The IPCC and their future climate scenarios

by Sebastian Hettrich

Whenever climate policies are mentioned and whether or not a particular policy supports the Paris Agreement, often the IPCC is referenced as well. But what actually is the IPCC? Who is behind the IPCC? What are they doing? And who are they funded by?

The Intergovernmental Panel on Climate Change (IPCC) is a United Nations body with 195 member states with the aim to gather the science behind the observed climate change [1]. In principle, it is a commission of thousands of the brightest climate scientists and experts in related fields who put together their research work on climate change, climate modelling, climate-related impacts, e.g. on biodiversity, health, or economy, as well as mitigation strategies. 

As such, they are not receiving any financial support from companies, private persons, environmentalist groups, or a single government. The scientists are volunteers with the intention to gather the latest scientific findings on climate change in order to provide the background knowledge for politics and society to tackle the issue.

While many IPCC assessment reports (AR) are kept very detailed and scientific, the IPCC also produces more accessible summaries for policy makers.

The assessment reports, in particular the AR 5, also provide four “what-if”-scenarios based on modelling results of multiple state-of-the-art climate simulation models [2]. 

The different scenarios are often referred to as RCPs, which stands for Representative Concentration Pathways. Their numbers,  2.6, 4.5, 6.0, or 8.5, refer to the additional wattage of solar radiation that is absorbed by the Earth per square metre without being reflected or re-emitted back into space (see our article here for more detail”). The additional scenario RCP1.9 did not find access to the IPCC AR5 as some of the models used were not able to achieve 1.9W/m² [3]. Thus the Paris agreement is based on the RCP2.6 to limit global warming at 2.0°C, with the hope of keeping it below 1.5°C (RCP1.9) [4].

These numbers can be examined in relation to a range of respective CO2-concentrations in the atmosphere, a range for the increase in global average temperature by 2100, the likelihood of achieving certain temperature goals, as well as the remaining carbon emission budget (see table 1).

Table 1: Comparison of the different scenarios and with preindustrial and current values

ScenarioPre-industrialTodayRCP1.9RCP2.6RCP4.5RCP6.0RCP8.5
W/m² energy absorption0N/A1.92.64.56.08.5
CO2 concentration in ppm280 [5]416 [6]< 430430-580 [2]580-720 [2]720-1000 [2]> 1000 [2]
additional warming by 2100 in °C in reference to 1850-1900 00.8-1.2 (1.0) [8]1.3 – 1.4[7]0.9 – 2.3 (1.6)[9]1.7 – 3.2 (2.4)[9]2.0 – 3.7 (2.8)[9]3.2 – 5.4 (4.3)[9]
Goals and their likelihoodAspirational goal of the Paris agreement< 1.5: 0–33%< 2.0: 33–66%< 3.0: 66–100%< 4.0: 66–100%[2]< 1.5: 0–33%< 2.0:0-50%< 3.0:50-100%< 4.0:66–100%[2]< 1.5: 0–33%< 2.0:0–33%< 3.0:0-50%< 4.0:66–100%[2]< 1.5:0–33%< 2.0:0–33%< 3.0:0–33%< 4.0:0-50%[2]
Cummulative CO2-Emissions (Mean) from 2012 until 2100 in GT631(2018: 420) [8]990 [IPCC, 2013], 1170 [8]2860 [9]3885 [9]6180 [9]
Leftover budget from 2021 until 2100 in GT
(2012 – 2019: 284 GT [10, [11)
313672254235675862

Like most things in science that are projections of the future, these values come with probabilities in which they are kept for which they are exceeded.

Figure 1 shows the four different pathways from AR5 being the blue lines for RCP2.6, RCP4.5, RCP6 and RCP8.5 from bottom to top. Also shown are the various simulation outcomes by different individual modelling runs in the fine blue and grey lines. The Intended Nationally Determined Contributions (INDCs) depict the point where governments intensify their climate conservation efforts [4].

On the right side of the diagram the resulting temperature range and their probabilities by 2100 are set in relation to the different scenarios. Interestingly, the Paris Agreement path only has a 33% chance of staying below a 2°C warming [4]. The 1.5°C goal is unlikely to be met: it has a 5% chance of meeting the goal.. 

Figure 1: A) Different emission pathways based on the IPCC AR5. Blue lines bottom to top represent the RCP scenarios 2.6, 4.5, 6.0 and 8.5, respectively. B) shows the probabilities for different temperature ranges of the respective RCP scenarios (Figure taken from [4]).

Consequently, the most likely, yet already hard to reach scenario to limit climate change to 1.5°C (as aimed by the Paris agreement) is the RCP1.9. Even though RCP2.6 has a small chance to stay below 1.5°C, it is more likely that the temperature rise  will stabilise somewhere between 2°C and 3°C [4]. Even though we are measuring currently even higher CO2 concentrations than for RCP 2.6, this goal could still be in reach until 2100, but it would require more ambitious climate protection policies than are currently planned.

References:

[1] IPCC, URL: https://www.ipcc.ch/about/, (last accessed 6 March 2021)
[2] IPCC, 2014: Climate Change 2014: Synthesis Report. Contribution of Working Groups I, II and III to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change [Core Writing Team, R.K. Pachauri and L.A. Meyer (eds.)]. IPCC, Geneva, Switzerland, 151 pp.
[3] Rogelj, J. et al. (2018), Scenarios towards limiting global mean temperature increase below 1.5C, Nature Climate Change, Vol. 8, pp. 325–332, doi:10.1038/s41558-018-0091-3, (last accessed 6 March 2021).
[4] Wuebbles, D.J., D.W. Fahey, K.A. Hibbard, B. DeAngelo, S. Doherty, K. Hayhoe, R. Horton, J.P. Kossin, P.C. Taylor, A.M. Waple, and C.P. Weaver, 2017: Executive summary. In: Climate Science Special Report: Fourth National Climate Assessment, Volume I [Wuebbles, D.J., D.W. Fahey, K.A. Hibbard, D.J. Dokken, B.C. Stewart, and T.K. Maycock (eds.)]. U.S. Global Change Research Program, Washington, DC, USA, pp. 12-34, doi:10.7930/J0DJ5CTG. 
[5] Eggleton, T., 2013, A short introduction to climate change, Cambridge University Press, ISBN: 987-1-107-61876-3.
[6] Global Monitoring Laboratory, Mauna Loa, Hawaii, URL:https://www.esrl.noaa.gov/gmd/ccgg/trends/, (last accessed 6 March 2021).
[7] Hausfather, Z., 2018, Explainer: How ‘Shared Socioeconomic Pathways’ explore future climate change, Carbon Brief, URL: https://www.carbonbrief.org/explainer-how-shared-socioeconomic-pathways-explore-future-climate-change, (last accessed 6 March 2021.)
[8] IPCC, 2018: Global Warming of 1.5°C. An IPCC Special Report on the impacts of global warming of 1.5°C above pre-industrial levels and related global greenhouse gas emission pathways, in the context of strengthening the global response to the threat of climate change, sustainable development, and efforts to eradicate poverty [Masson-Delmotte, V., P. Zhai, H.-O. Pörtner, D. Roberts, J. Skea, P.R. Shukla, A. Pirani, W. Moufouma-Okia, C. Péan, R. Pidcock, S. Connors, J.B.R. Matthews, Y. Chen, X. Zhou, M.I. Gomis, E. Lonnoy, T. Maycock, M. Tignor, and T. Waterfield (eds.)]. In Press.
[9] IPCC, 2013: Summary for Policymakers. 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.
[10] Our World in Data, URL: https://ourworldindata.org/grapher/cumulative-co-emissions?tab=table&stackMode=absolute&time=2011..latest&region=World, (last accessed 6 March 2021.)
[11] McSweeney, R., Tandon, A., 2020, Global Carbon Project: Coronavirus causes ‘record fall’ in fossil-fuel emissions in 2020, Carbon Brief, URL: https://www.carbonbrief.org/global-carbon-project-coronavirus-causes-record-fall-in-fossil-fuel-emissions-in-2020, (last accessed 6 March 2021.)
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