By Stephen Battersby for the Proceedings of the National Academy of Sciences.
Broadcast version by Kathryn Carley for Commonwealth News Service, reporting for the Pulitzer Center-Public News Service Collaboration.
As a phrase and as a promise, net zero has been a great success. Hundreds of countries have pledged to reduce their net greenhouse gas emissions to zero by around the middle of this century. So, too, have thousands of regions, cities, and companies. Net zero has become a beacon of hope, guiding us to climate safety.
But look closely, and the beacon becomes a little blurry. Some scientists argue that net zero might lead us to rely too heavily on technologies that capture CO2 from the air. That could bring dangerous delays and unwelcome side effects, and give fossil fuel producers leeway to keep pumping and polluting. And its allure may be obscuring our need to look beyond net zero to a more ambitious goal-a world of net-negative emissions.
Some climate scientists have ideas about how we could refine net zero to make it a more focused and effective target. Others say it should only be one part of a new climate narrative. "We don't think enough about net zero, what it means, and if it's the right goal," says environmental social scientist Holly Jean Buck, of the University at Buffalo in New York.
With the fate of the planet riding on the outcome, it's vital that governments and institutions are not led astray by their climate beacon-so the debate over net zero is more urgent than ever.
The Root of Zero
The idea of net zero is firmly based on climate science. In the 2000s, scientists worked out that if we stop pouring CO2 into the atmosphere, global average temperatures should roughly stabilize. That is because two effects of Earth's oceans happen to cancel out. Today, the atmosphere is kept relatively cool by the oceans. As seawater slowly warms, we lose that cooling effect, so if emissions fall to zero, we might expect the atmosphere to carry on warming for a few decades-a phenomenon known as thermal inertia. But the oceans also keep absorbing CO2, which should roughly balance the thermal inertia and keep temperatures steady.
Net zero took off in 2018, driven by the United Nations report "Global Warming of 1.5 °C." Three years earlier, the Paris Agreement had set out a goal to limit warming to well below 2 °C above pre-industrial levels and pursue efforts to limit it to 1.5 °C. The new report laid out how the world might try to hit the more ambitious end of that goal, based on models that combine climate and economic activity. It concluded that to avoid warming of more than 1.5 °C, we would not only have to cut emissions deeply, but also remove a lot of CO2 from the atmosphere. Such removal could balance any stubborn, ongoing sources of greenhouse gases, known as residual emissions. These might include CO2 from concrete manufacture, for example, or nitrous oxide from fertilizers. So instead of absolute zero emissions, the new goal aimed for net zero, which allows some residuals to be balanced by removal.
This was only possible because technologies that remove CO2 from the air had become feasible. "Targets through the years have tended to reflect the practicality at the time of reducing emissions," says climate ecologist Stephen Pacala at Princeton University in New Jersey. "When you could envision a practical path to zero net emissions without leaving the world in poverty-all of a sudden, humanity jumped on net zero as a target."
It has undoubtedly had a galvanizing effect. "Before this, few companies had climate targets at all," says Sam Fankhauser, a climate economist at the University of Oxford in the UK. "So this is a step in the right direction."
But that shouldn't be the end of the story. "Net zero comes from the science, so it's subject to change as we learn more," says climate economist Sabine Fuss at the Mercator Research Institute on Global Commons and Climate Change in Berlin, who was a lead author on the "Global Warming of 1.5 °C" report. Climate scientists agree that the concept holds several crucial ambiguities that need to be resolved.
Zero Sum
For a start, what is the best balance between cutting emissions and removing CO2? That depends on which emission sources will be too difficult to cut. But when Buck and her colleagues analyzed 50 national long-term climate strategies, they found that countries are inconsistent in how they consider residual emissions. "The risk is that governments put things that are expensive or politically inconvenient to abate into the 'residual box,'" the paper states. That makes it hard to know how much CO2 removal we need.
According to these strategies, the average residual emissions in developed countries will be 18% of current total emissions at the time of net zero. Extended to the whole world, that would imply annual removals of at least 12 billion tonnes of CO2.
Natural solutions, such as planting forests, can't come close to reaching this quantity on their own-and in a warming world, they will be increasingly vulnerable to fire, disease, and chain saws. So the assumption is that we will use a range of novel removal methods: using machines to suck CO2 directly from the atmosphere, for example, or burning biomass to generate energy while capturing and storing the CO2 emitted.
Most of these technologies operate at small scales today, collectively removing only about two million tonnes of CO2 per year. For now, most of them are expensive to operate. Some need a lot more research and development and may yet prove difficult to scale up. That's the first problem with asking too much of carbon removal: It might not have the capacity to meet such high demand, and then we would fail to hit net zero.
The second problem is unwanted side effects. Deployed at large scale, biomass-based CO2 removal could compete for land with agriculture or with rich ecosystems, which could push up global food prices or harm biodiversity. Other approaches are also likely to have snags, especially if stretched too far. Direct air capture requires a lot of energy, which must come from a very-low-carbon source not to be counterproductive. Enhanced weathering, which involves grinding certain types of rock to speed natural CO2-absorbing chemical reactions, could create air pollution.
Without defining the levels of reductions and removals that lead to net zero, there's no clear imperative for each country or company to cut its emissions to the bone. Instead, they might hope to pay others to remove lots of CO2 on their behalf. "Everyone thinks they will buy negative emissions from someone else," says climate scientist Bas van Ruijven at the International Institute for Advanced Systems Analysis in Laxenburg, Austria.
Worse, it seems increasingly likely that CO2 removal will have to go beyond merely balancing residuals. "Now it looks like we will need net negative to meet the Paris goal," says Fuss. That means removing more CO2 from the atmosphere than we put in. Researchers in the international ENGAGE project have developed models that include a range of sociopolitical constraints, such as the ability of governments to enforce climate legislation. These models project that climate warming will overshoot the 1.5 °C target by 2050. Reversing that overshoot would require several hundred gigatonnes of CO2 removal during this century. "So you cannot have an enormous amount of residual emission, as then you need an even more enormous amount of carbon removal," says van Ruijven, who is a member of the ENGAGE project.
It may be wise to go further and try to repair some of the damage we have done, dialing down global temperatures closer to pre-industrial levels and curbing the ocean acidification caused by absorbed CO2. That would, of course, require even more removals. Despite this, companies and countries are not yet planning to reach net negative.
In some quarters, net zero is seen as a final goal. This could leave the door open for fossil-fuel production to continue at high levels and for new infrastructure that could commit us to burning those fuels for decades to come. "We haven't focused enough on the phaseout of fossil fuels," says Buck. "If we only focus on emission at the point of combustion, then we are missing half the picture." The 2023 UN Climate Change Conference (known as COP28) alluded to this problem, calling for "transitioning away from fossil fuels in energy systems." But, this falls far short of a phaseout. "It is promising that they said something, but it could have been stronger," says Buck. "What you need is a plan and a lot of resources committed to phaseout."
Zero Clarity
Net zero holds a host of other ambiguities. "Today, everybody has their own idea of what net zero means," says Fuss. "So we should take a step back and refine the concept. It is really important to get all these things straight, so we are not fooling ourselves."
For example, it's unclear whether net zero should include climate feedback effects, such as CO
2 emitted by thawing permafrost. These could require vastly more removals to prevent temperatures from rising.
Nor does the target emphasize urgency. If governments are aiming for net zero in 2050, they might feel free to kick their heels for a while. But many mitigation measures will need decades to scale up, so "it's vital to reduce emission as much as possible in the short-term," says Fuss. "You don't break something just to then repair it."
Net zero doesn't yet specify the durability of removals, either. Today's emissions will linger for centuries, so they can't simply be balanced by a form of removal that is likely to last only years or even decades. As Fankhauser et al. write: "Achieving net zero through an unsustainable combination of fossil-fuel emissions and short-term removals is ultimately pointless."
The sum should also explicitly include any knock-on effects. For example, planting forests at high latitudes can be counterproductive because they create a darker landscape that absorbs more solar heat, melting local ice and snow.
Then there is the question of whether to include other greenhouse gases, such as methane, in the net-zero sum. Methane has a much shorter lifetime in the atmosphere, so attempting to cancel out methane emissions with CO
2 removal would tend to mean more warming in the short term, and less in the long run. That could be good or bad, depending on whether it takes us past climate tipping points.
Zooming in on Zero
How can we do better? The first thing is to decide what should be classed as a residual. "We should make sure that residual emissions are truly hard to abate," says Buck. Voluntary codes are starting to address that, including the net-zero corporate standard launched by the Science Based Targets initiative, which calls for residuals to be only 5-10% of a company's current emissions.
To get removals moving, Fuss thinks that we need higher prices on carbon emissions. "If we are asking people to remove, we are asking them to perform a public service," she says, "so we should be compensating them for extracting each tonne of CO
2."
Carbon pricing could also curb fossil fuel production. Pacala led a 2023 National Academies report on accelerating decarbonization, which, among other things, recommended an economy-wide carbon tax in the United States. He says that the 2022 Inflation Reduction Act (the nation's main policy tool for moving toward net zero) omitted any such tax in order to gain political traction.
Assuming that carbon removals can scale up fast enough, it will be vital to prove how much CO
2 they are removing, through monitoring, reporting, and verification (MRV) systems. That could be challenging. "MRV is hard enough with forests, where we already have decades of experience," says Buck. "With novel techniques, it's a big challenge, and I'm not sure it's solvable on a timescale of 20 years or so." But there are some promising signs. In November 2023, the European Parliament voted to adopt a new certification scheme for removals, aiming to boost their credibility and scale. Meanwhile, advances in remote sensing and machine learning could make MRV more achievable.
As well as trying to redefine net zero, perhaps nations and societies also need to take a step back and think more broadly about what to strive for. Buck thinks that net zero should become just one among a set of targets, including reductions in fossil-fuel production and enhancing the capacity of countries to implement the clean-energy transition. She also considers the term to be fundamentally unsatisfying, a piece of accountancy that is not compelling to most people. Perhaps the world needs a more inspiring climate narrative that comes not just from scientists, but also other groups. "We need to evolve broader languages," Buck says, "and make more effort to understand what would encourage people to change their lifestyles and consumption."
Fankhauser, meanwhile, cautions against focusing on climate impacts alone. "The risk is that we maximize natural systems for carbon uptake but compromise biodiversity and other ecosystem services," he says. "We need a holistic point of view."
Climate solutions should also avoid dumping pollution or costs disproportionately on disadvantaged communities. This isn't just a moral matter. "People are not going to go along with these changes unless they see benefits in their own lives," says Pacala, who points to the plight of coal miners in the United States and other workers whose jobs may be threatened by the energy transformation. "We have to manage the jobs of legacy workers, who were previously thrown under the bus," he says.
At the moment, there is no pithy phrase to sum up these diverse aims. "Net zero is powerful because it is two words," says Fankhauser. Adding more detail could spoil that rhetorical impact. Low-residual, urgent, all-greenhouse-gas net zero, aligned with biodiversity and poverty reduction-it hardly trips off the tongue. For now, at least, researchers and policymakers may have to stick with those two words, while carefully contemplating all the things that add up to zero.
Stephen Battersby wrote this article for the Proceedings of the National Academy of Sciences.
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