Eby Michael

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  • Article
    Climate-carbon cycle feedback analysis : results from the C4MIP model intercomparison
    (American Meteorological Society, 2006-07-15) Friedlingstein, Pierre ; Cox, P. ; Betts, Richard A. ; Bopp, Laurent ; von Bloh, W. ; Brovkin, V. ; Cadule, P. ; Doney, Scott C. ; Eby, Michael ; Fung, Inez Y. ; Bala, G. ; John, Jasmin G. ; Jones, C. D. ; Joos, Fortunat ; Kato, T. ; Kawamiya, M. ; Knorr, W. ; Lindsay, Keith ; Matthews, H. D. ; Raddatz, T. ; Rayner, Peter ; Reick, C. ; Roeckner, E. ; Schnitzler, K.-G. ; Schnur, R. ; Strassmann, K. ; Weaver, Andrew J. ; Yoshikawa, C. ; Zeng, Ning
    Eleven coupled climate–carbon cycle models used a common protocol to study the coupling between climate change and the carbon cycle. The models were forced by historical emissions and the Intergovernmental Panel on Climate Change (IPCC) Special Report on Emissions Scenarios (SRES) A2 anthropogenic emissions of CO2 for the 1850–2100 time period. For each model, two simulations were performed in order to isolate the impact of climate change on the land and ocean carbon cycle, and therefore the climate feedback on the atmospheric CO2 concentration growth rate. There was unanimous agreement among the models that future climate change will reduce the efficiency of the earth system to absorb the anthropogenic carbon perturbation. A larger fraction of anthropogenic CO2 will stay airborne if climate change is accounted for. By the end of the twenty-first century, this additional CO2 varied between 20 and 200 ppm for the two extreme models, the majority of the models lying between 50 and 100 ppm. The higher CO2 levels led to an additional climate warming ranging between 0.1° and 1.5°C. All models simulated a negative sensitivity for both the land and the ocean carbon cycle to future climate. However, there was still a large uncertainty on the magnitude of these sensitivities. Eight models attributed most of the changes to the land, while three attributed it to the ocean. Also, a majority of the models located the reduction of land carbon uptake in the Tropics. However, the attribution of the land sensitivity to changes in net primary productivity versus changes in respiration is still subject to debate; no consensus emerged among the models.
  • Article
    Historical and idealized climate model experiments : an intercomparison of Earth system models of intermediate complexity
    (Copernicus Publications on behalf of the European Geosciences Union, 2013-05-16) Eby, Michael ; Weaver, Andrew J. ; Alexander, K. ; Zickfeld, K. ; Abe-Ouchi, A. ; Cimatoribus, A. A. ; Crespin, E. ; Drijfhout, Sybren ; Edwards, N. R. ; Eliseev, A. V. ; Feulner, G. ; Fichefet, T. ; Forest, Chris E. ; Goosse, H. ; Holden, P. B. ; Joos, Fortunat ; Kawamiya, M. ; Kicklighter, David W. ; Kienert, H. ; Matsumoto, K. ; Mokhov, I. I. ; Monier, Erwan ; Olsen, Steffen M. ; Pedersen, J. O. P. ; Perrette, M. ; Philippon-Berthier, G. ; Ridgwell, Andy ; Schlosser, A. ; Schneider von Deimling, T. ; Shaffer, G. ; Smith, R. S. ; Spahni, R. ; Sokolov, Andrei P. ; Steinacher, M. ; Tachiiri, K. ; Tokos, K. ; Yoshimori, M. ; Zeng, Ning ; Zhao, F.
    Both historical and idealized climate model experiments are performed with a variety of Earth system models of intermediate complexity (EMICs) as part of a community contribution to the Intergovernmental Panel on Climate Change Fifth Assessment Report. Historical simulations start at 850 CE and continue through to 2005. The standard simulations include changes in forcing from solar luminosity, Earth's orbital configuration, CO2, additional greenhouse gases, land use, and sulphate and volcanic aerosols. In spite of very different modelled pre-industrial global surface air temperatures, overall 20th century trends in surface air temperature and carbon uptake are reasonably well simulated when compared to observed trends. Land carbon fluxes show much more variation between models than ocean carbon fluxes, and recent land fluxes appear to be slightly underestimated. It is possible that recent modelled climate trends or climate–carbon feedbacks are overestimated resulting in too much land carbon loss or that carbon uptake due to CO2 and/or nitrogen fertilization is underestimated. Several one thousand year long, idealized, 2 × and 4 × CO2 experiments are used to quantify standard model characteristics, including transient and equilibrium climate sensitivities, and climate–carbon feedbacks. The values from EMICs generally fall within the range given by general circulation models. Seven additional historical simulations, each including a single specified forcing, are used to assess the contributions of different climate forcings to the overall climate and carbon cycle response. The response of surface air temperature is the linear sum of the individual forcings, while the carbon cycle response shows a non-linear interaction between land-use change and CO2 forcings for some models. Finally, the preindustrial portions of the last millennium simulations are used to assess historical model carbon-climate feedbacks. Given the specified forcing, there is a tendency for the EMICs to underestimate the drop in surface air temperature and CO2 between the Medieval Climate Anomaly and the Little Ice Age estimated from palaeoclimate reconstructions. This in turn could be a result of unforced variability within the climate system, uncertainty in the reconstructions of temperature and CO2, errors in the reconstructions of forcing used to drive the models, or the incomplete representation of certain processes within the models. Given the forcing datasets used in this study, the models calculate significant land-use emissions over the pre-industrial period. This implies that land-use emissions might need to be taken into account, when making estimates of climate–carbon feedbacks from palaeoclimate reconstructions.
  • Preprint
    Response of a climate model to tidal mixing parameterization under present day and last glacial maximum conditions
    ( 2007-06-27) Montenegro, Álvaro ; Eby, Michael ; Weaver, Andrew J. ; Jayne, Steven R.
    Experiments with a climate model were conducted under present day and last glacial maximum conditions in order to examine the model’s response to a vertical mixing scheme based on internal tide energy dissipation. The increase in internal tide energy flux caused by a 120 m reduction in sea level had the expected effect on diffusivity values, which were higher under lower sea level conditions. The impact of this vertical diffusivity change on the Atlantic meridional overturning is not straightforward and no clear relationship between diffusivity and overturning is found. There exists a weak positive correlation between overturning and changes to the power consumed by vertical mixing. Most of the climatic response generated by sea level change was not related to alterations in the internal tide energy flux but rather to the direct change in sea level itself.