@Article{Hoffman_JGRB_20140201,
 author		= {Forrest M. Hoffman and James T. Randerson and Vivek K. Arora and Qing Bao and Patricia Cadule and Duoying Ji and Chris D. Jones and Michio Kawamiya and Samar Khatiwala and Keith Lindsay and Atsushi Obata and Elena Shevliakova and  Katharina D. Six and Jerry F. Tjiputra and Evgeny M. Volodin and Tongwen Wu},
 title		= {Causes and Implications of Persistent Atmospheric Carbon Dioxide Biases in {E}arth {S}ystem {M}odels},
 journal	= JGRB,
 volume		= 119,
 number		= 2,
 pages		= {141--162},
 doi		= {10.1002/2013JG002381},
 day		= 1,
 month		= feb,
 year		= 2014,
 abstract	= {The strength of feedbacks between a changing climate and future CO$_2$ concentrations are uncertain and difficult to predict using Earth System Models (ESMs). We analyzed emission-driven simulations---in which atmospheric CO$_2$ levels were computed prognostically---for historical (1850--2005) and future periods (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 CO$_2$ over the historical period with observations indicated that ESMs, on average, had a small positive bias in predictions of contemporary atmospheric CO$_2$. 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 CO$_2$ biases and future CO$_2$ levels for the multi-model ensemble. We used this relationship to create a contemporary CO$_2$ tuned model (CCTM) estimate of the atmospheric CO$_2$ trajectory for the 21$^\textnormal{st}$ century. The CCTM yielded CO$_2$ estimates of 600 $\pm$ 14 ppm at 2060 and 947 $\pm$ 35 ppm at 2100, which were 21 ppm and 32 ppm below the multi-model mean during these two time periods. Using this emergent constraint approach, the likely ranges of future atmospheric CO$_2$, CO$_2$-induced radiative forcing, and CO$_2$-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 CO$_2$ during the 21$^\textnormal{st}$ 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 CO$_2$ from Mauna Loa, our analysis suggests uncertainties in future climate projections can be reduced.}
}