(Bloomberg) -- A fifth of the Earth’s wetlands have been destroyed by humans over the past three centuries, a cumulative area larger than India, a comprehensive new study has found. Wetlands provide critical and historically under-recognized benefits to humanity, including flood defense, water storage and biodiversity protection, but their conversion for other uses has often proved more attractive in the short term than sustaining them.  The issue of wetland destruction has come into focus in recent months amid  global efforts to halt a catastrophic loss of species-rich ecosystems. Almost 200 countries agreed at the United Nations COP15 summit in Montreal in December to an ambitious and urgent program to halt biodiversity loss.The findings of the large-scale study, published Wednesday in the journal Nature, are substantially lower than previous estimates, some of which were based on extrapolating from regional wetland loss. Past assessments ranged so dramatically — anywhere from 28% to 87% — that they left the topic under a cloud of confusion for years.

The new study looks at inland wetlands only and not permanently inundated areas, tidal zones on coasts or marine wetlands near shore. These wetlands make up 6% of land and hold 12% of the Earth’s carbon. The sometimes-watery part of the world is home to about 40% of plant and animal species. Two of the top three targets identified in the framework agreed to in Montreal specifically include “inland waters” in their calls for restoring and conserving at least 30% of the planet’s land and waters by 2030. 

Drying out wetlands to grow crops has been responsible for 62% of the total loss since 1700, the study found. The researchers looked at data for three other major contributors: deforestation, the cultivation of yam, cassava, sugar cane and other wetland crops and the extraction of peat, a carbon- and nutrient-rich soil that can be used as a fertilizer or burned for energy. 

Peatlands are a particular concern, and existing stores in Siberia, Canada, the Amazon and the Congo play a key role in keeping biodiversity alive —  and planet-warming carbon in the ground. 

“The kinds of losses [of peatlands] we’ve seen in Indonesia for palm oil plantations would be devastating if they happened elsewhere in the tropics,” said Rob Jackson, a professor of Earth system science at Stanford University and an author of the study. “We don’t have that luxury.” 

Where historical data were insufficient, the researchers were able to use what they understood to model three other factors — the transformation of wetlands into pasture, rice fields or cities. 

Using 3,320 records dating back more than three centuries, the researchers were able to build a timeline of loss, in addition to a world map of former wetlands. In much of the world, human impacts on wetlands didn’t accelerate until after World War II, when governments drained parts of North America, Europe and China, mostly for agriculture. 

The top 15 countries contributing to the global trend are a mix of rich and poor economies. The US alone is responsible for more than 15% of the global loss, with China second at about 12%, followed by India, Russia and Indonesia. Pakistan, Malaysia, Russia and Germany each wiped out 3% to 4% of the global total within their borders.

The countries that converted the highest percentage of their own land looks very different. More than 90% of Ireland’s wetlands are gone, with Hungary, Lithuania, Germany, Italy, the UK and the Netherlands above 75% loss. 

The world’s greatest river systems have also lost high percentages of their accompanying natural systems. Wetland areas along the Yangtze and Danube have dropped about 75%, those by the Indus 65% and areas near the Mississippi by half. 

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