Climate

Africa’s “Great Green Wall” initiative is a proposed 8,000-kilometer line of trees meant to hold back the Sahara from expanding southward. New climate simulations looking to both the region’s past and future suggest this greening could have a profound effect on the climate of northern Africa, and even beyond.

By 2030, the project aims to plant 100 million hectares of trees along the Sahel, the semiarid zone lining the desert’s southern edge. That completed tree line could as much as double rainfall within the Sahel and would also decrease average summer temperatures throughout much of northern Africa and into the Mediterranean, according to the simulations, presented December 14 during the American Geophysical Union’s fall meeting. But, the study found, temperatures in the hottest parts of the desert would become even hotter.

Previous studies have shown that a “green Sahara” is linked to changes in the intensity and location of the West African monsoon. That major wind system blows hot, dry air southwestward across northern Africa during the cooler months and brings slightly wetter conditions northeastward during the hotter months.

Such changes in the monsoon’s intensity as well as its northward or southward extent led to a green Sahara period that lasted from about 11,000 to 5,000 years ago, for example (SN: 1/18/17). Some of the strongest early evidence for that greener Sahara of the past came in the 1930s, when Hungarian explorer László Almásy — the basis for the protagonist of the 1996 movie The English Patient — discovered Neolithic cave and rock art in the Libyan Desert that depicted people swimming.

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Past changes in the West African monsoon are linked to cyclical variations in Earth’s orbit, which alters how much incoming solar radiation heats up the region. But orbital cycles don’t tell the whole story, says Francesco Pausata, a climate dynamicist at the Université du Québec à Montréal who ran the new simulations. Scientists now recognize that changes in plant cover and overall dustiness can dramatically intensify those monsoon shifts, he says.

More vegetation “helps create a local pool of moisture,” with more water cycling from soil to atmosphere, increasing humidity and therefore rainfall, says Deepak Chandan, a paleoclimatologist at the University of Toronto who was not involved in the work. Plants also make for a darker land surface compared with blinding desert sands, so that the ground absorbs more heat, Chandan says. What’s more, vegetation reduces how much dust is in the atmosphere. Dust particles can reflect sunlight back to space, so less dust means more solar radiation can reach the land. Add it all up, and these effects lead to more heat and more humidity over the land relative to the ocean, creating a larger difference in atmospheric pressure. And that means stronger, more intense monsoon winds will blow.

The idea for Africa’s Great Green Wall came in the 1970s and ’80s, when the once-fertile Sahel began to turn barren and dry as a result of changing climate and land use. Planting a protective wall of vegetation to hold back an expanding desert is a long-standing scheme. In the 1930s, President Franklin Roosevelt mobilized the U.S. Forest Service and the Works Progress Administration to plant walls of trees from the Great Plains to Texas to slow the growth of the Dust Bowl. Since the 1970s, China has engaged in its own massive desert vegetation project — also nicknamed the Great Green Wall — in an attempt to halt the southward march of sand dunes from the Gobi Desert (SN: 7/9/21).

Led by the African Union, Africa’s Great Green Wall project launched in 2007 and is now roughly 15 percent complete. Proponents hope the completed tree line, which will extend from Senegal to Djibouti, will not only hold back the desert from expanding southward, but also bring improved food security and millions of jobs to the region.

What effect the finished greening might ultimately have on the local, regional and global climate has been little studied — and it needs to be, Pausata says. The initiative is, essentially, a geoengineering project, he says, and when people want to do any type of geoengineering, they should study these possible impacts.

To investigate those possible impacts, Pausata created high-resolution computer simulations of future global warming, both with and without a simulated wall of plants along the Sahel. Against the backdrop of global warming, the Great Green Wall would decrease average summertime temperatures in most of the Sahel by as much as 1.5 degrees Celsius.

But the Sahel’s hottest areas would get even hotter, with average temperatures increasing by as much as 1.5 degrees C. The greening would also increase rainfall across the entire region, even doubling it in some places, the research suggests.

These results are preliminary, Pausata says, and the data presented at the meeting were only for a high-emissions future warming scenario called RCP8.5 that may not end up matching reality (SN: 1/7/20). Simulations for moderate- and lower-emissions scenarios are ongoing.

The effects of greening the Sahara might extend far beyond the region, the simulations suggest. A stronger West African monsoon could shift larger atmospheric circulation patterns westward, influencing other climate patterns such as the El Niño Southern Oscillation and altering the tracks of tropical cyclones.

Chandan agrees that it’s important to understand just what impact such large-scale planting might have and notes that improvements in understanding what led to past changes in the Sahara are key to simulating its future. That the Great Green Wall’s impact could be far-ranging also makes sense, he says: “The climate system is full of interactions.” More

This was another year of bleak climate news. Record heat waves baked the Pacific Northwest. Wildfires raged in California, Oregon, Washington and neighboring states. Tropical cyclones rapidly intensified in the Pacific Ocean. And devastating flash floods inundated Western Europe and China. Human-caused climate change is sending the world hurtling down a road to more extreme weather events, and we’re running out of time to pump the brakes, the Intergovernmental Panel on Climate Change warned in August (SN: 9/11/21, p. 8).

The world needs to dramatically reduce its greenhouse gas emissions, and fast, if there’s any hope of preventing worse and more frequent extreme weather events. That means shifting to renewable sources of energy — and, importantly, decarbonizing transportation, a sector that is now responsible for about a quarter of the world’s carbon dioxide emissions.

But the path to that cleaner future is daunting, clogged with political and societal roadblocks, as well as scientific obstacles. Perhaps that’s one reason why the electric vehicle — already on the road, already navigating many of these roadblocks — swerved so dramatically into the climate solutions spotlight in 2021.

Just a few years ago, many automakers thought electric vehicles, or EVs, might be a passing fad, says Gil Tal, director of the Plug-in Hybrid & Electric Vehicle Research Center at the University of California, Davis. “It’s now clear to everyone that [EVs are] here to stay.”

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Globally, EV sales surged in the first half of 2021, increasing by 160 percent compared with the previous year. Even in 2020 — when most car sales were down due to the COVID-19 pandemic — EV sales were up 46 percent relative to 2019. Meanwhile, automakers from General Motors to Volkswagen to Nissan have outlined plans to launch new EV models over the next decade: GM pledged to go all-electric by 2035, Honda by 2040. Ford introduced electric versions of its iconic Mustang and F-150 pickup truck.

Consumer demand for EVs isn’t actually driving the surge in sales, Tal says. The real engine is a change in supply due to government policies pushing automakers to boost their EV production. The European Union’s toughened CO2 emissions laws for the auto industry went into effect in 2021, and automakers have already bumped up new EV production in the region. China mandated in 2020 that EVs make up 40 percent of new car sales by 2030. Costa Rica has set official phase-out targets for internal combustion engines.

In the United States, where transportation has officially supplanted power generation as the top greenhouse gas–emitting sector, President Joe Biden’s administration set a goal this year of having 50 percent of new U.S. vehicle sales be electric — both plug-in hybrid and all-electric — by 2030. That’s a steep rise over EVs’ roughly 2.5 percent share of new cars sold in the United States today. In September, California announced that by 2035 all new cars and passenger trucks sold in the state must be zero-emission.

There are concrete signs that automakers are truly committing to EVs. In September, Ford announced plans to build two new complexes in Tennessee and Kentucky to produce electric trucks and batteries. Climate change–related energy crises, such as the February failure of Texas’ power system, may also boost interest in EVs, Ford CEO Jim Farley said September 28 on the podcast Columbia Energy Exchange.

“We’re seeing more extreme weather events with global warming, and so people are looking at these vehicles not just for propulsion but for … other benefits,” Farley said. “One of the most popular features of the F-150 Lightning is the fact that you can power your house for three days” with the truck’s battery.

More to navigate

Although the EV market is growing fast, it’s still not fast enough to meet the Paris Agreement goals, the International Energy Agency reported this year. For the world to reach net-zero emissions by 2050 — when carbon emissions added to the atmosphere are balanced by carbon removal — EVs would need to climb from the current 5 percent of global car sales to 60 percent by 2030, the agency found.

As for the United States, even if the Biden administration’s plan for EVs comes to fruition, the country’s transportation sector will still fall short of its emissions targets, researchers reported in 2020 in Nature Climate Change. To hit those targets, electric cars would need to make up 90 percent of new U.S. car sales by 2050 — or people would need to drive a lot less.

And to truly supplant fossil fuel vehicles, electric options need to meet several benchmarks. Prices for new and used EVs must come down. Charging stations must be available and affordable to all, including people who don’t live in homes where they can plug in. And battery ranges must be extended. Average ranges have been improving. Just five or so years ago, cars needed a recharge after about 100 miles; today the average is about 250 miles, roughly the distance from Washington, D.C., to New York City. But limited ranges and too few charging stations remain a sticking point.

Today’s batteries also require metals that are scarce, difficult to access or produced in mining operations rife with serious human rights issues. Although there, too, solutions may be on the horizon, including finding ways to recycle batteries to alleviate materials shortages (SN: 12/4/21, p. 4).

EVs on their own are nowhere near enough to forestall the worst effects of climate change. But it won’t be possible to slow global warming without them.

And in a year with a lot of grim climate news — both devastating extreme events and maddeningly stalled political action — EVs offered one glimmer of hope.

“We have the technology. It’s not dependent on some technology that’s not developed yet,” Tal says. “The hope is that now we are way more willing to [transition to EVs] than at any time before.” More

In 1721, a Norwegian missionary set sail for Greenland in the hopes of converting the Viking descendants living there to Protestantism. When he arrived, the only traces he found of the Nordic society were ruins of settlements that had been abandoned 300 years earlier.

There is no written record to explain why the Vikings left or died out. But a new simulation of Greenland’s coastline reveals that as the ice sheet covering most of the island started to expand around that time, sea levels rose drastically, researchers report December 15 at the American Geophysical Union’s fall meeting in New Orleans.

These shifting coastlines would have inundated grazing areas and farmland, and could have helped bring about the end of the Nordic way of life in Greenland, says Marisa Borreggine, a geophysicist at Harvard University.

Greenland was first colonized by Vikings in 985 by a group of settlers in 14 ships led by Erik the Red, who had been banished from neighboring Iceland for manslaughter. Erik and his followers settled across southern Greenland, where they and their descendants hunted for seals, grazed livestock, built churches and traded walrus ivory with European mainlanders.

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The settlers arrived during what’s known as the Medieval Warm Period, when conditions across Europe and Greenland were temperate for a handful of centuries (SN: 7/24/19). But by 1350, the climate had started taking a turn for the worse with the beginning of the Little Ice Age, a period of regional cooling that lasted well into the 19th century.

Researchers have long speculated that a rapidly changing climate could have dealt a blow to Greenland’s Norse society. The island probably became much colder in the last 100 years of Norse occupation, says paleoclimatologist Boyang Zhao at Brown University in Providence, R.I, who was not involved in the new research. Lower temperatures could have made farming and raising livestock more difficult, he says. 

These lower temperatures would have had another impact on Greenland: the steady expansion of the island’s ice sheet, Borreggine and colleagues say.

Though rising sea levels usually go hand in hand with ice melting from ice sheets, oceans do not rise and fall uniformly in every place, Borreggine says. Around Greenland, sea level tends to rise when the ice sheet there grows.

This is for two main reasons: First, ice is heavy. The sheer weight of the ice sheet pushes the land it rests on down, meaning that as the ice sheet grows, more land is submerged. Second is gravity. Being massive, ice sheets exert some gravitational pull on nearby water. This makes the seawater around Greenland tilt upward toward the ice, meaning that water closer to the coast is higher than water in the open ocean. As the ice sheet grows, that pull becomes even stronger, and sea level close to the coast rises further.

Simulating the impact of the weight of the ice and its tug on Greenland’s waters, Borreggine and their colleagues found that sea level rose enough to flood the coast by hundreds of meters in some areas. Between the time the Vikings arrived and when they left, there was “pretty intense coastal flooding, such that certain pieces of land that were connected to each other were no longer connected,” they say.  

Today, some Viking sites are being inundated as a result of the overall rise in global sea level from climate change, which is being only marginally offset around Greenland by its melting ice sheet. Something similar could have happened back in the 14th and 15th centuries, destroying land that the Norse relied on for farming and grazing, Borreggine says.

“Previous theories about why Vikings left have really focused on the idea that they all died because it got really cold, and they were too dumb to adapt,” Borreggine says. But they say that archaeological digs have revealed a far more nuanced story, showing that Greenland’s Norse people did change their lifestyle by increasingly relying on seafood in the last century of their occupation.

But learning to adapt may have been too difficult in the face of an increasingly harsh landscape. The idea that rising sea levels may have been one of these challenges has merit, Zhao says, noting that the reasons why the Vikings disappeared from Greenland is nuanced.

As the climate changed, for example, these people may have also found themselves increasingly cut off from trade routes as the season for thick sea ice extended. And by the mid-14th century, the Black Plague was tearing through Europe, cutting into the Vikings’ biggest market for walrus ivory.

“Norse people came and left,” Zhao says. “But there are still a lot of unsolved questions,” including why exactly they left, he says.

The last written record of this society is a letter describing a wedding in 1408. A few years later, that couple moved to Iceland and started farming. Why the pair chose to leave is lost to history, but, as the new research suggests, sea level rise may have been part of the equation.  More

NEW ORLEANS — Warmer winters could make twisters more powerful.

Though tornadoes can occur in any season, the United States logs the greatest number of powerful twisters in the warmer months from March to July. Devastating winter tornadoes like the one that killed at least 88 people across Kentucky and four other states beginning on December 10 are less common. 

But climate change could increase tornado intensity in cooler months by many orders of magnitude beyond what was previously expected, researchers report December 13 in a poster at the American Geophysical Union’s fall meeting.

Tornadoes typically form during thunderstorms when warm, humid airstreams get trapped beneath cooler, drier winds. As the fast-moving air currents move past each other, they create rotating vortices that can transform into vertical, spinning twisters (SN: 12/14/18). Many tornadoes are short-lived, sometimes lasting mere minutes and traveling only 100 yards, says Jeff Trapp, an atmospheric scientist at the University of Illinois at Urbana-Champaign.

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Over the last 20 years, tornado patterns have shifted so that these severe weather events occur later in the season and across a broader range in the United States than before, Trapp says (SN: 10/18/18). But scientists have struggled to pin down a direct link between the twister changes and human-caused climate change.

Unlike hurricanes and other severe storm systems, tornadoes happen at such a small scale that most global climate simulations don’t include the storms, says Kevin Reed, an atmospheric scientist at Stony Brook University in New York who was not involved in the study.

To see how climate change may affect tornadoes, Trapp and colleagues started with atmospheric measurements of two historical tornadoes and simulated how those storm systems might play out in a warmer future.

The first historical tornado took place in the cool season on February 10, 2013, near Hattiesburg, Miss., and the second occurred in the warm season on May 20, 2013, in Moore, Okla. The researchers used a global warming simulation to predict how the twisters’ wind speeds, width and intensity could change in a series of alternative climate scenarios.

Both twisters would likely become more intense in futures affected by climate change, the team found. But the simulated winter storm was more than eightfold as powerful as its historical counterpart, in part due to a predicted 15 percent increase in wind speeds. Climate change is expected to increase the availability of warm, humid air systems during cooler months, providing an important ingredient for violent tempests.

“This is exactly what we saw on Friday night,” Trapp says. The unseasonably warm weather in the Midwest on the evening of December 10 and in the early morning of December 11 probably contributed to the devastation of the tornado that traveled hundreds of miles from Arkansas to Kentucky, he speculates.

Simulating how historical tornados could intensify in future climate scenarios is a “clever way” to address the knowledge gap around the effects of climate change on these severe weather systems, says Daniel Chavas, an atmospheric scientist at Purdue University in West Lafayette, Ind., who was not involved in the study.

But Chavas notes that this research is only one piece of a larger puzzle as researchers investigate how tornados might impact communities in the future.

One drawback of this type of simulation is it often requires direct measurements from a historical event, Reed says. That limits its prediction power to re-creating documented tornadoes rather than broadly forecasting shifts in large-scale weather systems.

Though the team based its predictions on only two previous tornados, Trapp says he hopes that adding more historical twisters to the analysis could provide more data for policy makers as well as residents of communities that may have to bear the force of intensifying tornadoes. More

The demise of a West Antarctic glacier poses the world’s biggest threat to raise sea levels before 2100 — and an ice shelf that’s holding it back from the sea could collapse within three to five years, scientists reported December 13 at the American Geophysical Union’s fall meeting in New Orleans.

Thwaites Glacier is “one of the largest, highest glaciers in Antarctica — it’s huge,” Ted Scambos, a glaciologist at the Boulder, Colo.–based Cooperative Institute for Research in Environmental Sciences, told reporters. Spanning 120 kilometers across, the glacier is roughly the size of Florida, and were the whole thing to fall into the ocean, it would raise sea levels by 65 centimeters, or more than two feet. Right now, its melting is responsible for about 4 percent of global sea level rise.

But a large portion of the glacier is about to lose its tenuous grip on the seafloor, and that will dramatically speed up its seaward slide, the researchers said. Since about 2004, the eastern third of Thwaites has been braced by a floating ice shelf, an extension of the glacier that juts out into the sea. Right now, the underbelly of that ice shelf is lodged against an underwater mountain located about 50 kilometers offshore. That pinning point is essentially helping to hold the whole mass of ice in place.

But data collected by researchers beneath and around the shelf in the last two years suggests that brace won’t hold much longer. Warm ocean waters are inexorably eating away at the ice from below (SN: 4/9/21; SN: 9/9/20). As the glacier’s ice shelf loses mass, it’s retreating inland, and will eventually retreat completely behind the underwater mountain pinning it in place. Meanwhile, fractures and crevasses, widened by these waters, are swiftly snaking through the ice like cracks in a car’s windshield, shattering and weakening it. 

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This deadly punch-jab-uppercut combination of melting from below, ice shattering and losing its grip on the pinning point is pushing the ice shelf to imminent collapse, within as little as three to five years, said Erin Pettit, a glaciologist at Oregon State University in Corvallis. And “the collapse of this ice shelf will result in a direct increase in sea level rise, pretty rapidly,” Pettit added. “It’s a little bit unsettling.”

Satellite data show that over the last 30 years, the flow of Thwaites Glacier across land and toward the sea has nearly doubled in pace. The collapse of this “Doomsday Glacier” alone would alter sea levels significantly, but its fall would also destabilize other West Antarctic glaciers, dragging more ice into the ocean and raising sea levels even more.

That makes Thwaites “the most important place to study for near-term sea level rise,” Scambos said. So in 2018, researchers from the United States and the United Kingdom embarked on a joint five-year project to intensively study the glacier and try to anticipate its imminent future by planting instruments atop, within, below it as well as offshore of it.

This pull-out-all-the-stops approach to studying Thwaites is leading to other rapid discoveries, including the first observations of ocean and melting conditions right at a glacier’s grounding zone, where the land-based glacier begins to jut out into a floating ice shelf. Scientists have also spotted how the rise and fall of ocean tides can speed up melting, by pumping warm waters farther beneath the ice and creating new melt channels and crevasses in the underside of the ice.

To better understand the rapid retreat of Thwaites Glacier, scientists drilled a hole through the ice at the glacier’s grounding zone, the region where the land-based glacier juts out into the sea to become a floating ice shelf. Heated water (heaters shown here) carved a borehole through the ice down to the grounding zone, allowing scientists to take the first ever measurements of ocean conditions in the region.PETER DAVIS/BAS

As Thwaites and other glaciers retreat inland, some scientists have pondered whether they might form very tall cliffs of ice along the edge of the ocean — and the potential tumble of such massive blocks of ice into the sea could lead to devastatingly rapid sea level rise, a hypothesis known as marine ice cliff instability (SN: 2/6/19). How likely researchers say such a collapse is depends on our understanding of the physics and dynamics of ice behavior, something about which scientists have historically known very little (SN: 9/23/20).

The Thwaites collaboration is also tackling this problem. In simulations of the further retreat of Thwaites, glaciologist Anna Crawford of the University of St. Andrews in Scotland and her colleagues found that if the shape of the land beneath the glacier dips deep enough in some places, that could lead to some very tall ice cliffs — but, they found, the ice itself might also deform and thin enough to make tall ice cliff formation difficult.

The collaboration is only at its halfway point now, but these data already promise to help scientists better estimate the near-term future of Thwaites, including how quickly and dramatically it might fall, Scambos said. “We’re watching a world that’s doing things we haven’t really seen before, because we’re pushing on the climate extremely rapidly with carbon dioxide emissions,” he added. “It’s daunting.” More

Wildfire smoke and urban air pollution bring out the worst in each other.

As wildfires rage, they transform their burned fuel into a complex chemical cocktail of smoke. Many of these airborne compounds, including ozone, cause air quality to plummet as wind carries the smoldering haze over cities. But exactly how — and to what extent — wildfire emissions contribute to ozone levels downwind of the fires has been a matter of debate for years, says Joel Thornton, an atmospheric scientist at the University of Washington in Seattle.

A new study has now revealed the elusive chemistry behind ozone production in wildfire plumes. The findings suggest that mixing wildfire smoke with nitrogen oxides — toxic gases found in car exhaust — could pump up ozone levels in urban areas, researchers report December 8 in Science Advances.

Atmospheric ozone is a major component of smog that can trigger respiratory problems in humans and wildlife (SN: 1/4/21). Many ingredients for making ozone — such as volatile organic compounds and nitrogen oxides — can be found in wildfire smoke, says Lu Xu, an atmospheric chemist currently at the National Oceanographic and Atmospheric Administration Chemical Sciences Laboratory in Boulder, Colo. But a list of ingredients isn’t enough to replicate a wildfire’s ozone recipe. So Xu and colleagues took to the sky to observe the chemistry in action.

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Through a joint project with NASA and NOAA, the researchers worked with the Fire Influence on Regional to Global Environments and Air Quality flight campaign to transform a jetliner into a flying laboratory. In July and August 2019, the flight team collected air samples from smoldering landscapes across the western United States. As the plane passed headlong through the plumes, instruments onboard recorded the kinds and amounts of each molecule detected in the haze. By weaving in and out of the smoke as it drifted downwind from the flames, the team also analyzed how the plume’s chemical composition changed over time.

Using these measurements along with the wind patterns and fuel from each wildfire sampled, the researchers created a straightforward equation to calculate ozone production from wildfire emissions. “We took a complex question and gave it a simple answer,” says Xu, who did the work while at Caltech.

As expected, the researchers found that wildfire emissions contain a dizzying array of organic compounds and nitrogen oxide species among other molecules that contribute to ozone formation. Yet their analysis showed that the concentration of nitrogen oxides decreases in the hours after the plume is swept downwind. Without this key ingredient, ozone production slows substantially.  

Air pollution from cities and other urban areas is chock full of noxious gases. So when wildfire smoke wafts over cityscapes, a boost of nitrous oxides could jump-start ozone production again, Xu says.

In a typical fire season, mixes like these could increase ozone levels by as much as 3 parts per billion in the western United States, the researchers estimate. This concentration is far below the U.S. Environmental Protection Agency’s health safety standard of 70 parts per billion, but the incremental increase could still pose a health risk to people who are regularly exposed to smoke, Xu says.

With climate change increasing the frequency and intensity of wildfires, this new ozone production mechanism has important implications for urban air quality, says Qi Zhang, an atmospheric chemist at the University of California, Davis who was not involved in the study (SN: 9/18/20). She says the work provides an “important missing link” between wildfire emissions and ozone chemistry.

The findings may also pose a challenge for environmental policy makers, says Thornton, who was not involved in the research. Though state and local authorities set strict regulations to limit atmospheric ozone, wildfire smoke may undermine those strategies, he says. This could make it more difficult for cities, especially in the western United States, to meet EPA ozone standards despite air quality regulations. More

It’s hard to imagine what Earth might look like in 2500. But a collaboration between science and art is offering an unsettling window into how ongoing climate change might transform now-familiar terrain into alien landscapes over the next few centuries.

These visualizations — of U.S. Midwestern farms overtaken by subtropical plants, of a dried-up Amazon rainforest, of extreme heat baking the Indian subcontinent — emphasize why researchers need to push climate projections long past the customary benchmark of 2100, environmental social scientist Christopher Lyon and colleagues contend September 24 in Global Change Biology.

Fifty years have passed since the first climate projections, which set that distant target at 2100, says Lyon, of McGill University in Montreal. But that date isn’t so far off anymore, and the effects of greenhouse gas emissions emitted in the past and present will linger for centuries (SN: 8/9/21).

To visualize what that future world might look like, the researchers considered three possible climate trajectories — low, moderate and high emissions as used in past reports by the United Nations’ Intergovernmental Panel on Climate Change — and projected changes all the way out to 2500 (SN: 1/7/20). The team focused particularly on impacts on civilization: heat stress, failing crops and changes in land use and vegetation (SN: 3/13/17).

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For all but the lowest-emission scenario, which is roughly in line with limiting global warming to “well under” 2 degrees Celsius relative to preindustrial times as approved by the 2015 Paris Agreement, the average global temperature continues to increase until 2500, the team found (SN: 12/12/15). For the highest-emissions scenario, temperatures increase by about 2.2 degrees C by 2100 and by about 4.6 degrees C by 2500. That results in “major restructuring of the world’s biomes,” the researchers say: loss of most of the Amazon rainforest, poleward shifts in crops and unlivable temperatures in the tropics.

The team then collaborated with James McKay, an artist and science communicator at the University of Leeds in England, to bring the data to life. Based on the study’s projections, McKay created a series of detailed paintings representing different global landscapes now and in 2500.

The team stopped short of trying to speculate on future technologies or cities to keep the paintings based more in realism than science fiction, Lyon says. “But we did want to showcase things people would recognize: drones, robotics, hybrid plants.” In one painting of India in 2500, a person is wearing a sealed suit and helmet, a type of garment that people in some high-heat environments might wear today, he says.

The goal of these images is to help people visualize the future in such a way that it feels more urgent, real and close — and, perhaps, to offer a bit of hope that humans can still adapt. “If we’re changing on a planetary scale, we need to think about this problem as a planetary civilization,” Lyon says. “We wanted to show that, despite the climate people have moved into, people have figured out ways to exist in the climate.”

2000 vs. 2500

High greenhouse gas emissions could increase average global temperatures by about 4.6 degrees Celsius relative to preindustrial times. As a result, extreme heat in India could dramatically alter how humans live in the environment. Farmers and herders, shown in 2000 the painting at left, may require protective clothing such as a cooling suit and helmet to work outdoors by 2500, as shown in the painting at right.

If greenhouse gas emissions remain high, the U.S. Midwest’s “breadbasket” farms, as seen below in 2000 in the painting at left, could be transformed into subtropical agroforestry regions by 2500, researchers say. The region might be dotted with some versions of oil palms and succulents, as envisioned in the painting at right, and rely on water capture and irrigation devices to offset extreme summer heat.

All: James McKay (CC-BY-ND) More

Over decades, centuries and millennia, the steady skyward climb of redwoods, the tangled march of mangroves along tropical coasts and the slow submersion of carbon-rich soil in peatlands has locked away billions of tons of carbon. 

If these natural vaults get busted open, through deforestation or dredging of swamplands, it would take centuries before those redwoods or mangroves could grow back to their former fullness and reclaim all that carbon. Such carbon is “irrecoverable” on the timescale — decades, not centuries — needed to avoid the worst impacts of climate change, and keeping it locked away is crucial.

Now, through a new mapping project, scientists have estimated how much irrecoverable carbon resides in peatlands, mangroves, forests and elsewhere around the globe — and which areas need protection.

The new estimate puts the total amount of irrecoverable carbon at 139 gigatons, researchers report November 18 in Nature Sustainability. That’s equivalent to about 15 years of human carbon dioxide emissions at current levels. And if all that carbon were released, it’s almost certainly enough to push the planet past 1.5 degrees Celsius of warming above preindustrial levels.

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“This is the carbon we must protect to avert climate catastrophe,” says Monica Noon, an environmental data scientist at Conservation International in Arlington, Va. Current efforts to keep global warming below the ambitious target of 1.5 degrees C require that we reach net-zero emissions by 2050, and that carbon stored in nature stays put (SN:12/17/18). But agriculture and other development pressures threaten some of these carbon stores.

To map this at-risk carbon, Noon and her colleagues combined satellite data with estimates of how much total carbon is stored in ecosystems vulnerable to human incursion. The researchers excluded areas like permafrost, which stores lots of carbon but isn’t likely to be developed (although it’s thawing due to warming), as well as tree plantations, which have already been altered (SN: 9/25/19). The researchers then calculated how much carbon would get released from land conversions, such as clearing a forest for farmland. 

That land might store varying amounts of carbon, depending on whether it becomes a palm oil plantation or a parking lot. To simplify, the researchers assumed cleared land was left alone, with saplings free to grow where giants once stood. That allowed the researchers to estimate how long it might take for the released carbon to be reintegrated into the land. Much of that carbon would remain in the air by 2050, the team reports, as many of these ecosystems take centuries to return to their former glory, rendering it irrecoverable on a timescale that matters for addressing climate change.

Releasing that 139 gigatons of irrecoverable carbon could have irrevocable consequences. For comparison, the United Nations’ Intergovernmental Panel on Climate Change estimates that humans can emit only 109 more gigatons of carbon to have a two-thirds chance of keeping global warming below 1.5 degrees C. “These are the places we absolutely have to protect,” Noon says.

Approximately half of this irrecoverable carbon sits on just 3.3 percent of Earth’s total land area, equivalent to roughly the area of India and Mexico combined. Key areas are in the Amazon, the Pacific Northwest, and the tropical forests and mangroves of Borneo. “The fact that it’s so concentrated means we can protect it,” Noon says.

Roughly half of irrecoverable carbon already falls within existing protected areas or lands managed by Indigenous peoples. Adding an additional 8 million square kilometers of protected area, which is only about 5.4 percent of the planet’s land surface, would bring 75 percent of this carbon under some form of protection, Noon says.

“It’s really important to have spatially explicit maps of where these irrecoverable carbon stocks are,” says Kate Dooley, a geographer at the University of Melbourne in Australia who wasn’t involved in the study. “It’s a small percentage globally, but it’s still a lot of land.” Many of these dense stores are in places at high risk of development, she says. 

“It’s so hard to stop this drive of deforestation,” she says, but these maps will help focus the efforts of governments, civil society groups and academics on the places that matter most for the climate. More