Raphael ThysIt’s the world’s most important climate goal: limiting the Earth’s warming to 1.5 degrees Celsius (2.7 Fahrenheit). It’s the aspiration of global agreements, and to inhabitants of some small island nations, the marker of whether their homes will continue to exist.
Keeping warming this low will help save the world’s coral reefs, preserve the Arctic’s protective sea ice layer and could avoid further destabilizing Antarctica and Greenland, staving off dramatic sea level rise.
But with the world having already warmed by more than 1.1 degrees Celsius (2 degrees Fahrenheit) above preindustrial temperatures, achieving the goal is in grave doubt.
To see what hope remains, The Washington Post examined over 1,200 different scenarios for climate change over the coming century, based on the models produced by the world’s leading climate scientists and considered in a key 2022 report of the U.N. Intergovernmental Panel on Climate Change.
Working with experts from the Potsdam Institute for Climate Impact Research in Germany, we explored the central features of these scenarios — how fast the world embraces clean energy, how quickly we can remove greenhouse gases from the atmosphere — and looked at how these in turn affect the planet’s temperature over the course of the century.
The results, as you will see, show a world that keeps inching closer to catastrophic climate change. But they also point a way toward a less hot future. The scenarios help show us what needs to be done — and what we can still do.
Each of these thin strands is based on sophisticated computer simulations that calculate how fast the economy and the population will grow, how quickly climate technology will advance, how rapidly we will cut our use of fossil fuels and then on top of that, the temperatures expected to result.
But when we look at those scenarios that have the temperature closing out the century below 1.5C, there is a big problem. With their dramatic plunges in greenhouse gas emissions levels by 2025 — just three years away — some of the scenarios, which were finalized in 2021 at the latest, increasingly conflict with reality. After all, the world just saw emissions rise in 2022.
So we first filtered these out, based on a formula provided by the Potsdam Institute researchers. The researchers also added a few other rules to prevent very unlikely near-term developments, like massive deployments of nuclear power by 2030.
There are two different kinds of scenarios that leave the planet, in the year 2100, below 1.5C of warming. One involves a “high overshoot,” but spending decades above 1.5C in such a world is an unsettling prospect. It raises the possibility, for instance, of the world experiencing dangerous tipping points and even calamities such as the irreversible loss of the West Antarctic ice sheet.
So it is worth focusing on those 26 scenarios that allow for only a “low” overshoot (or none at all). Many of these scenarios require the world, by mid-century, to go well beyond the popular “net zero” goal for fossil fuel emissions. Rather, the world will have to be removing more carbon dioxide from the atmosphere than it is putting in — “net negative.” And that will require the wide-scale deployment of nascent “carbon capture” technologies to remove what is already present, storing it underground, and likely also massive reforestation or other efforts to store carbon in the land itself.
To assess how realistic these scenarios are, The Post used a method developed by the Potsdam Institute researchers and their international colleagues to filter scenarios based on the kinds of future developments they anticipate and the speed of progress they show happening.
On five criteria, these scenarios are rated as “speculative,” “challenging” or “reasonable,” in order of increasing plausibility. In judging where to draw the lines, the scientists reviewed existing studies on topics such as energy transitions and carbon removal technologies, and applied their expert judgment to define the thresholds. They also had to remove a small number of scenarios that did not have enough information to be analyzed by these methods (about 10 percent of the total).
As we begin to apply these filters, we see how the paths to meeting 1.5C narrow. Let’s consider “reasonable” expectations for the world’s ability to remove carbon dioxide from the air and store it underground — which in many ways is the biggest variable affecting whether the world can still hold warming below 1.5C without a major overshoot. Then we’ll require scenarios to be “reasonable” on the four other dimensions too.
What the results suggest is that the world has probably run out of easy options to stay under 1.5C — or have a low overshoot.
So that leaves two sets of other scenarios to consider, if the world wants to end the century below 1.5C somehow.
First, if we still want to stay on a “low overshoot” path, we can consider what the Potsdam Institute researchers consider “challenging” scenarios, rather than “reasonable” ones. These scenarios assume the world will make even speedier progress on clean energy and carbon removal from the atmosphere.
What makes these scenarios work? One common theme is much more dramatic carbon removal from the atmosphere, storing it either underground or in forests and agricultural landscapes. The majority of these scenarios require us to be able to subtract over 7 billion tons per year from the atmosphere by 2050. This will require a huge scale up of interventions like carbon capture and storage, which only has an estimated capacity of about 43 million tons per year today. Capacity has roughly doubled in the past decade, but a far faster pace of change would be needed to achieve this outcome.
Some scenarios also require dramatic transformations of energy use thanks to a combination of renewables and vastly expanded energy efficiency. Many require the carbon intensity of energy use — how much CO2 is emitted per unit of energy consumed — to decrease by over 80 percent by 2050. This would require total or a near total phaseout of fossil fuels, widespread electrification of the world’s energy systems and major fuel shifts in transportation to electric vehicles or the use of other fuels such as hydrogen or biofuels, among other innovations.
Lila Warszawski, lead author of the Potsdam Institute study, and her colleagues argue that there is no one technology or solution that can be relied on to save the 1.5C goal.
“It’s hard, and you can’t put all your eggs in one basket,” Warszawski said. “It’s hard but, you still want to go for it.”
Theother choiceisto accept a fallback world in which the temperature significantly overshoots 1.5C during the century. With temperatures expected to be between 1.6 and 1.8C above preindustrial levels through the 2040s, 2050s and 2060s in most of these scenarios, that would raise the odds of unexpected climate catastrophes. But 16 scenarios allow for such a “high overshoot” and are considered “reasonable” by the Potsdam Institute experts on the five dimensions above.
How do these scenarios work? They still use carbon dioxide removal and storage, but at lower amounts — a few billion tons per year in coming decades from all sources. That’s out to the year 2050 — but to drive temperatures back down again in the second half of the century, the need for these interventions can be extremely large.
“Post 2050, these scenarios are really challenging because they require a very large amount of carbon dioxide removal to return temperature to below 1.5 degrees after a high overshoot,” said Elmar Kriegler, also a researcher at the Potsdam Institute and a leader of the analysis.
All these scenarios require a major takeover by renewable energy, such that the carbon intensity of powering our lives goes down by 64 to nearly 74 percent by 2050. But that is not as dramatic as the “challenging” scenarios that accomplish a low overshoot. The world’s demand for energy overall still grows in the “high overshoot” scenarios, but by a wide range, between 2 and 18 percent.
Not everybody will agree with these models — or, the cutoffs imposed by the Potsdam Institute researchers. Some experts are more optimistic about technology and humanity’s ability to innovate. Others point out that it is easy to imagine countries failing to achieve what is necessary to stay below 2C at all.
In the end, these are simply well-informed models of how the world will work. What’s more, we still have a limited understanding of how the climate system will respond to emissions.
At the U.N. Climate Change Conference late last month, world leaders reaffirmed the 1.5C goal. But these scenarios show that without dramatic action — action the leaders did not commit to taking — it most likely will not be possible.
Or at least, not without a major overshoot first. That is where the world is currently heading.
The initial scenarios considered here are the 1,202 emissions pathways for which warming levels could also be evaluated, considered by the IPCC in 2022 in its Sixth Assessment Report, Working Group III. The methods used to filter scenarios based on the five dimensions described above were published by Warszawski, Kriegler, and an international group of colleagues in Environmental Research Letters in 2021. For this article, the Potsdam Institute for Climate Impact Research scientists applied the same methodology to the scenarios considered by the IPCC in 2022.
Because some scenarios did not contain enough information to be considered, scientists were able to analyze 207 out of 230 scenarios, or 90 percent, that end the century below 1.5 degrees Celsius. This includes 87 out of 97 scenarios that limit warming to 1.5 degrees Celsius with limited or no overshoot. These are the scenarios considered in the charts above.
The researchers also imposed an additional screen, requiring that the scenarios not plunge too quickly below 2022 emissions levels by 2025, with 2022 emissions defined as the most recent estimate of fossil CO2 and cement emissions made by the Global Carbon Project. Scenarios were removed if they implied an emissions plunge of over 19 percent below 2022 levels in just 3 years (to less than ~29.65 Gt CO2). The scientists also applied some additional screening from the IPCC report to control for other questionable developments in the near-term future, such as not allowing large amounts of new nuclear power or carbon capture and storage.
The emissions and energy data that were used to calculate the five tests were interpolated to annual resolution. In most cases, the data in the database are provided in five-year intervals, extending to 10-year intervals after 2050.
Temperature curves shown in the charts above are the result of a climate model that takes the emissions results from Integrated Assessment Models as inputs, and calculates the probable temperatures that will result. The curves depict the median estimate of the resulting warming, which means that there is a 50 percent chance that the actual warming that the planet sees will be lower (or higher) than that value. Scenarios qualify as 1.5C scenarios (with no or a limited overshoot, or returning to 1.5C after a high overshoot) in the IPCC’s categorization if they have a greater than 50 percent chance of limiting warming to below 1.5 degrees Celsius by the end of the century.
The original scenario data are provided by the International Institute for Applied Systems Analysis’s AR6 Scenario Explorer and Database. The full citation is:
Edward Byers, Volker Krey, Elmar Kriegler, Keywan Riahi, Roberto Schaeffer, Jarmo Kikstra, Robin Lamboll, Zebedee Nicholls, Marit Sanstad, Chris Smith, Kaj-Ivar van der Wijst, Alaa Al Khourdajie, Franck Lecocq, Joana Portugal-Pereira, Yamina Saheb, Anders Strømann, Harald Winkler, Cornelia Auer, Elina Brutschin, Matthew Gidden, Philip Hackstock, Mathijs Harmsen, Daniel Huppmann, Peter Kolp, Claire Lepault, Jared Lewis, Giacomo Marangoni, Eduardo Müller-Casseres, Ragnhild Skeie, Michaela Werning, Katherine Calvin, Piers Forster, Celine Guivarch, Tomoko Hasegawa, Malte Meinshausen, Glen Peters, Joeri Rogelj, Bjorn Samset, Julia Steinberger, Massimo Tavoni, Detlef van Vuuren. AR6 Scenarios Database hosted by IIASA, International Institute for Applied Systems Analysis, 2022.