Originally published at Skeptical Science on July 7, 2015

Summary

The worst-case emissions pathway, RCP8.5, is a scenario that burns a huge amount of fossil fuels, especially coal. The model has sometimes been criticized as implausible because of its huge resource consumption and emissions of ~1700 billion tonnes of carbon (GtC) over the century. Those emissions are based in part on carbon cyclemodel assumptions, which recent work suggests may be too optimistic. New research shows that future plant growth may be restricted by nutrient availability, turning the land carbon sink into a source. Also, permafrost feedbacks (not considered in IPCC CMIP5 models) may also add significant emissions to the atmosphere under the RCP8.5 pathway. In addition, the latest research on the Amazon Basin reveals that the tropical forest carbon sinks may already be diminishing there. Together, these feedbacks suggest that the greenhouse gas concentrations in the RCP8.5 case could be achieved with ~400 GtC smaller human emissions, making the RCP8.5 worst-case scenario more plausible.

The climate models referred to  in the recent IPCC Fifth Assessment Report (AR5) are founded on one of four Representative Concentration Pathways or RCPs. The key word in RCP is concentration. In the RCPs, the concentration of greenhouse gases is fixed at different times in the future and the climate model (or general circulation model or GCM) uses those atmospheric concentrations to calculate future climate states. Underpinning the concentration pathways are socio-economic and emissions scenarios. There can be more than one underlying emissions scenario capable of producing the concentration pathway.

If you are unfamiliar with RCPs, check out the great guide that Graham Wayne wrote in August 2013 for Skeptical Science.

This way of modelling differs from previous approaches in which the starting point was a story or scenario about economic and social development that led to emissions. These emissions are run through a carbon-cycle model (which may be simple or complex) to produce atmospheric concentrations over time.

The schematic illustrates the differences in approach. The elements in red boxes are the prescribed inputs into the models, whereas the elements in blue ellipses are outputs. The advantage of the RCP prescribed-concentration approach is that the climate model outputs do not depend to the same degree on carbon-cycle models as they did in the emissions scenario method. The disadvantage is that there is no unique link between concentrations and emissions. The schematic is simplified in that there are feedbacks and loops in the processes that are not illustrated. Continue reading →