On a recent Saturday, some 100 volunteers gathered on a popular beach in Yokohama, wading in the shallows to plant strands of light-green eelgrass on the seabed.

What began as a project to restore the natural ecosystem along the coast of the city has taken on national importance: helping fight climate change as Japan aims to achieve carbon neutrality by 2050.

Japan, the world's fifth-largest emitter of greenhouse gases, covers a surface area smaller than California but has some of the longest coastlines in the world. That makes marine vegetation a viable method of capturing at least a fraction of the carbon dioxide it produces, scientists say.

"Over the course of this work, we've come to understand that it can absorb and store the carbon that causes climate change," said Keita Furukawa, marine scientist at the Association for Shore Environment Creation.

In a world first, Japan's most recent annual Greenhouse Gas Inventory, provided to the United Nations Framework Convention on Climate Change this month, factored the carbon absorbed by seagrass and seaweed beds into its calculations.

The Environment Ministry estimates that in the fiscal year that began April 2022, that amount of "blue carbon" — carbon that is naturally stored by marine and coastal ecosystems — was roughly 350,000 metric tons.

While that is just 0.03% of the 1.135 billion tons of CO2 equivalent greenhouse gases Japan emitted that year, blue carbon has taken on more importance as the country's forests age, absorbing less carbon dioxide than younger trees.

The amount of greenhouse gases absorbed by forests fell 17% over the five-year period to 2022, government data shows, and Japan has said it will make efforts on both land and in the sea to capture more carbon.

"If eelgrass were to grow in every shallow area of the sea it's possible for it to grow, I think it could absorb perhaps 10% or 20% of human emissions," Furukawa said.