IPCC’s Sixth Assessment Report: “No Climate Policy” Scenarios

by Nicole Gray

Early in August, the United Nations Intergovernmental Panel on Climate Change (IPCC) released the first part of their sixth assessment report (AR6): ‘Climate Change 2021: The Physical Science Basis’.  These assessment reports conduct a scientific, technical and socioeconomic analysis of the climate crisis and consist of four parts: three contributions from Working Groups plus a synthesis report [1]. They are designed to be descriptive rather than prescriptive – presenting findings to policymakers to inform decision making and intergovernmental negotiations rather than telling global leaders exactly what to do [1]. Released mere months before the 26th UN Climate Change Conference (COP26), the AR6 put huge pressure on policymakers to take urgent action on the climate crisis.

The sixth assessment report is the most comprehensive analysis of the climate crisis to ever be released [2]. It demonstrates what future impacts might be seen in five hypothetical policy and emissions scenarios, called ‘Shared Socioeconomic Pathways (SSPs)’. These possible climate futures range from a best-case scenario whereby emissions are net-negative by 2050 and warming limited to 1.5°C, to a worst-case scenario based on ‘business-as-usual’ and warming reaching 5°C by the end of the century [3].

This worst case, ‘no climate policy’ scenario seems unlikely. But the best-case scenario is also improbable, requiring emissions to fall by 25% by 2030 and 50% by 2050 [4]. The most likely emissions scenario is one of 3°C warming by the end of the century, which predicts sea-level rise of 44-75 centimetres by 2100, and potentially over 1 metre by 2150 [5,6]. Moreover, the IPCC report states that regardless of our emissions scenario, sea level rise will continue for centuries to millennia. Even if we can limit warming to 1.5°C, sea levels are predicted to rise by 2-3 metres over the next 2,000 years [6]. If warming reaches 2°C sea level rise will likely be between 2-6 metres, and at 5°C of warming, 19-22 metres [6].

Of course, all of these possible climate futures are based on a range of assumptions, and in reality, no one of these hypothetical pathways will be strictly adhered to – not all assumptions may hold true, and as policy and technology change so might our trajectory.  However, what is startling is that even if the most ambitious ‘green road’ (SSP1) pathway is taken, it is likely that the 1.5°C warming threshold (as set out in the 2015 Paris Agreement and signed by 196 countries), will be reached by 2040 [7,8]. Described as a ‘code red for humanity’, the report sends a clear and urgent message to policymakers regarding the need to mitigate and adapt [9].

In addition, the report emphasises that as emissions increase our natural carbon sinks will likely become less effective. So, not only is atmospheric CO2 increasing, the rate of removal is due to fall if we follow our most likely future trajectory (SSP-3) [6]. Throughout the pandemic we saw emissions fall, sparking optimism about the future of the climate crisis. However, two-thirds of the emissions that fell during the pandemic have now bounced back [10]. The IPCC sixth assessment report helps us realise these realities, laying the foundations for meaningful action to be taken.

In addition to scenario analyses, the AR6 highlighted the risk of compound events, recognising that extreme weather events such as heatwaves and droughts are more likely to occur simultaneously, as well as becoming more frequent in the future [11]. The report also considers how the impacts of the climate crisis will be affected by the characteristics of our future societies.  For example, by 2050 2.5 billion more people will live in cities [12]. The AR6 notes how cities intensify human-induced warming on a local basis, and also worsen the impacts of extreme weather events such as flooding due to the increase in impermeable surfaces [6]. The need for innovative solutions, such as permeable concrete that reduces the risk of flooding and mitigates against the ‘urban heat island’ effect, is clear thanks to the AR6 [13]. But, without intergovernmental action, these solutions may not be widely employed.

The future of the climate crisis ultimately depends on how quickly we transition to, and beyond, net zero. The IPCC sixth assessment report illustrated the current situation, future risks and potential impacts humanity might face. However, following COP26 many are critiquing global leaders for not doing enough. Whilst progress was made in Glasgow, estimates based on new commitments still put us on a  2.4°C trajectory [14]. There is clearly still a long way to go in tackling the climate crisis, but we should not lose hope. Under The Glasgow Climate Pact countries will be required to review their nationally determined contributirons at COP27 in Egypt, three years sooner than the Paris Agreement originally stated [15]. Hopefully, more ambitious commitments can be made in Egypt, and we can get on track for a more prosperous, Paris-aligned, global future.. 

Keep an eye out for articles on the other three components of the AR6, due for release later in 2022, which will delve deeper into impacts, adaptation, vulnerabilities and mitigation

Featured image courtesy of the IPCC


[1] IPCC. (2021) IPCC factsheet: what is the IPCC? https://www.ipcc.ch/news_and_events/docs/factsheets/FS_what_ipcc.pdf, accessed on 6 November 2021.
[2] Hersher, R. (2021). Climate Scientists Meet As Floods, Fires, Droughts And Heat Waves Batter Countries . https://www.npr.org/2021/07/26/1019433734/climate-scientists-meet-as-floods-fires-droughts-and-heat-waves-batter-countries?t=1636207986034 accessed on 6 November 2021.
[3] Coren, M.J. (2021). Scientists have finally added world politics to their climate models.  https://qz.com/2043909/ipcc-our-climate-change-future-will-be-determined-by-politics/ . Accessed on 6 November 2021.
[4] Henson, B. (2021). Key takeaways from the new IPCC report. https://yaleclimateconnections.org/2021/08/key-takeaways-from-the-new-ipcc-report/ . Accessed on 6 November 2021.
[5] Irfan, U. (2021). What’s the worst that could happen?  https://www.vox.com/22620706/climate-change-ipcc-report-2021-ssp-scenario-future-warming Accessed on 6 November 2021.
[6] IPCC, 2021: Summary for Policymakers. In: Climate Change 2021: The Physical Science Basis. Contribution of Working Group I to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change [MassonDelmotte, V., P. Zhai, A. Pirani, S.L. Connors, C. Péan, S. Berger, N. Caud, Y. Chen, L. Goldfarb, M.I. Gomis, M.Huang, K. Leitzell, E. Lonnoy, J.B.R. Matthews, T.K. Maycock, T. Waterfield, O. Yelekçi, R. Yu, and B. Zhou (eds.)].Cambridge University Press. In Press.
[7] Carbon Brief Staff (2021). In-depth Q&A: The IPCC’s sixth assessment report on climate science. https://www.carbonbrief.org/in-depth-qa-the-ipccs-sixth-assessment-report-on-climate-science . Accessed on 6 November 2021.
[8]Dröge, S. (2016). The Paris Agreement 2015: Turning point for the international climate regime. 
[9] McGrath, Matt (2021-08-09). “Climate change: IPCC report is ‘code red for humanity'”. BBC News. BBC. Archived from the original on 2021-08-13. Accessed on 6 November 2021.
[10] Psaropoulos, J. (2021). Climate crisis: The world has a long, hard climb to ‘net zero’. https://www.aljazeera.com/news/2021/10/31/the-world-has-a-long-hard-climb-to-net-zero-reports-find . Accessed on 6 November 2021.
[11] Tollefson, J. (2021). IPCC climate report: Earth is warmer than it’s been in 125,000 years. Nature 596, 171-172 (2021) doi: https://doi.org/10.1038/d41586-021-02179-1
[12] Un, D. E. S. A. (2018). Revision of world urbanization prospects. New York: United Nations Department of Economic and Social Affairs.
[13] Richards, D. R., & Edwards, P. J. (2018). Using water management infrastructure to address both flood risk and the urban heat island. International Journal of Water Resources Development, 34(4), 490-498.
[14] European Commision (2021). COP26: EU helps deliver outcome to keep the Paris Agreement targets alive. Glasgow. Accessed on 11 December 2021
[15] Harvey, F. (2021). What are the key points of the Glasgow climate pact? The Guardian. https://www.theguardian.com/environment/2021/nov/14/what-are-the-key-points-of-the-glasgow-climate-pact-cop26 Accessed on 11 December 2021. 
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