Scientists have mapped what they say is the largest peatland in the tropics, an area larger than New York State in the Congo Basin in Central Africa.

The peat, which consists of slowly decomposing vegetation in swamp forests, has been accumulating for more than 10,000 years. As in all peatlands, the vegetation is a natural storehouse of carbon taken from the atmosphere — in this case, about 30 billion metric tons of carbon, or roughly equivalent to the carbon in two decades of fossil fuel emissions in the United States.

“It’s astonishing to me that in 2017 we can be making these kinds of discoveries,” said Simon Lewis, a professor at the University of Leeds in England and an author of a study on the peatlands being published on Wednesday in the journal Nature.

Dr. Lewis and his colleagues first discovered the peatlands several years ago, working on a hunch that in the wetlands known as the Cuvette Centrale, which straddle the border of the Republic of Congo and the Democratic Republic of Congo, they would find peat in layers under the swamps. The vegetation, which is waterlogged year-round, lacks oxygen and other nutrients that would lead to its quick decomposition.