Microplastic and nanoplastic particles have invaded not only the soil and waters of our planet, but also its atmosphere. A new study published in Nature Climate Change shows that these airborne particles also contribute to global warming, especially when they are coloured. The discovery should therefore prompt researchers to include these new contributors to climate change in their models.

Microplastics and nanoplastics are particles less than one millimetre in size that come from plastic objects, such as bottles and bags, that have broken down in the environment after exposure to sunlight and through abrasion. 

Microplastics — a generic term that includes nanoplastics — found in the atmosphere absorb solar radiation and reflect some of it. When the difference between the solar energy received and absorbed and the energy reflected back into space, known as the radiative effect, is positive — that is, when the energy absorbed is greater than the energy returned — the particles help warm the air around them.

Solar radiation

A modeling study published in 2021 estimated that the radiative effect of microplastics in the atmosphere was minimal, even negligible, compared with that of all aerosols, such as dust and smoke suspended in the air. However, that study assumed that all microplastic particles were colourless. The reality is quite different: a large number of plastic objects are coloured. In a study published May 4, Chinese and US researchers examined how the characteristics of microplastics, such as their colour, size, composition and aging, affected the way they absorb and scatter solar radiation.

Using advanced optical techniques, they measured how different types of colored microplastics interacted with the different wavelengths of the light spectrum, from ultraviolet to infrared. They found that particles containing pigments absorbed much more light — 75 times more — than colourless particles.

The researchers also simulated the aging of plastics in the laboratory using ultraviolet lamps and observed that white particles tended to yellow, and therefore absorb more light, as they aged. Particles of other colours, particularly red ones, instead tended to whiten, increasing their albedo, or reflective power. Overall, the whitening of coloured particles roughly offset the yellowing of white ones.

In a second stage, the researchers combined these measurements of the optical properties of plastic particles with an estimate of their distribution in the atmosphere. They obtained that distribution through simulations of the atmospheric transport of microplastics and nanoplastics detected in various regions around the world. All this information was then integrated into a model to determine their radiative forcing and, by extension, their impact on the climate. The model indicated that airborne plastic microparticles and nanoparticles could produce an average global radiative forcing of about 0.039 watts per square metre. That is much less than the forcing from greenhouse gases, but is equivalent to 16 per cent of the warming effect of fine soot particles, also known as black carbon, which come from the incomplete combustion of wood, coal, oil and gas.

An “urgent” role to understand 

“Such a contribution is a substantial fraction, especially considering that microplastics and nanoplastics are a class of pollutants that has not yet been systematically included in climate assessments,” notes Gilberto Binda of the department of theoretical and applied sciences at the University of Insubria in Varese, Italy, in a perspective piece published in the same issue of Nature Climate Change.

The model also revealed significant spatial and temporal differences. These variations depended on sources of plastic emissions at the Earth’s surface and on the local effects of solar radiation. Ocean regions with large accumulations of plastic waste, such as the North Pacific Subtropical Gyre — whose ocean currents have helped concentrate plastic waste in what is known as the North Pacific garbage patch — show particularly high radiative forcing, potentially about five times higher than that of soot. Seasonal differences were also observed, particularly in polar regions, where stronger radiative effects were noted during summer periods when the sun never sets.

“This research identifies microplastics and nanoplastics as new contributors to atmospheric warming, suggesting that these pollutants may need to be considered as climate change actors in future climate assessments and policies,” Binda says. 

“Plastic production continues to rise worldwide, [so] understanding their role in the atmosphere is becoming increasingly urgent,” he notes.