Impacts of Temporal CO2 and Climate Trends on the Detection of Ocean Anthropogenic CO2 Accumulation

Abstract

The ocean is a critical reservoir in the carbon cycle for mitigating the effects of rising atmospheric CO2. While the oceanic sink is estimated to currently be 25% of annual CO2 emissions (eg. Le Quere et al. 2009), this is likely to decrease over the next 100 years as ocean temperatures increase and the ocean becomes saturated with CO2. In order to anticipate changes in oceanic uptake, we must improve our understanding of how much anthropogenic CO2 is currently being absorbed by the ocean. A common approach for estimating the oceanic uptake of anthropogenic carbon dioxide (Canthro ) depends on the linear approximation of oceanic dissolved inorganic carbon (DIC) from a suite of physical and biological ocean parameters, using an extended Multiple Linear Regression (eMLR). eMLR techniques take advantage of relationships between dissolved inorganic carbon (DIC) and hydrographic properties to filter out natural variability. This technique depends on a linear relationship between DIC and hydrographic properties and stable regression residuals through time. However, non-uniform and non-linear increases in DIC due to variable anthropogenic carbon uptake and long-term trends in the physical and biological state of the ocean may result in an increasingly non-linear relationship between DIC and hydrographic properties with time. In our study, the validity of these assumptions over the 21st century is tested using the Climate Community Systems Model (CCSM), a coupled carbon-climate model, to simulate natural and anthropogenic DIC. Findings demonstrate that the influence of both changing climate and changing chemistry beyond 2-4 decades invalidates the assumption that the residual fields will remain constant resulting in significant errors in the eMLR estimate of Canthro. This study determines that the eMLR method is unable to describe Canthro uptake for a sampling interval of greater than 30 years if the error is to remain below 20% for many regions in the Southern Ocean, tropical Atlantic Ocean, tropical Pacific Ocean, and North Atlantic Ocean. These results suggest that, for many regions of the ocean basins, hydrographic field programs such as CLIVAR have to be repeated at approximately decadal timescales in order to accurately predict the uptake of Canthro by the ocean if the eMLR method is used.