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GFDL’s ESM2 global coupled climate-carbon Earth System Models Part II: Carbon system formulation and baseline simulation characteristics

 

Dunne, J. P., J. G. John, E. N. Shevliakova, R. J. Stouffer, J. P. Krasting, S. L. Malyshev, P. C. D. Milly, L. T. Sentman, A. J. Adcroft, W. Cooke, K. A. Dunne, S. M. Griffies, R. W. Hallberg, M. J. Harrison, H. Levy, A. T. Wittenberg, P. J. Phillips, and N. Zadeh, Journal of Climate, 26(7). DOI:10.1175/JCLI-D-12-00150.1. 04/13.

Key Findings

  • GFDL has developed two new earth system models to study climate-carbon interactions on diurnal to millennial timescales
  • The models differ only in the physical ocean component – one the Modular Ocean Model version 4.1 pressure-based vertical coordinate, and the other the Generalized Ocean Layer Dynamics density-based coordinate. Comparison between the two allows us to assess the sensitivity of the coupled climate-carbon system to our assumptions about ocean formulation.
  • While the models demonstrate similar overall scale fidelity, they have important differences due to differences in oceanic ventilation rates (Part 1) ESM2M has a stronger biological carbon pump but weaker northward implied atmospheric CO2 transport than ESM2G.
  • The major advantages of ESM2G over ESM2M are: improved representation of surface Chlorophyll in the Atlantic and Indian oceans and thermocline nutrients and oxygen in the North Pacific. Additionally, changes in tree mortality parameters in ESM2G model produced a more realistic carbon accumulation in vegetation pools.
  • The major advantages of ESM2M over ESM2G are reduced nutrient and oxygen biases in the Southern and Tropical Oceans.
     

The primary objective of this work was to expand upon the capabilities of past GFDL models used to study climate on seasonal to centennial time scales by the addition of a comprehensive and interactive carbon cycle in the land, ocean and atmosphere to ‘close the carbon cycle’ in the same way we do for water and energy in a traditional climate model. While the primary contribution is in improving our ability to anticipate how earth system interactions will modulate the rate of increase of carbon dioxide in the atmosphere, the fact that the models require simulation of land and ocean ecosystems make them extremely valuable for a range of applications in ecosystem impacts and feedbacks as well. Our approach has been to develop two models with different ocean dynamical/physical cores while keeping all other components the same in order to test the sensitivity of our results to our assumptions inherent in our ocean configuration.

Relevance to NOAA science

This effort is a critical component of NOAA’s research into the future of the earth as a system under the influence of anthropogenic forcing to better understand how emissions of carbon dioxide from fossil fuels, land use decisions and climate and ecological interactions will determine future carbon dioxide levels and the corresponding climate change. These models are also critical to projection of the impacts of climate change and carbon dioxide fertilization and acidification on ecosystems.

Relevance to society

The models whose physical formulation and simulation characteristics are described here are intended to be supplied to the public as part of the Coupled Model Intercomparison Project Phase 5 in support of the IPCC Fifth Assessment.

Unique aspects of this study

This research is unique in utilizing GFDL’s highly successful CM2.1 climate model as a carbon model backbone, in incorporating GFDL’s state of the art ocean biogeochemical and terrestrial ecology models, and in comparing two models of starkly differing ocean physical configuration in the same configuration elsewhere.

Description of the methodology

We based the development of these new earth system models on GFDL’s highly successful CM2.1 climate model and made sure to maintain climate fidelity as interacting carbon system components were built in. We incorporated GFDL’s state of the art ocean biogeochemical model into two models of starkly differing ocean physical configuration and built a new terrestrial ecology model.

Known weaknesses or uncertainties

While state of the art in their design, these models suffer from many of the weaknesses typical of this class of model including the double ITCZ, weak Amazonian rainfall, and others. Fortunately, many of the weaknesses of these two models are opposing (e.g. one has weak El Nino, the other strong) that will allow us improved overall characterization of climate sensitivity.