The non-steady state oceanic CO2 signal: its importance, magnitude and a novel way to detect it

The role of the ocean has been pivotal in modulating rising atmospheric CO2 levels since the industrial revolution, sequestering nearly half of all fossil-fuel derived CO2 emissions. Net oceanic uptake of CO2 has roughly doubled between the 1960s (similar to 1 PgC yr(-1)) and 2000s (similar to 2 PgC yr(-1)), with expectations that it will continue to absorb even more CO2 with rising future atmospheric CO2 levels. However, recent CO2 observational analyses along with numerous model predictions suggest the rate of oceanic CO2 uptake is already slowing, largely as a result of a natural decadal-scale outgassing signal. This recent CO2 outgassing signal represents a significant shift in our understanding of the oceans role in modulating atmospheric CO2. Current tracer-based estimates for the ocean storage of anthropogenic CO2 assume the ocean circulation and biology is in steady state, thereby missing the new and potentially important non-steady state CO2 outgassing signal. By combining data-based techniques that assume the ocean is in a steady state, with techniques that constrain the net oceanic CO2 uptake signal, we show how to extract the non-steady state CO2 signal from observations. Over the entire industrial era, the non-steady state CO2 outgassing signal (similar to 13 +/- 10 Pg C) is estimated to represent about 9% of the total net CO2 inventory change (similar to 142 Pg C). However, between 1989 and 2007, the non-steady state CO2 outgassing signal (similar to 6.3 Pg C) has likely increased to be similar to 18% of net oceanic CO2 storage over that period (similar to 36 Pg C). The present uncertainty of our data-based techniques for oceanic CO2 uptake limit our capacity to quantify the non-steady state CO2 signal, however with more data and better certainty estimates across a range of diverse methods, this important and growing CO2 signal could be better constrained in the future.