A multibillion yuan reforestation project that started 20 years ago has helped raise forest coverage in environmentally fragile regions, diversify farmers' incomes and curb greenhouse gases, a senior official said on Thursday.

Zhang Jianlong, head of the National Forestry and Grassland Administration, said the landmark program to return cropland to forest and grassland has cost the central government more than 500 billion yuan ($72.6 billion) over the years.

The amount is nearly triple the cost of the Three Gorges Dam, the world's largest hydroelectric project, and makes it the world's most expensive ecological project.

"Benefits of all kinds are steadily presenting themselves," he told a working conference in Yan'an, Shaanxi province, which was among the pioneers of the nationwide reforestation effort.

Figures provided by the administration show 33.8 million hectares of farmland have been converted to forests and pastures since the project started in 1999, with 41 million farmers directly involved.

Forest coverage in regions included in the program has increased by an average of 4 percentage points in the past 20 years, the administration said.

The project was formed in the wake of the deadly floods that wreaked havoc in the Yangtze River and the Songhua River basins in 1998. More than 220 million people were affected.

The program, which encourages farmers to phase out farming and grazing practices in degraded farmlands, has helped restructure rural industry and boost income.

Subsidies were allotted to farmers, and training was also arranged for them to switch to businesses that could generate more stable incomes, like running apple orchards or vegetable greenhouses.

The administration said farmers who relinquished their lands have received an average subsidy of 9,000 yuan per family by the end of last year. Their per capita disposable income surged by 14.7 percent over the last decade.

A 2016 report said the annual value of the program's ecological contribution－including benefits to land nutrition, water conservation and air purification－is 1.38 trillion yuan.

Despite the environmental and financial benefits, Zhang said, the program has its own problems. Without comprehensive planning, the program has sometimes come into conflict with farmland protection. In addition, subsidies have been reduced over the last decade, and a solution has not been found to ensure that the land remains forested.

Zhang said his agency will improve the top-level design of the program to clarify what land should be reforested and with what method.