Prime Suspects

Asteroid or Comet Impact
Most experts agree that a massive object from outer space struck Earth near the Yucatan Peninsula at the close of the Cretaceous, ending the reign of the dinosaurs. Did an asteroid or icy comet also cause the Permian extinction 185 million years earlier? Here's the doomsday scenario: An impact triggers earthquakes, tsunamis, and a shockwave of heat, incinerating the surrounding landscape. Hot debris from the impact rains down over a wide region, igniting wildfires that burn for weeks on end. More importantly, the airborne dust and gases from the impact and the fires filter out sunlight for months, shutting down photosynthesis and dramatically cooling the Earth. The cold period is followed by extreme heat, as the skies clear of dust particles yet remain full of greenhouse gases. The extreme climatic shifts devastate life around the globe.

Volcanism
At the end of the Permian, over about one million years, a series of eruptions inundated an area the size of the continental U.S. with layer upon layer of lava and ash, leaving deposits up to four miles thick across what is today Siberia. In total, nearly a million cubic miles of magma was unleashed. In addition to the Permian wipeout, half-a-dozen extinction events in the past 250 million years are contemporaneous with similar but smaller-scale volcanism. Coincidence? Here's how eruptions could kill life far and wide: First, sulfurous gas and dust circle the Earth, blocking sunlight and creating storms of acid rain. Then years of cold are followed by decades of unbearable warming, as carbon dioxide and methane linger in the atmosphere.

Formation of Supercontinent
Earth's landmasses slowly move to shape new continents through time. Geologists in the 1970s devised this extinction scenario: the formation of the supercontinent Pangea (or "all Earth") decimates life in two ways. First, as species on separate landmasses and in different waters come together, they compete for resources, and the losing species die out. Second, Pangea's creation affects regional and global climates. Once-warm waters become intolerably cold. The vast interior of the continent experiences wild seasonal swings (think Siberia but worse). In short, most species face new stresses, and as some perish, the effect ripples through the web of life, killing many others. Since the 1970s, though, evidence has mounted that Pangea formed in the middle of the Permian, not at its end.

Glaciation
Global cooling and the spread of gargantuan glaciers likely caused the second largest mass extinction in Earth's history, the Ordovician, which took place 439 million years ago. Was it behind the Permian wipeout as well? Here's the picture: Growing glaciers pull water from the ocean and reduce the area of shallow continental shelves, which are home to the greatest diversity of marine plants and animals. Species compete fiercely for resources, and many lose out. On land, with giant glaciers encroaching, species unable to migrate toward proverbial greener pastures also perish. Geologists have found evidence in rock deposits throughout Europe and Asia, however, that sea level at the end of the Permian was rising, casting doubt on this theory.

Anoxic Oceans and Bacteria
The deep ocean as well as shallow marine habitats appear to have had low oxygen levels at the end of the Permian. This condition, called anoxia, could have been the linchpin in the extinction tale. For most marine life, the anoxia would have meant suffocation. But other life—particularly anaerobic bacteria that give off hydrogen sulfide—would have thrived. The hydrogen sulfide would have spread through the oceans, killing more species, and as it slowly fizzed out into the atmosphere, it would have poisoned life on land as well. Hydrogen sulfide in the atmosphere then would have damaged the ozone layer, opening paths for deadly ultraviolet radiation to reach the remaining life on Earth.

Methane Gas
Today, sediments on continental shelves contain vast amounts of methane, just as they did during the Permian. Some climate scientists fear that our present-day warming oceans could eventually release this methane into the atmosphere, with cataclysmic consequences. Methane, besides being toxic to most organisms, is also a potent greenhouse gas. At the end of the Permian, a great outpouring of methane could have caused runaway global warming and ended much of life on the planet. Even if this scenario turns out to be false, it holds a cautionary tale about how moderate global warming could suddenly turn severe.

One Expert's View
Learn why Doug Erwin thinks there are No Easy Answers.