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Causes and implications of persistent atmospheric carbon 
dioxide biases in Earth System Models

 

F. M. Hoffman^1,2, J. T. Randerson¹, V. K. Arora³, Q. Bao⁴, P. Cadule⁵, D. Ji⁶, C. D. Jones⁷, M. Kawamiya⁸, S. Khatiwala⁹, K. Lindsay¹⁰, A. Obata¹¹, E. Shevliakova¹², K. D. Six¹³, J. F. Tjiputra¹⁴, E.M. Volodin¹⁵, and T.Wu¹⁶

 

1 Department of Earth System Science, University of California, Irvine, California, USA,

2Climate Change Science Institute and Computational Earth Sciences Group, Oak Ridge National Laboratory, Oak Ridge, Tennessee, USA,

3Canadian Center for Climate Modeling and Analysis, Meteorological Service of Canada, Victoria, British Columbia, Canada,

4State Key Laboratory of Numerical Modeling for Atmospheric Sciences and Geophysical Fluid Dynamics, Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing, China,

5Institut Pierre Simon Laplace, Laboratoire des Sciences du Climat et de l’Environnement, Gif sur Yvette CEDEX, France,

6State Key Laboratory of Earth Surface Processes and Resource Ecology, College of Global Change and Earth System Science, Beijing Normal University, Beijing, China,

7Hadley Center, U.K. Met Office, Exeter, UK,

8Research Institute for Global Change, Japan Agency for Marine-Earth Science and Technology, Kanagawa, Japan,

9Lamont Doherty Earth Observatory, Columbia University, Palisades, New York, USA,

10Climate and Global Dynamics Division, National Center for Atmospheric Research, Boulder, Colorado, USA,

11Meteorological Research Institute, Japan Meteorological Agency, Ibaraki, Japan,

12Geophysical Fluid Dynamics Laboratory, Princeton University, Princeton, New Jersey, USA,

13Max Planck Institute for Meteorology, Hamburg, Germany,

14Uni Climate, Uni Research, Bergen, Norway,

15Institute of Numerical Mathematics, Russian Academy of Science, Moscow, Russia,

16 Climate System Modeling Division, Beijing Climate Center, China Meteorological Administration, Beijing, China

 

ABSTRACT

The strength of feedbacks between a changing climate and future CO2 concentrations is uncertain and difficult to predict using Earth System Models (ESMs). We analyzed emission-driven simulations—in which atmospheric CO2levels were computed prognostically—for historical (1850–2005) and future periods (Representative Concentration Pathway (RCP) 8.5 for 2006–2100) produced by 15 ESMs for the Fifth Phase of the Coupled Model Intercomparison Project (CMIP5). Comparison of ESM prognostic atmospheric CO2 over the historical period with observations indicated that ESMs, on average, had a small positive bias in predictions of contemporary atmospheric CO2. Weak ocean carbon uptake in many ESMs contributed to this bias, based on comparisons with observations of ocean and atmospheric anthropogenic carbon inventories. We found a significant linear relationship between contemporary atmospheric CO2 biases and future CO2levels for the multimodel ensemble. We used this relationship to create a contemporary CO2 tuned model (CCTM) estimate of the atmospheric CO2 trajectory for the 21st century. The CCTM yielded CO2estimates of 600±14 ppm at 2060 and 947±35 ppm at 2100, which were 21 ppm and 32 ppm below the multimodel mean during these two time periods. Using this emergent constraint approach, the likely ranges of future atmospheric CO2, CO2-induced radiative forcing, and CO2-induced temperature increases for the RCP 8.5 scenario were considerably narrowed compared to estimates from the full ESM ensemble. Our analysis provided evidence that much of the model-to-model variation in projected CO2 during the 21st century was tied to biases that existed during the observational era and that model differences in the representation of concentration-carbon feedbacks and other slowly changing carbon cycle processes appear to be the primary driver of this variability. By improving models to more closely match the long-term time series of CO2from Mauna Loa, our analysis suggests that uncertainties in future climate projections can be reduced.

 

KEY WORDS: Intergovernmental Panel on Climate Change (IPCC), greenhouse gases, terrestrial and 
oceanic carbon sinks, climate-carbon cycle feedbacks, climate warming, uncertainty quantification

 

PUBLISHED BY: JOURNAL OF GEOPHYSICAL RESEARCH: BIOGEOSCIENCES, 2014, 119(2): 141-162

 

SOURCE: http://onlinelibrary.wiley.com/doi/10.1002/2013JG002381/abstract