Rising atmospheric CO2 leads to large impact of biology on Southern Ocean CO2 uptake via changes of the Revelle factor - PubMed (original) (raw)

Rising atmospheric CO2 leads to large impact of biology on Southern Ocean CO2 uptake via changes of the Revelle factor

J Hauck et al. Geophys Res Lett. 2015.

Abstract

The Southern Ocean is a key region for global carbon uptake and is characterized by a strong seasonality with the annual CO2 uptake being mediated by biological carbon drawdown in summer. Here we show that the contribution of biology to CO2 uptake will become even more important until 2100. This is the case even if biological production remains unaltered and can be explained by the decreasing buffer capacity of the ocean as its carbon content increases. The same amount of biological carbon drawdown leads to a more than twice as large reduction in CO2(aq) concentration and hence to a larger CO2 gradient between ocean and atmosphere that drives the gas exchange. While the winter uptake south of 44°S changes little, the summer uptake increases largely and is responsible for the annual mean response. The combination of decreasing buffer capacity and strong seasonality of biological carbon drawdown introduces a strong and increasing seasonality in the anthropogenic carbon uptake.

Key points: Decrease of buffer capacity leads to stronger summer CO2 uptake in the futureBiology will contribute more to future CO2 uptake in Southern OceanSeasonality affects anthropogenic carbon uptake strongly.

Keywords: anthropogenic carbon; biological carbon draw down; buffer capacity; feedback process; ocean CO2 sink; ocean acidification.

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Figures

Figure 1

Figure 1

(first column) Monthly time series of CO2 uptake in the model simulations following the RCP8.5 scenario (RCP85, dark grey), in the scenario with constant atmospheric CO2 (CONST, equivalent to natural CO2 flux, blue) and the difference between the two simulations (anthropogenic CO2 flux, red) in MITgcm-REcoM-2 averaged over the full Southern Ocean (<30°S), and the subregions <58°S, 44°S–58°S, and 30°S–44°S as indicated in the figures. The annual mean is overlain in black. (second column) The mean seasonal cycle of CO2 uptake for the periods 2012–2031 (line) and 2081–2100 (dashed) in the same regions and model runs. January is shown as the first and again as the last month of the seasonal cycle.

Figure 2

Figure 2

Effect of buffer factor on CO2(aq) drawdown by biology calculated as in equation (4). Blue: CO2 drawdown with present-day buffer factor and biological carbon drawdown (gross primary production minus respiration minus remineralization). Red: future buffer factor and present-day biological carbon drawdown. Red, dashed: future buffer factor and future biological carbon drawdown. Present day: mean seasonal cycle 2012 to 2031, future: 2081 to 2100. Change in buffer factor has a much larger effect on CO2(aq) drawdown (and hence CO2 uptake) than increase in biological production. January is shown as the first and again as the last month of the seasonal cycle.

Figure 3

Figure 3

Monthly time series of Revelle factor (black, unitless) and buffer factor _γ_DIC (blue, μmol kg−1) [Egleston et al., 2010] for the RCP8.5 scenario in the period 2012 to 2100 and the regions <58°S, 44°S–58°S, and 30°S–44°S as indicated in the figures.

Figure 4

Figure 4

Buildup of Cant inventory (Pg C) relative to the reference year 2011 as simulated with the model (blue, RCP85 − CONST) and as calculated with the eMLR method from the RCP85 simulation using the parameters theta, alkalinity, and dissolved inorganic nitrogen (red). See supporting information for details of eMLR calculation.

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