Paleontologist Peter Ward’s book on mass extinctions and climate change provides a deep-time perspective that is both sobering and necessary. Under a Green Sky puts the present within a geological context while also making the climate crisis feel even more personal and pressing. Before getting that perspective in full, however, readers encounter several fetching narratives of paleontological and other scientific fieldwork across the globe. Captivating as they are, the stories are mostly used to set up later passages that aggressively dismantle an argument Ward clearly loathes: that most past mass extinctions — especially the Permian, some 250 million years ago — were caused by huge meteorite impacts. Ward takes scientists and the media to task for, in his mind, recklessly embracing impacts as the culprit du jour for nearly all prior mass extinctions, when an impact is clearly responsible for just one such die-off: the famous dinosaur-killer 65 million years ago.

Ward presents a powerful alternative model for explaining these extinctions. In short, an increase in carbon dioxide — from volcanism (in the past) or from humans (in the present) — warms the oceans enough to change circulation patterns. When this happens, sulfur-eating microbes sometimes thrive. These bacteria produce hydrogen sulfide, which, in sufficient quantities and under certain conditions, outgasses into the air, shreds the ozone layer, and poisons other living things. The warming also causes methane ice under the seas to melt and, well, burp, adding to the nasty mix. The end comes not in a bang but a stinky whimper.

Some may wish that Ward focused even more on scientists now plumbing the depths of these beguiling connections between current conditions and those of the past. In any case, we learn that tomorrow may mean palm trees in Seattle, nonstop hurricanes near the equator, and a hazy sky where amber dust blends with the wild blue yonder “to create a washed green tinge, a vomitous color.” Eventually, ocean patterns may turn the Earth into a desert world, or the conveyor belt that pushes warm water into the cold North Atlantic may simply shut off, producing the long-overdue return of an ice age.

Despite a sometimes hurried prose style, Ward’s book is replete with interesting anecdotes and clear scientific explanations. Readers discover why the climate actually is like a switch. Teaching us how to keep our fingers off the switch isn’t part of this book’s design, but Under a Green Sky provides a kind of core sample of ages past to help us make sense of what the present could hold for the future.