A new estimate of dryland forests suggests that the global forest cover is at least 9% higher than previously thought. The finding will help reduce uncertainties surrounding terrestrial carbon sink estimates.

Dryland biomes, where precipitation is more than counterbalanced by evaporation from surfaces and transpiration by plants, cover about 40% of the Earth's land surface.

These biomes contain some of the most threatened ecosystems, including biodiversity hotspots. However, previous estimates of dryland forests have been riddled with disparities, caused by issues such as differences in satellite spatial resolution, mapping approaches and forest definitions.

These disparities have led to major doubts about the reliability of global forest area estimates, and to questions about the real contribution made by forests to the global carbon cycle. Here, Jean-Francois Bastin et al. analyzed satellite data from Google Earth, using a detail sample pool of 213,795 0.5 hectare plots from around the globe.

Their new estimate of dryland forest is 40 to 47% higher than previous estimates, corresponding to 467 million hectares (Mha) of forest that have never been reported before.

This increases current estimates of global forest cover by at least 9%. These results explain the difference between recent global estimates of forest "land use" area (3890 Mha) and the area with a "land cover," the authors say.