By Jane Palmer for Ensia.
Broadcast version by Eric Galatas for Colorado News Connection reporting for the Solutions Journalism Network-Public News Service Collaboration
The summer of 2022 was tough for farmers in the American West: Hot, dry conditions led snow to melt early, reservoirs to run low and streams to pare down to mere trickles. For many, that meant less water to grow crops and reduced yields. But Byron Kominek, a farm manager near Longmont, Colorado, enjoyed an abundant harvest of peppers, tomatoes, squash and lettuces.
When his family farm stopped making a profit, Kominek installed solar panels on the plot and invited Sprout City Farms to grow crops beneath them. It's a setup known as agrivoltaics - where solar panels and agriculture occupy the same land - and the duo effectively harvests the sun twice, for both food and electricity.
Protected from the high midday sun, plants under panels become mini swamp coolers: As they open their pores to photosynthesize, water escapes from their leaves - creating a cooler microclimate. This reduction in heat increases the efficiency of the panels - even as the panels are sheltering the crops beneath from overexposure to the hot sun. Consequently, agrivoltaics can provide benefits to both farmers and electricity producers. In the past few years another possible advantage has come to the fore: crops grown under panels need less water.
"If you spilled your water in the shade versus the sun, where would it stay wet longer?" asks Greg Barron-Gafford, a University of Arizona professor who has helped set up the site as well as experimental agrivoltaic plots in Arizona, Africa and Israel.
At one such site, Biosphere 2 near Oracle, Arizona, Barron-Gafford has found that some crops beneath solar panels only need watering every couple of days, compared to every few hours for those grown in direct sunlight. Agrivoltaic cherry tomatoes proved 65% more water efficient than those grown under an open sky, for example, and the total fruit production doubled. Researchers are now scrutinizing how different spacings of panels are impacting the water needs of a range of crops in the hot, dry climate of the Sonoran Desert.
Meanwhile, at Kominek's farm, now called Jack's Solar Garden and the largest commercially active agrivoltaics system in the United States, the same scientists are testing which crops thrive under panels in the varied seasons of Colorado.
Barron-Gafford believes agrivoltaics could be help farmers in the West who want to keep farming in the face of climate change. "We want to adapt our food system to survive through periods of drought and warmer temperature changes, and that comes down to easing our dependence on irrigation," he says.
Harvest Hacking
Against the backdrop of Sonoran Desert scrub on a sunny but chilly November morning, Nesrine Rouini tends to seedlings under a canopy of solar panels at the Biosphere 2 site. A mere 9- by 18-meter (30- by 59-foot) garden, the experimental agrivoltaics site resembles an intensive care unit for plants - stakes bearing barcodes identify each new sprout, and a network of cables and wires runs along each seedbed to a centralized data logger. This Gordian knot of leads and controls transmits real-time data of a plant's living environment - soil moisture, temperature, solar radiation and a host of other variables - hour by hour to the research group in Tucson. Simultaneously, cameras track its growth from seedling to sprouting to flowering. A plant can't so much as open a stoma without its actions being spotted, relayed and recorded.
Once a week, Rouini, an agrivoltaics researcher on Barron-Gafford's team, visits the site and a control plot, which is not under panels. Using a handheld gas exchange device, she takes the pulse of each plant and checks how well it's coping with the shade or open sky. "This is how we found out that plants in the control plot experience midday depression and don't photosynthesize," Rouini says. "While the ones under the panels keep trucking along."
Irrigation on both plots starts at 7 a.m. By 9 a.m. the control plot soil already appears drier to the naked eye than earth under the panels. "Without shade the water just evaporates so much quicker," Rouini says.
Basic physics dictates that crops grown under panels need less water, but scientists still don't know how each crop will fare in each location and exactly how much water will be saved. Consequently, in Arizona, Colorado and a network of nearly 30 sites around the country, groups of researchers are trying to close that data gap.
Although scientists have studied the interaction between light and plants for decades, the novel shading regime of solar panels presents many unknowns, says Jordan Macknick, the lead energy-water-land analyst for the National Renewable Energy Laboratory and the principal investigator of the Innovative Solar Practices Integrated with Rural Economies and Ecosystems (InSPIRE) network of agrivoltaic sites. "The Holy Grail would be for any farmer to be able to pick a point on a United States map and retrieve information about what crops they could grow, the best configuration of panels and how much water they need," he says.
Focusing on Feasibility
In Colorado, Liza McConnell, Jack's solar research farm manager with Sprout City Farms, observed that the lettuces grown with half the amount of water administered to a control plot were only a little smaller and significantly sweeter than their sun-exposed equivalents. Celery, typically a high-water-use crop, also fared well under a reduced watering regime, as did smaller peppers, but the larger Anaheim peppers under panels didn't produce as much fruit as hoped. With the West in a prolonged drought and climate change taking its toll, people might have to adapt to not getting the exact type of pepper they want all year round McConnell says.
"In the face of climate change we need all options on the table," she says. "Agrivoltaics is not the only solution, but it is going to be one of the things that will help keep our communities safe and resilient."
Despite its many benefits, agrivoltaics may not be feasible for large-scale, single-crop farms that grow corn and soybeans and rely on using heavy machinery. Farming under solar panels is even challenging for farmers on their feet: McConnell equates it to farming on an obstacle course. On the flip side, the panels provide much needed shade for farmers on hot days.
"We're producing energy, we're producing food, we're conserving water, and we're building soil health that further conserves water and nutrients," McConnell says. "And then we're also protecting necessary human labor and quality of life for farmworkers."
The yield of certain crops, specifically warm-season peppers and tomatoes, might also be less under panels in Colorado, McConnell says. But these fruits only ripen at certain temperatures; if it's too hot, they won't ripen at all. "So a reduced yield is still better than having no tomatoes in the face of climate change," she says.
Macknick points out that the revenue that farmers can make by selling solar-generated power will more than compensate for any reductions in farm produce and that agrivoltaics could help farmers in the West and the Colorado River Basin be more financially resilient to droughts and climate change.
Another potential benefit of agrivoltaics is that it could open more land to farming, including Indigenous lands where food security and energy access have been issues. In hot and dry desert lands, for example, growing crops under panels can reduce the need for scarce water and increase productivity and feasibility for farming efforts. "Can some of these places now produce food because we've taken off that harsh edge of the environment?" Macknick poses.
Agrivoltaics also offers the potential to harvest and store rain so it can be used for irrigation. Gutters attached to the bottom of solar panels can capture rain and channel it into small reservoirs. But challenges exist in the execution. In Tucson, for example, water simply flows off water panels and gets wasted. "It would be good to think about how to set up guttering on panels to collect water intentionally and do it the right way," Barron-Gafford says.
Large-scale agrivoltaics endeavors will face plenty of challenges, and they won't be right for every farmer, Macknick says. But there's potential to improve yields of some crops while enhancing soil health, reducing water needs and producing power, too. "It is certainly going to play a growing role in farming," Macknick says. "I think we are going to see more and more of this."
Jane Palmer wrote this article for Ensia.
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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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