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

See 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."