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<jats:p>Stearns and van der Veen (Reports, 20 July 2018, p. 273) conclude that fast glacier sliding is independent of basal drag (friction), even where drag balances most of the driving stress. This conclusion raises fundamental physical issues, the most striking of which is that sliding velocity would be independent of stresses imparted through the ice column, including gravitational driving stress.</jats:p>

<jats:title>Off-target drug metabolism</jats:title> <jats:p> Anything humans swallow is exposed to the foraging and transforming activities of the gut microbiota. This applies to therapeutic drugs as well as food components and can be a major source of interpersonal variation in drug efficacy and toxicity. Zimmermann <jats:italic>et al.</jats:italic> found that individual drug responses depend on the genetics of an individual's microbiota. They explored the metabolism of nucleoside drugs (which are used as antivirals and antidepressants) in mice inoculated with a variety of mutant microbiota. They then modeled the pharmacokinetics in different body compartments and identified the host and microbe contributions. In some individuals, up to 70% of drug transformation can be ascribed to microbial metabolism. </jats:p> <jats:p> <jats:italic>Science</jats:italic> , this issue p. <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" related-article-type="in-this-issue" xlink:href="10.1126/science.aat9931">eaat9931</jats:related-article> </jats:p>

<jats:title>Reflections on cloud effects</jats:title> <jats:p> How much impact does the abundance of cloud condensation nuclei (CCN) aerosols above the oceans have on global temperatures? Rosenfeld <jats:italic>et al.</jats:italic> analyzed how CCN affect the properties of marine stratocumulus clouds, which reflect much of the solar radiation received by Earth back to space (see the Perspective by Sato and Suzuki). The CCN abundance explained most of the variability in the radiative cooling. Thus, the magnitude of radiative forcing provided by these clouds is much more sensitive to the presence of CCN than current models indicate, which suggests the existence of other compensating warming effects. </jats:p> <jats:p> <jats:italic>Science</jats:italic> , this issue p. <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" related-article-type="in-this-issue" xlink:href="10.1126/science.aav0566">eaav0566</jats:related-article> ; see also p. <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" issue="6427" page="580" related-article-type="in-this-issue" vol="363" xlink:href="10.1126/science.aaw3720">580</jats:related-article> </jats:p>

<jats:title>The case for endangerment</jats:title> <jats:p> In 2009, the U.S. Environmental Protection Agency (EPA) established the so-called “Endangerment Finding.” This defined a suite of six long-lived greenhouse gases as “air pollution.” Such air pollution was anticipated to represent a danger to the health and welfare of current and future generations. Thus, the EPA has the authority to regulate these gases under the rules of the U.S. Clean Air Act. Duffy <jats:italic>et al.</jats:italic> provide a comprehensive review of the scientific evidence gathered in the years since then. These findings further support and strengthen the basis of the Endangerment Finding. Thus, a compelling case has been made even more compelling with an enormous body of additional data. </jats:p> <jats:p> <jats:italic>Science</jats:italic> , this issue p. <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" related-article-type="in-this-issue" xlink:href="10.1126/science.aat5982">eaat5982</jats:related-article> </jats:p>

<jats:p> Minchew <jats:italic>et al</jats:italic> . take issue with our main conclusion that friction at the glacier bed does not control fast glacier flow. In this response, we further justify our methodology. We also point out that numerical studies referred to by Minchew <jats:italic>et al</jats:italic> . rely on inversions that are based on a sliding relation in which sliding speed is proportional to basal drag. Furthermore, observational studies referred to by Minchew <jats:italic>et al</jats:italic> . apply to glaciological settings that do not correspond to the terminal regions of Greenland outlet glaciers that we studied. </jats:p>