THE CHANGING CARBON CYCLE IN THE SOUTHERN OCEAN

Various human activities, including fossil fuel combustion and forest clearing, emit about eight petagrams (or billion tons) of carbon in the form of CO₂ into the atmosphere annually. The global ocean absorbs about two petagrams of CO₂, and about a half of that amount is absorbed by the Southern Ocean south of 30°S, thus slowing the rapid accumulation of CO₂ in the atmosphere. Partial pressure of CO₂ (pCO₂) is a measure of the chemical driving force for the CO₂ exchange between the ocean and the atmosphere. This paper discusses its space and time distribution over the Southern Ocean. The major sink zone for atmospheric CO₂ is located in a latitude belt between 30°S and 50°S, where the biological utilization of CO₂ and cooling of warm subtropical waters flowing southward produce low seawaterpCO₂. Strong winds in this zone also enhance the ocean's uptake. Although the source-sink conditions vary over a wide range through the seasons in the areas south of 50°S, this zone is a small sink on an annual average. Winter observations show that surface waterpCO₂ values in the source region for Antarctic Intermediate Water have increased at a rate faster than the atmospheric increase rate, suggesting that the ocean CO₂ sink intensity has been weakening for several decades and has changed from a net sink to a net source since 2005. The results of ocean general circulation-biogeochemistry model studies are found to be consistent with the observations.