What is an emission scenario?
An emissions scenario is an estimate of future emissions based on our understanding of natural sources of greenhouse gases and on assumptions about future socio-economic trends i.e. how much greenhouses gases will be released into the atmosphere by humans.
Because it isn’t clear exactly how global social and economic systems will respond to emissions reduction programs a range of scenarios are used to describe possible future trends in emissions of greenhouse gases into the atmosphere.
In 2010 the Intergovernmental Panel on Climate Change (IPCC) Special Report on Emission Scenarios (SRES) outlined emission scenarios that were used in the IPCC’s ensemble of climate models (CMIP3) for the IPCC’s Fourth Assessment Report (AR4).These ranged from A1FI (a future world of rapid economic growth with fossil-intensive technologies) to B1 (rapid transformation to a service and information economy with clean and resource-efficient technologies)1.
These scenarios provide a range of changes to the climate in response to the emissions: the A2 scenario is projected to result in warming by approximately 3.4C by 2100. The more optimistic B1 scenario projected a warming of the planet by 1.8C by 2100.
What is a representative concentration pathway (RCP)?
In 2013 the Intergovernmental Panel on Climate Change released the Fifth Assessment Report which was based on the fifth phase of the Coupled Model Intercomparison Project (CMIP5), which incorporates the latest versions of climate models and focuses on a new set of scenarios. These scenarios span a range of plausible radiative forcing scenarios called representative concentration pathways (RCPs). Radiative forcing, measured in watts per square metre (W/m²), is the extra heat the lower atmosphere will retain because of the presence of additional greenhouse gases and aerosols2.
RCPs take into account the impact of atmospheric concentrations of greenhouse gases and aerosols (such as sulfate and soot), along with the uncertainty in possible future emissions.
The RCPs span a wider range of possibilities than the SRES emission scenarios. The RCPs include a very low forcing level (RCP2.6), two medium stabilization scenarios (RCP4.5 and RCP6), and one very high baseline emission scenario (RCP8.5). The RCP8.5 pathway, which arises from little effort to reduce emissions and represents a failure to prevent warming by 2100, is similar to the highest SRES scenario.
Why did we use the SRES A2 emissions scenario for NARCliM?
The NSW and ACT Regional Climate Modelling project (NARCliM) uses a single emissions scenario, SRES A2 scenario. At the time that NARCLIM started, in 2010, the SRES emission scenarios underpinning the IPCC’s Third Assessment Report (2001) and Fourth Assessment Report (2007) were the only set of emission scenarios available at the time.
The SRES A2 emission scenario was selected for the NARCliM climate projections because the global emissions trajectory suggested that it was the most likely scenario. Recent publications have confirmed that we are tracking at the higher end of the A2 scenario3.
How does SRES A2 compare with the RCP future projections?
Although the IPCC’s Fifth Assessment Report (AR5) is based primarily on results from the CMIP5 modelling using RCPs, the report also uses results from the SRES CMIP3 modelling and therefore identifies similar scenarios from each set.
For the latter half of the 21st century, SRES A2 has a similar trajectory to that of RCP8.5, with both reaching about 8 W/m² by 2100 (Figure 1a). SRES A2 is also similar to RCP8.5 in terms of changes in global mean temperature (Figure 1b).
For RCP8.5, global mean surface temperatures for 2081 to 2100, compared with those for 1986 to 2005, are likely to be in the range of 2.6 C to 4.8°C higher, with a multi-model mean increase of 3.7°C4. The projected warming for SRES A2 for the 2090 to 2099 period, relative to 1980 to 1999, is given by IPCC AR4 as 2.0°C to 5.9°C, with a best estimate of 3.4°C5.
Figure 1. (a) Projected radiative forcing (W/m²) and (b) global mean surface temperature change (°C) over the 21st century, as determined by using the Special Report on Emissions Scenarios (SRES) and Representative Concentration Pathway (RCP) scenarios. (Temperature changes are decadal averages based on the model ensemble mean.)
- Intergovernmental Panel on Climate Change 2000. IPCC Special Report on Emissions Scenarios, Summary for Policymakers, https://www.ipcc.ch/pdf/special-reports/spm/sres-en.pdf
- Stocker, T F, D Qin, G K Plattner, L V Alexander, S K Allen, N L Bindoff, F M Bréon, J A Church, U Cubasch, S Emori, P Forster, P Friedlingstein, N Gillett, J M Gregory, D L Hartmann, E Jansen, B Kirtman, R Knutti, K Krishna Kumar, P Lemke, J Marotzke, V Masson-Delmotte, G A Meehl, I.I. Mokhov, S Piao, V Ramaswamy, D Randall, M Rhein, M Rojas, C Sabine, D Shindell, L D Talley, D G Vaughan and S P Xie, 2013: Technical Summary In: Climate Change 2013: The Physical Science Basis. Contribution of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change [Stocker, T F, D Qin, G K Plattner, M Tignor, S K Allen, J Boschung, A Nauels, Y Xia, V Bex and P M Midgley (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA, http://www.climatechange2013.org/images/report/WG1AR5_TS_FINAL.pdf
- Peters GP, Andrew RM, Boden T, Canadell JG, Ciais P, Le Quere C, Marland G, Raupach MR, Wilson C 2013 Commentary: The challenge to keep global warming below 2 degrees C Nature Climate Change, 3:4-6.
- IPCC, 2013: Climate Change 2013: The Physical Science Basis. Contribution of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change [Stocker, T F, D Qin, G K Plattner, M Tignor, S K Allen, J Boschung, A Nauels, Y Xia, V Bex and P M Midgley (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA, 1535 pp, doi:10.1017/CBO9781107415324.
- IPCC, 2007: Summary for Policymakers. In: Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change [Solomon, S, D Qin, M Manning, Z Chen, M Marquis, K B Averyt, M Tignor and H.L Miller (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA, https://www.ipcc.ch/pdf/assessment-report/ar4/wg1/ar4-wg1-spm.pdf