Earth was a hothouse 201 million years ago, leading to the Triassic-Jurassic mass extinction event. How did it recover? Why was there no runaway greenhouse effect?
David Bortin
Whittier, California, US
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A 鈥渞unaway greenhouse effect鈥� on Earth, such as the one that befell Venus long ago, would be initiated if carbon dioxide, methane and other greenhouse gases were to reach atmospheric concentrations so high (probably around 30,000 parts per million) that thermal radiation from the 鈥渉ot body鈥� planet couldn鈥檛 keep up with the influx of heat from the sun.
Earth would first become so hot as to be uninhabitable, then too hot even for liquid water. When the oceans boil, that added water vapour 鈥� another greenhouse gas 鈥� would create an irreversible 鈥渞unaway鈥� effect.
That didn鈥檛 happen at the end of the Triassic Period, and it isn鈥檛 imminent now. (Not that this is anything for us to worry about, since we will all be extinct long before it gets to that point anyway.)
What seems to have triggered the less-than-runaway greenhouse effect around 201 million years ago was a series of supermassive volcanic eruptions that either caused or were caused by the breakup of the pre-continental landmass called Pangaea.
These eruptions released so much carbon dioxide, hydrogen sulphide and methane that a 鈥減erfect storm鈥� of global warming, ocean acidification and lowered oxygen concentration transformed the environment too rapidly for organisms to adapt. Most terrestrial flora and fauna, and probably an even larger fraction of marine species, became extinct.
As with other extinction events (so far!), equilibrium is naturally and gradually retrieved, and the extinctions just expose ecological niches where the newly fittest can evolve and thrive.
Mike Follows
Sutton Coldfield, West Midlands, UK
Earth鈥檚 climate tends towards an equilibrium global mean surface temperature based on a range of boundary conditions such as the composition of our atmosphere and our distance from the sun.
There are many mechanisms of climate change that can nudge the global mean surface temperature away from this equilibrium.
The hothouse Earth at the end of the Triassic was most likely caused by volcanic eruptions of basalt rock over an area of 10 million square kilometres, known as the Central Atlantic magmatic province (CAMP).
The initial CAMP eruptions brought low-temperature magma to the surface. This released more sulphur dioxide, which created sulphuric acid particles that reflect sunlight. The resultant global cooling caused the mass extinction of 60 to 70 per cent of extant species.
The acid would have been rained out. Later eruptions were hotter and released more carbon dioxide, which led to an enhanced greenhouse effect and hothouse Earth conditions.
A warmer world is wetter, so more carbon dioxide is dissolved in rainwater, creating acid rain. This increases the weathering of rocks, which increases the concentration of bicarbonate and other ions that are flushed into the oceans.
In the oceans, calcium reacts with the bicarbonate ions to form calcium carbonate, which forms the bodies of coccolithophores and other marine plankton as well as coral polyps. When these organisms die, they fall to the ocean floor as marine snow, ending up as limestone.
Besides, with more carbon dioxide dissolving in the ocean, the population of photosynthesising phytoplankton increases, as does the flux of marine snow. The mass of terrestrial green plants also increases and, when they die, some form peat bogs. In other words, carbon is removed from the atmosphere and buried, which leads to cooling.
The late James Lovelock perceived Earth as a cybernetic system, with life nudging the climate towards the most suitable temperature. My hunch is that life speeds up this return to equilibrium, though it is still interminably slow.
At the end of the Triassic, the supercontinent called Pangaea was breaking apart, and this will also have influenced the climate. The polar Boreal Sea was kept warm by ocean currents passing through the Laurasian Seaway from the equatorial Tethys Sea.
A team of scientists, including Stephen Hesselbo at the University of Exeter in the UK, that a plume of magma lifted the crust to create the North Sea Dome in the middle of the Laurasian Seaway 174 million years ago. This constricted the poleward flow of heat and led to the subsequent global cooling.
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