The iron budget in ocean surface waters in the 20th and 21st centuries: projections by the Community Earth System Model version 1

摘要

We investigated the simulated iron budget in ocean surface waters in the1990s and 2090s using the Community Earth System Model version 1 and theRepresentative Concentration Pathway 8.5 future CO₂ emission scenario.We assumed that exogenous iron inputs did not change during the wholesimulation period; thus, iron budget changes were attributed solely tochanges in ocean circulation and mixing in response to projected globalwarming, and the resulting impacts on marine biogeochemistry. The modelsimulated the major features of ocean circulation and dissolved irondistribution for the present climate. Detailed iron budget analysis revealedthat roughly 70% of the iron supplied to surface waters in high-nutrient,low-chlorophyll (HNLC) regions is contributed by ocean circulation andmixing processes, but the dominant supply mechanism differed by region:upwelling in the eastern equatorial Pacific and vertical mixing in theSouthern Ocean. For the 2090s, our model projected an increased iron supplyto HNLC waters, even though enhanced stratification was predicted to reduceiron entrainment from deeper waters. This unexpected result is attributedlargely to changes in gyre-scale circulations that intensified the advectivesupply of iron to HNLC waters. The simulated primary and export productionin the 2090s decreased globally by 6 and 13%, respectively, whereasin the HNLC regions, they increased by 11 and 6%, respectively.Roughly half of the elevated production could be attributed to theintensified iron supply. The projected ocean circulation and mixing changesare consistent with recent observations of responses to the warming climateand with other Coupled Model Intercomparison Project model projections. Weconclude that future ocean circulation has the potential to increase iron supply toHNLC waters and will potentially buffer future reductions in oceanproductivity.