Arguments
Moving away from high-end emissions scenarios
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.
The article states,
"... growth in CO2 emissions slowed notably over the past decade ..."
When I plot atmospheric CO2 concentrations from 1970 to 2005 using 10-year moving averages, it shows an upward accelerating curve. When I plot the 10-year averages for 2010, 2015, and 2020 (which includes the data for 2025) the data points sit above the extrapolation of the 1970-2005 curve.
If "... growth in CO2 emissions slowed notably over the past decade ...", why is the Keeling curve currently accelerating upwards as fast or faster than it has since the 1970's?
Evan,
Because declared CO2 emissions are not the same as CO2 released into the atmosphere.
For example:
www.chathamhouse.org/2021/10/greenhouse-gas-emissions-burning-us-sourced-woody-biomass-eu-and-uk/annex-emissions-wood
and CO2 comes from other sources.
For example:
www.jpl.nasa.gov/news/new-nasa-study-tallies-carbon-emissions-from-massive-canadian-fires/
RickyO, thanks for your comments.
I am well aware of what you are saying, and my question was meant to prompt this kind of thinking in the reader. As you well know, the Keeling curver represents the net difference between all sources and sinks, both natural and anthropogenic.
The subtle suggestion i was attempting to make with my comment is to encourage the reader to follow the evolution of the Keeling Curve, because in the end, it is atmospheric GHG concentrations that will govern future warming, and not our documented GHG emissions budgets. I know the two are related, but the Keeling Curve represents reality, whereas emissions budgets, both measured and projected, represent only part of what controls future warming.
I apologize for saying that I included 2025 data in my curve. I meant to say 2024 data.
Evans graph shows atmospheric concentrations of CO2 continuing to increase and possibly even accelerating further , despite a reduction in the rate of emissions growth over the past ten years. According to Carbon brief: "Total global CO2 emissions have notably plateaued in the past decade (2015-24), growing at only 0.2% per year compared to the 1.9% rate of growth over the previous decade (2005-214) and the longer-term average growth rate of 1.7% between 1959 and 2014." This all seems perplexing.
I think there would be several possibilities why this is happening.
1). The possibility that reduction in the growth rate of emissions omits some sources of emissions as Ricky O points out such as burning wood pellets and the canadian wildfires. Also as the oceans warm and the permafrost melts, they release CO2 to the atmosphere. This process may be accelerating and thus negating the reduction in the growth rate of emissions, until emissions growth stops and emissions fall sufficiently.
2)The reduction in the growth rate of emissions is still within the margin of measuring error for growth in atmospheric concentrations so it hasnt shown up yet
3). There is no reduction in the growth rate of emissions because countries are not accurately reporting their emissions. However there are obvious ways of international agencies objectively assessing this such as data on the construction of renewable energy. I find it hard to believe that there is no reduction in emissions growth, although it is probably exaggerated to some extent.
Thanks.
Taking into account all of the above, it's plausible that we're burning through the remaining carbon budget faster than the calculations (budget minus emissions) show.
Let's assume that the emissions data are not far off. From a simple mass balance perspective, there are several possible explanations for atmospheric CO2 concentrations not following the same trend.
None of those seem like a Good Thing™.
Bob@6
Thanks for your comments. I agree with your comments and those of Nigel as well.
Natural CO2 emissions are about 25 times higher than anthropogenic emissions. As you note, half of our tiny emissions are typically reabsorbed by land and oceans. Therefore, it would not take much of an increase in the natural emissions, nor much of a decrease in the natural absorption rate, to cause an increase in the rate of CO2 accumulation in the atmosphere.
This is why I encourage people to follow the evolution of the Keeling Curve, rather than following projected emissions budgets. The Keeling Curve includes the net effect of all emissions (natural + human) minus all sinks (natural + human) to give us an accurate reading of how we're doing. Whether or not it is a pretty picture, it is an accurate picture.
Did I miss something Bob? To what are you referrring with the trade marked "Good Thing"?
Evan: the " Good Thing™" expression (capitalized, with the trademark symbol) is a measure of sarcasm, to ward off the usual "but it's all good for us" myth.
Bob,
I tend to keep an eye on the Global Trend in CO2, rather than stories of emissions cuts:
gml.noaa.gov/ccgg/trends/gl_trend.html
The el nino boost in the trend is clear to see in 2016 and 2024. Doesn't seem to be fading as quickly in this time around, though. Worth watching?
RickyO@9,
The CO2 graphs at the link you provide provide a possible explanation to the apparent acceleration of atmospheric CO2 concentration suggested by my plot of the data. The NOAA graph of global increase by decade (see below) shows that in the 1990's, the rate of increase decreased a small amount from that in the 1980's. Further, the increase from the 1960's to the 1970's was about 0.5 ppm, almost the same as what it was from the 2000's to the 2010's. Therefore, perhaps it is the decrease in the 1990's that affects my plot and suggests an apparent increase in the rate thereafter. Although I've seen this NOAA data before, thanks for bringing it to our attention again.
Lan, X., Tans, P. and K.W. Thoning: Trends in globally-averaged CO2 determined from NOAA Global Monitoring Laboratory measurements. Version Monday, 06-Jan-2025 10:06:16 MST https://doi.org/10.15138/9N0H-ZH07
RickyO:
El Nino/La Nina cycles are well-known to cause variations in both temperatures and natural CO2 uptake/emission.
So, yes, this needs to be considered in looking at trends (especially short-term).
It is worth noting that El Nino/La Nina cycles and their effect on atmospheric CO2 cycles is a "feature" of one of the common contrarian arguments that the overall rise in CO2 is natural (ie., not due to burning fossil fuels). Unfortunately, it is a "feature" that the people making that argument do not realize. Their method of analysis mistakes those short-term cyclical effects for a long-term trend, and they mistakenly think that the long-term upward trend in temperature is caused by the CO2 trend (rather than the correct interpretation that the CO2 trend is causing the temperature trend).
You can read more about that myth on this SkS page.
Evan @10
Your explanation for the apparent acceleration in the acceleration of CO2 levels recently looks right. Looks like its not a real acceleration.
I was curious why there was a flattening in the growth rate of atmospheric CO2 growth around 1980 - 2000. This coincides with a slowing in the rate of CO2 emissions growth over the same period, and a global flattening off in oil production from 1980 - 2000 approx. ( See links below). I recall this was the time period when smaller cars became popular so presumably the net result from flattening oil production was a slower rate of emissions growth.
Im not sure why oil production slowed over that 1980 - 2000 period, but it followed the OPEC oil crisis of the late 1970s which caused a temporary drop in oil production, and one source talked about a decline in output from the big existing oil fields in Saudi Arabia. But after the 2000s global oil production was back to business as usual, presumably as the OPEC oil embargo had ended, and new oil field discoveries were made, and there was Americas oil fracking boom.
www.statista.com/statistics/276629/global-co2-emissions/
ourworldindata.org/grapher/oil-production-by-country
Nigelj@11
I also recall that during the 1990's the UK was making a large-scale shift from using coal to using natural gas in their "Dash for Gas". Also, France made a large-scale shift towards nuclear power in the 1980's and 1990's. Then Mt. Pinatubo blew in 1991, and from what I understand, the temporary cooling caused a drop in atmospheric CO2 concentrations due a variety of factors, such as increased uptake by cooler oceans. All of this was a temporary bump, and there were likely other factors that you pointed out, which together caused a temporary slowdown in the buildup of atmospheric CO2.
https://en.wikipedia.org/wiki/Dash_for_Gas
https://en.wikipedia.org/wiki/Nuclear_power_in_France
Immediate and long-lasting effects of Mt. Pinatubo eruption
To put some other numbers into this interchange.
Regarding rates of emissions:-
The Global Carbon Project give data for the various fossil fuel types going way back. Their budgets back to1959 also show numbers for Land Use Change emissions (as well as the ocean absorption and the land absorption).
The 1980 wobble in emissions resulted from the 1970s oil crisis driving efficiency measures but the high 1970s oil price led to over-production and what was called the "1980s oil glut" thus ending the wobble. I'm not so sure about talk in that link of a slow-down in economic growth also being a factor as use of gas and coal doesn't seem to have shown any signs of this oil-use wobble and continued apace (as this OurWorldInData graph shows).
Regarding atmospheric levels:-
The Land Use Change emissions are a significant part of global emissions and when added to FF emissions allow the calculation of the Atmospheric Fraction (Af) which is the annual ration (Atmospheric increase)/(Man-made emissions). This has remained pretty constant since the 1960s altough there is no underlying reason for it**.
The land-based absorption provides the lion's share of the wobbles in the Af with El Niño the primary wobble-driver.
(**If an emissions-free world had a single emissions event, the annual absorption in Year 1 would be about 3% and through following non-emissions years the annual absorption would slowly decrease to zero over a millenium. How much atmospheric CO2 then remained would depend on the size of the emission - so roughly 25% remaining if the emission event was 600Gt(C), this the very rough size of our cumulative emissions to date.)