Free Astronomy Magazine May-June 2018

44 MAY-JUNE 2018 ASTROBIOLOGY bial activity in the coastal ocean which can deplete dissolved oxygen in the water col- umn (Diaz & Rosenberg, 2008), and recent syntheses suggests a global decline already of about 2% (Schmidtko et al., 2017; Ito et al., 2017). This in turn is leading to an ex- pansion of the oxygen minimum zones, greater ocean anoxia, and the creation of so-called ‘dead-zones’ (Breitburg et al., 2018)” . Schmidt and Frank also went so far as to suggest a geological layer that has charac- teristics very similar to those the Anthro- pocene will leave, that of the Paleocene/ Eocene transition 56 million years ago. Obviously, further checks will be needed to exclude any natural processes. Summing up, if on our planet there are traces of a previous industrial civilization, those traces are indirect and must be sought out in the form of alterations of the ecosystem in the sedimentary layers of epochs not farther than 400 million years ago and probably much closer to us. However, the two researchers also note that various natural phenomena may have mitigated those traces, and that even a few natural events can mimic them. Furthermore, it is not possible to predict the thickness of the sediments in which the markers hide, a value that can vary both due to the speed with which sedi- ments settle and the length of the period of disfig- urement of the ecosystem. In this regard, we conclude with a keen considera- tion of Schmidt and Frank: “There is an in- teresting para- dox in consider- ing the Anthro- pogenic foot- print on a geological timescale. The longer human civilization lasts, the larger the sig- nal one would expect in the record. How- ever, the longer a civilization lasts, the more sustainable its practices would need to have become in order to survive. The more sustainable a society (e.g. in energy generation, manufacturing, or agriculture) the smaller the footprint on the rest of the planet. But the smaller the footprint, the less of a signal will be embedded in the ge- ological record. Thus the footprint of civi- lization might be self-limiting on a rela- tively short time-scale.” ! T he authors of the Silurian Hypothesis: Gavin Schmidt, NASA GISS (above) and Adam Frank, University of Rochester.