Does atmospheric CO2 seasonality play an important role in governing the air-sea flux of CO2?

[摘要] The amplitude, phase, and form of the seasonal cycle of atmospheric CO₂concentrations varies on many time and space scales (Peters et al., 2007).Intra-annual CO₂ variation is primarily driven by seasonal uptake andrelease of CO₂ by the terrestrial biosphere (Machta et al., 1977; Buchwitzet al., 2007), with a small (Cadule et al., 2010; Heimannet al., 1998), butpotentially changing (Gorgues et al., 2010) contribution from the ocean.Variability in the magnitude, spatial distribution, and seasonal drivers ofterrestrial net primary productivity (NPP) will be induced by, amongst otherfactors, anthropogenic CO₂ release (Keeling et al., 1996), land-use change(Zimov et al., 1999) and planetary orbital variability, and will lead tochanges in CO₂^atm seasonality. Despite CO₂^atm seasonality being adynamic and prominent feature of the Earth System, its potential to drivechanges in the air-sea flux of CO₂ has not previously (to the best of myknowledge) been explored. It is important that we investigate the impact ofCO₂^atm seasonality change, and the potential for carbon-cycle feedbacksto operate through the modification of the CO₂^atm seasonal cycle,because the decision had been made to prescribe CO₂^atm concentrations(rather than emissions) within model simulations for the fifth IPCC climateassessment (Taylor et al., 2009). In this study I undertake ocean-modelsimulations within which different magnitude CO₂^atm seasonal cycles areprescribed. These simulations allow me to examine the effect of a change inCO₂^atm seasonal cycle magnitude on the air-sea CO₂ flux. I then usean off-line model to isolate the drivers of the identified air-sea CO₂flux change, and propose mechanisms by which this change may come about.Three mechanisms are identified by which co-variability of the seasonalcycles in atmospheric CO₂ concentration, and seasonality in sea-iceextent, wind-speed and ocean temperature, could potentially lead to changesin the air-sea flux of CO₂ at mid-to-high latitudes. The sea-ice drivenmechanism responds to an increase in CO₂^atm seasonality by pumpingCO₂ into the ocean, the wind-speed and solubility-driven mechanisms, byreleasing CO₂ from the ocean (in a relative sense). The relativeimportance of the mechanisms will be determined by, amongst other variables,the seasonal extent of sea-ice. To capture the described feedbacks withinearth system models, CO₂^atm concentrations must be allowed to evolvefreely, forced only by anthropogenic emissions rather than prescribedCO₂^atm concentrations; however, time-integrated ocean simulations implythat the cumulative net air-sea flux could be at most equivalent to a few ppmCO₂^atm. The findings presented here suggest that, at least underpre-industrial conditions, the prescription of CO₂^atm concentrationsrather than emissions within simulations will have little impact on themarine anthropogenic CO₂ sink.

[学科分类] 地球化学与岩石