Clouds form when water vapor—an invisible gas in the atmosphere—sticks to tiny floating particles, such as dust, and turns into liquid water droplets or ice crystals. In a newly published study, we show that microplastic particles can have the same effects, producing ice crystals at temperatures 5° to 10° Celsius (9° to 18° Fahrenheit) warmer than droplets without microplastics.

This suggests that microplastics in the air may affect weather and climate by producing clouds in conditions where they would not form otherwise.

We are atmospheric chemists who study how different types of particles form ice when they come into contact with liquid water. This process, which occurs constantly in the atmosphere, is called nucleation.

Clouds in the atmosphere can be made up of liquid water droplets, ice particles, or a mixture of the two. In clouds in the mid- to upper atmosphere where temperatures are between 32° and minus 36° F (0° to minus 38° C), ice crystals normally form around mineral dust particles from dry soils or biological particles, such as pollen or bacteria.

Microplastics are less than 5 millimeters wide—about the size of a pencil eraser. Some are microscopic. Scientists have found them in Antarctic deep seas, the summit of Mount Everest, and fresh Antarctic snow. Because these fragments are so small, they can be easily transported in the air.

Why it matters

Ice in clouds has important effects on weather and climate because most precipitation typically starts as ice particles.

Many cloud tops in nontropical zones around the world extend high enough into the atmosphere that cold air causes some of their moisture to freeze. Then, once ice forms, it draws water vapor from the liquid droplets around it, and the crystals grow heavy enough to fall. If ice doesn’t develop, clouds tend to evaporate rather than causing rain or snowfall.

While children learn in grade school that water freezes at 32° F (0° C), that’s not always true. Without something to nucleate onto, such as dust particles, water can be supercooled to temperatures as low as minus 36° F (minus 38° C) before it freezes.

For freezing to occur at warmer temperatures, some kind of material that won’t dissolve in water needs to be present in the droplet. This particle provides a surface where the first ice crystal can form. If microplastics are present, they could cause ice crystals to form, potentially increasing rain or snowfall.

Clouds also affect weather and climate in several ways. They reflect incoming sunlight away from Earth’s surface, which has a cooling effect, and absorb some radiation that is emitted from Earth’s surface, which has a warming effect.

The amount of sunlight reflected depends on how much liquid water vs. ice a cloud contains. If microplastics increase the presence of ice particles in clouds compared with liquid water droplets, this shifting ratio could change clouds’ effect on Earth’s energy balance.

The Earth constantly receives energy from the Sun and reflects it back into space. Clouds have both warming and cooling effects in this process.

Credit: NOAA

How we did our work

To see whether microplastic fragments could serve as nuclei for water droplets, we used four of the most prevalent types of plastics in the atmosphere: low-density polyethylene, polypropylene, polyvinyl chloride, and polyethylene terephthalate. Each was tested both in a pristine state and after exposure to ultraviolet light, ozone, and acids. All of these are present in the atmosphere and could affect the composition of the microplastics.

We suspended the microplastics in small water droplets and slowly cooled the droplets to observe when they froze. We also analyzed the plastic fragments’ surfaces to determine their molecular structure, since ice nucleation could depend on the microplastics’ surface chemistry.

For most of the plastics we studied, 50 percent of the droplets were frozen by the time they cooled to minus 8° F (minus 22° C). These results parallel those from another recent study by Canadian scientists, who also found that some types of microplastics nucleate ice at warmer temperatures than droplets without microplastics.

Exposure to ultraviolet radiation, ozone, and acids tended to decrease ice nucleation activity on the particles. This suggests that ice nucleation is sensitive to small chemical changes on the surface of microplastic particles. However, these plastics still nucleated ice, so they could still affect the amount of ice in clouds.

What still isn’t known

To understand how microplastics affect weather and climate, we need to know their concentrations at the altitudes where clouds form. We also need to understand the concentration of microplastics compared with other particles that could nucleate ice, such as mineral dust and biological particles, to see whether microplastics are present at comparable levels. These measurements would allow us to model the impact of microplastics on cloud formation.

Plastic fragments come in many sizes and compositions. In future research, we plan to work with plastics that contain additives, such as plasticizers and colorants, as well as with smaller plastic particles.

This article is republished from The Conversation under a Creative Commons license. Read the original article.

Enlarge / The cycle of building and breaking up of supercontinents seems to drive long-term climate trends. (credit: Walter Myers/Stocktrek Images)

Global temperature records go back less than two centuries. But that doesn't mean we have no idea what the world was doing before we started building thermometers. There are various things—tree rings, isotope ratios, and more—that register temperatures in the past. Using these temperature proxies, we've managed to reconstruct thousands of years of our planet's climate.

But going back further is difficult. Fewer proxies get preserved over longer times, and samples get rarer. By the time we go back past a million years, it's difficult to find enough proxies from around the globe and the same time period to reconstruct a global temperature. There are a few exceptions, like the Paleocene-Eocene Thermal Maximum (PETM), a burst of sudden warming about 55 million years ago, but few events that old are nearly as well understood.

Now, researchers have used a combination of proxy records and climate models to reconstruct the Earth's climate for the last half-billion years, providing a global record of temperatures stretching all the way back to near the Cambrian explosion of complex life. The record shows that, with one apparent exception, carbon dioxide and global temperatures have been tightly linked. Which is somewhat surprising, given the other changes the Earth has experienced over this time.

Enlarge / Artist's conception of the state of the Earth during its global glaciations. (credit: NASA)

Earth has gone through many geologic phases, but it did have one striking period of stasis: Our planet experienced a tropical environment where algae and single-celled organisms flourished for almost 2 billion years. Then things changed drastically as the planet was plunged into a deep freeze.

It was previously unclear when Earth became a gargantuan freezer. Now, University College London researchers have found evidence in an outcrop of rocks in Scotland, known as the Port Askaig Formation, that show evidence of the transition from a tropical Earth to a frozen one 717 million years ago. This marks the onset of the Sturtian glaciation and would be the first of two "snowball Earth" events during which much of the planet’s surface was covered in ice. It is thought that multicellular life began to emerge after Earth thawed.

Found in the Scottish islands known as the Garvellachs, this outcrop within the Port Askaig Formation is unique because it offers the first conclusive evidence of when a tropical Earth froze over—underlying layers that are a timeline from a warmer era to a frigid one. Other rocks that formed during the same time period in other parts of the world lack this transitional evidence because ancient glaciers most likely scraped it off.

Enlarge / Image of some of the atmospheric circulation seen during NeuralGCM runs. (credit: Google)

Right now, the world's best weather forecast model is a General Circulation Model, or GCM, put together by the European Center for Medium-Range Weather Forecasts. A GCM is in part based on code that calculates the physics of various atmospheric processes that we understand well. For a lot of the rest, GCMs rely on what's termed "parameterization," which attempts to use empirically determined relationships to approximate what's going on with processes where we don't fully understand the physics.

Lately, GCMs have faced some competition from machine-learning techniques, which train AI systems to recognize patterns in meteorological data and use those to predict the conditions that will result over the next few days. Their forecasts, however, tend to get a bit vague after more than a few days and can't deal with the sort of long-term factors that need to be considered when GCMs are used to study climate change.

On Monday, a team from Google's AI group and the European Centre for Medium-Range Weather Forecasts are announcing NeuralGCM, a system that mixes physics-based atmospheric circulation with AI parameterization of other meteorological influences. Neural GCM is computationally efficient and performs very well in weather forecast benchmarks. Strikingly, it can also produce reasonable-looking output for runs that cover decades, potentially allowing it to address some climate-relevant questions. While it can't handle a lot of what we use climate models for, there are some obvious routes for potential improvements.