Projecting manmade climate change: scenarios to 2050

By Judith Curry

Stop using the worst-case scenario for climate change — more realistic scenarios make for better policy.

The International Energy Agency has just published a document ‘NetZero by 2050: A Roadmap for the Global Energy Sector.‘  This document provides a comprehensive assessment of the challenges of reaching NetZero carbon emissions by 2050, along with clear milestones for meeting this challenge.

The IEA report describes their analysis of the trajectory that our emissions is currently on. Policies that have actually been implemented (STEPS) versus the trajectory that would be achieved if all countries met their current commitments (APC) are shown in the diagram below. The implication of the IEA STEP scenario is that if policies that have already been implemented are maintained, the global carbon dioxide emissions three decades from now will be similar to what they are today. 

How do the IEA emissions scenarios compare with those used by the Intergovernmental Panel on Climate Change (IPCC) in their projections of future climate change?  A brief description of the emissions scenarios used by the IPCC is provided here for reference. 

The Representative Concentration Pathways (RCPs) are a set of four climate scenarios for the end of the 21st century. The RCPs were formulated for use in the 2014 Intergovernmental Panel on Climate Change (IPCC) Fifth Assessment Report and the CMIP5 climate model simulations, to reflect different potential climate outcomes – RCP2.6, RCP4.5, RCP6.0 and RCP8.5.
The number (e.g. 8.5) reflects the additional radiative forcing (in Watts per square meter) in 2100 from greenhouse gas emissions and other factors, relative to pre-industrial times. To date, radiative forcing relative to pre-industrial levels is ~2.5 Watts per square meter. 

For the forthcoming IPCC Sixth Assessment Report (AR6) and the CMIP6 climate model simulations, Shared Socioeconomic Pathways (SSPs) are formulated from five socio-economic and technological trajectories that reflect pathways that the world could follow in the 21st century. Each pathway has a baseline in which no climate policies are enacted after 2010. Additional SSP scenarios are linked to climate policies to generate different outcomes for the end
 of the 21st century. A subset of SSP scenarios has been selected for the IPCC AR6, with radiative forcing of 1.9, 2.6, 3.4, 4.5, 6.0, 7.0 or 8.5 Watts per square meter in 2100.  While the SSP nomenclature is more recent, the scientific literature and journalists continue to mostly use the RCP nomenclature.

In comparing the IEA scenarios with the IPCC scenarios, we see that the value for 2020 is higher for the IPCC (38 – 42 GtCO2/yr) than for the IEA (34 GtCO2/yr, which is the best available estimate for 2020). The IPCC scenarios – both for CMIP5 and CMIP6 – are higher than the IEA projections for RCP8.5, RCP7.0, RCP6.0 and RCP4.5.  Out to 2050, RCP6.0 and RCP4.5 show similar, nearly flat trends that are comparable to the IEA STEP scenario.  

The most striking aspect of the comparison between the IPCC and IEA scenarios to 2050 is the strong divergence of RCP8.5 from the IEA scenario, with RCP8.5 emissions values more than twice as high as the IEA STEP scenario at 2050.

RCP8.5 was formulated to explore an extreme outcome that is judged by energy analysts to be extremely unlikely. However, RCP8.5 is commonly referred to as the  ‘business as usual’ scenario. Referring to RCP8.5 as ‘business as usual’ implies that it is probable in the absence of stringent emissions mitigation. The IPCC, the U.S. National Climate Assessment and a majority of published papers have centered their analyses on RCP8.5 as a reference scenario against which climate impacts and policies are evaluated.  Further, RCP8.5 is being used by the insurance sector for projecting climate change impacts and also by state and local governments for regional adaptation planning.

Over the past several years, there has been substantial debate over RCP8.5 – whether it is plausible or even possible, and whether it should be used for policy-making purposes. The 8.5 scenarios can only emerge under a very narrow range of circumstances, comprising a severe course change from recent energy use. Both the CMIP5 and CMIP6 8.5 scenarios have drawn criticism particularly regarding assumptions around future coal use, requiring 6.5 times more coal use in 2100 than today – an amount larger than some estimates of recoverable coal reserves.  A recent elicitation of energy experts gives RCP8.5 only a 5% chance of occurring among all of the possible no-policy baseline scenarios; the likelihood of RCP8.5 becomes much lower when recent and future commitments for policy actions are considered. 

In evaluating these scenarios, it is important to recognize that predicting future emissions is inherently uncertain, particularly as the time horizon increases. Poorly understood carbon feedbacks (such as methane emissions from thawing permafrost) could lead to higher forcing levels. However, such speculative feedbacks are unlikely to arise from the relatively modest warming expected between now and 2050. Another source of uncertainty relates to emissions from land use change, which is estimated to account for 5-15% of total emissions. But even with these uncertainties, RCP8.5 is an extremely unlikely, if not impossible, scenario for the 21st century.

We should rightly approach projections far into the future with humility and acknowledge that there is a great deal of uncertainty.  However, for 30-year projections to 2050, which is a key time scale of relevance to the insurance industry and for local adaptation, the range of plausible scenarios can be narrowed from the complete menu of IPCC emissions scenarios. 

Climate impact assessments are being biased in an alarming direction by continued inclusion, and especially sole reliance, on RCP8.5.  For climate change to 2050, RCP4.5 and RCP6.0 are the most likely of the IPCC scenarios, and should be the focus of impact assessments for the insurance sector and for local adaptation planning over the next several decades.



Previous blog posts:

Is RCP8.5 an impossible scenario?

What’s the worst case? Emissions/concentration scenarios

Non-technical articles:

Worst climate scenario probably wind happen, scientists say

Carbon Brief: Explainer: The high-emissions RCP8.5 global warming scenario

Global CO2 emissions are on the brink of a long plateau

Pielke Jr: In 2020 climate science needs to hit the reset button

How billionaires Tom Steyer and Michael Bloomberg corrupted climate science

If climate scenarios are wrong for 2020, can they get 2100 right?

Breakthrough Inst: A 3C world is now ‘business as usual’

Recent journal publications:

Hausfather and Peters: Emissions – the ‘business as usual’ story is misleading

Ritchie and Dowlatabadi: Why do climate change scenarios return to coal?

Pielke and Ritchie: Systemic misuse of scenarios in climate research and assessment

Burgess et al. IPCC baseline scenarios have over-projected CO2 emissions and economic growth

Kriegler et al.: Fossil fueled development (SSP5): An energy and resource intensive scenario for the 21st century

Wang et al.: Implications of fossil fuel supply constraints on climate change projections: A supply-side analysis

Christensen et al.: Uncertainty in forecasts of long-run economic growth

Hausfather and Peters: RCP8.5 is a problematic scenario for near term emissions

Schramm et al. Reply: RCP8.5 is neither problematic nor misleading

Bauer et al.: Shared Socioeconomic Pathways of the Energy Sector – Quantifying the Narratives

Gidden et al.: Global emissions pathways under different socioeconomic scenarios for CMIP6: a dataset of harmonized emissions trajectories through the end of the century

O’Neill et al: The Scenario Model Intercomparison Project

Riahi et al.: RCP8.5 – a scenario of comparatively high greenhouse gas emissions

Riahi et al.: Shared Socioeconomic Pathways and their energy, land use, and greenhouse gas implications: An overview

Ritchie and Dowlatabadi: Defining climate change scenario characteristics with a phase space of cumulative primary energy and carbon intensity8

Van Vuuren et al.: Representative concentration pathways: an overview

This article appeared on the Climate Etc. website at https://judithcurry.com/2021/05/19/projecting-manmade-climate-change-scenarios-to-2050/


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