This post is about how much warming we should expect on current policy and assuming emissions stop at 2100. We argue the risk of extreme warming (>6 degrees) conditional on these assumptions now looks much lower than it once did.
Crucially, the point of this post is about the direction of an update, not an absolute assessment of risk -- indeed, the two of us disagree a fair amount on the absolute risk, but strongly agree on the direction and relative magnitude of the update.
The damage of climate change depends on three things:
- How much we emit
- The warming we get, conditional on emissions
- The impact of a given level of warming.
The late and truly great economist Martin Weitzman argued for many years that the catastrophic risk from climate change was greater than commonly recognised. In 2015, Weitzman, along with Gernot Wagner, an economist now at New York University, released Climate Shock, which argued that the chance of more than 6 degrees of warming is worryingly high. Using the International Energy Agency’s estimate of the most likely level of emissions on current policy, and the IPCC’s estimate of climate sensitivity, Wagner and Weitzman estimated that the chance of more than 6 degrees is 11%, on current policy.
In recent years, the chance of more than 6 degrees of warming on current policy has fallen quite substantially for two reasons:
- Emissions now look likely to be lower
- The right tails of climate sensitivity have become thinner
1. Good news on emissions
For a long time the climate policy and impacts community was focused on one possible ‘business as usual’ emissions scenario known as Representative Concentration Pathway 8.5 (RCP8.5), a worst case against which climate action would be compared. Each representative concentration pathway can be paired with a socioeconomic story of how the world will develop in key areas such as population, income, inequality and education. These are known as ‘shared socioeconomic pathways’ (SSPs).
The latest IPCC report outlines five shared socioeconomic pathways. The only one that is compatible with RCP8.5 is a high economic growth fossil fuel-powered future called Shared Socioeconomic Pathway 5 (SSP5). In combination, SSP5 and RCP8.5 is called ‘SSP5-8.5’. On SSP5-8.5, we would emit a further 2.2 trillion tonnes of carbon by 2100, on top of the 0.65 trillion tonnes we have emitted so far. For reference, we currently put about 10 billion tonnes of carbon into the atmosphere from fossil fuel burning and industry. The other emissions pathways are shown below:
However, for a variety of reasons, SSP5-RCP8.5 now looks increasingly unlikely as a ‘business as usual’ emissions pathway. There are several reasons for this. Firstly, the costs of renewables and batteries have declined extremely quickly. Historically, models have been too pessimistic on cost declines for solar, wind and batteries: out of nearly 3,000 Integrated Assessment Models, none projected that solar investment costs (different to the levelised costs shown below) would decline by more than 6% per year between 2010 and 2020. In fact, they declined by 15% per year.
This means that renewables will play an increasing role in energy supply in the future. In part for this reason, energy systems models now suggest that high fossil fuel futures are much less likely. For example, the chart below shows emissions on current policies and pledged policies, according to the International Energy Agency.
The chart above from Hausfather and Peters (2020) relies on IEA models of future energy systems. These may still be too pessimistic on renewables. You can find the IEA’s cost assumptions here. They show the levelised cost of solar falling by 40% between now and 2030. But if historical trends continue, we should actually expect costs to decline by 89%. Trends may not continue, perhaps because we may be reaching saturation for renewables capacity additions, which drive cost declines. But there seems a decent chance that the cost declines will continue.
Fundamentally, existing mainstream economic models of climate change consistently fail to model exponential cost declines, as shown on the chart below. The left pane below shows historical declines in solar costs compared to Integrated Assessment Model projections of costs. The pane on the right shows the cost of solar compared to Integrated Assessment Model assessments of ‘floor costs’ for solar - the lowest that solar could go. Real world solar prices have consistently smashed through these supposed floors.
Secondly, SSP5-RCP8.5 assumes an enormous expansion in the use of coal, which looks very unlikely in part due to the decline in the cost of renewables and the abundance of natural gas driven by hydraulic fracturing, and in part because countries tend to transition away from coal as they get richer. For example, here is the increase in per capita coal use on different emissions scenarios:
For comparison, China burned what was widely seen to be a prodigious amount of coal from 2000 onwards, but that is dwarfed by the increase in coal use projected on SSP5-8.5. Indeed, total fossil fuel use on SSP5-RCP8.5 by 2100 exceeds some estimates of total recoverable fossil fuel resources. The International Energy Agency claims that coal use peaked in 2014 and is now in structural decline.
Thirdly, in order for us to follow SSP5-RCP8.5, there would have to be very fast economic growth and technological progress, but meagre progress on low carbon technologies. This does not seem very plausible. In order to reproduce SSP5-8.5 with newer models, the models had to assume that average global income per person will rise to $140,000 by 2100 and also that we would burn large amounts of coal.
It is difficult to imagine that in such a cornucopia, there would not also be a lot of progress on low carbon technology, and that countries would have greatly increasing willingness to pay to protect the environment.
Fourthly, climate policy has strengthened substantially over the last few years. Countries representing 66% of global CO2 emissions have committed to achieving net-zero emissions by 2050. Most importantly, China has pledged to get to net zero by 2060. Some of these commitments, such as that of the UK, are enshrined in law. Even if those targets are missed by a decade or more, this is a sharply different trajectory than RCP8.5.
For all these reasons, RCP8.5 now looks much less likely. Even the Integrated Assessment Models that were used to generate different versions of RCP8.5, known as MESSAGE and REMIND, now suggest that a moderate emissions trajectory is the most likely outcome, on current policy. Probabilistic assessments of emissions scenarios are thin on the ground, but recent studies suggest that the chance of RCP8.5 is now well below 5%. See for example, this from Liu and Raftery (2021):
Various other recent studies suggest that on current policy, we are most likely to follow something in the range of RCP2.6 to RCP6, with RCP4.5 now the most likely scenario. Moreover, this is all assuming that current policy stays as it is. But it now looks like climate policy is going to strengthen in the future.
If you would like to read more on emissions scenarios, we would recommend the following papers:
- Hausfather and Peters, ‘Emissions – the ‘business as usual’ story is misleading’, Nature, 2020.
- Hausfather, ‘Flattening the Curve of Future Emissions’ Breakthrough Institute, August 2021.
- Ritchie and Dowlatabadi, ‘The 1000 GtC coal question: Are cases of vastly expanded future coal combustion still plausible?’ Energy Economics, 2017.
- Ritchie and Dowlatabadi, ‘Why do climate change scenarios return to coal?’ Energy, 2017.
2. Good news on climate sensitivity
The main focus of climate science has been to quantify what is known as equilibrium climate sensitivity, which is the warming we get after a doubling of CO2 concentrations, ignoring the effect that the melting of the ice sheets (which occurs over millennia) might have on temperatures. The new IPCC report finds that the uncertainty around equilibrium climate sensitivity has now narrowed, as shown by this chart from Carbon Brief.
In the 2013-14 IPCC report, the upper 5% bound on equilibrium climate sensitivity was 6 degrees, now it is 5 degrees.
In spite of its prominence, equilibrium climate sensitivity is not actually a very useful metric. It measures the warming we get on the assumption that CO2 concentrations reach a certain level and then stay there indefinitely. But, once emissions stop, CO2 concentrations would actually decline due to natural uptake of CO2, initially by the oceans. Atmospheric CO2 would look a bit like this after a given level of emissions:
The red line is the result we get after the release of a pulse of 5 trillion tonnes of carbon and the blue line is what happens after the release of 1 trillion tonnes of carbon. In both cases, CO2 concentrations peak and then slowly decline, returning to their natural state after hundreds of thousands of years.
CO2 concentrations would only stay at a certain level indefinitely if there were a very precise low level of CO2 emissions sustained over many centuries to precisely compensate for ocean CO2 uptake. This is unlikely to happen in the real world.
For this reason, a more useful metric is the Transient Climate Response to Cumulative Emissions, which measures the warming we get for a given amount of cumulative emissions. As it turns out, there is a near-linear relationship between cumulative emissions and global warming, as the chart below shows:
Once emissions stop, temperature would stay roughly constant for 100 years before slowly declining over hundreds of thousands of years.
In any case, these climate sensitivity metrics are related and the narrowing of uncertainty about the equilibrium climate sensitivity has fed through into the transient climate response to cumulative emissions. In the 2013-14 IPCC report, the 66% confidence range for the transient climate response to cumulative emissions was 0.8°C to 2.5°C per trillion tonnes of carbon. In the latest IPCC report, this has narrowed to 1.0°C to 2.3°C per trillion tonnes of carbon. The IPCC does not give a 5% to 95% range for the transient climate response to cumulative emissions, but we think given the update on equilibrium climate sensitivity, we should expect the narrowing of the 5% to 95% range to be greater still.
The main reason that uncertainty about climate sensitivity has narrowed is that formal Bayesian methods that incorporate all lines of evidence have recently been used to estimate it. So, this is a victory for the Reverend Bayes and the Marquis de Laplace, not for expensive climate models. This is discussed at length in the outstanding (and long but readable) Sherwood et al (2020) ‘An Assessment of Earth's Climate Sensitivity Using Multiple Lines of Evidence’.
3. What is the risk of extreme warming?
Now, we can bring all of this together. It is not all that easy to estimate how much warming we might get from a given amount of cumulative CO2 emissions. We not only need to predict CO2 emissions from fossil fuel and industry, but also from deforestation and other forms and land use change, and in addition we need to account for non-CO2 greenhouse gases like methane. One thing we can do is to start with projections of which representative emissions pathways we might follow and then use the IPCC’s estimate of how much warming we get on those pathways.
Studies that try to project future emissions suggest that the most likely pathway is now something around RCP4.5 (with a range from RCP2.6 to RCP6). According to the IPCC, this suggests that something like 2.5 to 3 degrees of warming relative to pre-industrial levels is now the most likely outcome, on current policy. (The world has already warmed by around 1 degree since the Industrial Revolution). The upper 95% bound for warming is around 2.4 to 4 degrees
(The ‘very likely range’ is the 5% to 95% range).
So, the chance of more than 6 degrees now seems to be well below 1%, much lower than the 11% estimated by Wagner and Weitzman.
Several conclusions, clarifications and talking points follow from this.
Firstly, on the assumption that the direct or indirect global catastrophic risk (defined as killing >10% of the global population or doing equivalent damage) of climate change depends on warming of more than 6 degrees, the global catastrophic risk from climate change is at least an order of magnitude lower than previously thought. If you think 4 degrees of warming would be a global catastrophic risk, then that risk is also considerably lower than previously thought: where once it was the most likely outcome, the chance is now arguably lower than 5%. None of this is to say that climate change is solved. But we need to acknowledge progress when it occurs and those of us trying to find the best ways to make a difference on the margin should adjust priorities accordingly. This is also not to say that warming of 2-4 degrees would not be bad. We have ample reason to transition to a zero carbon economy from the effects of climate change and also of air pollution.
Secondly, this illustrates the importance of the neglectedness of a problem. Due to environmental activism, governments and the private sector have spent trillions of dollars on climate change over the years. This is now starting to yield fruit. If the same amount of effort went into dealing with pandemics, biorisk would also fall a lot.
Thirdly, recent progress illustrates the value of an innovation-led approach to climate change, which Johannes first introduced to the EA community via John and Hauke Hillebrandt. Solar, wind and batteries have declined dramatically in cost. The lifetime cost of electric cars is set to be lower than petrol or diesel cars within five years. In many places, countries will start to choose these technologies simply because they are cheaper and better, not because they care about the climate. Because most (>85%) emissions in the future will come from outside the West, innovation is a uniquely tractable strategy because it provides leverage on global emissions without coordination.
Fourthly, it is not a foregone conclusion that renewables will take over the entire energy supply. Way et al (2021), projects that renewables, batteries and hydrogen (as the long-term storage option) will take over the global energy supply and save us money, within 20 years if cost declines continue. It is a brilliant paper on cost reductions for modular technologies, but it only examines cost barriers to the scale-up of renewables. But the main barriers to the scale-up of renewables so far have been social and political opposition surrounding land use, value deflation and grid integration. Renewables and the associated transmission infrastructure take-up a lot of land, and so may run into one of the few iron laws of politics: NIMBYs always win.
As this shows, even in renewables-loving Germany, solar capacity additions have stalled for several years despite radically falling prices. Germany built solar when it was expensive and is building less as it has become cheap, showing the severity of value deflation, grid integration and related issues and the relative non-informativeness of PV module prices alone.
One way to reduce the risk that decarbonisation efforts fail is by encouraging a wider range of low carbon energy technologies, such as nuclear fission, nuclear fusion and enhanced geothermal.
Finally, this analysis does not give a full picture of climate risk because it only explores the most likely emissions scenarios conditional on current policy up to 2100. But technological progress might be slower than we expect and there might be backsliding on climate policy perhaps due to tensions between the Great Powers. Emissions might also continue past 2100. It is important to think about the probability of very high emissions over long time periods. That, however, is the subject for another post.
Thanks to Luca Righetti, Matthew Ives of the Oxford Martin School, Zeke Hausfather and Leslie Abrahams of Clean Air Task Force for discussion.
1. Wagner and Weitzman, Climate Shock, Table 3.1.
2. IPCC, Climate Change 2021: The Physical Science Basis, Summary for Policymakers, Assessment Review 6, Figure SPM.7.
3. Global CO2 emissions are around 36 billion tonnes, which equals around 10 billion tonnes of carbon. (3.667 tonnes of CO2 = 1 tonne of carbon). https://ourworldindata.org/co2-emissions
4. “Sound energy investments require reliable forecasts. As illustrated in Figure 2(a), past projections of present renewable energy costs by influential energy-economy models have consistently been much too high. (“Projections” are forecasts conditional on scenarios, so we use the terms interchangeably.) The inset of the figure gives a histogram of 2,905 projections by integrated assessment models, which are perhaps the most widely used type of global energy-economy models19,20,21,22, for the annual rate at which solar PV system investment costs would fall between 2010 and 202019. The mean value of these projected cost reductions was 2.6%, and all were less than 6%. In stark contrast, during this period solar PV costs actually fell by 15% per year. Such models have consistently failed to produce results in line with past trends3,23” Way et al, ‘Empirically grounded technology forecasts and the energy transition’, Oxford Martin School, 2021: p.3.
5. On SSP5-8.5, we would burn a further 2.2 trillion tonnes of carbon. IPCC, Climate Change 2021: The Physical Science Basis, Assessment Review 6, Summary for Policymakers: Figure SPM.7. Mohr et al (2015) estimate that ultimately recoverable fossil fuel resources are less than 1.6 trillion tonnes of carbon. S. H. Mohr et al., ‘Projection of World Fossil Fuels by Country’, Fuel 141 (1 February 2015): Table 2, https://doi.org/10.1016/j.fuel.2014.10.030.
6. “Covid-19 has catalysed a structural fall in global coal demand” IEA, World Energy Outlook 2020
7. Zeke Hausfather, ‘Flattening the Curve of Emissions’, Breakthrough Institute, 2021. https://thebreakthrough.org/issues/energy/flattening-the-curve-of-future-emissions
8. For an overview, see Hausfather, ‘Flattening the Curve of Future Emissions’ Breakthrough Institute, 2021.
9. “In order to calculate an ECS, which is defined here to include all feedback processes except ice sheets, the approach of Rohling et al. (2012) can be used.” IPCC, Climate Change 2021: The Physical Science Basis, Assessment Review 6, Chapter 7 p. 102.
10. It also ignores the feedback effects from melting ice sheets.
11. As mentioned, it is not clear whether this is even technologically possible.
12. “Physically this can be understood by realizing that the ECS is a theoretical quantity representing the warming that would occur only if atmospheric concentrations of greenhouse gases were held constant indefinitely while the climate system was allowed to come into equilibrium. Such a ‘constant radiative forcing’ scenario would require a very precise low level of emission of CO2 sustained over many centuries to precisely compensate for ocean CO2 uptake. This is clearly not a particularly policy-relevant scenario.” Richard Millar et al., ‘The Cumulative Carbon Budget and Its Implications’, Oxford Review of Economic Policy 32, no. 2 (2016): 323–42.
13. “The Zero Emissions Commitment (ZEC) is the change in global mean temperature expected to occur following the cessation of net CO2 emissions and as such is a critical parameter for calculating the remaining carbon budget… Overall, the most likely value of ZEC on multi-decadal timescales is close to zero, consistent with previous model experiments and simple theory.” Andrew H. MacDougall et al., ‘Is There Warming in the Pipeline? A Multi-Model Analysis of the Zero Emissions Commitment from CO2’, Biogeosciences 17, no. 11 (15 June 2020): Figure 3b. https://doi.org/10.5194/bg-17-2987-2020.
14. “The transient climate response to cumulative carbon emission (TCRE) is likely between 0.8°C to 2.5°C per 1000 PgC (high confidence), for cumulative carbon emissions less than about 2000 PgC until the time at which temperatures peak” IPCC AR5
15. “In the literature, units of °C per 1000 PgC are used, and the AR6 reports the TCRE likely range as 1.0°C to 2.3°C per 1000 PgC in the underlying report, with a best estimate of 1.65°C.” IPCC AR6.
16. Hausfather, ‘Flattening the Curve of Future Emissions’ Breakthrough Institute, August 2021; Hausfather ‘Blog: The New IEA Report Shows How We Are Flattening the Curve of Future Emissions’, October 2021; Climate Action Tracker.