Reconstruction of the history of anthropogenic CO2 concentrations in the ocean

S Khatiwala,F Primeau,T Hall

doi:10.1038/nature08526

Abstract

The release of fossil fuel CO(2) to the atmosphere by human activity has been implicated as the predominant cause of recent global climate change. The ocean plays a crucial role in mitigating the effects of this perturbation to the climate system, sequestering 20 to 35 per cent of anthropogenic CO(2) emissions. Although much progress has been made in recent years in understanding and quantifying this sink, considerable uncertainties remain as to the distribution of anthropogenic CO(2) in the ocean, its rate of uptake over the industrial era, and the relative roles of the ocean and terrestrial biosphere in anthropogenic CO(2) sequestration. Here we address these questions by presenting an observationally based reconstruction of the spatially resolved, time-dependent history of anthropogenic carbon in the ocean over the industrial era. Our approach is based on the recognition that the transport of tracers in the ocean can be described by a Green's function, which we estimate from tracer data using a maximum entropy deconvolution technique. Our results indicate that ocean uptake of anthropogenic CO(2) has increased sharply since the 1950s, with a small decline in the rate of increase in the last few decades. We estimate the inventory and uptake rate of anthropogenic CO(2) in 2008 at 140 +/- 25 Pg C and 2.3 +/- 0.6 Pg C yr(-1), respectively. We find that the Southern Ocean is the primary conduit by which this CO(2) enters the ocean (contributing over 40 per cent of the anthropogenic CO(2) inventory in the ocean in 2008). Our results also suggest that the terrestrial biosphere was a source of CO(2) until the 1940s, subsequently turning into a sink. Taken over the entire industrial period, and accounting for uncertainties, we estimate that the terrestrial biosphere has been anywhere from neutral to a net source of CO(2), contributing up to half as much CO(2) as has been taken up by the ocean over the same period.

Highlights

The release of fossil fuel CO2 to the atmosphere by human activity has been implicated as the predominant cause of recent global climate change[1]
Much progress has been made in recent years in understanding and quantifying this sink, considerable uncertainties remain as to the distribution of anthropogenic CO2 in the ocean, its rate of uptake over the industrial era, and the relative roles of the ocean and terrestrial biosphere in anthropogenic CO2 sequestration
Our results indicate that ocean uptake of anthropogenic CO2 has increased sharply since the 1950s, with a small decline in the rate of increase in the last few decades

Summary

SUMMARY

The combined GLODAP/WOA05 straints, one for each tracer, of databases the form:. Cðx’, t’ÞGðx, t; x’, t’Þ, where C is the observed tracer concentration, and l its radioactive decay rate (non-zero only for 14C). L. et al The ocean sink for anthropogenic CO2. Ocean measurements and models of carbon sources and sinks. How accurate is the estimation of anthropogenic carbon in the ocean? W. Inferring the concentration of anthropogenic carbon in the ocean from tracers. Le Quere, C. et al Saturation of the Southern Ocean CO2 sink due to recent climate change. J. Response of the global carbon cycle to human-induced changes in the Southern Hemisphere winds. E. et al Inverse estimates of anthropogenic CO2 uptake, transport, and storage by the ocean. K. et al Evaluation of ocean carbon cycle models with data-based metrics. M. Transport times and anthropogenic carbon in the subpolar North Atlantic Ocean. Estimates of anthropogenic carbon in the Indian Ocean with allowance for mixing and timevarying air-sea CO2 disequilibrium.

29. Atmospheric Trace Gases

Findings

METHODS