IPCC AR5, the representative concentration pathways

One of the most important events expected for this 2014 is the official release of the Intergovernmental Panel on Climate Change (IPCC) fifth assessment report (AR5). The IPCC began working on the AR5 in 2010. Between 2011 and 2013 some drafts were published on the internet. In September 2013 the working group 1 (The physical science basis) completed its report.The working group 2 (Impacts, adaptation and vulnerability) and the working group 3 (Mitigation of climate change) are expected to finish this year aloing with the nfinal synthesis report. but by this years it is expected that the final official version to be released.
Up to the last year we used to talk about the Special Report on Emissions Scenarios (SRES), but the AR5 uses a new term: Representative Concentration Pathway (RCP).
AR5 relies on the Coupled Model Intercomparison Project Phase 5 (CMIP5), an international effort among the climate modeling community to coordinate climate change experiments. Most of the CMIP5 and Earth System Model (ESM) simulations for AR5 were performed with prescribed CO2 concentrations reaching 421 ppm (RCP2.6), 538 ppm (RCP4.5), 670 ppm (RCP6.0), and 936 ppm (RCP 8.5) by the year 2100. (

Image 1. Cover of the IPCC-AR5

What is the RCP
They are prescribed pathways for greenhouse gas and aerosol concentrations, together with land use change, that are consistent with a set of broad climate outcomes used by the climate modelling community.
The pathways are characterised by the radiative forcing produced by the end of the 21st century. Radiative forcing is the extra heat the lower atmosphere will retain as a result of additional greenhouse gases, measured in Watts per square meter (W/m2).
Image 2. Temperature differences by the end of the 21st century
Source: Zeeburgnieuws

The RCPs span a wider range of possibilities than the SRES marker scenarios used in the modelling for the IPCC 3rd and 4th Assessment.
RCPs start with atmospheric concentrations of greenhouse gases rather than socioeconomic processes. This is important because every modelling step from a socioeconomic scenario to climate change impacts adds uncertainty. By starting with concentrations, there are fewer steps to impacts and therefore less cumulative uncertainty in impact assessments. This way uncertainty is shared more evenly among the various components. RCP also includes mitigation and adaptation policies. The figure shows a comparison of the CO2 projections according to SRES and RCP.
Image 3. Relation between RCO and SRES scenarios
Source: WMO

References & Further Reading
IPCC 5th assessment report
Representative concentration pathways

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