Climate

If a tree farts in the forest, does it make a sound? No, but it does add a smidge of greenhouse gas to the atmosphere.

Gases released by dead trees — dubbed “tree farts” — account for roughly one-fifth of the greenhouse gases emitted by skeletal, marshy forests along the coast of North Carolina, researchers report online May 10 in Biogeochemistry. While these emissions pale in comparison with other sources, an accurate accounting is necessary to get a full picture of where climate-warming gases come from.

A team of ecologists went sniffing for tree farts in ghost forests, which form when saltwater from rising sea levels poisons a woodland, leaving behind a marsh full of standing dead trees. These phantom ecosystems are expected to expand with climate change, but it’s unclear exactly how they contribute to the world’s carbon budget.

“The emergence of ghost forests is one of the biggest changes happening in response to sea level rise,” says Keryn Gedan, a coastal ecologist at George Washington University in Washington, D.C., who was not involved in the work. “As forests convert to wetlands, we expect over long timescales that’s going to represent a substantial carbon sink,” she says, since wetlands store more carbon than forests. But in the short term, dead trees decay and stop taking up carbon dioxide through photosynthesis, “so that’s going to be a major greenhouse gas source.”

Sign Up For the Latest from Science News

Headlines and summaries of the latest Science News articles, delivered to your inbox

To better understand how ghost forests pass gas into the atmosphere, the researchers measured greenhouse gases wafting off dead trees and soil in five ghost forests on the Albemarle-Pamlico Peninsula in North Carolina. “It’s kind of eerie” out there, says Melinda Martinez, a wetland ecologist at North Carolina State University in Raleigh.

But Martinez ain’t afraid of no ghost forest. In 2018 and 2019, she measured CO2, methane and nitrous oxide emissions from dead trees using a portable gas analyzer she toted on her back. “I definitely looked like a ghostbuster,” she says.

Wetland ecologist Melinda Martinez totes a portable gas analyzer on her back to measure the “tree farts” emitted by a ghost forest tree. A tube connects the gas analyzer to an airtight seal around the trunk of the tree.M. Ardón

Soils gave off most of the greenhouse gases from the ghost forests. Each square meter of ground emitted an average 416 milligrams of CO2, 5.9 milligrams of methane and 0.1 milligrams of nitrous oxide per hour. On average, dead trees released about 116 milligrams of CO2, 0.3 milligrams of methane and 0.04 milligrams of nitrous oxide per square meter per hour — totaling about one-fourth the soil’s emissions.

Measuring greenhouse gases from the trees is “kind of measuring the last breath of these forests,” says Marcelo Ardón, an ecosystems ecologist and biogeochemist at North Carolina State University. The dead trees “don’t emit a ton, but they are important” to a ghost forest’s overall emissions.

Ardón coined the term “tree farts” to describe the dead trees’ greenhouse gas emissions. “I have an 8-year-old and an 11-year-old, and fart jokes are what we talk about,” he explains. But the analogy has a biological basis, too. Actual farts are caused by microbes in the body; the greenhouse gases emitted by ghost forests are created by microbes in the soil and trees.

In the grand scheme of carbon emissions, ghost forests’ role may be minor. Tree farts, for instance, have nothing on cow burps (SN: 11/18/15). A single dairy cow can emit up to 27 grams of methane — a far more potent greenhouse gas than CO2 — per hour. But accounting for even minor sources of carbon is important for fine-tuning our understanding of the global carbon budget, says Martinez (SN: 10/1/19). So it would behoove scientists not to turn up their noses at ghost tree farts.   More

Winter usually kills most forest fires. But in the boreal woods that encircle the far North, some fires, like zombies, just don’t die. 

The first broad scientific look at overwintering “zombie fires” reveals these rare occurrences can flare up the year after warmer-than-normal summers and account for up to 38 percent of the total burn area in some regions, researchers report online May 19 in Nature. As climate change accelerates in boreal forests, the frequency of zombie fires could rise and exacerbate warming by releasing more greenhouse gases from the region’s soils, which may house twice as much carbon as Earth’s atmosphere (SN: 4/11/19).

Zombie fires hibernate underground. Blanketed by snow, they smolder through the cold, surviving on the carbon-rich fuel of peat and boreal soil and moving very slowly — just 100 to 500 meters over the winter. Come spring, the fires reemerge near the forest they previously charred, burning fresh fuel well before the traditional fire season starts. Until now, these zombie fires have remained relatively mysterious to science, known mostly from firefighter anecdotes.

Strange coincidences on satellite images, however, got the attention of earth systems scientist Rebecca Scholten and her colleagues. “My adviser noticed that some years, new fires were starting very close to the previous year’s fire,” says Scholten, of Vrije University Amsterdam. This is unusual, she says, since boreal fires are usually sparked by random lightning or human activity. Local fire managers confirmed that these were the same fires, prompting the researchers to wonder just how often fires overwinter.

Sign Up For the Latest from Science News

Headlines and summaries of the latest Science News articles, delivered to your inbox

To find evidence of underground fires, the researchers combined firefighter reports with satellite images of Alaska and northern Canada captured from 2002 to 2018. They looked for blazes that started close to the scars left the previous year and that began before midsummer, when lightning-sparked fires usually occur.

The team found that zombie fires are rare, accounting for 0.8 percent of the total area burned by forest fires in these regions over those 16 years, but there was lots of variability. In 2008, one zombie fire burned approximately 13,700 hectares in Alaska, about 38 percent of all burned areas that year in that state. Zombie fires were more likely to occur, and burn larger swaths of land, after warmer summers that allowed fires to reach deeper into the soil, the researchers found.

Boreal forests are warming faster that the global average and “we’re seeing more hot summers and more large fires and intense burning,” Scholten says. That might set the stage for zombie fires to play a bigger role.

“This is a really welcome advance which could help fire management,” says Jessica McCarty, a geographer at Miami University in Oxford, Ohio, who wasn’t involved in the study. Understanding when zombie fires are more likely to occur could help firefighters identify these areas early, she says, protecting fragile landscapes that house a lot of climate warming gases.

“Some of these soils are thousands of years old,” McCarty says. While “areas we thought were fire resistant are now fire prone” due to climate change, she says, better fire management can make a difference. “We’re not helpless.” More

Over the last four decades, a highly organized, well-funded campaign powered by the fossil fuel industry has sought to discredit the science that links global climate change to human emissions of carbon dioxide and other greenhouse gases. These disinformation efforts have sown confusion over data, questioned the integrity of climate scientists and denied the scientific consensus on the role of humans.

Such disinformation efforts are outlined in internal documents from fossil fuel giants such as Shell and Exxon. As early as the 1980s, oil companies knew that burning fossil fuels was altering the climate, according to industry documents reviewed at a 2019 U.S. House of Representatives Committee on Oversight and Reform hearing. Yet these companies, aided by some scientists, set out to mislead the public, deny well-established science and forestall efforts to regulate emissions.

But the effects of climate change on extreme events such as wildfires, heat waves and hurricanes have become hard to downplay (SN: 12/19/20 & SN: 1/2/21, p. 37). Not coincidentally, climate disinformation tactics have shifted from outright denial to distraction and delay (SN: 1/16/21, p. 28).

As disinformation tactics evolve, researchers continue to test new ways to combat them. Debunking by fact-checking untrue statements is one way to combat climate disinformation. Another way, increasingly adopted by social media platforms, is to add warning labels flagging messages as possible disinformation, such as the labels Twitter and Facebook (which also owns Instagram) began adding in 2020 regarding the U.S. presidential election and the COVID-19 pandemic.

Sign Up For the Latest from Science News

Headlines and summaries of the latest Science News articles, delivered to your inbox

At the same time, Facebook was sharply criticized for a change to its fact-checking policies that critics say enables the spread of climate disinformation. In 2019, the social media giant decided to exempt posts that it determines to be opinion or satire from fact-checking, creating a potentially large disinformation loophole.

In response to mounting criticism, Facebook unveiled a pilot project in February for its users in the United Kingdom, with labels pointing out myths about climate change. The labels also point users to Facebook’s climate science information center.

For this project, Facebook consulted several climate communication experts. Sander van der Linden, a social psychologist at the University of Cambridge, and cognitive scientist John Cook of George Mason University in Fairfax, Va., helped the company develop a new “myth-busting” unit that debunks common climate change myths — such as that scientists don’t agree that global warming is happening.

Cook and van der Linden have also been testing ways to get out in front of disinformation, an approach known as prebunking, or inoculation theory. By helping people recognize common rhetorical techniques used to spread climate disinformation — such as logical fallacies, relying on fake “experts” and cherry-picking only the data that support one view — the two hope to build resilience against these tactics.

This new line of defense may come with a bonus, van der Linden says. Training people in these techniques could build a more general resilience to disinformation, whether related to climate, vaccines or COVID-19.

Science News asked Cook and van der Linden about debunking conspiracies, collaborating with Facebook and how prebunking is (and isn’t) like getting vaccinated. The conversations, held separately, have been edited for brevity and clarity.

We’ve seen both misinformation and disinformation used in the climate change denial discussion. What’s the difference?

van der Linden: Misinformation is any information that’s incorrect, whether due to error or fake news. Disinformation is deliberately intended to deceive. Then there’s propaganda: disinformation with a political agenda. But in practice, it’s difficult to disentangle them. Often, people use misinformation because it’s the broadest category.

Has there been a change in the nature of climate change denialism in the last few decades?

Cook: It is shifting. For example, we fed 21 years of [climate change] denial blog posts from the U.K. into a machine learning program. We found that the science denialism misinformation is gradually going down — and solution misinformation [targeting climate policy and renewable energy] is on the rise [as reported online in early March at SocArXiv.org].

As the science becomes more apparent, it becomes more untenable to attack it. We see spikes in policy misinformation just before the government brings in new science policy, such as a carbon pricing bill. And there was a huge spike before the [2015] Paris climate agreement. That’s what we will see more of over time.

How do you hope Facebook’s new climate change misinformation project will help?

Cook: We need tech solutions, like flagging and tagging misinformation, as well as social media platforms downplaying it, so [the misinformation] doesn’t get put on as many people’s feeds. We can’t depend on social media. A look behind the curtain at Facebook showed me the challenge of getting corporations to adequately respond. There are a lot of internal tensions.

van der Linden: I’ve worked with WhatsApp and Google, and it’s always the same story. They want to do the right thing, but don’t follow through because it hurts engagement on the platform.

But going from not taking a stance on climate change to taking a stance, that’s a huge win. What Facebook has done is a step forward. They listened to our designs and suggestions and comments on their [pilot] test.

We wanted more than a neutral [label directing people to Facebook’s information page on climate change], but they wanted to test the neutral post first. That’s all good. It’ll be a few months at least for the testing in the U.K. phase to roll out, but we don’t yet know how many other countries they will roll it out to and when. We all came on board with the idea that they’re going to do more, and more aggressively. I’ll be pleasantly surprised if it rolls out globally. That’s my criteria for success.

Scientists have been countering climate change misinformation for years, through fact-checking and debunking. It’s a bit like whack-a-mole. You advocate for “inoculating” people against the techniques that help misinformation spread through communities. How can that help?

van der Linden: Fact-checking and debunking is useful if you do it right. But there’s the issue of ideology, of resistance to fact-checking when it’s not in line with ideology. Wouldn’t life be so much easier if we could prevent [disinformation] in the first place? That’s the whole point of prebunking or inoculation. It’s a multilayer defense system. If you can get there first, that’s great. But that won’t always be possible, so you still have real-time fact-checking. This multilayer firewall is going to be the most useful thing.

You’ve both developed online interactive tools, games really, to test the idea of inoculating people against disinformation tactics. Sander, you created an online interactive game called Bad News, in which players can invent conspiracies and act as fake news producers. A study of 15,000 participants reported in 2019 in Palgrave Communications showed that by playing at creating misinformation, people got better at recognizing it. But how long does this “inoculation” last?

van der Linden: That’s an important difference in the viral analogy. Biological vaccines give more or less lifelong immunity, at least for some kinds of viruses. That’s not the case for a psychological vaccine. It wears off over time.

In one study, we followed up with people [repeatedly] for about three months, during which time they didn’t replay the game. We found no decay of the inoculation effect, which was quite surprising. The inoculation remained stable for about two months. In [a shorter study focused on] climate change misinformation, the inoculation effect also remained stable, for at least one week.

John, what about your game Cranky Uncle? At first, it focused on climate change denial, but you’ve expanded it to include other types of misinformation, on topics such as COVID-19, flat-earthism and vaccine misinformation. How well do techniques to inoculate against climate change denialism translate to other types of misinformation?

Cook: The techniques used in climate denial are seen in all forms of misinformation. Working on deconstructing [that] misinformation introduced me to parallel argumentation, which is basically using analogies to combat flawed logic. That’s what late night comedians do: Make what is obviously a ridiculous argument. The other night, for example, Seth Meyers talked about how Texas blaming its [February] power outage on renewable energy was like New Jersey blaming its problems on Boston [clam chowder].

My main tip is to arm yourself with awareness of misleading techniques. Think of it like a virus spreading: You don’t want to be a superspreader. Make sure that you’re wearing a mask, for starters. And when you see misinformation, call it out. That observational correction — it matters. It makes a difference. More

Rivers ravaged by a lengthy drought may not be able to recover, even after the rains return. Seven years after the Millennium drought baked southeastern Australia, a large fraction of the region’s rivers still show no signs of returning to their predrought water flow, researchers report in the May 14 Science.

There’s “an implicit assumption that no matter how big a disturbance is, the water will always come back — it’s just a matter of how long it takes,” says Tim Peterson, a hydrologist at Monash University in Melbourne, Australia. “I’ve never been satisfied with that.”

The years-long drought in southeastern Australia, which began sometime between 1997 and 2001 and lasted until 2010, offered a natural experiment to test this assumption, he says. “It wasn’t the most severe drought” the region has ever experienced, but it was the longest period of low rainfall in the region since about 1900.

Peterson and colleagues analyzed annual and seasonal streamflow rates in 161 river basins in the region from before, during and after the drought. By 2017, they found, 37 percent of those river basins still weren’t seeing the amount of water flow that they had predrought. Furthermore, of those low-flow rivers, the vast majority — 80 percent — also show no signs that they might recover in the future, the team found.

Sign Up For the Latest from Science News

Headlines and summaries of the latest Science News articles, delivered to your inbox

Many of southeastern Australia’s rivers had bounced back from previous droughts, including a severe but brief episode in 1983. But even heavy rains in 2010, marking the end of the Millennium drought, weren’t enough to return these basins to their earlier state. That suggests that there is, after all, a limit to rivers’ resilience.

What’s changed in these river basins isn’t yet clear, Peterson says. The precipitation post drought was similar to predrought precipitation, and the water isn’t ending up in the streamflow, so it must be going somewhere else. The team examined various possibilities: The water infiltrated into the ground and was stored as groundwater, or it never made it to the ground at all — possibly intercepted by leaves, and then evaporating back to the air.

But none of these explanations were borne out by studies of these sites, the researchers report. The remaining, and most probable, possibility is that the environment has changed: Water is evaporating from soils and transpiring from plants more quickly than it did predrought.

Peterson has long suggested that under certain conditions rivers might not, in fact, recover — and this study confirms that theoretical work, says Peter Troch, a hydrologist at the University of Arizona in Tucson. Enhanced soil evaporation and plant transpiration are examples of such positive feedbacks, processes that can enhance the impacts of a drought. “Until his work, this lack of resilience was not anticipated, and all hydrological models did not account for such possibility,” Troch says.

“This study will definitely inspire other researchers to undertake such work,” he notes. “Hopefully we can gain more insight into the functioning of [river basins’] response to climate change.”

Indeed, the finding that rivers have “finite resilience” to drought is of particular concern as the planet warms and lengthier droughts become more likely, writes hydrologist Flavia Tauro in a commentary in the same issue of Science. More

Coastal mangrove forests are carbon storage powerhouses, tucking away vast amounts of organic matter among their submerged, tangled root webs.

But even for mangroves, there is a “remarkable” amount of carbon stored in small pockets of forest growing around sinkholes on Mexico’s Yucatan Peninsula, researchers report May 5 in Biology Letters. These forests can stock away more than five times as much carbon per hectare as most other terrestrial forests.

There are dozens of mangrove-lined sinkholes, or cenotes, on the peninsula. Such carbon storage hot spots could help nations or companies achieve carbon neutrality — in which the volume of greenhouse gas emissions released into the atmosphere is balanced by the amount of carbon sequestered away (SN: 1/31/20).

At three cenotes, researchers led by Fernanda Adame, a wetland scientist at Griffith University in Brisbane, Australia, collected samples of soil at depths down to 6 meters, and used carbon-14 dating to estimate how fast the soil had accumulated at each site. The three cenotes each had “massive” amounts of soil organic carbon, the researchers report, averaging about 1,500 metric tons per hectare. One site, Casa Cenote, stored as much as 2,792 metric tons per hectare.

Mangrove roots make ideal traps for organic material. The submerged soils also help preserve carbon. As sea levels have slowly risen over the last 8,000 years, mangroves have kept pace, climbing atop sediment ported in from rivers or migrating inland. In the cave-riddled limestone terrain of the Yucatan Peninsula, there are no rivers to supply sediment. Instead, “the mangroves produce more roots to avoid drowning,” which also helps the trees climb upward more quickly, offering more space for organic matter to accumulate, Adame says.

As global temperatures increase, sea levels may eventually rise too quickly for mangroves to keep up (SN: 6/4/20). Other, more immediate threats to the peninsula’s carbon-rich cenotes include groundwater pollution, expanding infrastructure, urbanization and tourism. More

A sudden zag in which way the North Pole was drifting in the 1990s probably stemmed in large part from glacial melt caused by climate change, a new study suggests.

The locations of Earth’s geographic poles, where the planet’s axis pierces the surface, aren’t fixed. Instead, they wander in seasonal and near-annual cycles, largely driven by weather patterns and ocean currents (SN: 4/15/03). But in addition to moving about in relatively tight swirls just a few meters across, the poles drift over time as the planet’s weight distribution shifts and alters its rotation around its axis.

Before the mid-1990s, the North Pole had been drifting toward the western edge of Canada’s Ellesmere Island. But then the pole veered eastward by about 71 degrees toward the northeastern tip of Greenland. It’s continued to head that way, moving about 10 centimeters per year. Scientists aren’t quite sure why this shift occurred, says Suxia Liu, a hydrologist at the Institute of Geographic Sciences and Natural Resources Research in Beijing.

Liu and colleagues checked how well the polar drift trends matched data from previous studies on glacial melt worldwide. In particular, glacier melt in Alaska, Greenland and the southern Andes accelerated in the 1990s (SN: 9/30/20). The timing of that melting, as well as the effects it would have had on Earth’s mass distribution, suggests that glacial melt induced by climate change helped trigger the change in polar drift, the team reports in the April 16 Geophysical Research Letters.

The team’s analysis shows that while glacier melting can account for much of the change in polar drift, it doesn’t explain all of it. So other factors must be at play. With copious irrigation, for example, groundwater pumped from aquifers in one region can end up in an ocean far away (SN: 10/9/19). Like glacial melt, water management alone can’t explain the North Pole’s tack, the team reports, but it can give the Earth’s axis a substantial nudge.

The findings “reveal how much human activity can have an impact on changes to the mass of water stored on land,” says Vincent Humphrey, a climate scientist at the University of Zurich not involved in this study. And they show how large these mass shifts can be, he says. “They’re so big that they can change the axis of the Earth.” More

The under-ice trek of an autonomous underwater vehicle is giving scientists their first direct evidence for how and where warm ocean waters are threatening the stability of Antarctica’s vulnerable Thwaites Glacier. These new data will ultimately help scientists more accurately project the fate of the glacier — how quickly it is melting and retreating inland, and how far it might be from complete collapse, the team reports April 9 in Science Advances.

“We know there’s a sick patient out there, and it’s not able to tell us where it hurts,” says Eric Rignot, a glaciologist at the University of California, Irvine who was not involved in the new study. “So this is the first diagnosis.”

Scientists have eyed the Florida-sized Thwaites Glacier with mounting concern for two decades. Satellite images reveal it has been retreating at an alarming rate of somewhere between 0.6 to 0.8 kilometers per year on average since 2001, prompting some to dub it the “doomsday glacier.” But estimates of how quickly the glacier is retreating, based on computer simulations, vary widely from place to place on the glacier, Rignot and other researchers reported in Science Advances in 2019. Such uncertainty is the biggest difficulty when it comes to future projections of sea level rise (SN: 1/7/20).

The primary culprit for the rapid retreat of Thwaites and other Antarctic glaciers is known: Relatively warm ocean waters sneak beneath the floating ice shelves, the fringes of the glaciers that jut out into the ocean (SN: 9/9/20). This water eats away at the ice shelves’ underpinnings, points where the ice is anchored to the seafloor that buttress the rest of the glacier against sliding into the sea.

Sign Up For the Latest from Science News

Headlines and summaries of the latest Science News articles, delivered to your inbox

Scientists have used satellite data to roughly map out what lies beneath the Thwaites ice shelf. Three deep channels carved into the seafloor snake beneath a vast water-filled cavity 120 kilometers across. But without direct measurements of the chemistry and paths the water takes to reach Thwaites’ underbelly, it’s been impossible to know where the threatening water is really coming from, how warm it is, and where it’s attacking the ice, says Anna Wåhlin, a physical oceanographer at the University of Gothenburg in Sweden.

In February and March 2019, Wåhlin and her colleagues sent the AUV Ran to traverse two of the deep channels. Gliding about 50 meters above the seafloor, the AUV collected the first direct measurements of temperature, salinity and oxygen levels in the water. From those measurements, the team was able to trace the origins of different parcels of water mixing beneath Thwaites.

Based on its chemical makeup, some of the warm water came from neighboring Pine Island Bay. “We were very surprised,” because Pine Island Bay wasn’t previously thought to be a major player in the future of Thwaites, Wåhlin says. The water mass from there was near the bottom of the cavity, about 500 meters deep, and was both less salty than the surrounding seawater and several degrees Celsius warmer than the freezing point. That’s an unstable situation, likely to create turbulence, and increasing the potential for erosion of the ice, Wåhlin says.

The find also suggests that what happens in Pine Island Bay doesn’t necessarily stay in Pine Island Bay — and that the fate of Thwaites may be closely intertwined with that of the Pine Island Glacier, another rapidly-melting river of ice, Wåhlin says. Together, the two glaciers are responsible for most of the ice and water that Antarctica is currently shedding. But while Thwaites is still pinned to the seafloor in some places, which slows its slide into the sea, those underpinnings are long gone for Pine Island, she says.

In April, scientists identified three tipping points for the precarious Pine Island glacier, thresholds it might cross as climate conditions evolve that would lead to phases of rapid, irreversible retreat. The third and final threshold, prompted by a roughly 1.2 degree Celsius increase in the temperature of ocean waters compared with current ocean temperatures, would drive the glacier to complete collapse, the team found.

An upcoming expedition Wåhlin and others are planning for January 2022 will use two AUVs to explore much farther into the cavity beneath Thwaites. Ideally, the AUVS will get several hundred kilometers closer to the shore, all the way to the grounding line, where the base of the glacier rests on land.

“That’s the key down the line,” Rignot says. Observing how water masses are interacting with the glacier’s grounding line will be crucial to understanding the future of the glacier, he says. “That’s the place where melting makes the most difference to the glacier’s stability.”

And there’s a lot that researchers still don’t know about the vast water cavity beneath Thwaites ice shelf, including its precise dimensions and the best places for AUVs to explore, he adds. “We are only just at the beginning.” More

Among the biggest questions for climate change forecasters is how atmospheric aerosols shape clouds, which can help cool the planet. Now, a new study finds that one promising strategy for understanding how aerosols and clouds interact can overestimate the cooling ability of pollution-generated clouds by up to 200 percent, researchers report in the Jan. 29 Science.
“Clouds in general, and how aerosols interact with the climate, are a big uncertainty in climate models,” says Franziska Glassmeier, an atmospheric scientist at Delft University of Technology in the Netherlands. Scientists know that aerosols — both natural, as from volcanoes, and human-caused, as from pollution — can change a cloud’s thickness, ability to scatter sunlight or how much rainfall it produces. But these complicated physical effects are difficult to simulate, so scientists have sought real-world examples to study these effects.
Enter ship tracks. Exhaust belched out of massive cargo ships crossing the oceans can form these bright lines of clouds. The tiny exhaust particles act as cloud nuclei: Water vapor condenses on the particles to form cloud droplets, the watery stuff of clouds. Ship tracks are “this prime example where we can see this cause and effect,” Glassmeier says. “Put in particles, and you can see the clouds get brighter.” Brighter clouds means that they are reflecting even more sunlight back into space.
Visible and measurable by satellite, the tracks offer a potential window into how larger-scale industrial pollution around the globe might be altering the planet’s cloudscape — and perhaps how such clouds might affect the climate. Satellite-derived analyses of ship tracks involve measuring the density of the water droplets in the clouds from the images, and calculating how the brightness of the clouds changes over time.

Sign Up For the Latest from Science News

Headlines and summaries of the latest Science News articles, delivered to your inbox

To assess how well ship tracks actually represent the overall impact of pollution on clouds, Glassmeier and her colleagues compared the cooling effect of ship track clouds with that of simulated pollution-derived clouds, such as might occur over a city. In particular, the researchers wanted to simulate how both the thickness and the brightness of the clouds — and therefore their cooling effect — might evolve over time, as a result of processes like rainfall and evaporation.
The problem, the team found, is that the ship tracks don’t tell the whole story. Ship tracks are short-lived, because the source of pollution is always on the move. But industrial pollution doesn’t tend to happen in a brief pulse: Instead, there is a steady influx of particles to the atmosphere. And that difference in inputs affects how natural clouds respond over time.
In both the ship track studies and the simulations of industrial pollution, clouds initially brighten and produce a cooling effect. That’s because, in both cases, the addition of abundant aerosol particles to the atmosphere gives water vapor numerous surfaces on which to condense, creating many small water droplets that form this brighter cloud and reflect incoming radiation.
After a few hours, however, as a ship moves on, the ship track goes away, and the pulse of pollution ceases, Glassmeier says. The initial brief bit of cooling subsides as the preexisting natural clouds return to their original, nonpolluted state.
But in the case of industrial pollutants, the natural clouds don’t return to their original state, the simulations show. Rather, the pollutants hasten the clouds’ demise. That’s because the tinier aerosol-seeded droplets begin to evaporate more quickly than larger, natural cloud droplets would. This increased evaporation thins the original cloud, allowing more heat through than if the pollutants never arrived. And that can ultimately have an overall warming, rather than cooling, effect on the climate, the team says.
“There is this timescale effect that needs to be taken into account,” Glassmeier says. Relying solely on ship track data to understand all sources of pollution misses this gradual thinning effect. “I wouldn’t throw all the ship track data away; we just need to interpret it in a new way.” Current climate models tend to omit this thinning effect, she says.
The new study is “really useful for helping to interpret aerosol-cloud relationships in satellite data,” says Edward Gryspeerdt, an atmospheric physicist at Imperial College London who was not involved in the study. It “demonstrates that the cloud response to aerosols is not instant, but evolves over time.”
Scientists have been aware that ship tracks may not lead to cooling, says Graeme Stephens, an atmospheric scientist at NASA’s Jet Propulsion Laboratory in Pasadena, Calif. For example, Stephens notes that he and others have previously found that ship tracks can speed up cloud thinning by increasing the rate of evaporation at the tops of the clouds, while at the same time suppressing rainfall, which maintains some of the cloud’s thickness. These two competing responses make determining a cloud’s ultimate fate tricky.
But what ship tracks can do is act as “a controlled laboratory of sorts,” Stephens says. They “offer us a way to examine aerosol influences on clouds in a direct, concrete way.” More