What if we don’t need “the” solution to human-caused climate change, but a smorgasbord of solutions? That’s the idea favored by Nathan Johnson, a postdoctoral researcher at Imperial College London and lead author of a paper published on March 5 in the journal Science.
Building on landmark work from two decades ago, the new paper divvies up the gargantuan challenge of reducing climate-warming greenhouse gas emissions by splitting the task into a less individually daunting set of potential approaches. These are called “wedges” – manageable slices of a much bigger carbon-reduction pie.
As the paper explains: “People can build personalized decarbonization pathways by choosing a portfolio of these strategies.”
The authors provide 36 wedge strategies that together can be mixed and matched into more than 6 trillion combinations able to limit global warming to 1.5°C, the goal the world set in the 2015 Paris Agreement. Still more wedges could be devised to meet the tastes of policymakers and the public. In other words, to use a British phrase, we’re spoilt for choice.
Each of the wedges crafted by Johnson and coauthor Iain Staffell, an associate professor at Imperial College London, represents about 4% of current global emissions. When scaled linearly out to 2050, each wedge would mitigate – or avoid – 2 billion metric tons of emissions per year by 2050, including carbon dioxide and other major greenhouse gases.
How many wedges would be needed to meet various goals?
- Current policies already adopted by the world’s governments (such as through the Paris Agreement) equate to about 17 wedges’ worth, compared to doing nothing. That would roughly stabilize emissions through midcentury. However, simply stabilizing emissions could allow global temperatures to rise at least 2.5°C above preindustrial levels.
- If the aim is to stay within the much-discussed 1.5°C threshold (a long-term benchmark we’re already close to exceeding), then another 20 wedges would need to be implemented.
- Upping this to 25 wedges would be enough to bring about a net-zero-emission world by 2050.
As Johnson and Staffell see it, their updated wedge framework is a choose-your-own-adventure mode of addressing climate change. They believe it’s well-suited to the present moment, despite the wholesale retrenchment in U.S. climate policy now unfolding. In light of this titanic withdrawal, and the fact that domestic U.S. emissions of carbon dioxide have dropped to around 13% of the global total, an increasing amount of the heavy lifting – and leading – may fall to other nations.
Read: After a major blow to U.S. climate regulations, what comes next?
“The framework doesn’t prescribe who should lead or which options to pick,” Johnson said in an email. “It simply shows what the world must deliver, collectively, to limit warming to a given level.”
Johnson and Staffell included 36 strategies for wedge building from across the spectrum of climate-friendly behaviors, from energy efficiency and cleaner fuels to regenerative land management and CO2 capture and storage. Some of the options go well beyond those included in integrated assessment models, which have long been favored in climate policy circles. While those models are powerful tools, Johnson said, they’re not designed to incorporate consumer-driven approaches such as changing diet or travel habits. Moreover, their black-box design can be intimidating to laypeople.
“The value of the updated wedges,” Johnson said, “is that they put well-known and less-familiar options, lower- and higher-risk options, and technological, nature-based and behavioral options on the same footing.”
More wedges, more choices
The new study’s roots go back to an influential 2004 analysis also published in Science. In that paper, Princeton University biologist Stephen Pacala and physicist Robert Socolow unveiled their stabilization-wedge paradigm. Pacala and Socolow showed that seven out of 15 proposed “wedges” – each about three times larger than the ones newly put forth by Johnson and Staffell – would be enough to bring global CO2 emissions down from a business-as-usual track to a stabilized path. In other words, if all seven wedges were fully realized, then the annual emissions by 2054 would be unchanged from 2004. And those wedges wouldn’t have to be pulled from pie in the sky: Each one would employ technology that was already available.
The 2004 paper made an immediate splash, finding widespread use in both education and policy circles.
“It was regarded as countering a lot of negativity,” Socolow said in an interview. He added that basing the original wedges on already-existing technology was an intentional choice – largely in response to officials at the time who bemoaned what they saw as a lack of workable solutions.
The choice of a 50-year window was another conscious framing. As Pacala and Socolow noted in their paper, “It is the length of a career, the lifetime of a power plant, and an interval for which the technology is close enough to envision.”
Back in the mid-2000s, when the first Pacala-Socolow paper came out, global CO2 emissions were jumping by 3 to 5% per year. At that point, the idea of merely stabilizing emissions looked hugely ambitious. The world has since managed to pull back slightly on those headlong business-as-usual growth rates. In fact, from 2013 to 2016, CO2 emissions held virtually steady, with global economic output still managing to climb. However, in the wake of a pandemic-induced dip, emissions have been rising by closer to 1-2% per year. That’s still a lot of emissions growth, especially since the percentage is drawn from a larger base than 20 years ago.
Another big change since the mid-2000s is increased concern about the profound risks posed if carbon emissions aren’t cut quickly enough. That growing awareness led to eventual criticism of the original wedges, as summarized by Socolow: “You guys are not ambitious enough. Flat emissions for 50 years is immoral.” As Socolow explained, a downward arrow shown in their original wedge diagram from mid-century onward was meant to convey that action does need to move beyond merely stabilizing emissions – but as they saw it, stabilization was a hefty enough challenge to deal with at that point.
“An overly difficult goal can generate measures to reach that goal that are dangerous themselves,” Socolow said. “The cure can be worse than the disease.”
Like the original wedges, Johnson and Staffell’s updated framework doesn’t dictate any particular policy. However, it does allow for charting paths well beyond stabilizing emissions. A user could piece together strategies to move the world, wedge by wedge, toward stabilizing global temperatures around 1.5°C and even reaching net-zero emissions. An online game-style tool developed by the authors that debuted with the paper this week allows users to do just that.
Elegant simplicity
Johnson first encountered the idea of stabilization wedges while working on his master’s degree at Imperial College in the late 2010s. In a course that Johnson’s coauthor Staffell was teaching, the students were asked to come up with their own decarbonization pathways as a class exercise.
“I was immediately drawn to the elegant simplicity and the focus on putting people at the heart of the decisions we make around climate change mitigation,” Johnson said.
In his multi-pronged doctoral thesis, Johnson reviewed progress on the original Pacala-Socolow wedges and crafted a forward-looking update. The review paper appeared in Environmental Research Letters in 2021; the updated wedge framework is the focus of the new Science paper.
At times, the can-do spirit of the wedge concept has run up against the let’s-do-it-later mindset of modern industrial society. When Socolow reevaluated the wedge concept in 2011, he found that the world now needed nine rather than seven of the original 2004 wedges to stabilize emissions. And in a 2013 paper called “Rethinking wedges,” a group led by Steven Davis at the University of California, Irvine, found that 19 of those original wedges would be needed to achieve nearly-net-zero emissions by 2050.
“No matter the number required, wedges can still simplify and quantify the challenge,” Davis and colleagues concluded. “But the problem was never easy.”
In their 2021 look-back at the original wedges, Johnson and Imperial College London coauthors Robert Gross and Staffell found less-than-encouraging progress. By their reckoning, the world was on track to achieve only about one to two of the original 2004 wedges by midcentury, instead of the seven needed to stabilize emissions.
More-efficient personal transport is one bright spot that’s emerged, thanks to improved gas mileage, reduced driving, and the boom in electric vehicles. Wind and solar power are also surging even more than expected. Offsetting these big gains for carbon reduction are a decline in nuclear power, a surge in deforestation (most notably in the 2010s), and a growth in emissions from buildings, especially in faster-growing developing countries.
Here’s how several areas of active and potential progress fit into the updated wedge framework:
- One wedge of solar power requires 70 gigawatts of capacity to be built each year through 2050. For a wedge of wind power, it’s 30 gigawatts. Yet in 2024 alone, the world built 600 gigawatts of solar and 120 gigawatts of wind power. If sustained, this pace would put the world on course for nine wedges of solar power and four wedges of wind power by mid-century.
- In contrast, clean hydrogen production and carbon capture and storage would have to increase by more than a hundredfold over 2024 levels to achieve just one wedge each.
- Reducing consumption could make a real difference, but it would also require real effort. One wedge each could be managed if flying were to decrease by 70% (roughly the same global reduction as seen during the COVID pandemic), or if average meat consumption dropped by 30% (akin to a typical person skipping a quarter-pound burger every two or three days).
- Deforestation would need to drop by a sustained 40%. That may sound ambitious, but Johnson notes it’s roughly the same percentage achieved in Brazil in the first six months after President Luiz Inácio Lula da Silva took office in 2023.
The whole pie at once, or slice by slice?
More than two decades after his landmark paper with Stephen Pacala, Rob Socolow is still motivated by the drive to keep climate change action accessible. He finds himself distressed by “the tremendous amount of polarization” that’s emerged. In a 2020 essay for Daedalus called “Witnessing for the Middle to Depolarize the Climate Change Conversation,” Socolow invoked one way to view wedges: “The conversation about solutions is actually many parallel conversations, each focused on a single important pathway.”
Though Socolow has a few methodological differences with Johnson – for example, he still prefers the 50-year versus 30-year time window – the two had productive dialogue as the new framework was developed.
“I love that the wedges are useful in getting conversations going,” Socolow said. He said he views the effort to accomplish each wedge as a campaign in itself. Yet he added: “How fast can you create a global campaign? We’re learning how hard it is – harder than we might have imagined.”
Some of those who scoff at incrementalist policy might find the wedge approach unsatisfying. After all, a wedge is by definition an increment.
“Big transitions don’t happen overnight,” Johnson countered. “You can’t jump from capturing a few million metric tons of CO2 to billions, or installing a heat pump in every home, in a single year. Technologies, supply chains, and public adoption take time to scale.”
As Johnson sees it, the wedge approach is flexible enough to bring in the transformative changes sought by harder-green activists.
“The wedges help keep realism while still allowing for more radical pathways,” he said. “Choosing more wedges means a steeper emissions decline and faster, deeper changes across more parts of the economy.”
For example, wedges could be devised to represent actions that pull carbon out of the atmosphere and into oceans, or reductions in the amount of methane that livestock emit. Even a concerted effort to reduce population growth by strengthening women’s access to education and health care could be translated into one or more wedges.
“Many of the strategies we identify require individual behavior change, and all require general public support,” Johnson and colleagues conclude. “Fostering informed opinion from an empowered public relies on people possessing a firm grounding in the options for decarbonization. Our framework can be understood broadly and used to quickly construct and debate pathways for mitigating climate change.”
Jeff Masters contributed to this post.
Republish our articles for free, online or in print, under a Creative Commons license.
