The non-steady state oceanic CO 2 signal: its importance, magnitude and a novel way to detect it
The role of the ocean has been pivotal in modulating rising atmospheric CO 2 levels since the industrial revolution, sequestering nearly half of all fossil-fuel derived CO 2 emissions. Net oceanic uptake of CO 2 has roughly doubled between the 1960s (~1 Pg C yr −1) and 2000s (~2 Pg C yr −1), with expectations that it will continue to absorb even more CO 2 with rising future atmospheric CO 2 levels. However, recent CO 2 observational analyses along with numerous model predictions suggest the rate of oceanic CO 2 uptake is already slowing, largely as a result of a natural decadal-scale outgassing signal. This recent CO 2 outgassing signal represents a significant shift in our understanding of the oceans role in modulating atmospheric CO 2. Current tracer-based estimates for the ocean storage of anthropogenic CO 2 assume the ocean circulation and biology is in steady state, thereby missing the new and potentially important "non-steady state" CO 2 outgassing signal. By combining data-based techniques that assume the ocean is in a steady state, with techniques that constrain the net oceanic CO 2 uptake signal, we show how to extract the non-steady state CO 2 signal from observations. Over the entire industrial era, the non-steady state CO 2 outgassing signal (~13 ± 10 Pg C) is estimated to represent about 9% of the total net CO 2 inventory change (~142 Pg C). However, between 1989 and 2007, the non-steady state CO 2 outgassing signal (~6.3 Pg C) has likely increased to be ~18% of net oceanic CO 2 storage over that period (~36 Pg C). The present uncertainty of our data-based techniques for oceanic CO 2 uptake limit our capacity to quantify the non-steady state CO 2 signal, however with more data and better certainty estimates across a range of diverse methods, this important and growing CO 2 signal could be better constrained in the future.