One day in 2016, a British glaciologist named John Moore attended a meeting in Cambridge, England, that included a presentation about a glacier on Greenland’s west coast. Typically referred to by its Danish name, Jakobshavn, but also known by its Greenlandic moniker, Sermeq Kujalleq, the glacier functions as a kind of drain, situated on the edge of Greenland’s massive ice sheet, that moves 30 billion to 50 billion tons of icebergs off the island every year. These icebergs, some of them skyscraper-size, calve regularly from the glacier front, crash into a deep fiord and float west into Disko Bay. Then they drift out into the North Atlantic, break apart and melt. The intense activity here, as well its breathtaking location, have earned the area a designation as a UNESCO World Heritage site and made it a powerful attraction for Greenland’s small but vibrant tourist trade.
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For scientists, though, Jakobshavn elicits urgency. Glaciologists have identified it as one of the fastest-deteriorating glaciers in the world. And as waves lap higher on the shores of cities like Miami Beach and New York, this far-off ice is partly the reason. Jakobshavn alone was responsible for 4 percent of the rise in global sea levels during the 20th century. It probably contains enough ice to ultimately push sea levels up at least another foot.
What struck Moore that day in Cambridge was not only Jakobshavn’s potential collapse but also the way its ice interacts with the surrounding water. A slide at the meeting showed how warm water nearly a thousand feet below the surface flows from the Atlantic into Disko Bay and eventually makes its way into the fiord and to the glacier front, where it carves away at and weakens the ice. Moore noted something interesting at the bottom of the bay’s entrance: the warm water flows over a sill, a ridge rising several hundred feet above the ocean floor and just over three miles long, akin to a threshold that crosses the floor of a doorway between two rooms. “It’s kind of a pity that sill isn’t just a bit higher,” Moore thought. “Because then it would stop the warm water from coming in and hitting the glacier.” Not long after, he wondered: What if someone made the sill higher?
For the next year, Moore mulled over that question. How hard would that be? How expensive? And how effectively could a raised sill halt the influx of warm water and slow Jakobshavn’s shrinkage?
In 2018, Moore and his colleague Michael Wolovick published an article that proposed looking into building a sea wall 100 meters high, or about 328 feet, on the floor of Disko Bay. Raising the sill, using gravel and sand, could reduce the warm water in the fiord and allow Jakobshavn to thicken naturally and stabilize. Moore believed that such a sea wall might not only decrease Greenland’s contribution to sea-level rise; it might also serve as a trial run for far grander ambitions. If the idea proved workable in the Arctic, it could be translated to Antarctica, where much larger glaciers in the Amundsen Sea, especially one known as Thwaites, threaten to raise sea levels substantially. “Should we spend vast sums to wall off all the world’s coasts,” Moore and Wolovick asked in Nature, “or can we address the problem at its source?”
As a practical matter, the plan was audacious, perhaps more of a thought experiment than a feasible option. A sea wall at Jakobshavn might cost some $500 million; at Thwaites, one might run to more than $50 billion. Moore also believed that the latter could represent the most difficult construction project in human history. And such a geoengineering effort would almost certainly pose problems beyond costs and logistics.
Credit…Illustration by Catalogtree
“Geoengineering” commonly refers to human interventions in Earth’s natural systems in order to reap societal benefits even in the face of unclear risks. Some geoengineering ideas, like crushing rocks and dispersing the dust to absorb CO2 from the air — a practice known as “enhanced weathering” — are already being tried. Others, like injecting particulates into the upper atmosphere to reflect sunlight and cool the Earth (much as a volcanic eruption might), have so far proved too contentious to field-test.
In general, geoengineering seeks to reduce the impacts of climate change and to buy us more time as we transition to a zero-carbon world. Such projects also confront advocates with extraordinary challenges of engineering and financing — as well as political, cultural and ethical obstacles. The glacial barrier proposed by Moore and Wolovick is a case in point. It is novel, expensive, complex, potentially risky and controversial. But as the pandemic receded, the two scientists turned their focus to determining if their idea could become more than a hypothetical.
About a year ago, I began talking with Moore about his progress. We first met in person in September, in Rovaniemi, Finland, where Moore works as a professor of glaciology at the University of Lapland. As we sat outside his building, he traced the evolution of his glacier-conservation idea. He was encouraged, he told me, by early philanthropic support for his research into an underwater barrier. (The support had come from a Scandinavian billionaire, he said, in addition to academic institutions.) And recent computer modeling, which he previewed for me, suggested that an underwater barrier could be beneficial in West Antarctica. “I think it’s superoptimistic as a result,” he said.
Moore thought the path ahead would take at least a decade, though, as he progressed to larger sites, starting deep in a Norway fiord, then moving on to Greenland. Then maybe he could begin in Antarctica. An installation near Thwaites — considering its bad winter weather and logistical complications — might take another decade. “No one has really said lately that it’s supercrazy,” he said, laughing. Still, some glaciologists had voiced strong opposition as well as skepticism. And Moore and a team of associates didn’t yet know whether native Greenlanders would find his plans acceptable, or whether the political treaty in Antarctica would allow for construction.
He was nevertheless certain that his project remained viable. “The usual argument against doing any kind of geoengineering is that we have to do mitigation, mitigation, mitigation — those are the three arrows in the quiver,” Moore said. But mitigating our carbon emissions might not do much to halt the collapse of some threatened glaciers. Why not do mitigation and intervention, Moore asked, to avoid catastrophe?
He had reached a point, he said, where he wanted to see if there were good reasons not to pursue his ideas. “Let’s try and find the red flag,” he said, meaning the risks that might follow from creating an underwater barrier. And if we can’t find them, he added, he was determined to move forward.
As we get closer to reaching the point where Earth’s temperatures are 1.5 degrees Celsius greater than they were in preindustrial times — a level we are likely to hit by the end of this decade — it seems increasingly clear that an age of geoengineering, both in prospect and in practice, has arrived. The resulting projects can often require complexity and sophistication. To disperse reflective particles to cool the Earth, for instance, may require manufacturing a fleet of specialized high-altitude aircraft. But geoengineering efforts can be low-tech too. Painting urban roofs white to cool buildings is one example; covering permafrost or glaciers with blankets to keep them cool is another. One afternoon Moore and I discussed whether fencing in the edges of Antarctic glaciers could catch snow and keep it from blowing out to sea, thereby building up drifts to thicken the ice.
David Keith, a former Harvard professor who has been a leading advocate of researching the potential risks and benefits of putting particulates into the upper atmosphere, recently began organizing the Climate Systems Engineering initiative at the University of Chicago; one priority is to systematically consider the practical engineering challenges of various climate interventions. Keith told me he has been excited by the global investments in clean energy over the past few years, which surpass $1 trillion, as well as by the increased efforts to remove carbon from the atmosphere by means other than trees. (He is a founder of a company, Carbon Engineering, that is focused on direct air capture.) “It’s not like, will deployment happen?” Keith says, referring to certain kinds of geoengineering. “Deployment is happening.”
And yet, it’s largely happening on an ad hoc basis. No single entity organizes geoengineering research projects or evaluates potential risks and effects; nor is there a clear process by which governments or other entities decide whether they are sensible or safe or affordable. Instead, academics like Moore are usually left to push their ideas forward, independently, and hope they find funding and momentum. In Keith’s view, the Arctic barrier idea is promising (“the best one I’ve seen yet for glaciers”) but might require at least a decade of study to understand its true costs and benefits.
The need to evaluate the risks associated with geoengineering can make for wide gaps between research and deployment. In a recent paper, Moore’s colleague Wolovick, who now works at the Alfred Wegener Institute in Germany, noted that “humanity is a very long way from being able to implement any sort of targeted glacial engineering.” Recently, during one of our conversations, he mentioned that there are “so many scientific unknowns here in terms of the glaciology, the oceanography, the materials science, the marine biology.” All of them need to be studied in extraordinary detail — along with the necessary engineering studies. And, Wolovick added: “It’s vital that we’re not perceived as just rushing into things recklessly. If we rush into it too fast, then people would rightly be skeptical.”
Wolovick sees a glacial barrier taking more time to develop than Moore does. He told me that he would not be surprised if it requires 20 years to learn how to install one in Antarctica. But he’s convinced that it’s important to have that understanding of engineering requirements and ecosystem impacts in case big glaciers continue to degrade steadily over those 20 years. If we aren’t prepared, at some panic point in the future a poorly studied geoengineering scheme might be attempted — with disastrous results. An underwater barrier might push warm water away from one vulnerable glacier to another, for instance, hastening the collapse of a different glacier.
There have been some early attempts to evaluate various geoengineering schemes. Last summer, Moore collaborated with academics from the University of the Arctic, with which about 200 universities are affiliated, to survey and, preliminarily, to rank 61 geoengineering ideas. The criteria included technical readiness, “scalability,” cost versus benefits and whether a project could be easily reversed if it posed a danger. Lars Kullerud, the university’s president, spearheaded the study, which was published in October. When we met in Oslo, he told me his goal was to “remove the taboo” from geoengineering. Such projects, he said, would not preclude eliminating carbon emissions, which he sees as the only long-term solution to arresting climate change. But he said he is urgently interested in “emergency measures” for the Arctic, which appears to be warming four times as fast as the rest of the Earth.
The study’s geoengineering ideas were scored on a scale of 1 to 3 in 12 categories, with a higher average number indicating more promise. Pumping water on glaciers, for example, got a score of 1.25; brightening Arctic clouds with seawater to enhance reflectivity a 2.27. Moore and Wolovick’s anchored sea barrier to preserve glaciers scored a respectable 2.10. No score exceeded 2.5.
What helped was that the idea had evolved from a hard berm of gravel and stone to a flexible curtain. When Moore and I first spoke last year, he was thinking of something like what’s found in a big walk-in freezer, where strips of plastic separate warm air from cold. Except this one would float up from the sea bottom, which is where the salty warm water circulates, rather than hang down from buoys. It could be made of polyethylene, “which is very slippery,” Moore points out, “and that’s very important when you have these big icebergs moving past.” As Moore was pondering that approach, Kullerud was wondering if they needed to draw on far greater engineering expertise. “In my job I travel a lot,” he told me, “and everywhere I went, I simply asked, ‘If you want to build something big under the water, where do you find competence?’ Because we cannot take a bunch of academics from a university and make them actually implement such a thing. Even professors in engineering, they’re very theoretical.”
Most everyone he spoke with suggested he look in his own backyard, in Norway, to a global engineering conglomerate called Aker Solutions. “These are the big guys who build big ugly stuff in the water for the oil industry,” he said. “They are the ones who understand this.”
A few years ago, I camped near the face of the Jakobshavn glacier with a team of researchers led by David Holland, an oceanographer at New York University who has visited Greenland almost every summer since 2007. Flown in by helicopter, we spent a week on a scarp of dusty land at the ice sheet’s edge, as the sun circled in the sky, all day and night. We kept constant watch on the glacier: a blue-white wall several miles in length, looming 300 feet above the surface of the fiord and descending 3,000 feet below. It rumbled and cracked off huge pieces of ice — the reports sounded like gunshots.
Holland periodically dropped probes into the fiord to measure water temperatures. He has done similar work at Thwaites. In his view, water temperature is the key to how so-called “ocean-terminating” glaciers like Jakobshavn and Thwaites shed ice from the land and raise sea levels. It’s almost like a control knob. Holland recently told me that Jakobshavn’s fiord is about one degree Celsius warmer than it was before the 1990s — it’s now about four degrees Celsius above freezing — and that the waters abutting Thwaites, which are the same temperature, have probably also warmed significantly. With years of observations behind him, he says, the evidence is now relatively robust “that warm waters do impact ocean outlet glaciers — and that the warm water itself seems to be attributable to changing winds, which are arguably attributable to increased greenhouse gases.”
A number of glaciologists I spoke with view Moore’s idea for protecting glaciers as technically or ethically problematic. Twila Moon, the deputy lead scientist at the National Snow and Ice Data Center, in Boulder, Colo., told me that she’s concerned by how geoengineering, in general terms, can be regarded as a climate-change solution when it only addresses — at best — some of its impacts. Of more direct concern, Moon told me, seabed curtains in the Arctic might affect ecosystems and fisheries. And because Greenland is melting on its surface, where the air is getting warmer, she says, slowing iceberg production from glaciers like Jakobshavn might not have much effect on global sea levels. Ian Joughin, a glaciologist at the University of Washington who has followed the decline of Thwaites for two decades, stressed to me the difficulty of its location: At Thwaites, there is no infrastructure of any kind for at least 500 miles. “It’s just a very hard area,” he says. “It’s even hard for ships to get in there to put in a single mooring.”
Holland is philosophical about Moore’s idea. “With Greenland’s it’s like, Should you do it?” he told me. “But Antarctica is more like, Could you do it?” He has doubts about the latter. “We’ve struggled to get there, and it takes years to plan things. There are no second chances for anything. When you’re there, you’re far away from everything, and you have a small window of operations in the months of January and February when the temperature’s warm enough — and then boom, it’s cold again.” On the other hand, he adds, there were few “pressure points” on Earth as important as Thwaites: “Thwaites could add two feet to sea levels — but then it could trigger its neighbor glaciers to add several more feet,” he says. “So you look at it and say: ‘Well, the cost to the world is Florida’s gone. Bangladesh is gone. Lots of little cities around the world are gone. Big cities, too, like Shanghai.’ I mean, the cost events are just so large it’s incomprehensible.”
Moore realizes the area around Thwaites is as forbidding as any location on Earth. “So this is something when people say, you know, it’s really difficult working in Antarctica, I say: yeah, well, I have abandoned ship because it’s been stuck in the ice,” he says, referring to his first trip there. “I do know how difficult it is.” Still, he acknowledges that scientific expertise does not necessarily mean expertise in ocean engineering. It’s why he supported Kullerud’s efforts to consult outsiders from Aker Solutions.
In September, I spent a day at Aker’s main office, on the outskirts of Oslo. In addition to engineering oil and gas installations in the North Sea, the firm designs platforms for deep-sea wind turbines and offshore fish farms. Two of Aker’s engineers, Ole Wroldsen and Dorthe Julie Kirkeby, attended a meeting that Moore and Kullerud convened in Iceland a year ago, where they explored ideas for the curtain. Wroldsen told me that when he first heard about the rock wall, he balked. Constructing it could cause a lot of damage to the seabed, he thought, and it would probably be destroyed by large icebergs floating out to sea. Plastic curtains weren’t OK, either. “It’s a good idea, but it’s a bad solution,” Wroldsen says. “You don’t put plastic into the ocean, right? We’re in a fight against plastic in the ocean.”
Designing and building structures for extreme ocean environments is a rarefied field. In the North Sea, Aker Solutions must build platforms to withstand waves of 100 feet in one of the harshest working environments in the world. But as Wroldsen and Kirkeby told me — and later showed me, in a glass-walled wave tank at Aker — the energy of a wave on the ocean floor is a fraction of what it is at sea level. This gave them encouragement that a seafloor curtain in Greenland and Antarctica, two extremely harsh environments, could survive storms and tidal forces.
Materials present their own challenges. If rocks and plastic are not suitable, then what is? Wroldsen wonders if a tight mesh of sisal or abaca, durable natural fibers that sailors have used for centuries, might be strong and slippery enough. Aker’s preliminary ideas have evolved into something that resembles a gigantic, tightly knitted tennis net. Their version would rise upward from the ocean floor for at least several hundred feet and stretch lengthwise, probably in paneled sections, for several miles in Greenland or 50 miles or so in Antarctica. It would be anchored by weights at various points but include gaps to allow nutrients and fish to pass underneath. It would flex “like a swinging saloon door,” as Wroldsen puts it. For installation, a curtain could be loaded onto ships and dropped to the sea bottom.
This design is only a first crack at the engineering, Wroldsen cautions, and other approaches have been explored in academic papers by Moore’s colleagues. But Wroldsen told me he now believes it is possible to manufacture and install an effective curtain, which was not his initial reaction to the idea. What if it didn’t function right, I asked, or had unintended consequences for marine life? The curtain would need to be buoyant, Wroldsen replied, so in theory you could cut it from its moorings, let it float to the surface and gather it back up. Then you could go back to the drawing board.
The physical impediments to geoengineering schemes can be daunting, but political and cultural objections can be even harder to resolve. Antarctica — lacking countries, and permanent residents — is governed by a complex international treaty. Any seabed curtain project there would very likely need the support of most of the more than two dozen countries with voting power. In Greenland, a similar intervention would depend on the permission of Inuit society, who make up a majority of the population.
Over the past year, Moore’s colleagues seemed to be making progress, though incremental at best. At the University of Lapland in Rovaniemi, Ilona Mettiäinen, an environmental social scientist, told me that because this project would be the first of its kind, going ahead with it would probably raise serious economic and social challenges. And the possibility of interrupting the production of Jakobshavn’s iconic icebergs adds to the sensitivities. Tourism and fisheries are the primary concerns. “We wouldn’t want to see this rich place, or any site, be harmed by such a curtain,” she said, acknowledging that local concerns — not global ones — would shape whether Greenlanders would give permission to a proposal like Moore’s. Lars Kullerud framed the situation for me this way: Destroying an ecosystem in the Arctic means your idea will not go anywhere — “even if you save the world,” he said.
Greenlanders’ main concern right now is not climate change, says Carl Egede Bøggild, a glaciologist working at the education ministry in Nuuk, Greenland’s capital city. The island is focused on becoming financially and politically independent from Denmark. In trying to expand their economy, he says, Greenlanders might ask of Moore’s curtain: “Why should we pay for your Western-created problem of CO2 emissions?” Sara Olsvig, a former member of the Greenlandic and Danish parliaments who now serves as the international chairwoman of the nonprofit Inuit Circumpolar Council, echoes this concern. Olsvig told me that Greenlanders are wary when outsiders talk about taking action for some greater good. “That is exactly what we have been experiencing as Indigenous peoples of the Arctic,” she says, “that our lands were colonized, our societies were colonized, in the greater good of someone else.”
As the Greenlanders’ position seemed to be hardening into opposition this fall, Moore made plans to take the next steps. As Kullerud put it to me, “You can make computer models forever, but in the end, you are going to have to try to build something, somewhere.” Next year, Moore would like to conduct an experiment outdoors spanning tens of meters — in a small fiord in Norway, perhaps — to try out curtain materials. A larger test might follow, perhaps hundreds of meters in length, in Svalbard, a small Arctic archipelago controlled by Norway. And then — a few years from now, at the very earliest — he and his team could try to install something larger in Greenland. As Moore had come to doubt Jakobshavn’s residents would give permission (“a perfectly respectable outcome,” he acknowledged), he began to focus on another big fiord, in remote northeast Greenland, where there is an enormous outlet glacier known as 79 North, named for its latitude. “There are no settlements up there,” he told me. “It wouldn’t necessarily be trying to balance the cultures and the politics of it.”
Several times I asked Moore and his colleagues a simple question: Who will pay? A serious oceanography study easily runs to $10 million, $20 million, $30 million, Kullerud told me. “That kind of thing universities can handle, that’s not the problem.” And even small-scale tests in fiords might not be so difficult to finance through some combination of academic and philanthropic funds; a Swedish billionaire named Frederik Paulsen Jr. paid for the group’s conclave in Reykjavik last year. Collecting hundreds of millions of dollars, though, is more complicated — but that will probably be necessary. Just building the ships, including icebreakers, needed for Antarctica would be a staggering expense, as much as $10 billion.
A rough cost-benefit analysis for the curtain has been Moore and Wolovick’s central argument since the beginning: Spend money locally, save money globally. A recent study put the future global costs from both the damages of sea-level rise and new coastal defenses at many hundreds of billions of dollars. Moore estimates that a curtain in Greenland that averts a rise of perhaps one-tenth of a meter might save $5 billion a year, and a curtain in Antarctica that prevents a two-meter rise might save $100 billion a year. The curtains could quickly pay for themselves.
Moore and his colleagues are thus pushing a relatively new economic idea: that maintaining the integrity of Greenland’s and Antarctica’s ice sheets — considering them as a “global good,” as Moore puts it — is worthwhile for the world’s coastal countries. His argument echoes the case made for the value of the Amazon rainforest. In recent years, through a United Nations-sponsored effort known as REDD+, industrialized countries have paid governments for rainforest conservation and, at least in theory, to avert carbon emissions. Similarly, nations most vulnerable to sea-level rises could help fund a curtain. Marianne Hagen, a former government official in Norway who now works with Moore and Kullerud on the curtain project, mentioned the idea that some funding for a curtain could be raised through insurance payments made by coastal homeowners or businesses.
Moore considers these efforts to be crucial — but they go beyond his expertise. “I’m trying not to do everything at once,” he told me one day, sighing. He would rather work on the science, he said, and to a lesser degree the engineering. And if those show promise, he has other experts ready to figure out the money and the politics.
In early October, David Keith’s new initiative at the University of Chicago sponsored a two-day workshop on glacial geoengineering. The day before it began, I happened to run into Moore and Wolovick at a cafe on campus. Sitting at a picnic table in the sun and sipping from a can of stout, Moore called me over and said, “I’m really enthusiastic about some modeling we got for Thwaites.” These were computer projections, done by a colleague at Dartmouth, that simulated how a curtain might prevent the flow of warm water from hitting the glacier. But some recent data for Jakobshavn did not look as good, Moore added. He grimaced and shook his head. This was not wholly unexpected: A few weeks earlier, when we met in Finland, he was concluding that the air and water in southwestern Greenland were getting too warm for a curtain to be truly effective. He would have to look elsewhere on the island. “You know, 20 years ago it might have made sense to do something,” he said of Jakobshavn, “but it’s too late now.”
Moore and Wolovick addressed about 30 researchers the next day, patiently explaining their ideas. As I sat listening to the group debate the curtain, the meeting seemed less about identifying specific problems than about airing a frequent refrain: Did we yet know enough about how these glaciers interact with the ocean, or how a seabed curtain could alter marine life? One participant asked a particularly pointed question: “Suppose we spend $50 billion on this, and it doesn’t work?” Yet even the question of what it meant for the curtain to “work” was difficult to answer. What the future would be like without a seabed curtain — versus what it would be like with one — was not yet knowable.
By the end of the day, two predominant positions seemed to be in tension with each other: The curtain was an intriguing response to a monumental problem, and it was a frustratingly incomplete one. But it occurred to me that the curtain also serves as a good example of how our responses to ideas that at first seem hard to accept, including many geoengineering examples, can evolve over time. Even with so many scientific and engineering questions still unanswered, Moore and Wolovick’s project, which I first heard about five years before, had advanced significantly in a very short period. And Moore has said he had found no red flags.
But the Earth had not stopped warming. Perhaps the essential question about geoengineering is whether it is riskier for humanity to focus exclusively on reducing emissions in the coming years, or on adapting to a hotter world, or whether we should simultaneously expand our research into climate interventions. “I just don’t agree with Greta Thunberg on this,” Moore had told me in Finland, referring to the Swedish environmental activist who has spoken out against research into solar geoengineering. Nor does he think that the risks of “moral hazard” — that by investing heavily in ways to alleviate the impacts of climate change we lose the motivation or resources to prevent its causes — are reason to pause his glacier research.
For his part, Moore believes we should be prepared for sea-level rises that might well exceed, in speed and volume, the estimates from the Intergovernmental Panel on Climate Change, which calculates that by the end of this century, oceans will be between 1.6 feet and 3.2 feet higher. What’s more, even if the world slashes CO2 emissions drastically over the coming decades (a prospect that today seems unlikely), that may not solve the problem. Two recent academic papers — one on Greenland, the other on Antarctica — suggest that a low-emissions future may not improve the health of some crumbling glaciers. Their ice may crash into the oceans anyway.
One of the people running the Chicago conference was a glaciologist named Doug MacAyeal, an elder statesman in the discipline who, at age 69, had just retired. “I was a young glaciologist in the 1970s when we thought it was utterly preposterous that changes in ice sheets could affect sea levels in any time less than 1,000 years,” MacAyeal told me. Back then, that consensus was being challenged only on the fringe — especially by a glaciologist named John H. Mercer, of Ohio State, who in 1978 was the first to connect the possibility of a catastrophic collapse in West Antarctica to climate change. But the idea met with ferocious resistance, MacAyeal pointed out. It required decades of studies and data to validate that glaciers in West Antarctica could indeed slide into the ocean within a human lifetime.
MacAyeal told me he sees the curtain proposal in a comparable way. It’s not because he’s sure it will work (he is not), but because he thinks it faces similar disbelief. To stand next to an active glacier, he said, “is like standing next to a monster. It’s so big, so terrifying. And to think a human could intervene in that, and change it? It’s just something that almost seems inconceivable.” It may not be considered inconceivable a few years from now. “We don’t know if we’ll ever get there,” he added, “but the question needs to get into the open.”
Jon Gertner has been writing about science and technology for the magazine since 2003. He last wrote about the U.S. space force. Olaf Otto Becker is a photographer in Germany whose work focuses on documenting the visible traces of human overpopulation left behind in nature.