The non-steady state oceanic CO_2 signal: its importance, magnitude and a novel way to detect it

摘要

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.