by Judith Curry
Stop using worst-case scenarios for climate change – more realistic scenarios lead to better policy.
The International Energy Agency just published a document’NetZero to 2050: Roadmap for the global energy industry.‘This document provides a comprehensive assessment of the challenges of reaching NetZero’s carbon footprint by 2050, along with clear milestones for meeting this challenge.
The IEA report describes their analysis of the trajectory our emissions are currently in. Actual implemented policies (STEPS) versus the trajectory that would be achieved if all countries met their current commitments (APCs) are shown in the chart below. The implication of the IEA STEP scenario is that if the policies that have been implemented are maintained, global carbon dioxide emissions three decades from now will be similar to today.
How do the IEA’s 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 emission scenarios used by the IPCC is provided here for reference.
Representative Focused Pathways (RCPs) are a set of four climate scenarios for the end of the 21st century. The RCPs were developed for use in the Fifth Assessment Report of the Intergovernmental Panel on Change. Climate (IPCC) and simulate the CMIP5 climate model, to reflect different potential climate outcomes – RCP2.6, RCP4.5, RCP6.0 and RCP8.5. The number (eg 8.5) reflects the additional radiative force (in Watts per square meter) in 2100 from greenhouse gas emissions and other factors, compared with pre-industrial times. So far the radiant force relative to pre-industrial levels is ~2.5 Watts per square meter.
For the upcoming IPCC Sixth Assessment Report (AR6) and climate model simulation CMIP6, the Shared Socio-Economic Paths (SSPs) build from five socio-economic trajectories and technology reflects the paths the world could take in the 21st century. Each path has a baseline where no climate policy was enacted after 2010. Additional SSP scenarios are provided. linked with climate policies to produce different outcomes for the end of the 21st century. A subset of SSP scenarios was selected for IPCC AR6, with irradiance 1.9, 2.6, 3.4, 4.5, 6.0, 7.0 or 8.5 Watts per square meter by 2100. While the SSP nomenclature is more recent, scientific literature and journalists continue to use it primarily use RCP nomenclature.
When we compare the IEA scenarios with the IPCC scenarios, we find that the value for 2020 is higher for IPCC (38 – 42 GtCO2/year) compared to the IEA (34 GtCO2/yr, is the best estimate available for 2020). The IPCC scenarios – both for CMIP5 and CMIP6 – outperform the IEA projections for RCP8.5, RCP7.0, RCP6.0 and RCP4.5. By 2050, RCP6.0 and RCP4.5 show similar, nearly constant, trends comparable to the IEA STEP scenario.
The most striking aspect of the comparison between the IPCC and IEA scenarios to 2050 is the strong difference between RCP8.5 and the IEA scenario, with the RCP8.5 emission value more than twice as high as the scenario. IEA STEP in 2050.
RCP8.5 was developed to explore an extreme outcome that is considered extremely unlikely by energy analysts. However, RCP8.5 is often referred to as a ‘business as usual’ scenario. Referring to RCP8.5 as ‘business as usual’ means that it is possible in the absence of strict emission reductions. IPCC, US National Climate Assessment and the majority of published papers have focused their analyzes on RCP8.5 as a reference scenario for assessing climate policy and impacts. . Furthermore, RCP8.5 is being used by the insurance industry to forecast climate change impacts as well as by state and local governments for regional adaptation planning.
Over the years, there has been much debate about RCP8.5 – whether it is reasonable or even doable and whether it should be used for policy-making purposes. 8.5 scenarios are only possible under a very narrow set of circumstances, including a dramatic shift from recent energy use. Both the CMIP5 and CMIP6 8.5 scenarios have come under particular criticism regarding assumptions around future coal use, which would require 6.5 times more coal use in 2100 than today – an amount greater than some estimates of recoverable coal reserves. A recent poll of energy experts showed that RCP8.5 has only a 5% chance of occurring out of all possible no-policy baselines; the likelihood of RCP8.5 becoming much lower when recent and future commitments to policy actions are considered.
When evaluating these scenarios, it is important to realize that predicting future emissions is inherently uncertain, especially as the time horizon increases. Not well understood carbon feedbacks (such as methane emissions from thawing permafrost) can lead to higher coercive levels. However, such speculative responses are unlikely to arise given the relatively modest warming expected between now and 2050. Another source of uncertainty concerns emissions from changing use land use, 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 properly approach projections of the future with humility and acknowledging that there are many uncertainties. However, for the 30-year forecast to 2050, which is an important time scale relevant to the insurance industry and for local adaptation, the range of plausible scenarios can be narrowed down from the complete menu of IPCC emission scenarios.
Climate impact assessments are being skewed in an alarming way by continued inclusion, and in particular, reliance solely 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 few decades.
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Presenter:
Past blog posts:
Is RCP8.5 an impossible scenario?
What is the worst case scenario? Emission/concentration scenarios
Non-technical articles:
Scientists say worst possible climate scenario with wind
Carbon summary: Explainer: High Emissions RCP8.5 Global Warming Scenario
Global CO2 emissions are on the verge of a long plateau
Pielke Jr: In 2020, climate science needs to hit the reset button
How billionaires Tom Steyer and Michael Bloomberg screwed up climate science
If the climate scenarios are wrong for 2020, can they get 2100 right?
Breakthrough version: A 3C world is now ‘business as usual’
Recent magazine publications:
The Godfather and Peters: Emissions – misleading ‘business as usual’ story
Ritchie and Dowlatabadi: Why do climate change scenarios go back to coal?
Pielke and Ritchie: Systematic misuse of scenarios in climate research and assessment
Burgess et al. IPCC baseline scenarios with CO2 emissions and economic growth exceeding expectations
Kriegler et al: Fossil fuel development (SSP5): An energy and resource-intensive scenario for the 21st century
Wang and associates: The effects of fossil fuel supply constraints on climate change projections: A supply-side analysis.
Christensen et al: Uncertainty in long-term economic growth forecast
The Godfather and Peters: RCP8.5 is a problematic scenario for short-term emissions
Schramm et al. Answer: RCP8.5 is no problem and not misleading
Bauer et al: Shared socioeconomic paths of the energy industry – Quantifying the stories
O’Neill et al: A project to compare scenario models
Riahi and associates: RCP8.5 – a relatively high GHG emission scenario
Riahi and associates: Shared socioeconomic pathways and their energy, land use and greenhouse gas impacts: An overview
Ritchie and Dowlatabadi: Determination of climate change scenario features with phase space of accumulated primary energy and carbon intensity8
Van Vuuren et al: Representative focus pathways: an overview
