Twenty-First-Century Compatible CO2 Emissions and Airborne Fraction Simulated by CMIP5 Earth System Models under Four Representative Concentration Pathways

Journal of Climate. 02/2013; 26:4398-4413.

 

Chris Jones, Eddy Robertson, Vivek Arora, Pierre Friedlingstein, Elena Shevliakova, Laurent Bopp, Victor Brovkin, Tomohiro Hajima, Etsushi Kato,Michio Kawamiya, Spencer Liddicoat, Keith Lindsay, Christian H. Reick,Caroline Roelandt, Joachim Segschneider, Jerry Tjiputra.

Met Office Hadley Centre, Exeter, United Kingdom and

Canadian Centre for Climate Modelling and Analysis, Environment Canada, University of Victoria, Victoria, British Columbia, Canada and

College of Engineering, Mathematics and Physical Sciences, University of Exeter, Exeter, EX4 4QF, United Kingdom and

Department of Ecology and Evolutionary Biology, Princeton University, Princeton, New Jersey and

LSCE, IPSL, CEA, UVSQ, CNRS, Gif-sur-Yvette, France and

Max Planck Institute for Meteorology, Hamburg, Germany and

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

Center for Global Environmental Research, National Institute for Environmental Studies, Tsukuba, Japan and

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

Geophysical Institute, University of Bergen, Bergen, Norway.

 

Abstract

 

The carbon cycle is a crucial Earth system component affecting climate and atmospheric composition. The response of natural carbon uptake to CO₂ and climate change will determine anthropogenic emissions compatible with a target CO₂pathway. For phase 5 of the Coupled Model Intercomparison Project (CMIP5), four future representative concentration pathways (RCPs) have been generated by integrated assessment models (IAMs) and used as scenarios by state-of-the-art climate models, enabling quantification of compatible carbon emissions for the four scenarios by complex, process-based models. Here, the authors present results from 15 such Earth system GCMs for future changes in land and ocean carbon storage and the implications for anthropogenic emissions. The results are consistent with the underlying scenarios but show substantial model spread. Uncertainty in land carbon uptake due to differences among models is comparable with the spread across scenarios. Model estimates of historical fossil-fuel emissions agree well with reconstructions, and future projections for representative concentration pathway 2.6 (RCP2.6) and RCP4.5 are consistent with the IAMs. For high-end scenarios (RCP6.0 and RCP8.5), GCMs simulate smaller compatible emissions than the IAMs, indicating a larger climate–carbon cycle feedback in the GCMs in these scenarios. For the RCP2.6 mitigation scenario, an average reduction of 50% in emissions by 2050 from 1990 levels is required but with very large model spread (14%–96%). The models also disagree on both the requirement for sustained negative emissions to achieve the RCP2.6 CO₂ concentration and the success of this scenario to restrict global warming below 2°C. All models agree that the future airborne fraction depends strongly on the emissions profile with higher airborne fraction for higher emissions scenarios.

 

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Additional Information: 

Climate change assessments talk a lot about climate targets – here we look at exactly how we must adjust our carbon-dioxide emissions in order to meet any target.

IPCC Assessment Reports examine the evidence base for climate change and also present the implications of future emissions of CO2 and other greenhouse gases under a range of possible socio-economic/technological scenarios. The so-called RCP scenarios (Representative Concentration Pathways) used for the 5^th Assessment Report are intended to show possible future pathways of global climate forcing from greenhouse gases spanning from a very low (RCP2.6) to very high (RCP8.5) future radiative forcing with two intermediate scenarios.

In particular the RCP2.6 is of interest as the resulting global climate change during the 21st century is at or close to the UN’s target of 2 degrees – a level chosen to avoid the worst effects of dangerous climate change.

The models used to create these future pathways have to translate human carbon emissions into increases in the atmospheric burden of CO2. This involves making assumptions, often at a global or very large regional scale, about natural sources and sinks of carbon from ecosystems and how these will change in the future. However, if natural sinks of CO2 increase in the future (for example due to higher CO2 levels) then we will be able to emit more CO2 to stay within a given target. Conversely, if natural sinks weaken (for example in response to rising temperatures or changes in precipitation patterns) then we will have to make greater emissions reductions to achieve the same target.

In this paper we use a range of complex, process-based Earth System Models which include fine regional detail of how the natural carbon cycle responds to environmental changes. These models allow us to quantify the important effects of increasing CO2 and changes in climate on natural sources and sinks and therefore what human emissions are compatible with given climate targets. The models were part of an internationally coordinated exercise known as CMIP5 (Coupled Model Intercomparison Project – Phase 5) as detailed here:

http://cmip-pcmdi.llnl.gov/cmip5/index.html

 

The analysis here was drawn upon during the preparation of the IPCC AR5 report, especially for Working group 1 chapter 6 (carbon cycle) and chapter 12 (future projections). It is one paper from a special collection of CMIP5 carbon cycle results published in the journal of climate here:

http://journals.ametsoc.org/page/C4MIP