Northern Hemisphere permafrost extent: Drylands, glaciers and sea floor. Comment to the paper: Obu, J., et al. 2019. Northern Hemisphere permafrost map based on TTOP modeling for 2000–2016 at 1 km2 scale, Earth Science Reviews, 193, 299–316

The article published by Obu et al. (2019) estimating the occurrence of permafrost over a Northern Hemisphere. The results published differ from those presented in previous works. This comment highlights the errors introduced in that study, and in a positive note, its cause and proposed solution. The problem remains beyond capabilities of computed models or empirical research. Its solution lies in using the correct categories and land classifications, and clearly defining the medium subjected to freezing. In particular, the correct classification of ice plays a decisive role here. To provide a full picture of the occurrence of permafrost on the Earth surface, three different media must be considered: 1) exposed land, traditionally understood as "dry land", 2) glaciers and ice sheets together with its bed and 3) the sea floor (continental shelves). The biggest weakness of the published study is not including glaciated areas in their estimations. The authors do not take either a clear position in this crucial matter, despite a radical divergence of opinions has emerged in recent years. Whether and in what way Greenland or other glaciated areas are covered by permafrost remains a pressing issue, as the final determination of the range of the permafrost worldwide depends on it. Until then, any criteria or benchmark used will continue to be ambiguous and open to discussion, maintaining the discrepancy at millions of square kilometers. [ABSTRACT FROM AUTHOR]