In the Wake of Deeper Convection: Nonsteady State Anthropogenic Carbon in the Greenland Sea.

We evaluate changes in dissolved inorganic carbon (DIC) in the Greenland Sea between 2002 and 2016, a period characterized by increasing convection depths. We find a mid‐depth maximum in anthropogenic carbon (Cant) accumulation that occurred as waters at these depths were rejuvenated by deeper reaching convection; broadly, these waters have caught up with the atmospheric CO2 rise that had happened between the last time they were ventilated and 2002 and also tracked the atmospheric CO2 rise 2002–2016. The overlying waters only tracked the atmospheric CO2 rise 2002–2016. The mid‐depth maximum in Cant accumulation was not evident in estimates generated with commonly used multiple linear regression (MLR) methods. We analyze the reasons why and show that the eMLR(C*) method may not fully capture nonsteady state changes in Cant when applied along a single hydrographic section as done here. This nonsteady component equates to redistribution of C*, whose spatial gradients in the Greenland Sea are dominated by Cant. We also show that the regular extended multiple linear regression method is sensitive to loss of spatial DIC gradients, which now happens as more and more Cant enters the ocean. Our findings demonstrate that MLR‐based estimates of the Cant accumulation rate should not be taken at face value in highly dynamical ocean regions, such as the Greenland Sea, and the need for also considering the total change in DIC and how this is affected by natural processes. Further investigations into the ability of MLR methods to reproduce nonsteady state changes in Cant are encouraged. Plain Language Summary: The ocean holds vast quantities of carbon. Each year this inventory increases as the ocean absorbs a quarter of our CO2 emissions. Keeping track of ocean carbon is a key climate change research priority. Observations from the Greenland Sea indicate at first glance a steady rise in DIC concentrations in the upper approximately 1,500–2,000 m of the water column, roughly equal to what one would expect from the atmospheric CO2 rise. This is unusually deep compared to the rest of the global ocean but reflects the deep‐water formation that occurs in this region. A closer inspection of the data, however, reveals that the seemingly uneventful rise in carbon in this region is the net result of several counteracting processes. In response to deeper convection, mid‐depth waters have lost inorganic carbon generated by the remineralization of organic matter, natural carbon. This has been counteracted by an unusually large rise in their content of man‐made, or anthropogenic carbon. Widely adopted methods for estimating decadal rises in anthropogenic carbon struggle to quantify these changes, such that our ability to detect the nature of effects of climate variability and change on the efficiency of the ocean carbon sink can be questioned. Key Points: Deeper convection caused a mid‐depth maximum in the rate of anthropogenic carbon increase in the Greenland Sea from 2002 to 2016The mid‐depth maximum in anthropogenic carbon accumulation was not evident in estimates generated with multiple linear regression methodsNonsteady state anthropogenic carbon accumulation may bias the eMLR(C*) method when applied along a single hydrographic section [ABSTRACT FROM AUTHOR]