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    How much does eating meat affect nations’ greenhouse gas emissions?

    The food we eat is responsible for an astounding one-third of global greenhouse gas emissions caused by human activities, according to two comprehensive studies published in 2021.

    “When people talk about food systems, they always think about the cow in the field,” says statistician Francesco Tubiello, lead author of one of the reports, appearing in last June’s Environmental Research Letters. True, cows are a major source of methane, which, like other greenhouse gases, traps heat in the atmosphere. But methane, carbon dioxide and other planet-warming gases are released from several other sources along the food production chain.

    Before 2021, scientists like Tubiello, of the Food and Agriculture Organization of the United Nations, were well aware that agriculture and related land use changes made up roughly 20 percent of the planet’s greenhouse gas emissions. Such land use changes include cutting down forests to make way for cattle grazing and pumping groundwater to flood fields for the sake of agriculture.

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    But new modeling techniques used by Tubiello and colleagues, plus a study from a group at the European Commission Tubiello worked with, brought to light another big driver of emissions: the food supply chain. All the steps that take food from the farm to our plates to the landfill — transportation, processing, cooking and food waste — bring food-related emissions up from 20 percent to 33 percent.

    To slow climate change, the foods we eat deserve major attention, just like fossil fuel burning, says Amos Tai, an environmental scientist at the Chinese University of Hong Kong. The fuller picture of food-related emissions demonstrates that the world needs to make drastic changes to the food system if we are to reach international goals for reducing global warming.

    Change from developing countries

    Scientists have gained a clearer understanding of global human-related emissions in recent years through databases like EDGAR, or Emissions Database for Global Atmospheric Research, developed by the European Union. The database covers every country’s human-emitting activities, from energy production to landfill waste, from 1970 to the present. EDGAR uses a unified methodology to calculate emissions for all economic sectors, says Monica Crippa, a scientific officer at the European Commission’s Joint Research Centre.

    Crippa and colleagues, with help from Tubiello, built a companion database of food system–related emissions called EDGAR-FOOD. Using that database, the researchers arrived at the same one-third estimate as Tubiello’s group.

    Crippa’s team’s calculations, reported in Nature Food in March 2021, split food system emissions into four broad categories: land (including both agriculture and related land use changes), energy (used for producing, processing, packaging and transporting goods), industry (including the production of chemicals used in farming and materials used to package food) and waste (from unused food).

    The land sector is the biggest culprit in food system emissions, Crippa says, accounting for about 70 percent of the global total. But the picture looks different across different nations. The United States and other developed countries rely on highly centralized megafarms for much of their food production; so the energy, industry and waste categories make up more than half of these countries’ food system emissions.

    In developing countries, agriculture and changing land use are far greater contributors. Emissions in historically less developed countries have also been rising in the last 30 years, as these countries have cut down wild areas to make way for industrial farming and started eating more meat, another major contributor to emissions with impacts across all four categories.

    As a result, agriculture and related landscape shifts have driven major increases in food system emissions among developing countries in recent decades, while emissions in developed countries have not grown.

    For instance, China’s food emissions shot up by almost 50 percent from 1990 to 2018, largely due to a rise in meat-eating, according to the EDGAR-FOOD database. In 1980, the average Chinese person ate about 30 grams of meat a day, Tai says. In 2010, the average person in China ate almost five times as much, or just under 150 grams of meat a day.

    Top-emitting economies

    In recent years, Crippa says, six economies, the top emitters, have been responsible for more than half of total global food emissions. These economies, in order, are China, Brazil, the United States, India, Indonesia and the European Union. The immense populations of China and India help drive their high numbers. Brazil and Indonesia make the list because large swaths of their rainforests have been cut down to make room for farming. When those trees come down, vast amounts of carbon flow into the atmosphere (SN: 7/3/21 & 7/17/21, p. 24).

    The United States and the European Union are on the list because of heavy meat consumption. In the United States, meat and other animal products contribute the vast majority of food-related emissions, says Richard Waite, a researcher at the World Resources Institute’s food program in Washington, D.C.

    Waste is also a huge issue in the United States: More than one-third of food produced never actually gets eaten, according to a 2021 report from the U.S. Environmental Protection Agency. When food goes uneaten, the resources used to produce, transport and package it are wasted. Plus, the uneaten food goes into landfills, which produce methane, carbon dioxide and other gases as the food decomposes.

    Meat consumption drives emissions

    Climate advocates who want to reduce food emissions often focus on meat consumption, as animal products lead to far greater emissions than plants. Animal production uses more land than plant production, and “meat production is heavily inefficient,” Tai says.

    “If we eat 100 calories of grain, like maize or soybeans, we get that 100 calories,” he explains. All the energy from the food is delivered directly to the person who eats it. But if the 100 calories’ worth of grain is instead fed to a cow or a pig, when the animal is killed and processed for food, just one-tenth of the energy from that 100 calories of grain goes to the person eating the animal.

    Methane production from “the cow in the field” is another factor in meat consumption: Cows release this gas via their manure, burps and flatulence. Methane traps more heat per ton emitted than carbon dioxide, Tubiello says. So emissions from cattle farms can have an outsize impact (SN: 11/28/15, p. 22). These livestock emissions account for about one-third of global methane emissions, according to a 2021 U.N. report.

    Shifting from meats to plants

    U.S. residents should consider how they can shift to what Brent Kim calls “plant-forward” diets. “Plant-forward doesn’t mean vegan. It means reducing animal product intake, and increasing the share of plant foods that are on the plate,” says Kim, program officer at the Johns Hopkins Center for a Livable Future.

    Kim and colleagues estimated food emissions by diet and food group for 140 countries and territories, using a similar modeling framework to EDGAR-FOOD. However, the framework includes only the food production emissions (i.e. agriculture and land use), not processing, transportation and other pieces of the food system incorporated in EDGAR-FOOD.

    Producing the average U.S. resident’s diet generates more than 2,000 kilograms of greenhouse gas emissions per year, the researchers reported in 2020 in Global Environmental Change. The group measured emissions in terms of “CO2 equivalents,” a standardized unit allowing for direct comparisons between CO2 and other greenhouse gases like methane.

    Going meatless one day a week brings down that figure to about 1,600 kilograms of CO2 equivalents per year, per person. Going vegan — a diet without any meat, dairy or other animal products — cuts it by 87 percent to under 300. Going even two-thirds vegan offers a sizable drop to 740 kilograms of CO2 equivalents.

    Kim’s modeling also offers a “low food chain” option, which brings emissions down to about 300 kilograms of CO2 equivalents per year, per person. Eating low on the food chain combines a mostly plant-based diet with animal products that come from more climate-friendly sources that do not disturb ecological systems. Examples include insects, smaller fish like sardines, and oysters and other mollusks.

    Tai agrees that not everybody needs to become a vegetarian or vegan to save the planet, as meat can have important cultural and nutritional value. If you want to “start from the biggest polluter,” he says, focus on cutting beef consumption.

    But enough people need to make these changes to “send a signal back to the market” that consumers want more plant-based options, Tubiello says. Policy makers at the federal, state and local levels can also encourage climate-friendly farming practices, reduce food waste in government operations and take other actions to cut down the resources used in food production, Waite says.

    For example, the World Resources Institute, where Waite works, is part of an initiative called the Cool Food Pledge, in which companies, universities and city governments have signed on to reduce the climate impacts of the food they serve. The institutions agree to track the food they purchase every year to ensure they are progressing toward their goals, Waite says.

    Developed countries like the United States — which have been heavy meat consumers for decades — can have a big impact by changing food choices. Indeed, a paper published in Nature Food in January shows that if the populations of 54 high-income nations switched to a plant-focused diet, annual emissions from these countries’ agricultural production could drop by more than 60 percent. More

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    Coastal cities around the globe are sinking

    Coastal cities around the globe are sinking by up to several centimeters per year, on average, satellite observations reveal. The one-two punch of subsiding land and rising seas means that these coastal regions are at greater risk for flooding than previously thought, researchers report in the April 16 Geophysical Research Letters.

    Matt Wei, an earth scientist at the University of Rhode Island in Narragansett, and colleagues studied 99 coastal cities on six continents. “We tried to balance population and geographic location,” he says. While subsidence has been measured in cities previously, earlier research has tended to focus on just one city or region. This investigation is different, Wei says. “It’s one of the first to really use data with global coverage.”

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    Wei and his team relied on observations made from 2015 to 2020 by a pair of European satellites. Instruments onboard beam microwave signals toward Earth and then record the waves that bounce back. By measuring the timing and intensity of those reflected waves, the team determined the height of the ground with millimeter accuracy. And because each satellite flies over the same part of the planet every 12 days, the researchers were able to trace how the ground deformed over time.

    The largest subsidence rates — up to five centimeters per year —are mostly in Asian cities like Tianjin, China; Karachi, Pakistan; and Manila, Philippines, the team found. What’s more, one-third, or 33, of the analyzed cities are sinking in some places by more than a centimeter per year.

    That’s a worrying trend, says Darío Solano-Rojas, an earth scientist at the National Autonomous University of Mexico in Mexico City who was not involved in the research. These cities are being hit with a double whammy: At the same time that sea levels are rising due to climate change, the land is sinking (SN: 8/15/18). “Understanding that part of the problem is a big deal,” Solano-Rojas says.

    Wei and his colleagues think that the subsidence is largely caused by people. When the researchers looked at Google Earth imagery of the regions within cities that were rapidly sinking, the team saw mostly residential or commercial areas. That’s a tip-off that the culprit is groundwater extraction, the team concluded. Landscapes tend to settle as water is pumped out of aquifers (SN: 10/22/12).

    But there’s reason to be hopeful. In the past, cities such as Shanghai and Indonesia’s Jakarta were sinking by more than 10 centimeters per year, on average. But now subsidence in those places has slowed, possibly due to recent governmental regulations limiting groundwater extraction. More

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    Climate change intensified deadly storms in Africa in early 2022

    Climate change amped up the rains that pounded southeastern Africa and killed hundreds of people during two powerful storms in early 2022.

    But a dearth of regional data made it difficult to pinpoint just how large of a role climate change played, scientists said April 11 at a news conference.

    The findings were described in a study, published online April 11, by a consortium of climate scientists and disaster experts called the World Weather Attribution network.

    A series of tropical storms and heavy rain events battered southeast Africa in quick succession from January through March. For this study, the researchers focused on two events: Tropical Storm Ana, which led to flooding in northern Madagascar, Malawi and Mozambique in January and killed at least 70 people; and Cyclone Batsirai, which inundated southern Madagascar in February and caused hundreds more deaths.

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    To search for the fingerprints of climate change, the team first selected a three-day period of heavy rain for each storm. Then the researchers tried to amass observational data from the region to reconstruct historical daily rainfall records from 1981 to 2022.

    Only four weather stations, all in Mozambique, had consistent, high-quality data spanning those decades. But, using the data on hand, the team was able to construct simulations for the region that represented climate with and without human-caused greenhouse gas emissions.

    The aggregate of those simulations revealed that climate change did play a role in intensifying the rains, Izidine Pinto, a climatologist at the University of Cape Town in South Africa, said at the news event. But with insufficient historical rainfall data, the team “could not quantify the precise contribution” of climate change, Pinto said.

    The study highlights how information on extreme weather events “is very much biased toward the Global North … [whereas] there are big gaps in the Global South,” said climate scientist Friedericke Otto of Imperial College London.

    That’s an issue also highlighted by the Intergovernmental Panel on Climate Change. The IPCC cites insufficient Southern Hemisphere data as a barrier to assessing the likelihood of increasing frequency and intensity of tropical cyclones beyond the North Atlantic Ocean (SN: 8/9/21). More

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    A UN report says stopping climate change is possible but action is needed now

    It doesn’t have to be this way. 

    The world already has the know-how and tools to dramatically reduce emissions from fossil fuels — but we need to use those tools immediately if we hope to forestall the worst impacts of climate change. That’s the message of the third and final installment of the massive sixth assessment of climate science by the United Nations’ Intergovernmental Panel on Climate Change, which was released April 4.

    “We know what to do, we know how to do it, and now it’s up to us to take action,” said sustainable energy researcher Jim Skea of Imperial College London, who cochaired the report, at a news event announcing its release. 

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    Earth is on track to warm by an average of about 3.2 degrees Celsius above preindustrial levels by the end of the century (SN: 11/26/19). Altering that course and limiting warming to 1.5 degrees or even 2 degrees means that global fossil fuel emissions will need to peak no later than the year 2025, the new report states. 

    Right now, meeting that goal looks extremely unlikely. National pledges to reduce fossil fuel emissions to date amount to “a litany of broken climate promises,” said United Nations Secretary-General António Guterres at the event. 

    The previous two installments of the IPCC’s sixth assessment described how climate change is already fueling extreme weather events around the globe — and noted that adaptation alone will not be enough to shield people from those hazards (SN: 8/9/21; SN: 2/28/22).

    The looming climate crisis “is horrifying, and I don’t want to sugarcoat that,” says Bronson Griscom, a forest ecologist and the director of Natural Climate Solutions at the environmental organization Conservation International, based in Arlington, Va. 

    But Griscom, who was not an author on the new IPCC report, says its findings also give him hope. It’s “what I would call a double-or-nothing bet that we’re confronted with right now,” he says. “There [are] multiple ways that this report is basically saying, ‘Look, if we don’t do anything, it’s increasingly grim.’ But the reasons to do something are incredibly powerful and the tools in the toolbox are very powerful.”

    Tools in the toolbox

    Those tools are strategies that governments, industries and individuals can use to cut emissions immediately in multiple sectors of the global economy, including transportation, energy, building, agriculture and forestry, and urban development. Taking immediate advantage of opportunities to reduce emissions in each of those sectors would halve global emissions by 2030, the report states. 

    Consider the transportation sector, which contributed 15 percent of human-related greenhouse gas emissions in 2019. Globally, electric vehicle sales have surged in the last few years, driven largely by government policies and tougher emissions laws for the auto industry (SN: 12/22/21). 

    If that surge continues, “electric vehicles offer us the greatest potential [to reduce transportation emissions on land], as long as they’re combined with low or zero carbon electricity sources,” Inger Andersen, the executive director of the United Nations Environment Programme, said at the news event. But for aviation and long-haul shipping, which are more difficult to electrify, reduced carbon emissions could be achieved with low-carbon hydrogen fuels or biofuels, though these alternatives require further research and development.

    Then there are urban areas, which are contributing a growing proportion of global greenhouse gas emissions, from 62 percent in 2015 to between 67 and 72 percent in 2020, the report notes. In established cities, buildings can be retrofitted, renovated or repurposed to make city layouts more walkable and provide more accessible public transportation options. 

    And growing cities can incorporate energy-efficient infrastructure and construct buildings using zero-emissions materials. Additionally, urban planners can take advantage of green roofs, urban forests, rivers and lakes to help capture and store carbon, as well as provide other climate benefits such as cleaner air and local cooling to counter urban heat waves (SN: 4/3/18). 

    Meanwhile, “reducing emissions in industry will involve using materials and energy more efficiently, reusing and recycling products and minimizing waste,” Diana Ürge-Vorsatz, the vice chair of the IPCC’s Working Group III, said at the news event. 

    As for agriculture and forestry, these and other land-use industries contribute about 22 percent of the world’s greenhouse gas emissions, with half of those emissions coming from deforestation (SN: 7/13/21). So reforestation and reduced deforestation are key to flipping the balance between CO₂ emissions and removal from the atmosphere (SN: 7/9/21; SN: 1/3/22). But there are a lot of other strategies that the world can employ at the same time, the report emphasizes. Better management of forests, coastal wetlands, grasslands and other ecosystems, more sustainable crop and livestock management, soil carbon management in agriculture and agroforestry can all bring down emissions (SN: 7/14/21). 

    The report also includes, for the first time in the IPCC’s reports, a chapter on the “untapped potential” of lifestyle changes to reduce emissions. Such changes include opting for walking or cycling or using public transportation rather than driving, shifting toward plant-based diets and reducing air travel (SN: 5/14/20). 

    Those lifestyle changes could reduce emissions by 40 to 70 percent by 2050, the report suggests. To enable those changes, however, government policies, infrastructure and technology would need to be in place. 

    Government policies are also key to financing these transformational changes. Globally, the investment in climate-related technologies needs to ramp up, and quickly, to limit warming below 2 degrees C, the report states. Right now, investments are three to six times lower than they need to be by 2030. And a combination of public and private investments will be essential to aiding the transition away from fossil fuels and toward renewable energy in developing nations (SN: 1/25/21). 

    Future strategies

    Still, reducing emissions alone won’t be enough: We will need to actively remove carbon from the atmosphere to achieve net zero emissions and keep the planet well below 2 degrees C of warming, the report notes. “One thing that’s clear in this report, as opposed to previous reports, is that carbon removal is going to be necessary in the near term,” says Simon Nicholson, director of the Institute for Carbon Removal Law and Policy at American University in Washington, D.C., who was not involved in the report. 

    Such strategies include existing approaches such as protecting or restoring carbon dioxide–absorbing forests, but also technologies that are not yet widely available commercially, such as directly capturing carbon dioxide from the air, or converting the gas to a mineral form and storing it underground (SN: 12/17/18). 

    These options are still in their infancy, and we don’t know how much of an impact they’ll have yet, Nicholson says. “We need massive investment now in research.”

    An emphasis on acting “now,” on eliminating further delay, on the urgency of the moment has been a recurring theme through all three sections of the IPCC’s sixth assessment report released over the last year. What impact these scientists’ stark statements will have is unclear.

    But “the jury has reached a verdict, and it is damning,” U.N. Secretary-General Guterres said. “If you care about justice and our children’s future, I am appealing directly to you.” More

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    Forests help reduce global warming in more ways than one

    When it comes to cooling the planet, forests have more than one trick up their trees.  

    Tropical forests help cool the average global temperature by more than 1 degree Celsius, a new study finds. The effect stems largely from forests’ capacity to capture and store atmospheric carbon (SN: 11/18/21). But around one-third of that tropical cooling effect comes from several other processes, such as the release of water vapor and aerosols, researchers report March 24 in Frontiers in Forests and Global Change.

    “We tend to focus on carbon dioxide and other greenhouse gases, but forests are not just carbon sponges,” says Deborah Lawrence, an environmental scientist at the University of Virginia in Charlottesville. “It’s time to think about what else forests are doing for us besides just absorbing carbon dioxide.”

    Researchers already knew that forests influence their local climates through various physical and chemical processes. Trees release water vapor through pores in their leaves — a process called evapotranspiration — and, like human sweating, this cools the trees and their surroundings. Also, uneven forest canopies can have a cooling effect, as they provide an undulating surface that can bump hot, overpassing fronts of air upward and away. What’s more, trees generate aerosols that can lower temperatures by reflecting sunlight and seeding clouds.

    But on a global scale, it wasn’t clear how these other cooling benefits compared with the cooling provided by forests’ capturing of carbon dioxide, Lawrence says.

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    So she and her colleagues analyzed how the complete deforestation of different regions would impact global temperatures, using data gathered from other studies. For instance, the researchers used forest biomass data to determine how much the release of carbon stored by those forests would warm the global temperature. They then compared those results with other studies’ estimates of how much the loss of other aspects of forests — such as evapotranspiration, uneven canopies and aerosol production — affected regional and global temperatures.

    The researchers found that in forests at latitudes from around 50° S of the equator to 50° N, the primary way that forests influenced the global average temperature was through carbon sequestration. But those other cooling factors still played large roles.

    Forests located from 30° N to 30° S provided alternative benefits that cool the planet by over 0.3 degrees C, about half as much cooling as carbon sequestration provided. And the bulk of that cooling, around 0.2 degrees C, came from forests in the core of the tropics (within 10° of the equator). Canopy topography generally provided the greatest cooling, followed by evapotranspiration and then aerosols.

    Forests in the far north, however, appear to have a net warming effect, the team reports. Clearing the boreal forests — which stretch across Canada, Alaska, Russia and Scandinavia — would expose more snow cover during the winter. This would decrease ground level temperatures because snow reflects much of the incoming sunlight back into the sky. Still, the researchers found that altogether, the world’s forests cool the global average temperature about 0.5 degrees C.

    The findings suggest that global and regional climate action efforts should refrain from focusing solely on carbon emissions, Lawrence says. “There’s this whole service that tropical forests are providing that simply are not visible to us or to policy makers.”

    The research shows that clearing tropical forests robs us of many climate-cooling benefits, says Gabriel de Oliveira, a geographer from the University of South Alabama in Mobile. But deforestation isn’t the only way that humans impair forests’ cooling ability, he says. Many forests are damaged by fires or selective logging, and are less able to help with cooling (SN: 9/1/21). It would be useful to consider how forest degradation, in addition to deforestation, impacts regional and global climate temperatures, de Oliveira says, to assess the impact of restoring and protecting forests (SN: 7/13/21). “It’s cool to see beyond carbon dioxide, but it’s also very important to see beyond deforestation.” More

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    Smoke from Australia’s intense fires in 2019 and 2020 damaged the ozone layer

    Towers of smoke that rose high into the stratosphere during Australia’s “black summer” fires in 2019 and 2020 destroyed some of Earth’s protective ozone layer, researchers report in the March 18 Science.

    Chemist Peter Bernath of Old Dominion University in Norfolk, Va., and his colleagues analyzed data collected in the lower stratosphere during 2020 by a satellite instrument called the Atmospheric Chemistry Experiment. It measures how different particles in the atmosphere absorb light at different wavelengths. Such absorption patterns are like fingerprints, identifying what molecules are present in the particles.

    The team’s analyses revealed that the particles of smoke, shot into the stratosphere by fire-fueled thunderstorms called pyrocumulonimbus clouds, contained a variety of mischief-making organic molecules (SN: 12/15/20). The molecules, the team reports, kicked off a series of chemical reactions that altered the balances of gases in Earth’s stratosphere to a degree never before observed in 15 years of satellite measurements. That shuffle included boosting levels of chlorine-containing molecules that ultimately ate away at the ozone.

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    Ozone concentrations in the stratosphere initially increased from January to March 2020, due to similar chemical reactions — sometimes with the contribution of wildfire smoke — that produce ozone  pollution at ground level (SN: 12/8/21). But from April to December 2020, the ozone levels not only fell, but sank below the average ozone concentration from 2005 to 2019.

    Earth’s ozone layer shields the planet from much of the sun’s ultraviolet radiation. Once depleted by human emissions of chlorofluorocarbons and other ozone-damaging substances, the layer has been showing signs of recovery thanks to the Montreal Protocol, an international agreement to reduce the atmospheric concentrations of those substances (SN: 2/10/21).

    But the increasing frequency of large wildfires due to climate change — and their ozone-destroying potential — could become a setback for that rare climate success story, the researchers say (SN: 3/4/20). More

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    Even the sea has light pollution. These new maps show its extent

    The first global atlas of ocean light pollution shows that large swaths of the sea are squinting in the glare of humans’ artificial lights at night.

    From urbanized coastlines along the Persian Gulf to offshore oil complexes in the North Sea, humans’ afterglow is powerful enough to penetrate deep into many coastal waters, potentially changing the behaviors of creatures that live there, researchers report December 13 in Elementa: Science of the Anthropocene. Regional and seasonal differences — such as phytoplankton blooms or sediment from rivers — also affect the depth to which light penetrates.

    Artificial lights are known to affect land dwellers, such as by swelling or shrinking certain insect populations, or by making it harder for sparrows to fight off West Nile virus (SN: 3/30/21; SN: 8/31/21; SN: 1/19/18). But the bright lights of coastal cities, oil rigs and other offshore structures can also create a powerful glow in the sky over the sea.

    To assess where this glow is strongest, marine biogeochemist Tim Smyth of Plymouth Marine Laboratory in England and colleagues combined a world atlas of artificial night sky brightness created in 2016 with ocean and atmosphere data (SN: 6/10/16). Those data include shipboard measurements of artificial light, satellite data collected monthly from 1998 to 2017 to estimate the prevalence of light-scattering phytoplankton and sediment, and computer simulations of how different wavelengths of light move through the water.

    Not all species are equally sensitive to light, so to assess impact, the team focused on copepods, ubiquitous shrimplike creatures that are a key part of many ocean food webs. Like other tiny zooplankton, copepods use the sun or the winter moon as a cue to plunge en masse to the dark deep, seeking safety from surface predators (SN: 1/11/16; SN: 4/18/18).

    Humans’ nighttime light has the most impact in the top meter of the water, the team found. Here, artificial light is intense enough to cause a biological response across nearly 2 million square kilometers of ocean, an area roughly that of Mexico. Twenty meters down, the total affected area shrinks by more than half to 840,000 square kilometers.

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    Sunlight helps clean up oil spills in the ocean more than previously thought

    Sunlight may have helped remove as much as 17 percent of the oil slicking the surface of the Gulf of Mexico following the 2010 Deepwater Horizon spill. That means that sunlight plays a bigger role in cleaning up such spills than previously thought, researchers suggest February 16 in Science Advances.

    When sunlight shines on spilled oil in the sea, it can kick off a chain of chemical reactions, transforming the oil into new compounds (SN: 6/12/18). Some of these reactions can increase how easily the oil dissolves in water, called photodissolution. But there has been little data on how much of the oil becomes water-soluble.

    To assess this, environmental chemists Danielle Haas Freeman and Collin Ward, both of Woods Hole Oceanographic Institution in Massachusetts, placed samples of the Macondo oil from the Deepwater Horizon spill on glass disks and irradiated them with light using LEDs that emit wavelengths found in sunlight. The duo then chemically analyzed the irradiated oil to see how much was transformed into dissolved organic carbon.

    The most important factors in photodissolution, the researchers found, were the thickness of the slick and the wavelengths of light. Longer wavelengths (toward the red end of the spectrum) dissolved less oil, possibly because they are more easily scattered by water, than shorter wavelengths. How long the oil was exposed to light was not as important.

    Though the team didn’t specifically test for seasonal or latitude differences, computer simulations based on the lab data suggested that those factors, as well as the oil’s chemical makeup, also matter.

    The researchers estimate irradiation helped dissolve from 3 to 17 percent of surface oil from the Deepwater Horizon spill, comparable to processes such as evaporation and stranding on coastlines. What impact the sunlight-produced compounds might have on marine ecosystems, however, isn’t yet known.  More