How Airborne Microplastics Affect Climate Change
Microplastics—minuscule bits of bottles, bags, synthetic fibers and other plastic waste that have broken up in the environment—are influencing Earth’s climate as they circulate through the atmosphere. Like other aerosol particles, both natural and synthetic, microplastics seem to have an overall cooling effect (albeit a small one), according to the first study to look at the possible climate effects of airborne microplastics. The study’s authors and other researchers say the findings, published on Wednesday in Nature, show the urgent need to get a better handle on how much plastic debris is in the air, where it is and what it is made of in order to better pin down its climatic influence.
Microplastics are yet another kind of particle that humans are adding to the atmosphere “that has a climate impact. And that is big, and that is important, and we need to start accounting for that” when examining factors that affect Earth’s climate, says Deonie Allen, a microplastics researcher at the University of Strathclyde in Scotland. “This is the paper that opens that door,” adds Allen, who was not involved in the new study but has worked with the authors on other research.
All kinds of plastic waste crumble into smaller and smaller pieces when exposed to sunlight, wind, rain and other environmental conditions. Plastic’s generally low density means these fragments can easily be picked up by winds and blown around the world. In recent years, scientists have even found microplastics on remote mountain peaks and in the Arctic.
It occurred to researchers—including Laura Revell, an atmospheric scientist at New Zealand’s University of Canterbury and one of the new study’s authors—that all those particles swirling around the globe would intercept sunlight, as do other aerosols such as dust, sulfates and black carbon. This ultimately influences temperatures on Earth’s surface. Sulfates, for example, scatter radiation, exerting a cooling effect. Black carbon, on the other hand, absorbs visible and infrared radiation, warming the atmosphere.
But unlike sulfates or black carbon, plastic is not one material but hundreds. It encompasses a multitude of different polymers, as well as the chemicals and pigments that are added to them. Microplastic particles also come in a wide array of sizes and shapes. “That makes them particularly tricky,” Revell says. Her team’s study only considered noncolored fragments and fibers shed by synthetic fabric because they were the only materials for which the researchers had information on radiative properties. These particles scatter ultraviolet and visible light and absorb infrared light. When the scientists included these interactions in global climate models, they could estimate the particles’ net impact on Earth’s energy balance—which was a very slight cooling. The study estimated so-called effective radiative forcing (ERF), a measure of changes in Earth’s energy balance. Microplastics had an ERF of about –0.75 milliwatt per square meter, whereas all other aerosols have an ERF between –0.71 and –0.14 watt per square meter. (There are 1,000 milliwatts in one watt.) At the global level, warming from greenhouse gases in the atmosphere overwhelms these cooling influences.
But microplastics could have localized cooling or warming effects depending on how they vary from place to place: there are higher concentrations over some cities, for example. “The regional effects of aerosols can be significant” even when the overall global effect is low, says climate scientist Bjørn Samset, who studies aerosols at the Center for International Climate Research in Oslo and was not involved with the new study.
The exact effect on temperature can vary depending on how many particles are involved, how high in the atmosphere they are and numerous other variables. Because Revell and her co-authors wanted to take a first stab at addressing the question of climate influence, they assumed a uniform concentration of one microplastic particle per cubic meter of air throughout the lowest layer of the atmosphere. Even the limited concentration studies done to date show huge variations, however, from as low as 0.01 particle per cubic meter over parts of the Pacific Ocean to as high as 5,550 particles per cubic meter over Beijing. Studies have used different methods of sampling and detection, some of which miss the smallest plastic particles. In the studies that have used more sensitive methods, the tiniest particles made up half of what was found. And scientists do not yet know how many microplastics may be present at higher levels of the atmosphere, where their effects could be different.
Factoring in pigments and other additives could also change the effect they have. Pigments, for example, would generally increase light absorption, which would tend to warm the atmosphere. Revell says there is simply not yet enough information available to draw such conclusions. And then there are organic materials that could alter things by glomming on to plastic particles, as well as the ways these particles may interact with other atmospheric chemicals or influence cloud formation. “We still don’t know much about how they actually behave in the atmosphere,” Revell says.
Though the overall effect she and her colleagues have calculated is small, compared with that of other aerosols, “it is big enough to be quantified,” Allen says, adding that this shows the need to fund more and better monitoring of atmospheric microplastics. Rather than microplastics being a separate problem, she says, the results “securely cement [them] in the climate change argument.”