J. Kasting, R. Kopparapu, R. Ramirez, and C. Harman. (2013)cite arxiv:1312.1328Comment: To be published in Proceedings of the National Academy of Sciences (PNAS) special issue on Exoplanets. Supplementary information is available at http://www3.geosc.psu.edu/~ruk15/publications/ms.pdf.