"Forests cover 31 percent of the world’s land surface, just over 4 billion hectares. (One hectare = 2.47 acres.) This is down from the pre-industrial area of 5.9 billion hectares. According to data from the U.N. Food and Agriculture Organization, deforestation was at its highest rate in the 1990s, when each year the world lost on average 16 million hectares of forest—roughly the size of the state of Michigan. At the same time, forest area expanded in some places, either through planting or natural processes, bringing the global net loss of forest to 8.3 million hectares per year. In the first decade of this century, the rate of deforestation was slightly lower, but still, a disturbingly high 13 million hectares were destroyed annually. As forest expansion remained stable, the global net forest loss between 2000 and 2010 was 5.2 million hectares per year." (2012)
MATH: 4 billion ha - 5.9 billion ha = -32% change
The world has lost one third of its forest cover since pre-industrial times.
Emily E. Adams, "World Forest Area Still on the Decline," Earth Policy Institute, Eco-Economy Indicators, Forest Cover, last modified August 31, 2012, accessed June 9, 2014, http://www.earth-policy.org/indicators/C56.