μatm (Kawaguchi et al, 2013; McNeil & Matear, 2007, 2008). Coupled intercomparison projects (CMIP5) show that low latitude waters could experience a decrease in pH from pre‐industrial levels of 8.17–7.77 by 2100, or 880 to 930 μatm CO 2 (Hartin, Bond‐Lamberty, Patel, & Mundra, 2016). Only under the lowest CO 2 emissions scenario with stringent mitigation will these changes be avoided (IPCC, 2019).…”

pyhector: A Python interface for the simple climate model Hector

“…The terrestrial carbon cycle includes primary production and respiration fluxes. The ocean carbon cycle circulates carbon via a simplified thermohaline circulation, calculating air-sea fluxes as well as the marine carbonate system (Hartin et al 2016).…”

“…The terrestrial carbon cycle includes primary production and respiration fluxes. The ocean carbon cycle circulates carbon via a simplified thermohaline circulation, calculating air-sea fluxes as well as the marine carbonate system (Hartin et al 2016).The model input is time series of greenhouse gas emissions; as example scenarios for these the Pyhector package contains the Representative Concentration Pathways (RCPs) 2 . These were developed to cover the range of baseline and mitigation emissions scenarios and are widely used in climate change research and model intercomparison projects.…”

Impacts of Observational Constraints Related to Sea Level on Estimates of Climate Sensitivity

“…Hector (v2.0), the reduced complexity model used in this study, is an open source, object-oriented, simple global climate carbon cycle model (Hartin et al, 2015;Schwarber et al, 2019). It has been applied to ocean acidification projections and sensitivities (Hartin et al, 2016) and is the default carbon cycle climate module of the Global Change Assessment Model (Calvin et al, 2018). Bakker et al, 2017 that includes contributions from thermal expansion, glaciers, and polar ice sheets.…”