Carbon dioxide, one of the main greenhouse gases linked to climate change, is not just a global problem, but a local one too, according to a recent study.

Scientists from the Monterey Bay Aquarium Research Institute (MBARI) report that CO₂ emissions from Silicon Valley and Salinas Valley are blowing over to Monterey Bay, a region renowned for its rich marine life. They suspect this CO₂ will get absorbed by the ocean, making the waters more acidic and unfavorable for many sea creatures.

“This is a first-of-its-kind study to talk about CO₂ from urban sources affecting acidification in coastal waters,” said Richard Feely, a chemical oceanographer with the National Oceanic and Atmospheric Administration, who wasn’t involved in the research. “One might expect to see the same thing occur along the East Coast and places where there is high industrial activity near the coast.”

In urban hotspots, local emissions from transportation or large industries create CO₂ “domes”. This localized CO₂ concentration can hover over cities during early mornings, increasing urban air temperatures and affecting human health.

In the case of coastal urban hubs, land breezes can transport these CO₂ domes offshore. This process is especially important during winters, when seaward-flowing winds predominate and are stronger. In winters, water is warmer than land, and this temperature difference drives winds from land to sea.

At research stations in Monterey Bay, MBARI scientists began noticing high values of CO₂ in the air between 2013 and 2018. Expected to range around 400 parts per million, air CO₂ levels in Monterey Bay peaked at 450 on many occasions.

“At first they looked like randomly spaced peaks,” said lead researcher Devon Northcott. “But if you looked closely they seemed to align early in the morning, and more than that, they align with when they’re coming off the land.”

This was their first clue alluding to land-based pollution.

In the early days of making air and ocean CO₂ measurements, MBARI scientists used sensors anchored to research ships. They associated high air CO₂ levels to pollution from the ship. “We discarded those high values,” said Francisco Chavez, biological oceanographer at MBARI.

But in the last five years, they made measurements from sensors fitted on fixed moorings and a self-driving surface-water robot. The moorings are located at three locations near Monterey’s coast and the robot journeys back and forth from Moss Landing to 31 miles into the sea, collecting data every hour.

“What was most surprising was the consistency (in elevated air CO₂ values) that we observed,” Chavez said. “We saw it over many days in the winter, and over many winters.”

Simultaneous measurements of wind direction and speed allowed his team to track the location of CO₂’s origin. Also, based on wind flow, they figured that elevated CO₂ air rarely travels beyond 60 miles offshore.

“These (CO₂ emissions) were coming from Hecker Pass, which sort of connects us to Silicon Valley, and from the Salinas Valley,” Chavez said. His team considers human activities in the areas, as well as night-time CO₂ respiration from crops as possible contributors.

In addition to global CO₂, the ocean now has localized CO₂ added to its absorption budget. But how much does this additional pollution amount to?

Researchers estimate a 20 percent increase in CO₂ entering worldwide coastal waters. This is assuming that the process seen in Monterey Bay occurs similarly along all coasts.

“Twenty percent is really significant and important,” Feely said.

CO₂ enhancements can make ocean water more acidic. And this is bad news for shelled ocean creatures like pteropods, which are microscopic marine snails devoured by salmon, as well as mussels and sea urchins. The acidity dissolves their shells.

Feely, however, cautions that these are early estimates. “They need to be substantiated with measurements in other coastal areas,” he said.

MBARI’s research team hopes for the same. They’d like to see scientists in other parts of the world make similar measurements. “We want to know if the same processes occur all over the world,” Chavez said. And not just for CO₂, but other greenhouse gases like nitrogen dioxide too, he reckons.

Scott Doney, an oceanographer at the University of Virginia who wasn’t involved in the study, envisions further opportunities for inquiry. “What I’d like to see moving forward is trying to figure out what the sources are,” he said. This would mean “dissecting” the CO₂ dome and understanding the exact contribution of agriculture, transportation or industries.

High-resolution data from satellites like NASA’s Orbiting Carbon Observatory or isotopic fingerprints of greenhouse gases may provide answers.