Ships Could Cut Carbon Emissions by Half

A shipboard system using limestone and seawater could reduce maritime carbon dioxide emissions by up to 50%, according to research published in Science Advances.

The technology mimics a natural ocean process, converting CO2 from ship exhaust into bicarbonate—a safe compound that naturally exists in seawater and remains stable for tens of thousands of years.

Scientists at USC and Caltech, collaborating with startup Calcarea, developed the system to address one of the world’s most challenging decarbonization sectors. Maritime shipping accounts for nearly 3% of global greenhouse gas emissions, yet current solutions like low-carbon fuels and electrification remain expensive or impractical for long-distance voyages.

Nature-Inspired Chemistry

“What’s beautiful about this is how simple it is,” explains William Berelson, the Paxson H. Offield Professor in Coastal and Marine Systems at USC and study co-author. “We’re speeding up a process the ocean already uses to buffer CO2—but doing it on a ship, and in a way that can meaningfully reduce emissions at scale.”

The process works as ships move through seawater. CO2 from exhaust gets absorbed into water pumped onboard, making it slightly acidic. This water then passes through limestone beds, where acid reacts with rock to form bicarbonate. The treated water, now stripped of CO2, flows back into the ocean.

Lab Results Scale to Ship Size

Researchers tested the system using controlled laboratory experiments with seawater, limestone, and CO2. Their findings showed strong agreement between experimental results and theoretical predictions, giving confidence for scaling up to vessel-sized operations.

Key performance metrics include:

• Conversion efficiency of 20-35% in laboratory conditions
• Counterflow design achieving 74% instantaneous CO2 removal compared to 40% for parallel flow
• Ship-scale reactors requiring less than 1% of total vessel volume
• Limestone consumption of 12 shipping containers per day for a 10,000-container vessel

“We wanted to show that we not only understood the chemistry—we could also predict how much CO2 would be neutralized,” Berelson notes. This predictive capability allowed researchers to model real-world ship applications.

Ocean Safety Validation

Advanced ocean modeling examined what happens when bicarbonate-rich water returns to the sea. Simulations tracked a hypothetical ship traveling repeatedly between China and Los Angeles over 10 years, discharging treated water along the route.

Results showed negligible impact on ocean pH and chemistry. Surface alkalinity and dissolved inorganic carbon increased by less than 1.4% after a decade of continuous operation—well within natural variation ranges.

“We see our approach as a complementary strategy that could help ships reduce their environmental impact without major design overhauls,” said Jess Adkins, co-founder and CEO of Calcarea and professor at Caltech.

From Laboratory to Ocean

The technology addresses a critical need in maritime decarbonization. Current solutions face significant barriers: alternative fuels remain expensive, while electrification works only for short routes. The limestone-seawater system could integrate with existing vessels without extensive modifications.

Calcarea is already in discussions with commercial shippers about pilot programs. The company previously announced a collaboration with Lomar Shipping’s venture lab to commercialize the technology.

“Scalability is built into our design,” Adkins explains. “We’re engineering a system that can integrate with existing vessels and be adopted fleetwide. By working directly with industry partners, we’re accelerating the path from lab to ocean.”

Real-World Impact Potential

The researchers estimate widespread adoption could cut shipping-related CO2 emissions in half. For a typical 10,000-container vessel traveling at 15 knots, the system would require four reaction units of 600 cubic meters each—a modest footprint compared to cargo space.

Ocean modeling revealed another benefit: ships naturally create turbulence that quickly mixes treated water with surrounding seawater, diluting it by a factor of 710 within minutes. This rapid mixing prevents CO2 from escaping back to the atmosphere.

“This is the kind of scale we need if we’re going to make a real dent in global emissions,” Berelson concludes. “It’s not going to happen overnight, but it shows what’s possible.” The technology offers hope for an industry struggling to find practical decarbonization solutions while maintaining global trade efficiency.