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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.

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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

An analysis of the successes and failures of past electrical power projects across Africa suggests the continent isn’t likely to go green before 2030. More

The New Climate WarMichael E. MannPublic Affairs, $29
Sometime around the fifth century B.C., the Chinese general and military strategist Sun Tzu wrote in his highly quotable treatise The Art of War, “If you know the enemy and know yourself, you need not fear the result of a hundred battles.”
In The New Climate War, climate scientist Michael Mann channels Sun Tzu to demystify the myriad tactics of “the enemy” — in this case, “the fossil fuel companies, right-wing plutocrats and oil-funded governments” and other forces standing in the way of large-scale action to combat climate change. “Any plan for victory requires recognizing and defeating the tactics now being used by inactivists as they continue to wage war,” he writes.
Mann is a veteran of the climate wars of the 1990s and early 2000s, when the scientific evidence that the climate is changing due to human emissions of greenhouse gases was under attack. Now, with the effects of climate change all around us (SN: 12/21/20), we are in a new phase of those wars, he argues. Outright denial has morphed into “deception, distraction and delay.”
Such tactics, he says, are direct descendants of earlier public relations battles over whether producers or consumers must bear ultimate responsibility for, say, smoking-related deaths. When it comes to the climate, Mann warns, an overemphasis on individual actions could eclipse efforts to achieve the real prize: industrial-scale emissions reductions.

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He pulls no punches, calling out sources of “friendly fire” from climate advocates who he says divide the climate community and play into the “enemy’s” hands. These advocates include climate purists who lambaste scientists for flying or eating meat; science communicators who push fatalistic visions of catastrophic futures; and idealistic technocrats who advocate for risky, pie-in-the-sky geoengineering ideas. All, Mann says, distract from what we can do in the here and now: regulate emissions and invest in renewable energy.
The New Climate War’s main focus is to combat psychological warfare, and on this front, the book is fascinating and often entertaining. It’s an engrossing mix of footnoted history, acerbic political commentary and personal anecdotes. As far as what readers can do to assist in the battle, Mann advocates four strategies: Disregard the doomsayers; get inspired by youth activists like Greta Thunberg; focus on educating the people who will listen; and don’t be fooled into thinking it’s too late to take action to change the political system.
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2020 is in a “dead heat” with 2016 for the hottest year on record, scientists with NASA and the National Oceanic and Atmospheric Administration announced January 14.
Based on ocean temperature data from buoys, floats and ships, as well as temperatures measured over land at weather stations around the globe, the U.S. agencies conducted independent analyses and arrived at a similar conclusion.
NASA’s analysis showed 2020 to be slightly hotter, while NOAA’s showed that 2016 was still slightly ahead. But the differences in those assessments are within margins of error, “so it’s effectively a statistical tie,” said NASA climatologist Gavin Schmidt of the Goddard Institute for Space Studies in New York City at a Jan. 14 news conference.
NOAA climate scientist Russell Vose, who is also based in New York City, described in the news conference the extreme warmth that occurred over land last year, including a months-long heat wave in Siberia (SN: 12/21/20). Europe and Asia recorded their hottest average temperatures on record in 2020, with South America recording its second warmest.
It’s possible that 2020’s temperatures in some areas might have been even higher if not for massive wildfires. Vose noted that smoke lofted high into the stratosphere as a result of Australia’s intense fires in early 2020 may have slightly decreased temperatures in the Northern Hemisphere, though this is not yet known (SN: 12/15/20).
The ocean-climate pattern known as the El Niño Southern Oscillation can boost or decrease global temperatures, depending on whether it’s in an El Niño or La Niña phase, respectively, Schmidt said (SN: 5/2/16). The El Niño phase was waning at the start of 2020, and a La Niña was starting, so the overall impact of this pattern was muted for the year. 2016, on the other hand, got a large temperature boost from El Niño. Without that, “2020 would have been by far the warmest year on record,” he said.
But placed in the bigger picture, these rankings “don’t tell the whole story,” Vose said. “The last six to seven years really stand out above the rest of the record, suggesting the kind of rapid warming we’re seeing. [And] each of the past four decades was warmer than the one preceding it.” More

A more acidic ocean could give some species a glow-up.
As the pH of the ocean decreases as a result of climate change, some bioluminescent organisms might get brighter, while others see their lights dim, scientists report January 2 at the virtual annual meeting of the Society for Integrative and Comparative Biology.
Bioluminescence is de rigueur in parts of the ocean (SN: 5/19/20). The ability to light the dark has evolved more than 90 times in different species. As a result, the chemical structures that create bioluminescence vary wildly — from single chains of atoms to massive ringed complexes.
With such variability, changes in pH could have unpredictable effects on creatures’ ability to glow (SN: 7/6/10). If fossil fuel emissions continue as they are, average ocean pH is expected to drop from 8.1 to 7.7 by 2100. To find out how bioluminescence might be affected by that decrease, sensory biologist Tom Iwanicki and colleagues at the University of Hawaii at Manoa gathered 49 studies on bioluminescence across nine different phyla. The team then analyzed data from those studies to see how the brightness of the creatures’ bioluminescent compounds varied at pH levels from 8.1 to 7.7.

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As pH drops, the bioluminescent chemicals in some species, such as the sea pansy (Renilla reniformis), increase light production twofold, the data showed. Other compounds, such as those in the sea firefly (Vargula hilgendorfii), have modest increases of only about 20 percent. And some species, like the firefly squid (Watasenia scintillans), actually appear to have a 70 percent decrease in light production.
For the sea firefly — which uses glowing trails to attract mates — a small increase could give it a sexy advantage. But for the firefly squid — which also uses luminescence for communication — low pH and less light might not be a good thing.
Because the work was an analysis of previously published research, “I’m interpreting this as a first step, not a definitive result,” says Karen Chan, a marine biologist at Swarthmore College in Pennsylvania who wasn’t involved in the study. It “provides [a] testable hypothesis that we should … look into.”
The next step is definitely testing, Iwanicki agrees. Most of the analyzed studies took the luminescing chemicals out of an organism to test them. Finding out how the compounds function in creatures in the ocean will be key. “Throughout our oceans, upward of 75 percent of visible critters are capable of bioluminescence,” Iwanicki says. “When we’re wholescale changing the conditions in which they can use that [ability] … that’ll have a world of impacts.” More

The COVID-19 pandemic wasn’t just a shock to the human immune system. It was also a shock to the Earth system, dramatically changing the air quality in cities around the globe.
As countries around the globe struggled to contain the disease, they imposed temporary shutdowns. Scientists are now sifting through data collected by satellite and on the ground to understand what this hiatus in human activities can tell us about the atmospheric cocktail that generates city pollution. Much of this preliminary data was shared at the American Geophysical Union annual meeting in December.
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It was already known that peoples’ activities were curtailed enough to result in a dramatic drop in emissions of greenhouse gases in April, as well as a dip in the seismic noises produced by humans (SN: 5/19/20; SN: 7/23/20). That quiet period didn’t last, though, and carbon dioxide emissions began to climb back upward by the summer. April 2020 saw a drop of about 17 percent in global monthly CO2 emissions from fossil fuels, but by year’s end, annual CO2 emissions for the globe were only 7 percent lower than they were in 2019. That reduction was too brief, compared with the hundreds of years that the gas can linger in Earth’s atmosphere, to put a dent in the planet’s atmospheric CO2 level (SN: 8/7/20).

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But in addition to briefly reducing emissions of climate-warming gases, this abrupt halt in many human activities — particularly commuter traffic — also created an unprecedented experiment for scientists to examine the complicated chemistry of atmospheric pollutants in cities. By altering the usual mix of pollutants hovering over cities, the shutdowns may help scientists better understand another longstanding misery for human health: poor air quality in many cities.
That’s not to say that the pandemic has a silver lining, says Jessica Gilman, a tropospheric chemist at the National Oceanic and Atmospheric Administration in Boulder, Colo. “Misery is no solution to our global environmental challenges.”
But there’s now a wealth of data from cities around the globe on how the pandemic altered regional or local concentrations of the precursors of ozone, a primary component of smog. Those precursors include nitrogen oxides and volatile organic compounds — both produced by traffic — as well as methane, produced by the oil and gas industry. With satellites, scientists are also able to assess how levels of these pollutants changed around the globe.
Building a global picture of altered city pollution is no easy task, though. Researchers are finding that the pandemic’s impact on levels of various pollutants was highly regional, affected by differences in wind and rain as well as by photochemical interactions with sunlight — the intensity of which also changes with the season.  
That stark variety of regional effects was evident in, for example, the different post-pandemic ozone levels in Denver and New York City. Nitrogen oxide gases produced by traffic are a powerful precursor to cities’ elevated ozone levels, which can damage the lungs and trigger respiratory ailments. The United States has made strides in reducing these gases over the last few decades — but there hasn’t been a corresponding drop in ozone levels, Dan Jaffe, an environmental chemist at the University of Washington Bothell, reported at the meeting on December 9.
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The shutdowns gave researchers some insight into why, Jaffe says. From March 15 through July 23, New York City had a 21 percent decrease in nitrogen dioxide, one of several nitrogen oxide gases, in comparison with 2019 levels. Although the shutdowns were more stringent during the spring months, it turned out that summertime reductions in nitrogen dioxide were most strongly linked to the city’s change in ozone levels, the researchers found. “We see very strong reduction in summertime ozone this year,” Jaffe said at the meeting, citing unpublished data.
That’s because in the summer months, heat and sunlight react with the precursor gases in the atmosphere, like nitrogen dioxide, creating a toxic cocktail. This kind of insight can be a boon to policy makers in a non-pandemic year, suggesting that nitrogen oxide regulations should focus most strongly on the summer, Jaffe says. “It’s really good evidence that NOx reductions extending into July in 2020 had an important impact.”
In Denver, however, ozone didn’t drop so consistently — possibly because wildfires were beginning to rage across the U.S. West by the end of the summer (SN: 12/21/20). The fires produce nitrogen oxides, carbon monoxide and fine particles that can also help to increase ground-level ozone.
“There are different patterns in different cities,” Jaffe says. “There are a lot of factors to sort out, and a lot of work to be done.” Armed with a wealth of new data from 2020, scientists hope to be able to make some headway. More