The estimates, based on field emission studies carried out over two decades, also show that the latest year studied, 2015, saw levels of carbon emissions from peat amounting to two-thirds of that from the burning of fossil fuels, the production of cement, and gas flaring in Malaysia and Indonesia.

Although Jukka Miettinen of the National University of Singapore believes the estimates are "not surprising", they should draw attention to the contribution of peatland oxidation to global greenhouse-gas emissions.

"The constantly occurring peat oxidation emissions should not be forgotten," he said. "They may not have attracted as much attention [as] the highly publicized peatland fire emissions, but overall the two sources have been comparable in magnitude over the past decade."

Tropical peatlands are thought to contain as much carbon as the entire above-ground biomass of the Amazon rainforest, and most of those peatlands are in Southeast Asia. Thanks to human intervention, however, the peatlands have turned from being a carbon sink to a carbon source as they are either burned or drained – the latter allowing the peat to decompose, oxidize and release carbon dioxide.

Quantifying the levels of carbon emissions has not been an easy task. Gas fluxes from peatlands are affected by various factors, from the level of the water table to the types of fertilizer present in the environment. Accurate estimates require long-term field studies.

In 2014, the Intergovernmental Panel on Climate Change synthesized the results from field-emission studies of tropical peatlands. Miettinen and colleagues used this synthesis in conjunction with an updated land-cover map to revise peatland emissions in the western part of insular Southeast Asia.

The researchers found that cumulative carbon emissions since 1990 were of the order of 2.5 gigatonnes, with 2015 emissions at around 146 megatonnes per year. Industrial plantations were responsible for 44% of the emissions, while small-holders were responsible for 34%, meaning that 78% in total were from managed land cover.

"The estimates published in [our] paper are somewhat lower than earlier estimates, [which resulted from] conservative IPCC emission factors, but they are within a broad range estimated earlier," said Miettinen.

Miettinen added that one reason peat fires receive more attention is that they are highly visible, generating lots of smog. But there have been positive developments in policy-making circles. "The countries in the region are increasingly considering measures to reduce their peat-derived carbon emissions," he said, "including the protection and rehabilitation of remaining peat swamp forests, and the implementation of best-practice water management in drained peatlands."

The findings are published in Environmental Research Letters (ERL).