Mapping the world's dry forests: The extent of forest area in dryland habitats, which occupy more than 40% of Earth's land surface, is uncertain compared with that in other biomes. Bastin et al. provide a global estimate of forest extent in drylands, calculated from high-resolution satellite images covering more than 200,000 plots. Forests in drylands are much more extensive than previously reported and cover a total area similar to that of tropical rainforests or boreal forests. This increases estimates of global forest cover by at least 9%, a finding that will be important in estimating the terrestrial carbon sink. Science , this issue p. 635

Schepaschenko et al . question our findings, claiming that we did not refer to all existing maps and that we did not account for all sources of uncertainty. In our response, we detail our selection criteria for reference maps, which clarify why the work of Schepaschenko et al . was not used, and we explain why our uncertainty assessment is complete and how it was misunderstood by Schepaschenko et al .

Griffith et al . do not question the quality of our analysis, but they question our results with respect to the definition of forest we employed. In our response, we explain why the differences we report result from a difference of technique and not of definition, and how anyone can adapt—as we did—our data set to any forest definition and tree cover threshold of interest.

De la Cruz et al . question the reliability of our results, claiming that we do not refer to the most appropriate spatial extent of drylands. In our response, we explain why we chose an existing and internationally recognized delineation of drylands among several options, and why our findings are due to a difference of remote sensing technique and not to the definition of drylands we have selected.

While access to energy is one of the most prominent global public goods, undisciplined pursuit of energy resources can produce serious public bads in respect of environmental impact. The case of Costa Rica illustrates both the dependency and the vulnerability involved, and provides a clear example of the benefits of ‘emissions trading’, if an international regulatory agreement to be achieved.