Exploring the Role of Ocean Heat and Carbon Uptake in Determining the Linear Relationship between Global Warming and Cumulative CO2 Emissions

Abstract:

Recent studies showed an approximately linear relationship between global mean temperature change and cumulative CO₂ emissions. The ratio between temperature change and cumulative emissions, referred to as Transient Climate Response to cumulative Emissions (TCRE), is approximately constant over a wide range of cumulative CO₂ emissions but differs between climate models. Thus besides being potentially useful for estimating global warming for a certain amount of CO₂ emissions, the TCRE is also a benchmark for model-intercomparison as it combines both physical and biogeochemical model characteristics. However, the underlying physical and bio-geochemical mechanisms behind the constancy of the TCRE are not well known. It has been suggested that ocean heat and carbon uptake are governed by the same mechanisms, determining the constancy of the TCRE. However, there are reasons to question this hypothesis as atmospheric CO₂ levels and in turn temperature response also depend on land carbon uptake. This study explores the role of ocean heat and carbon uptake in the approximate constancy of the TCRE, but also takes into account the role of land carbon uptake. For this study the University of Victoria Earth System Climate Model (UVic ESCM), a model of intermediate complexity, is used. The UVic ESCM includes a simple 1-dimensional atmosphere, a dynamic 3-dimensional ocean coupled to a sea-ice model, and a land-surface scheme coupled to a dynamic vegetation model. All components include a carbon cycle. Simulations are performed whereby the effect of different ocean mixing parameterizations on ocean heat and carbon uptake and the TCRE under CO₂ forcing is explored. The results of these model simulations give insight into the mechanisms behind the constancy of the TCRE.