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  • Tony Luong
  • The climate optimist

    Susan Solomon’s research pinpointed how CFCs caused the Antarctic ozone hole—and later showed that the Montreal Protocol is helping to mend it. She’s convinced we can make progress on addressing climate change, too.

    One night in 1986, Susan Solomon, a young researcher with the US National Oceanic and Atmospheric Administration (NOAA), stood in subzero temperatures on the roof of McMurdo Station, the US research center in Antarctica. Solomon was adjusting the mirrors mounted there to capture moonlight and direct it to a visible absorption spectrograph in the lab below. Her goal was to measure the concentrations of different compounds in the atmosphere above Antarctica, in order to make sense of the large hole in the ozone layer that had developed there.

    She wasn’t wearing goggles, because she needed to see very clearly, and as she squinted to make sure the beams of moonlight were angled properly toward the spectrograph, her eyes watered slightly. In an instant, her tears froze between the top and bottom eyelids of one eye, sealing it shut. But it was a clear night, and Solomon wasn’t willing to walk away from the opportunity to collect what she recalls as “the perfect moon data.” So she kept working.

    This story is part of the March/April 2019 Issue of the MIT News magazine
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    Once back inside, she was able to open her eye again when her tears melted. And the data she gathered that night and over two months in Antarctica would change our understanding of how chlorofluorocarbons, released into the atmosphere from refrigerants and a range of other consumer products, damage the ozone layer, which helps protect Earth from ultraviolet radiation. In response to this and other scientific work, an international agreement limited and then banned the use of CFCs. Thirty years later, Solomon was also the first to clearly demonstrate that, thanks to this change, the Antarctic ozone hole has slowly begun to heal.

    “It was exciting to come up with a theory about ozone destruction in 1986 and then, 30 years later, see that we, the people of this planet, have solved the problem,” she says.

    The recovery of the ozone layer is “the most positive recent response to a global environmental crisis we can tell our students about,” says Ross Salawitch, an expert on environmental chemistry at the University of Maryland.

    Solomon’s experience has imbued her with optimism about the world’s ability to confront environmental threats. At MIT, where she has served on the faculty since 2012, she teaches a class on science, politics, and environmental policy, which culminates in a discussion of climate change. Her students “come in not believing we can do anything about environmental problems, because that’s what they know,” she says. “They’re millennials. They haven’t lived through the kind of era of policymakers and scientists working together and getting stuff done that I personally have been blessed to experience.”

    Solomon is quick to acknowledge that climate change poses tougher political challenges than ozone depletion, because fossil-fuel consumption is so integral to the world economy. Still, she argues that by studying past environmental successes, her students will come to understand “what is actually going to have to happen to make progress.”

    Explaining the ozone hole
    Solomon is an explorer by temperament and a chemist by training. As a child of nine or 10 years old, she was inspired to become a scientist by watching The Undersea World of Jacques Cousteau. Fascinated by chemistry in high school, where she did a science fair project analyzing the composition of gaseous mixtures, she studied chemistry as an undergrad at the Illinois Institute of Technology in Chicago and then atmospheric chemistry at Berkeley, where she received her PhD in 1981. She then took a position at NOAA, modeling perturbations in the stratosphere.

    In 1974, the chemists F. Sherwood Rowland and Mario Molina had shown how chlorine could destroy ozone. They had proposed a series of gas phase reactions in which small amounts of chlorine trigger the conversion of large amounts of ozone, or O3, to ordinary molecular oxygen, O2. Rowland and Molina, who was later a professor at MIT, had also identified chlorofluo­rocarbons as a source of chlorine in the atmosphere, released when the sun’s energy broke up otherwise stable CFC molecules. (Rowland, Molina, and an atmospheric chemist named Paul Crutzen later shared a Nobel Prize for work related to ozone depletion.) Initially, researchers anticipated that the ozone layer would thin by roughly 5 to 10% over the course of 100 years. On the basis of these predictions, policymakers began to call for change in the use of chlorofluorocarbons, and countries banned their use as propellants in aerosol sprays beginning in 1978.

    In 1985, however, British scientists discovered that the issue of ozone destruction was playing out in ways Rowland and Molina’s work had not predicted. Specifically, they identified a hole in stratospheric ozone much larger than anyone had anticipated—and opening up each spring over Antarctica, which no theory had forecast. “It was a quite a blockbuster,” says Solomon, who immediately began to study potential explanations.

    Although researchers understood how chlorine could cause ozone destruction, they couldn’t explain the sudden appearance of the ozone hole. Nor did they understand why the hole had shown up above Antarctica, far from the main sources of CFC pollution.

    Intent on solving the mystery, Solomon focused on the specific height range where the reaction was occurring and homed in on the presence there of polar stratospheric clouds—iridescent clouds that form in the stratosphere at extremely low temperatures. (They’re most prevalent in Antarctica, which is much colder than the Arctic.) She hypothesized that these clouds were providing tiny, icy surfaces on which a crucial initiating reaction could take place once sunlight arrived with the Antarctic spring.

    “People thought that all the reactions would have to happen in the gas phase,” she says. “They didn’t think surface chemistry mattered in the stratosphere, and the idea that the clouds could do anything was heretical.”

    Solomon developed a quantitative model of her theory, which she published in the Journal of Geophysical Research. “It wasn’t the whole story,” she says. “But it did help to explain why the ozone hole appeared in the stratosphere, why it occurred near the South Pole, and why it happened in the spring.”

    At the same time, Solomon wanted to substantiate her theory with real-world data. So in 1986, she led a scientific expedition to Antarctica to investigate the chemistry firsthand. Knowing that sunlight contributes to the process of ozone destruction, she and her team arrived at McMurdo Station that August, at the end of the Antarctic winter, when the continent was still shrouded in darkness nearly 24 hours a day. “As the sun starts trying to get above the horizon, around noon local time, you have hours and hours of dawn, so the sky is these beautiful deep purples and blues and later reds as the sun gets higher,” she recalls. “There isn’t a lot of wildlife in the winter, although the seals still drag themselves out and try to get some sun, even when it’s dark.”

    In 1986, Solomon traveled to Antarctica to collect data that backed up her theory of how ozone destruction was happening and why it was so pronounced above that continent.
    National Oceanic and Atmospheric Administration

    Solomon’s team brought with them a small number of scientific instruments, including a visible spectrograph, which enabled them to analyze light from the sun, the moon, and the sky. By measuring how strongly different wavelengths of light were absorbed as they passed through the atmosphere, the group hoped to determine the concentrations of key molecules present overhead. These molecules included ozone and chlorine dioxide, which is formed indirectly when ozone reacts with chlorine. The team got a construction crew to cut a hole in the roof of a cabin at McMurdo so that the moonlight could reach the spectrograph, which was inside. They also had the crew build the makeshift platform on the roof on which Solomon stood to position the mirrors. When the wind swirled about, she had to steady them by hand. “I really had to hang on to them because they were shaking,” she says.

    The team worked 18 to 20 hours a day, especially when the moon was up. “It was absolutely nuts,” she says. They slept for a few hours at a time on cots and ate a lot of sandwiches. (“The great thing is that food doesn’t spoil down there, and there are no insects,” she says.)

    The spectral data that Solomon and her team gathered provided support for the theory that chlorine originally locked up in chlorofluorocarbons is released by way of a surface reaction between hydrochloric acid and chlorine nitrate on polar stratospheric clouds.

    That chlorine in turn reacts with ozone to produce chlorine monoxide and goes on to deplete the ozone. Further buttressing her theory, Solomon’s expedition found that stratospheric concentrations of hydrochloric acid were low and those of chlorine monoxide and chlorine dioxide were high. (They also used balloons to measure concentrations of ozone directly and found that it, too, was severely depleted in the stratosphere, as expected.) The following year, Solomon returned to McMurdo Station to gather more data. A team from NASA, which included Jim Anderson, also flew from Punta Arenas, Chile, into the Antarctic ozone hole to measure the chemical compounds present.

    During a follow-up expedition in 1987, she also found time to visit with Emperor penguins. In her office at NOAA, she later posed next to a globe showing Antarctica.
    National Oceanic and Atmospheric Administration

    Ultimately, says Solomon, “every single measurement lined up and told the story in the same way,” proving that chlorofluorocarbons were responsible for the hole.

    Solomon’s success in identifying the initiating reaction that took place on these polar stratospheric clouds “was the key contribution,” says Anderson, now a professor of physical chemistry at Harvard, “and it was very, very important.”

    In 1987, an international agreement called the Montreal Protocol addressed the problem threatening the ozone layer. Initially, 46 countries agreed to freeze chlorofluorocarbons at their current levels. Subsequently, they decided to phase out the compounds entirely, first in the developed world and then in the developing world. By 2015, the agreement had been signed by every member of the United Nations, 197 signatories in total. “We basically had to reinvent how we did air conditioning and refrigeration throughout the world,” Maryland’s Ross Salawitch says. “It required immense global cooperation.” The agreement was also successful because it encouraged developing countries to participate in a way that would benefit them. Thanks to “great diplomacy by the State Department,” he adds, “it wasn’t just the First World saying, ‘You’re going to have to do this in a more costly fashion.’ Instead, it was ‘Why don’t you build your own factories—we’ll share knowledge with you, and you can participate in this new and growing market for replacement gases.’”

    Assessing climate-­change research
    Over time, Solomon also turned her attention to climate change, studying how chlorofluorocarbons and ozone trap heat in the troposphere (the atmospheric layer between the ground and the stratosphere), where ozone is considered a pollutant because it contributes to smog. In 1994, she participated in the Intergovernmental Panel on Climate Change (IPCC), coauthoring a report on radiative forcing, which is the balance between the energy entering Earth’s atmosphere from the sun and the energy radiated back out into space. She continued to play a consistent role on the IPCC as well as on separate international assessments of ozone. In 2002, she was invited to serve as scientific cochair for the IPCC report on global climate change. “I knew that it would be a huge amount of work and would expose me to the political sphere in a much more intimate way,” Solomon says. But she agreed to do it because she knew getting countries to collaborate on climate policy would depend on a shared understanding of the science involved: “If we have our own science and the Saudis and Chinese and Russians each have their own science, the chances of the diplomats agreeing on something are going to be pretty small.”

    Susan Solomon figured out that the polar stratospheric clouds above Antarctica provided the icy surface needed to kick off the chain of chemical reactions creating a hole in the ozone layer.
    Tony Luong

    In her role on the IPCC report, Solomon worked with some 150 of the best climate scientists in the world, helping to synthesize current research related to climate change. Finding areas of consensus required a mix of science and diplomacy. “You’re up there calling on Sweden and Botswana and the US and whoever else, and they’re making their points, and you have to make sure that whatever they suggest is consistent with the science,” she says. “You’re trying to make it clearer and clearer, especially for a broad audience. And if that’s not what they’re trying to do and they’re trying to play games, you have to shut it down in a graceful way.”

    In 2007, the group produced a textbook-size tome, which Solomon keeps above her desk, and which garnered attention for stating, for the first time, that “warming is unequivocal.” (Solomon herself came up with that wording, based on the researchers’ thorough examination of the research.) It also said that most of the warming of the past 50 years was “very likely due to human activity.” (The report defined “very likely” as meaning that the conclusion was 90% certain.) The year the report was published, the IPCC and former vice president Al Gore shared the Nobel Peace Prize for their efforts. A photo in Solomon’s office shows Gore and members of the IPCC group meeting with President George W. Bush in the Oval Office.

    In the years that followed, Solomon continued to work on climate change. In 2009, she published a paper showing that some of the effects of carbon dioxide, which takes a long time to dissipate from the atmosphere, are probably irreversible. Other researchers had conducted a set of experiments modeling where the carbon currently in the atmosphere would end up if emissions came to a halt. After examining this work, Solomon noticed that in all the models, Earth’s surface temperature did not significantly decrease for a thousand years, even in the absence of new carbon dioxide emissions. “On a human time scale, a thousand years is pretty close to irreversible,” she says. “I realized that that story had not been told clearly enough in a simple paper. I also wanted to understand why it was true.”

    She ultimately concluded that the key factor was the ocean, which is slow to warm but stores heat—and therefore warms the atmosphere—for long periods of time. In addition, her team found that even without further emissions of carbon dioxide, sea levels would also continue to rise for hundreds of years. And her lab at MIT would later reach a similar conclusion for even short-lived greenhouse gases like methane. Her research raises questions about whether vulnerable island nations and coastal populations can still be saved from the consequences of climate change.

    “There is more sea-level rise coming for Florida—and elsewhere in the world—even if we stop emitting,” she says.

    In other climate-related work, Solomon and her team explored how volcanic eruptions affect ozone depletion. In 2011, they found that even eruptions that appear to cause damage only at the local level can still fling enough sulfur into the stratosphere to put it into circulation there, where it can have global consequences. Solomon “was the first to look at the impact of smaller volcanic eruptions on climate,” says Catherine Wilka, a fourth-year graduate student in Solomon’s lab, “and now our group has extended that to look at the impact of those eruptions on the chemistry of ozone.” As it turns out, “there’s not a lot of weather in the stratosphere, so light, small particles from volcanic eruptions tend to stay there for a long time.” They provide more surface area for the reactions that trigger ozone depletion, similar to the processes that Solomon identified decades ago on polar stratospheric clouds.

    What would Solomon do?


    Here’s her advice on how to make progress in tackling global warming.
    After showing how CFCs were behind the Antarctic ozone hole, Susan Solomon observed how diplomats and scientists from around the world collaborated to address it. She identified three factors—she calls them the Three Ps—that must hold true to allow progress on environmental problems.

    1. It must be personal. People need to feel that the problem affects them in their daily lives. She says that is beginning to happen with climate change, at least in some parts of the world—though not yet in the developed world to the extent that would drive the point home.

    2. It’s got to be perceptible. “We do a better job on smog and acid rain and other problems we can see with our own eyes,” she says. Island nations that are already dealing with rising sea levels, diminishing fresh water supplies, and the effects of fiercer hurricanes are more motivated to address climate change today than larger nations where climate change has had less visible impact.

    3. The strategy must be practical. People need to believe that solutions exist and are feasible. “There’s too much of a perception out there that the alternatives to fossil fuels are completely impractical,” she says. “I’m seeing more and more evidence that that’s not the case.”

    Bottom line: successfully addressing climate change will require more innovation—and more widespread understanding of how serious the problem is. “Alternatives like wind, hydropower, and solar already are cheaper than fossil in some places, but not everywhere,” Solomon says. But as researchers focus on minimizing carbon emissions and improving methods to capture and store clean energy, she says, it’s also important to engage in a broader conversation about climate change: “People need to understand the problem better, in order to have a shared discussion about what is at risk and what we as a society think about how much risk is too much.”

    Potential for positive change
    In 2016, Solomon published the first paper showing clear signs that the Antarctic ozone layer is starting to heal, thanks to reductions in the emission of chlorofluorocarbons. That means human populations will have greater protection from ultraviolet radiation, which is known to cause cataracts, skin cancer, and other ills. And while the picture of ozone recovery is complicated by the effects of recent volcanic eruptions, Solomon’s paper showed that “if we remove that effect, ozone depletion isn’t as bad as it used to be,” Salawitch says. Dozens of other papers have now provided corroborating evidence, he adds, and “we can make fairly definitive statements that the Antarctic ozone hole is on the road to recovery.”

    Indeed, ozone recovery is one of the environmental success stories that Solomon highlights in her teaching. Megan Lickley, SM ’12, a fourth-year PhD student who works with Solomon, says they “talk a lot about whether previous successes are useful” in understanding what may happen with climate change, the most pressing issue of the day. She adds that Solomon is “engaging and extremely good at getting people to talk about their interpretations of policy arguments and what can be done in the future.”

    Recent news related to global climate change has been bleak—from the rapid rise in sea levels to catastrophic wildfires in California. The US government’s fourth National Climate Assessment, released by the Trump administration on the Friday of Thanksgiving weekend in 2018, warned that if significant steps are not taken to reduce emissions, the world will experience ever more dangerous heat waves, fires, floods, and hurricanes. These events are likely to cause crop failures, more frequent wildfires, and severe disruptions in supply chains and trade. Together, the changes could reduce the GDP of the United States alone 10% by 2100.

    But even in the face of dispiriting data, Solomon continues to focus on figuring out how to fix things. She tells her students that making headway on climate change depends on getting people to understand how the problem will affect them personally—and to believe there are practical solutions. And she’s hopeful about the progress she’s seeing.

    “Climate change won’t be solved until alternative energies become more widely adopted, but they are already becoming adopted at an incredibly astonishing pace,” she says. “Admittedly, it may not be as fast as it would need to be to hold temperatures to one and a half degrees—that would really be Herculean. But we can bend the warming curve.”

    Solomon notes that even though solar and wind power are not yet competitive in many parts of the United States, in many other places, clean-energy alternatives are becoming cheaper than fossil fuels. Certainly, there’s a good deal more work to be done. Innovation must continue to bring down the costs of clean energy and improve the battery technology needed to store it. And the challenge of converting existing infrastructure to cleaner power sources will be massive.

    “It’s not going to be immediate, but it’s happening,” she says. “Things are changing in the right direction.” 

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    In 1986, Solomon traveled to Antarctica to collect data that backed up her theory of how ozone destruction was happening and why it was so pronounced above that continent.
    National Oceanic and Atmospheric Administration
    During a follow-up expedition in 1987, she also found time to visit with Emperor penguins. In her office at NOAA, she later posed next to a globe showing Antarctica.
    National Oceanic and Atmospheric Administration
    During a follow-up expedition in 1987, she also found time to visit with Emperor penguins. In her office at NOAA, she later posed next to a globe showing Antarctica.
    National Oceanic and Atmospheric Administration
    Susan Solomon figured out that the polar stratospheric clouds above Antarctica provided the icy surface needed to kick off the chain of chemical reactions creating a hole in the ozone layer.
    Tony Luong
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