Long-term surface pCO2 trends from observations and models

Jerry F Tjiputra,Are Olsen,Laurent Bopp,Andrew Lenton,Benjamin Pfeil,Tilla Roy,Joachim Segschneider,Ian Totterdell,Christoph Heinze

doi:10.3402/tellusb.v66.23083

Abstract

We estimate regional long-term surface ocean pCO2 growth rates using all available underway and bottled biogeochemistry data collected over the past four decades. These observed regional trends are compared with those simulated by five state-of-the-art Earth system models over the historical period. Oceanic pCO2 growth rates faster than the atmospheric growth rates indicate decreasing atmospheric CO2 uptake, while ocean pCO2 growth rates slower than the atmospheric growth rates indicate increasing atmospheric CO2 uptake. Aside from the western subpolar North Pacific and the subtropical North Atlantic, our analysis indicates that the current observation-based basin-scale trends may be underestimated, indicating that more observations are needed to determine the trends in these regions. Encouragingly, good agreement between the simulated and observed pCO2 trends is found when the simulated fields are subsampled with the observational coverage. In agreement with observations, we see that the simulated pCO2 trends are primarily associated with the increase in surface dissolved inorganic carbon (DIC) associated with atmospheric carbon uptake, and in part by warming of the sea surface. Under the RCP8.5 future scenario, DIC continues to be the dominant driver of pCO2 trends, with little change in the relative contribution of SST. However, the changes in the hydrological cycle play an increasingly important role. For the contemporary (1970–2011) period, the simulated regional pCO2 trends are lower than the atmospheric growth rate over 90% of the ocean. However, by year 2100 more than 40% of the surface ocean area has a higher oceanic pCO2 trend than the atmosphere, implying a reduction in the atmospheric CO2 uptake rate. The fastest pCO2 growth rates are projected for the subpolar North Atlantic, while the high-latitude Southern Ocean and eastern equatorial Pacific have the weakest growth rates, remaining below the atmospheric pCO2 growth rate. Our work also highlights the importance and need for a sustained long-term observing strategy to continue monitoring the change in the ocean anthropogenic CO2 sink and to better understand the potential carbon cycle feedbacks to climate that could arise from it.

Highlights

The observed large-scale patterns of pressure of CO2 gas (pCO2) are well reproduced by the models with maximum values found in the equatorial Pacific and minimum values in subpolar North Atlantic, western North Pacific, and parts of the Southern Ocean
In the eastern equatorial Pacific where the pCO2 maximum is associated with upwelling of old dissolved inorganic carbon (DIC)-rich water masses, the pCO2 is considerably underestimated by the HadGEM2-ES and overestimated by the CESM1-BGC
We have investigated the change in the ocean pCO2 trends relative to the atmospheric pCO2 trends over the historical and future periods, based on latest ESM simulations (CMIP5) and available upper ocean observations (1970Á 2011)

Summary

Introduction

The atmospheric CO2 concentration recorded at the Mauna Loa observatory has recently passed 400 ppm (http://www. esrl.noaa.gov/gmd/ccgg/trends/weekly.html) and is signifi-creativecommons.org/licenses/by/4.0/), allowing third parties to copy and redistribute the material in any medium or format and to remix, transform, and build upon the material for any purpose, even commercially, provided the original work is properly cited and states its license.(page number not for citation purpose)J. Creativecommons.org/licenses/by/4.0/), allowing third parties to copy and redistribute the material in any medium or format and to remix, transform, and build upon the material for any purpose, even commercially, provided the original work is properly cited and states its license. One of the most evident and direct indicators of the ocean carbon uptake is the increasing partial pressure of CO2 gas (pCO2) in the surface oceans, which has been shown to increase globally and only slightly lag behind the atmospheric CO2 partial pressure (e.g. Sabine et al, 2013). The deviation of ocean pCO2 trends from the atmospheric trend can be used to infer information about the nature of the ocean sink of anthropogenic CO2. The opposite is true when the ocean pCO2 trend is less than that of the atmospheric pCO2. The long-term evolution of surface pCO2 reflects part of the ocean’s integrated response to the ongoing climate change

Results

Discussion

Conclusion