Our new paper on IPCC scenarios is now posted as a preprint. Details below and I have summarized it in a new column at Forbes.
Burgess, M. G., Ritchie, J., Shapland, J., & Pielke, R., Jr. (2020, February 18). IPCC baseline scenarios over-project CO2 emissions and economic growth. https://doi.org/10.31235/osf.io/ahsxw SocArXiv
Scenarios used by the Intergovernmental Panel on Climate Change (IPCC) are central to climate science and policy. A recent Nature commentary found observed trends and International Energy Agency (IEA) projections of global CO2 emissions substantially diverging from high-emission scenarios such as RCP8.5, which are often treated as equivalent to ‘business as usual’ in climate research and assessment. Here, we quantify the bases for this divergence by comparing “baseline” (or “no policy”) scenario projections of key fossil-fuel CO2 emission drivers to observations from 2005-2017, and also to projections through 2040 from world energy outlooks. We find most of the baseline scenarios used in the Fifth (AR5) and designed for the forthcoming Sixth (AR6) IPCC Assessment Reports have over-projected per-capita GDP growth—severely in most developing regions, and slightly in other regions—and have slightly over-projected carbon intensity (CO2 emissions/primary energy) in most regions. These baseline scenarios will likely continue to over-project carbon intensity through 2040 and beyond, in part due to unrealistic assumptions about fossil-fuel expansion. Long-term economic growth outlooks lack consensus among economists, but we argue that most of these baseline scenarios will likely continue over-projecting per-capita GDP to at least 2040 due to inertia. Our results inform the rapidly evolving discussion on uses of scenarios in climate science and policy.
We find that most of the scenarios of the IPCC – and not just the most extreme baseline highlighted by Hausfather and Peters – over-projected global carbon dioxide emissions (from FF). Specifically, real-world observations of carbon dioxide emissions from 2005 to 2017 grew at a slower rate than 83% of AR5 (Working Group 3 baseline) scenarios and 73% of AR6 (SSP baseline) scenarios. This can be seen in the figure below.
The divergence between the real-world observations of carbon dioxide emissions and the baseline scenarios of the IPCC is expected to widen over the next several decades. According to projections of major energy outlooks, by 2040 carbon dioxide emissions (from FF) may fall below the entire range of IPCC AR5 and AR6 baseline scenarios, even assuming that no new major climate policy efforts are undertaken between now and then.
There is both good news and bad news to be found in our decomposition of the reasons behind the divergence between the real-world and the scenarios.
The good news is obvious: carbon dioxide emissions are not growing as fast as was expected across most models and across all regions of the world. One important reason for this is that the world’s energy consumption is becoming less carbon intensive, meaning that our energy supply is becoming cleaner faster than was expected. Scholars will no doubt debate what role enacted climate policies may have played in this trend, but it is clear that a large part of the trend is due to factors beyond climate policy alone, such as the boom in natural gas from fracking and corresponding decreasing competitiveness of coal.
The challenge of dramatically decarbonizing the global economy remains huge, but the fact that emissions are not increasing as fast as expected should be received as welcome good news.
The bad news is that many of the scenarios of the IPCC have over-projected global per-capita GDP growth, across all regions of the world. Economic growth confers many benefits, and is a stated goal of politicians across the world. With respect to climate policy, if economic activity is not advancing as fast as anticipated, it could create potential headwinds for more aggressive climate policies.
While economists disagree about the potential for global growth to 2100, it seems unlikely that in the coming decades the world can “catch-up” to projected growth rates of AR5 and AR6 baseline scenarios, since that would require rates of growth from 2020 to 2040 much higher than assumed in most scenarios. In the real world, projections of the IMF and the possible impact of the coronavirus on near-term GDP growth rates suggests that slower than projected per capita GDP growth is set to persist in the coming years.
If so, then this would mean that global carbon dioxide emissions (from FF) will continue to diverge from baseline scenarios of the IPCC. To the extent that GDP growth rates may be over-projected in leading energy assessments, this would mean even lower carbon dioxide emissions than found in their projections.
Why does our analysis matter?
If the scenarios of the IPCC offer, in effect, road maps that help us to shape our possible pathways into the future, then we are at risk of proceeding with out-of-date information. Flawed maps can lead to unexpected dead ends and challenges in reaching desired destinations.
In climate research, everything from projection of near-term (to mid-century) climate impacts to cost-benefit analysis of the near-term costs and benefit of mitigation will be impacted adversely by utilizing overly aggressive projections of carbon dioxide emissions. Similarly, work that depends upon projections of population, per capita GDP, energy intensity and carbon intensity will be impacted by models that no longer correspond with the real world.
Such impacts will grow increasingly significant as the real-world diverges from corresponding characterizations of the real-world found in models. As we conclude, “scenarios are drifting away from policy relevance.” Ultimately, there is no good reason why climate science and policy should continue to rely on models that are now out-of-date with respect to the evolution of the real world. Climate science and policy are far too important not to update scenarios and provide policy makers with more accurate maps to help guide how we travel into the future.