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

Corinne A Hartin,Pralit Patel,Anupriya Mundra,Benjamin Bond-Lamberty

doi:10.5194/bg-13-4329-2016

Abstract

Abstract. Continued oceanic uptake of anthropogenic CO2 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 (ΩAr) 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 CO2 concentrations – Q10 (terrestrial respiration temperature response) as well as changes in ocean circulation, while changes in ΩAr saturation levels are sensitive to changes in ocean salinity and Q10. 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.

Highlights

Human activities have led to increasing anthropogenic emissions of greenhouse gases to the atmosphere
In the first decade of the 21st century CO2 emissions from anthropogenic sources and land-use changes accounted for ∼ 9 Pg C yr−1, with future emission projections of up to 28 Pg C yr−1 by 2100 under Representative Concentration Pathway (RCP) 8.5 (Riahi et al, 2011)
In this study we focus on the upper ocean high- and low-latitude inorganic carbon chemistry under RCP 8.5

Summary

Introduction

Human activities have led to increasing anthropogenic emissions of greenhouse gases to the atmosphere. The world’s oceans have played a critical role in lessening the effects of climate change by absorbing 25–30 % of the total anthropogenic carbon emissions since 1750 (Le Quéré et al, 2013; Sabine et al, 2011). In response to this increasing atmospheric burden of CO2 and increasing oceanic uptake, the oceans are experiencing both physical and biogeochemical changes: surface and deep water warming, reduced subsurface oxygen, and a reduction in calcium carbonate saturation levels and pH (Doney, 2010).

Results

Discussion

Conclusion