UTOPIA in code: charting the fate of our plastic world

To answer these questions, Professor Matthew MacLeod and postdoctoral researcher Prado Domercq developed UTOPIA, an open-source mass balance model that simulates how plastics move through, settle in, and transform within Earth’s systems. UTOPIA stands for " mUltimedia uniT world OPen-source model for mIcroplAstic "—a sprawling research tool that ties together the world’s oceans, soils, skies, sediments, and more.

“I strongly believe that one of the most important things about a model or a project is that it has a nice name and a memorable short name,” says MacLeod. “UTOPIA gives this sort of vision—that you're creating a world that is not impacted by plastic.”

From lakes to the world

The project began as a collaboration between MacLeod and environmental chemist Antonia Praetorius, sparked by a call for proposals on modelling plastic pollution. Building on MacLeod’s earlier work in chemical mass balance, the team initially focused on aquatic systems—developing a model for lakes and rivers, called Full Multi. But soon, it became clear that the story of plastic pollution extended far beyond water.

Domercq joined the project at that point, bringing her background in nanoparticle emissions and fate modelling. “At the beginning I was more focused on nanoparticles,” she explains. “But I started seeing the similarities to nano- and microplastics in terms of that they behave as particles, not chemicals. So you look at the fate in a different way.” She continues: “There’s a lot of knowledge in many different silos—analytical development, lab studies, monitoring campaigns—and with this kind of modelling, you can try to look at the bigger picture and put everything together.”

That global perspective became UTOPIA: a model not only of plastic transport, but also of its fragmentation, aggregation, biofouling, sinking, remobilisation, and eventual degradation into carbon dioxide or burial in sediments. It covers 17 environmental compartments and considers both mass and particle number, reflecting two very different ways plastic exerts its effects.

A demonstration of UTOPIA

During the interview, MacLeod walks us through the user interface—built with support from the Swedish Research Council-funded infrastructure InfraVis. He inputs a scenario: 100 grams per second of PVC (polyvinyl chloride) entering coastal surface waters.
Because PVC is dense, it doesn’t float—it sinks.

“You throw it into the ocean, it just sinks into the sediments: 98.6% of it,” he says.

When the scenario is changed to polyethylene, a lighter plastic, the outcome shifts dramatically: more material stays afloat, travels further, and some is even returned to shore.

“There’s quite a bit on the beaches,” Domercq adds. “It comes back from the waves—washed back up.”

MacLeod scrolls through the output. “Now it’s more exciting,” he says. “Whereas before you had 98% [in sediments], now only 5% does that.” Instead, a much larger share ends up in the open ocean, and about 2% is buried into the beach.” 

Then he pulls up a heat map showing different sizes and aggregation states. “The big plastic particles float and float away,” he explains. “But once they fragment, they actually sink down.” As the particles get smaller, their vertical distribution is no longer dominated by density. “They behave more like a gas, or like a suspended particle.” It’s a quiet transformation: what starts as a single floating item becomes a mix of forms—some sinking, some circulating, and some returning to shore.

One powerful feature is the ability to toggle between mass-based and particle-based views.

“On a mass basis, way less than 1% of the plastic is in the atmosphere. But on a particle number basis, it’s two orders of magnitude larger,” MacLeod explains. “It’s still a small amount—but much larger on a particle basis.”

Why it matters

Plastic may eventually break down into carbon dioxide, but the journey is long—and during that time, it can cause harm.

“In between, it’s a particle interfering with other processes, potentially,” says MacLeod. “There’s still a lot of unknowns about what happens to it in between.” He continues: “The analytical chemistry is getting better and better. So people are able to measure plastic in all kinds of places—and they keep finding it everywhere they look: in human bodies, in Arctic ice, sediments… And we still don’t fully understand what it’s doing in all these places.”

Some impacts are visible and immediate—like birds mistaking plastic for food.

“This is just wrong,” says MacLeod. “And we shouldn’t do this.”

Building on uncertainty

Behind UTOPIA’s clean interface lies a vast architecture: 21 interconnected sub-models, each representing a process like fragmentation, sedimentation, or atmospheric transport. Each must work across five particle size bins and four aggregation states, totaling 420 process pathways.

“Behind what we just looked at is actually 21 separate process-based models,” Domercq explains. “And each of them has to work across multiple sizes and aggregation types.”

To keep the model useful despite uncertainty, UTOPIA is hosted on GitHub, with every sub-model open to improvement and discussion.

“The idea is that scientists working on a process—say, sea spray aerosol mobilisation—can look at our model and say, ‘Hey, this part’s wrong,’ and help us fix it.”

From predictions to policy—and back again

Can UTOPIA influence policy? Maybe not directly—but it can provide the foundation.

“Right now it’s mostly for scientists—to run scenarios, generate hypotheses, improve understanding,” MacLeod says. “But you could, in theory, use it for forecasting under different policy actions.”

UTOPIA has already been used in teaching and scenario analysis. One important insight is the concept of “toxic debt”—even if we stop all plastic emissions today, microplastic concentrations could continue to rise due to legacy material breaking down.Domercq believes the tool could eventually help plastic designers too.

“We’ve identified five key properties that matter most: density, fragmentation pattern, fragmentation time, shape, and the timescale for discorporation,” she says. “In theory, you could use UTOPIA in reverse—input your environmental goals and see which combinations of properties would get you there.”

A new standard for the Plastic Age

As the world negotiates a global plastics treaty, UTOPIA offers a way to go beyond visual metaphors and toward quantitative understanding.

“We’re not forecasting disaster,” says MacLeod. “We’re creating a framework to understand what’s already happening—and we’re inviting others to help make it better.”

On World Environment Day, with its rallying cry to “beat plastic pollution”, this work reminds us: it’s not enough to clean up the beach. We need to understand the invisible particles that linger, accumulate, and migrate.

UTOPIA may not save the world from plastic—but it just might help us learn how.

Access UTOPIA

Key concepts in environmental pollution and plastic fate

Some of the terms used in this story are common in environmental science and pollution modelling. Here's a quick guide to help you follow along:

Mass balance model

A method scientists use to track how much of a substance enters the environment, how it moves, and where it ultimately ends up.

Microplastics

Plastic fragments smaller than 5 millimeters, often formed as larger plastics break apart.

Nanoplastics

Ultra-small plastic particles—measured in billionths of a meter—that result from ongoing fragmentation.

Fragmentation

The process by which plastic breaks down into smaller and smaller pieces, due to sunlight, mechanical forces, or chemical weathering.

Sink

A part of the environment where pollutants accumulate and stay for long periods—such as ocean sediments, soils, or deep lakes.

Exposure

The potential for organisms (including humans) to come into contact with a pollutant, either through air, water, soil, or food.