For centuries human societies have viewed wetlands as wastelands to be “reclaimed” for higher uses. China began large-scale alteration of rivers and wetlands in 486 B.C. when it started constructing the Grand Canal, still the longest canal in the world. The Dutch drained wetlands on a large scale beginning about 1,000 years ago, but more recently have restored many of them. As a surveyor and land developer, George Washington led failed efforts to drain the Great Dismal Swamp on the border between Virginia and North Carolina.
Today many modern cities around the world are built on filled wetlands. Large-scale drainage continues, particularly in parts of Asia. Based on available data, total cumulative loss of natural wetlands is estimated to be 54 to 57 percent – an astounding transformation of our natural endowment.
Vast stores of carbon have accumulated in wetlands, in some cases over thousands of years. This has reduced atmospheric levels of carbon dioxide and methane – two key greenhouse gases that are changing Earth’s climate. If ecosystems, particularly forests and wetlands, did not remove atmospheric carbon, concentrations of carbon dioxide from human activities would increase by 28 percent more each year.
Source: Fight climate change with more swamps | EarthSky.org