Representative Concentration Pathway

A Representative Concentration Pathway (RCP) is a greenhouse gas concentration (not emissions) trajectory adopted by the IPCC. Four pathways were used for climate modeling and research for the IPCC fifth Assessment Report (AR5) in 2014. The pathways describe different climate futures, all of which are considered possible depending on the volume of greenhouse gases (GHG) emitted in the years to come. The RCPs – originally RCP2.6, RCP4.5, RCP6, and RCP8.5 – are labelled after a possible range of radiative forcing values in the year 2100 (2.6, 4.5, 6, and 8.5 W/m2, respectively).[1][2][3] Since AR5 the original pathways are being considered together with Shared Socioeconomic Pathways: as are new RCPs such as RCP1.9, RCP3.4 and RCP7.[4]

All forcing agents' atmospheric CO2-equivalent concentrations (in parts-per-million-by-volume (ppmv)) according to the four RCPs used by the fifth IPCC Assessment Report to make predictions.

Concentrations

The RCPs are consistent with a wide range of possible changes in future anthropogenic (i.e., human) GHG emissions, and aim to represent their atmospheric concentrations.[5] Despite characterizing RCPs in terms of inputs, a key change from the 2007 to the 2014 IPCC report is that the RCPs ignore the carbon cycle by focusing on concentrations of greenhouse gases, not greenhouse gas inputs.[6] The IPCC studies the carbon cycle separately, predicting higher ocean uptake of carbon corresponding to higher concentration pathways, but land carbon uptake is much more uncertain due to the combined effect of climate change and land use changes.[7]

The four RCPs are consistent with certain socio-economic assumptions but are being substituted with the shared socioeconomic pathways which are anticipated to provide flexible descriptions of possible futures within each RCP. The RCP scenarios superseded the Special Report on Emissions Scenarios projections published in 2000 and were based on similar socio-economic models.[8]

RCPs

RCP 1.9

RCP1.9 is a pathway that limits global warming to below 1.5 °C, the aspirational goal of the Paris Agreement.[4]

RCP 2.6

RCP 2.6 is a "very stringent" pathway.[4] According to the IPCC, RCP 2.6 requires that carbon dioxide (CO2) emissions start declining by 2020 and go to zero by 2100. It also requires that methane emissions (CH4) go to approximately half the CH4 levels of 2020, and that sulphur dioxide (SO2) emissions decline to approximately 10% of those of 1980–1990. Like all the other RCPs, RCP 2.6 requires negative CO2 emissions (such as CO2 absorption by trees). For RCP 2.6, those negative emissions would be 2 Gigatons of CO2 per year (GtCO2/yr).[9] RCP 2.6 is likely to keep global temperature rise below 2 °C by 2100.[10]

RCP 3.4

RCP3.4 represents an intermediate pathway between the “very stringent” RCP2.6 and less stringent mitigation efforts associated with RCP4.5.[11] As well as just providing another option a variant of RCP3.4 includes considerable removal of greenhouse gases from the atmosphere.[4]

RCP 4.5

RCP 4.5 is described by the IPCC as an intermediate scenario.[10] Emissions in RCP 4.5 peak around 2040, then decline.[12]

According to the IPCC, RCP 4.5 requires that carbon dioxide (CO2) emissions start declining by approximately 2045 to reach roughly half of the levels of 2050 by 2100. It also requires that methane emissions (CH4) stop increasing by 2050 and decline somewhat to about 75% of the CH4 levels of 2040, and that sulphur dioxide (SO2) emissions decline to approximately 20% of those of 1980–1990. Like all the other RCPs, RCP 4.5 requires negative CO2 emissions (such as CO2 absorption by trees). For RCP 4.5, those negative emissions would be 2 Gigatons of CO2 per year (GtCO2/yr).[9] RCP 4.5 is more likely than not to result in global temperature rise between 2 degrees C, and 3 degrees C, by 2100 with a mean sea level rise 35% higher than that of RCP 2.6.[13] Many plant and animal species will be unable to adapt to the effects of RCP 4.5 and higher RCPs.[14]

RCP 6

In RCP 6, emissions peak around 2080, then decline.[12]

RCP 7

RCP7 is a baseline outcome rather than a mitigation target.[4]

RCP 8.5

In RCP 8.5 emissions continue to rise throughout the 21st century.[12] Since AR5 this has been thought to be very unlikely, but still possible as feedbacks are not well understood.[15] RCP8.5, generally taken as the basis for worst-case climate change scenarios, was based on what proved to be overestimation of projected coal outputs. The RCP8.5 scenario has thus become relatively unlikely, with one report calling it "increasingly implausible with each passing year."[16] Despite its relative unlikelihood, RCP8.5 remains useful for its aptness in both tracking historical total cumulative CO2 emissions and predicting mid-century (and earlier) emissions based on current and stated policies.[17]

Projections based on the RCPs
21st century

Mid- and late-21st century (2046–2065 and 2081–2100 averages, respectively) projections of global warming and global mean sea level rise from the IPCC Fifth Assessment Report (IPCC AR5 WG1) are tabulated below. The projections are relative to temperatures and sea levels in the late-20th to early-21st centuries (1986–2005 average). Temperature projections can be converted to a reference period of 1850–1900 or 1980–99 by adding 0.61 or 0.11 °C, respectively.[18]

AR5 global warming increase (°C) projections[18]
2046–20652081–2100
ScenarioMean and
likely range		Mean and
likely range
RCP2.61.0 (0.4 to 1.6)1.0 (0.3 to 1.7)
RCP4.51.4 (0.9 to 2.0)1.8 (1.1 to 2.6)
RCP61.3 (0.8 to 1.8)2.2 (1.4 to 3.1)
RCP8.52.0 (1.4 to 2.6)3.7 (2.6 to 4.8)

Across all RCPs, global mean temperature is projected to rise by 0.3 to 4.8 °C by the late-21st century.

AR5 global mean sea level (m) increase projections[18]
2046–20652081–2100
ScenarioMean and
likely range		Mean and
likely range
RCP2.60.24 (0.17 to 0.32)0.40 (0.26 to 0.55)
RCP4.50.26 (0.19 to 0.33)0.47 (0.32 to 0.63)
RCP60.25 (0.18 to 0.32)0.48 (0.33 to 0.63)
RCP8.50.30 (0.22 to 0.38)0.63 (0.45 to 0.82)

Across all RCPs, global mean sea level is projected to rise by 0.26 to 0.82 m by the late-21st century.

23rd century

AR5 also projects changes in climate beyond the 21st century. The extended RCP2.6 pathway assumes sustained net negative anthropogenic GHG emissions after the year 2070.[5] "Negative emissions" means that in total, humans absorb more GHGs from the atmosphere than they release. The extended RCP8.5 pathway assumes continued anthropogenic GHG emissions after 2100.[5] In the extended RCP 2.6 pathway, atmospheric CO2 concentrations reach around 360 ppmv by 2300, while in the extended RCP8.5 pathway, CO2 concentrations reach around 2000 ppmv in 2250, which is nearly seven times the pre-industrial level.[5]

For the extended RCP2.6 scenario, global warming of 0.0 to 1.2 °C is projected for the late-23rd century (2281–2300 average), relative to 1986–2005.[19] For the extended RCP8.5, global warming of 3.0 to 12.6 °C is projected over the same time period.[19]

See also

References
  1. "Representative Concentration Pathways (RCPs)". IPCC. Retrieved 13 February 2019.
  2. Richard Moss; et al. (2008). Towards New Scenarios for Analysis of Emissions, Climate Change, Impacts, and Response Strategies (PDF). Geneva: Intergovernmental Panel on Climate Change. p. 132.
  3. Weyant, John; Azar, Christian; Kainuma, Mikiko; Kejun, Jiang; Nakicenovic, Nebojsa; Shukla, P.R.; La Rovere, Emilio; Yohe, Gary (April 2009). Report of 2.6 Versus 2.9 Watts/m2 RCPP Evaluation Panel (PDF). Geneva, Switzerland: IPCC Secretariat.
  4. "Explainer: How 'Shared Socioeconomic Pathways' explore future climate change". Carbon Brief. 2018-04-19. Retrieved 2020-03-04.
  5. Collins, M., et al.: Section 12.3.1.3 The New Concentration Driven RCP Scenarios, and their Extensions, in: Chapter 12: Long-term Climate Change: Projections, Commitments and Irreversibility (archived 16 July 2014), in: IPCC AR5 WG1 2013, pp. 1045–1047
  6. IPCC 2013: Technical Summary (PDF) (Report). the uncertainty is now estimated to be smaller than with the AR4 method for long-term climate change, because the carbon cycle–climate feedbacks are not relevant for the concentration-driven RCP projections
  7. IPCC AR5- Technical Summary- TFE.7 Carbon Cycle Perturbation and Uncertainties (PDF) (Report). With very high confidence, ocean carbon uptake of anthropogenic CO2 emissions will continue under all four Representative Concentration Pathways (RCPs) through to 2100, with higher uptake corresponding to higher concentration pathways. The future evolution of the land carbon uptake is much more uncertain, with a majority of models projecting a continued net carbon uptake under all RCPs, but with some models simulating a net loss of carbon by the land due to the combined effect of climate change and land use change. In view of the large spread of model results and incomplete process representation, there is low confidence on the magnitude of modelled future land carbon changes.
  8. Ward, James D.; Mohr, Steve H.; Myers, Baden R.; Nel, William P. (December 2012). "High estimates of supply constrained emissions scenarios for long-term climate risk assessment". Energy Policy. 51: 598–604. doi:10.1016/j.enpol.2012.09.003.
  9. https://ar5-syr.ipcc.ch/topic_futurechanges.php Box 2.2
  10. https://ar5-syr.ipcc.ch/topic_futurechanges.php Box 2.2, figure 1
  11. "Explainer: How 'Shared Socioeconomic Pathways' explore future climate change". Carbon Brief. April 19, 2018.
  12. Figure 2, in Meinshausen & others 2011, p. 223
  13. https://ar5-syr.ipcc.ch/topic_summary.php table SPM.1
  14. https://ar5-syr.ipcc.ch/topic_futurechanges.php 2.3.1
  15. "BBC World Service - The Inquiry, Have our climate models been wrong?". BBC. Retrieved 2020-03-05.
  16. Hausfather, Zeke; Peters, Glen (29 January 2020). "Emissions – the 'business as usual' story is misleading". Nature. 577 (7792): 618–20. doi:10.1038/d41586-020-00177-3. PMID 31996825.
  17. Schwalm, Christopher R.; Glendon, Spencer; Duffy, Philip B. (2020-08-18). "RCP8.5 tracks cumulative CO2 emissions". Proceedings of the National Academy of Sciences. 117 (33): 19656–19657. doi:10.1073/pnas.2007117117. ISSN 0027-8424. PMID 32747549.
  18. IPCC: Table SPM-2, in: Summary for Policymakers (archived 16 July 2014), in: IPCC AR5 WG1 2013, p. 21
  19. Collins, Matthew, et al.: Executive summary, in: Chapter 12: Long-term Climate Change: Projections, Commitments and Irreversibility (archived 16 July 2014), in: IPCC AR5 WG1 2013, p. 1033

Note: The following references are cited in this article using Template:Harvard citation no brackets:

  • Meinshausen, M.; et al. (November 2011), "The RCP greenhouse gas concentrations and their extensions from 1765 to 2300 (open access)", Climatic Change, 109 (1–2): 213–241, doi:10.1007/s10584-011-0156-z.
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