Posted on 20 January 2025 by Zeke Hausfather

This is a re-post from the Climate Brink

I have a new paper out today in the journal Dialogues on Climate Change exploring both the range of end-of-century climate outcomes in the literature under current policies and the broader move away from high-end emissions scenarios. Current policies are defined broadly as policies in place today and a continuation of trends in technology costs, but no additional climate policy enacted for the remainder of the century.

The figure below shows the literature summary I put together (as of fall 2024), which includes estimates of current policy outcomes (in red), outcomes where countries meet their 2030 Paris Agreement nationally determined contributions (in orange), constrained estimates using socioeconomic factors (or other factors) to try and estimate most likely end-of-century trajectories including future policy (in grey), and net-zero commitments made by countries (in blue).

 These suggest a median estimate of future warming under current policies of 2.7C in 2100 (with a 5th-95th percentile range of central estimates spanning 2.3C to 3C). Adding in emissions uncertainties and climate system uncertainties gives a much wider range of 1.9C at the low end to 3.7C at the high end. Current policies represent something of a moving target, which complicates the interpretation of a review of recent literature; those studies from 2021 may lag behind the policy and technology environment of 2024, for example.

The push to examine the range of outcomes consistent with current policy (and a rapidly growing literature on the topic) allows us to better constrain the upper bound of plausible scenarios today. In particular, the range of current policy scenarios in the literature largely preclude emissions pathways in high-end scenarios like RCP8.5 (Riahi et al., 2011), SSP3–7.0, or SSP5–8.5 (Riahi et al., 2017) in the absence of an active reversal of current policy and current technology trends.

I’ve included a more detailed excerpt of the article below, but I’d encourage folks to read the whole (open access) piece here, as its written to be accessible to a more general audience.

A move away from high-end emissions scenarios

This move away from high-end emissions scenarios in the literature reflects a broader recognition that the world is undergoing an energy transition away from a future of continuing fossil fuel expansion (IEA, 2023). Fifteen years ago many researchers argued that “business as usual” would likely lead to a world 4C or 5C above pre-industrial levels by 2100 (Sokolov et al., 2009). Today the world is in a very different place; growth in CO₂ emissions slowed notably over the past decade (Friedlingstein et al., 2023), and emissions are projected to plateau in coming years under current policies and commitments (IEA, 2023). Global investments in clean energy topped $1.8 trillion in 2023, nearly double the level of global investments in fossil fuels (IEA, 2023).

High emissions scenarios assume a 21st century dominated by coal; however, global coal usage has been relatively flat since 2013, and is forecast to decline over the remainder of the century (IEA, 2023). There are also likely fundamental resource limits to the degree of coal expansion seen in RCP8.5 and SSP5–8.5 (Ritchie and Dowlatabadi, 2017), as well as overly optimistic assumptions of future economic growth (Burgess et al., 2023).

The reduced plausibility of high-end emissions scenarios has been widely recognized in recent years. The recent IPCC AR6 WG3 report (Riahi et al., 2022) noted that “high-end scenarios have become considerably less likely since AR5 but cannot be ruled out.” They also clarified that these do not represent current policy scenarios, but rather a world that actively reverses past progress, pointing out that “RCP8.5 and SSP5–8.5 do not represent a typical ‘business-as-usual’ projection but are only useful as high end, high-risk scenarios.”

The reassessment of probable emissions outcomes in recent years has sparked a debate about the extent to which this was driven by climate policy and technological development. There is a tendency to assume in hindsight that past high emissions scenarios were clearly unrealistic at the time. However, there was a commonly held view in the late 2000s and early 2010s that the world was heading to around 4C warming by 2100 under current policy scenarios (Sokolov et al., 2009). Research at the time criticized assumptions of “spontaneous” decarbonization in baseline emissions scenarios as “optimistic at best and unachievable at worst” (Pielke et al., 2008).

At the same time, it is clear that the highest end of emissions scenarios found in the literature (e.g., RCP8.5 and SSP5–8.5) were misinterpreted by much of the community as “business-as-usual” when they were never intended to reflect the median no-policy baseline scenario (Hausfather and Peters, 2020). For example, RCP8.5 was designed to reflect the 90th percentile of baseline scenarios in the literature (van Vuuren et al., 2011), with outcomes consistent with RCP6.0 deemed approximately equally likely in the absence of climate policy interventions. The median baseline scenario at the time resulted in closer to 7 W/m² forcing (and ∼4C median warming) rather than the ∼4.5C warming found in RCP8.5 and 4.7C warming in SSP5–8.5. Prior to the 2015 Paris Agreement, more modest baseline warming estimates were published by both the IEA (3.5C) and Climate Action Tracker (3.6C) (CAT, 2023; IEA, 2023).

Ultimately, the degree to which the improvement in probable 21st century emissions outcomes was due to progress in driving down the costs of clean energy and climate policy interventions vs. implausible assumptions of high future emissions is to a large degree unknowable given its dependence on counterfactual assumptions. It is hard to rule out the possibility that the 21st century could have ended up dominated by coal—as seemed much more plausible from the vantage point of the mid-2000s—even if it is clearly quite unlikely today.