As dusk settles over Cambridge on a midwinter evening, Dan Cziczo stops for a moment to take in a spectacular view. It’s sunset, and just above the horizon, streaks of red and orange bleed into deeper swaths of purple and blue as clouds of every type stretch across the darkening sky. Cotton-ball puffs of cumulus blend with a blanketed layer of stratus, and thin, featherlike threads of cirrus trail overhead. For anyone taking a break from work to look west along the Charles River, the sight is a stunner.

For Cziczo, a 42-year-old atmospheric scientist at MIT, the view, in a way, is his work. Cziczo studies cloud formation, and he sees clouds—cirrus in particular—as a key to answering a crucial question: exactly how much will Earth warm up in the near future?

The best answer scientists have come up with so far is still uncertain—anywhere from 1 to 5 °C, depending on the amount of greenhouse gases that humans add to the atmosphere. In parts of the world such increases could mean rising seas, stronger storms, and damaging fires and floods. With every degree of warming, scientists predict up to 15 percent reductions in crop yields, 15 percent decreases in the area covered by Arctic sea ice, 10 percent increases in rainfall during the heaviest storms, and 400 percent increases in areas burned by wildfires in the western United States. That means the difference between one and five degrees of warming is quite significant.

In 2007, in a report issued by the Nobel Prize–winning Intergovernmental Panel on Climate Change, scientists from around the world concluded that much of the uncertainty in climate projections has to do with clouds. The scientists noted that while clouds may block solar radiation from entering the atmosphere, the conditions under which they form, and the extent to which they actually cool the planet by reflecting that radiation away, is very poorly understood. Further complicating matters, a warmer Earth holds more moisture, which could increase the total volume of clouds.

To reduce the uncertainty in climate projections, Cziczo and his research group at MIT are studying subjects such as aerosols, or airborne particles, which act as “seeds” that help clouds form. As particles like dust float up into the atmosphere, they provide a surface on which water vapor may condense or freeze, forming a fine mist that from a distance can appear puffy, layered, or wispy, depending on a region’s temperature and relative humidity.

“Different particles and clouds act differently, and understanding this balance is really how we’re going to increase the certainty [of climate projections],” Cziczo says. “Pinning it down to say, ‘Are we getting one degree or three degrees of warming?’ That’s the kind of thing we’re trying to figure out.”

Seeing through cirrus
Tonight, Cziczo is catching the cloud display from an enviable perch: the roof of MIT’s 21-story Cecil and Ida Green Building, the tallest building in Cambridge.

The roof has long been an ideal site for atmospheric study, housing instruments that measure wind speed, relative humidity, and temperature. On occasion, Cziczo, an associate professor in the Department of Earth, Atmospheric, and Planetary Sciences, will bring his students up here to take instrument readings, using the data to figure out whether and where clouds will form.

This time, however, he’s just here for the view.

“If you look through the sunset, you can see the higher clouds, the sort of wispy ones,” Cziczo says as he points out cirrus clouds in the distance. “They make these cool filaments … their Greek name has to do with horsehair or mare’s tails, and those are the ones we’ve been studying lately, because of their importance in climate.”

Cirrus clouds form four to 12 kilometers above Earth’s surface in the upper portion of the troposphere, the lowest layer of the atmosphere. At such altitudes, water vapor can freeze around particles, forming ice crystals. The resulting ice clouds, as cirrus clouds are also known, are usually the first cloud layer that sunlight meets as it makes its way to the surface. The ice crystals act as tiny reflectors that scatter sunlight. It’s thought that clouds in general may reflect enough sunlight back into space to offset between half and three-quarters of the warming caused by greenhouse gases such as carbon dioxide. The net impact of cirrus clouds, however, is unclear: while they shield the planet from incoming sunlight, they also trap radiation trying to escape from its surface.

To know exactly what role cirrus clouds play, Cziczo says, it’s important to understand how they form—specifically, what particles, or aerosols, are naturally seeding them.

As the sun sets, he heads down to his lab on the 13th floor, where two glass tubes, partially enclosed in a metal casing, are brewing up clouds. The setup, which he helped build, is called a cloud chamber. By adjusting the temperature and relative humidity in the chamber, researchers can create perfect conditions for the formation of cloud droplets or ice crystals. The only missing ingredient is an ideal seed on which clouds may form.

Sifting for seeds
Cziczo has been testing various aerosols to see which will most readily form clouds in the chamber. By feeding these different aerosols into the chamber as he mimics weather conditions in certain parts of the world, he hopes to determine what particles are causing cloud formation in those regions. To demonstrate, he takes out a small jar of gray powder, mineral dust collected in Wisconsin.

“Let’s make a dust storm,” he says, and waves the open jar in front of two nozzles, which take air into each glass tube. The tubes are too small to generate visible clouds, so Cziczo uses a system of lasers to measure whether the water vapor has coalesced into droplets large enough to be considered cloud particles.

Next, Cziczo dries the cloud droplets by sending them through a small compartment filled with desiccants similar to what’s in the packets found in shoeboxes. He and his colleagues can then analyze them to determine the exact composition of the cloud seed.

The cloud chamber is small enough to be packed up and taken to any part of the world to sample directly from a region’s atmosphere, which Cziczo says is a big advantage. Scientists may find that while a certain aerosol is excellent at seeding clouds in the lab, that aerosol isn’t found at the altitude where it might form clouds in nature. It is generally assumed that biological material is a fantastic substance for forming ice clouds, he says, noting that some types of pollen generate clouds remarkably well in his chamber. “But when you go in the field, you realize it’s just not present in the upper troposphere in large numbers, so it can’t have a large effect on clouds. If you just sampled on the ground, you might fool yourself into thinking that it’s important.” So he has made a point of including both field studies and lab work in his group’s research.

Sitting in ice clouds
Over the past 15 years, Cziczo has visited mountaintops in search of the kinds of aerosols likely to be found throughout the upper troposphere. As a postdoc at the University of Colorado and the National Oceanic and Atmospheric Administration, he made trips to Storm Peak Laboratory, in north central Colorado, where he sampled high-altitude clouds with an early version of the cloud chamber. That experience prepared him for a research and teaching position at the Swiss Federal Institute of Technology in Zurich, and then for a literal high point in his career: a stint sampling clouds at the Sphinx Observatory, a remote research station built along the spine of the Bernese Alps. Named for its sphinx-like architecture, it is one of the highest land-based observatories in the world at more than 11,000 feet above sea level. At this altitude, mixed-phase clouds—which are similar to cirrus clouds—can blanket the peaks.

The site, which has been called “the Top of Europe,” is a tourist attraction by day, when people travel up by train—electrically powered so as not to taint scientists’ measurements with exhaust. At night, however, the tourists take off, and the researchers bunk down.

“The first night, nobody sleeps,” Cziczo recalls. “You get a pounding headache, and you can feel your heart beating. It takes a couple days to acclimate, but after that, it’s amazing … at times, you’re actually sitting in ice clouds.”

After his time in Switzerland, Cziczo continued his work back in the United States at the Pacific Northwest National Laboratory. In 2011, he moved east to join the faculty at MIT.

In March 2011, he and his students took the cloud chamber to the Johnson Space Flight Center in Houston, where they mounted it to the nose of an old B-57 bomber. The plane, which was flown in the 1950s during reconnaissance missions, has since been repurposed as a research aircraft and is now used for projects such as a NASA field campaign called the Mid-latitude Airborne Cirrus Properties Experiment (MACPEX). The plane flies as high as 63,000 feet, making it perfect for sampling cirrus clouds, though it can be tricky to predict when they might appear.

In a period of six weeks, the team collected cloud samples over the Gulf of Mexico and the desert Southwest. Analyzing their composition showed that mineral dust, such as sand kicked up from a desert storm, accounted for about 60 percent of the aerosols in those clouds. The researchers also found that between 8 and 25 percent of cloud-forming dust particles contained lead. What they didn’t find was perhaps more surprising: biological material such as pollen and spores, or carbon emitted from smokestacks. While researchers have seen carbon and pollen readily form clouds in the lab, this type of aerosol accounted for less than 1 percent of cirrus cloud particles in Cziczo’s findings.

Researchers hope such experiments will help pin down exactly which aerosols form cirrus clouds and, more important, whether those aerosols are released by human activity. For example, Cziczo says that while mineral dust is a natural substance, made largely of dirt and sand blown off Earth’s surface, humans have significantly changed the amount of it in the atmosphere.

“When you change land uses, when you get rid of forests to create cropland, or you till under crops … you’re perturbing mineral dust,” he says. “So it’s a natural particle, but there’s more of it because of manmade activities. And it looks like it’s one of these things that’s forming ice clouds.” The lead-containing cloud dust the team found probably came from human activity as well: sources like aircraft tailpipes, coal-­burning power plants, and leaded gasoline that wasn’t phased out worldwide until the mid-1990s. Although he’s certainly not advocating further pollution, Cziczo acknowledges that “global warming would be much worse if it wasn’t for the human addition of particles to the atmosphere.”

“In the past, climate assessment groups have really not addressed whether anthropogenic activity might be affecting ice clouds, even though they are known to be important in the climate,” says Jon Abbatt, a professor of atmospheric chemistry at the University of Toronto. “That’s what’s special about Dan’s work. He has the capabilities to assess whether there are anthropogenic signatures in ice clouds. That’s the starting point for trying to make an assessment of ice formation as it relates to climate change.”

Measuring together
Cziczo and a growing community of atmospheric scientists hope that identifying the basics of cloud formation wipes out any remaining uncertainty about global warming. In addition to their experimental work, they are developing climate models that incorporate cloud formation. The data they collect will help make such models much more precise, although there are significant challenges to overcome: most models simulate climate by dividing the globe into a grid, averaging weather data over squares that are, at the finest resolution, 100 square kilometers. Incorporating cloud data at the level of fine aerosols would require enormous computational power.

Chien Wang, senior research scientist in MIT’s Center for Global Change Science, is working with Cziczo to find ways to fit this fine-particulate data into large-scale climate models. “Dan’s lab and field work can obviously help us to improve our model to better simulate the linkage between aerosols and ice clouds, and their climate effects,” Wang says. “I’m very glad that we can have him in house.”

Cziczo’s work may also help overcome another major obstacle in the field. Researchers in disparate groups tend to build their own cloud chambers, and the measurements from one may not be comparable to those from another. The instrument that Cziczo helped engineer was recently licensed by a company in Colorado, which is manufacturing it as the first commercial ice-cloud chamber. Model number 001 has a place of honor on his MIT lab bench, and other researchers have placed orders for more units.

Back in his MIT office, Cziczo looks out his window, a wide view that takes in the Boston skyline and a few stray clouds above. Occasionally he takes pictures of cloud formations, or interesting contrails from passing planes, and asks his students to identify the type of cloud and where it might have formed. It’s an exercise born of plain wonder as much as scientific curiosity.

“As a kid I was always sort of fascinated with clouds and flying and things like that,” he remembers. “I think I get more joy out of it now, because I understand some of it, and I still try to look outside and figure things out.”

In addition to studying clouds in Earth’s atmosphere, Dan Cziczo is investigating those that may form on Mars. Though the Martian atmosphere is too thin to support life, recent images from NASA’s Mars Reconnaissance Orbiter have shown carbon dioxide snow, precipitated from clouds.

To find out what might be forming those clouds, which looked to Cziczo like “diamond dust,” he and his students are growing clouds under Mars-like conditions in the lab. They recently made a trip to the largest cloud chamber in the world, the Aerosol Interaction and Dynamics in the Atmosphere (AIDA) facility in Karlsruhe, Germany—an old, repurposed nuclear reactor whose core has been replaced with a three-story-tall chamber. Scientists from around the world use the massive chamber to observe large-scale effects they could not see in benchtop models.

Cziczo’s team contacted NASA for samples of dust thought to be similar in composition to dust on Mars (it was actually collected from U.S. deserts) and placed them in the cloud chamber, adjusting its temperature and relative humidity to levels that have been observed on Mars. The experiment successfully formed a water-ice cloud.

Cziczo hopes to continue this new extraterrestrial branch of his research, which he says was inspired partly by the atmospheric images taken by the Mars Reconnaissance Orbiter and NASA’s Phoenix lander.

“You can see ice crystals falling out of the atmosphere,” he says. “And it’s funny, because the first time I saw those images, they looked like clouds in Earth’s atmosphere.”

–J.C.