Marine Ecosystem Dynamics and Biogeochemical Cycling in the Community Earth System Model [CESM1(BGC)]: Comparison of the 1990s with the 2090s under the RCP4.5 and RCP8.5 Scenarios

Abstract

The authors compare Community Earth System Model results to marine observations for the 1990s and examine climate change impacts on biogeochemistry at the end of the twenty-first century under two future scenarios (Representative Concentration Pathways RCP4.5 and RCP8.5). Late-twentieth-century seasonally varying mixed layer depths are generally within 10 m of observations, with a Southern Ocean shallow bias. Surface nutrient and chlorophyll concentrations exhibit positive biases at low latitudes and negative biases at high latitudes. The volume of the oxygen minimum zones is overestimated.The impacts of climate change on biogeochemistry have similar spatial patterns under RCP4.5 and RCP8.5, but perturbation magnitudes are larger under RCP8.5. Increasing stratification leads to weaker nutrient entrainment and decreased primary and export production (.30% over large areas). The global-scale decreases in primary and export production scale linearly with the increases in mean sea surface temperature. There are production increases in the high nitrate, low chlorophyll (HNLC) regions, driven by lateral iron inputs from adjacent areas. The increased HNLC export partially compensates for the reductions in non-HNLC waters (;25% offset). Stabilizing greenhouse gas emissions and climate by the end of this century (as in RCP4.5) will minimize the changes to nutrient cycling and primary production in the oceans. In contrast, continued increasing emission of CO 2 (as in RCP8.5) will lead to reduced productivity and significant modifications to ocean circulation and biogeochemistry by the end of this century, with more drastic changes beyond the year 2100 as the climate continues to rapidly warm.

Mechanisms rectifying the annual mean response of tropical Atlantic rainfall to precessional forcing

“…This is a fully-coupled, global climate model consisting of interactive atmosphere (CAM4), ocean (POP2), sea ice (CICE4) and land (CLM4) components. CESM1 was developed from the Community Climate System Model (CCSM) version 4 (Gent et al 2011); its enhancements include the incorporation of biogeochemical cycles and atmospheric chemistry (Hurrell et al 2013;Moore et al 2013). The atmospheric model was run at T31 resolution, which corresponds to 3.75° by 3.75° horizontal resolution, with 26 vertical levels, while the ocean horizontal resolution is 3° by 3° with 60 vertical levels.…”

Representing northern peatland microtopography and hydrology within the Community Land Model

“…The final simulation increased air temperature by +9 K and specific humidity by 30 %, which is lower than the constant RH scenario. This final humidity setting is based on the projection of CESM under RCP8.5 scenario at the end of 21st century (Moore et al, 2013). We note that the warming and constant Q scenario most closely represents the planned experimental manipulation at SPRUCE, since there will be no water vapor additions.…”

Global-scale variations of the ratios of carbon to phosphorus in exported marine organic matter

“…Present Earth System Models (which assume fixed Redfield stoichiometry) suggest declines in carbon productivity and export over the twenty-first century, due in part to expanding oligotrophic regions 3,4,28 . Our results suggest that the more efficient carbon export in these regions would partially offset these expected declines in production and export.…”