Life on Earth does not enjoy change, and climate change is something it likes least of all. Every aspect of an organism’s life depends on climate, so if that variable changes, everything else changes too – the availability of food and water, the timing of migration or hibernation, even the ability of bodily systems to keep running.
Species can adapt to gradual changes in their environment through evolution, but climate change often moves too quickly for them to do so. It’s not the absolute temperature, then, but the rate of change that matters. Woolly mammoths and saber-toothed tigers thrived during the Ice Ages, but if the world were to shift back to that climate overnight, we would be in trouble.
Put simply, if climate change is large enough, quick enough, and on a global scale, it can be the perfect ingredient for a mass extinction. This is worrying, as we are currently at the crux of a potentially devastating period of global warming, one that we are causing. Will our actions cause a mass extinction a few centuries down the line? We can’t tell the future of evolution, but we can look at the past for reference points.
There have been five major extinction events in the Earth’s history, which biologists refer to as “The Big Five”. The Ordovician-Silurian, Late Devonian, Permian-Triassic, Late Triassic, Cretaceous-Tertiary…they’re a bit of a mouthful, but all five happened before humans were around, and all five are associated with climate change. Let’s look at a few examples.
The most recent extinction event, the Cretaceous-Tertiary (K-T) extinction, is also the most well-known and extensively studied: it’s the event that killed the dinosaurs. Scientists are quite sure that the trigger for this extinction was an asteroid that crashed into the planet, leaving a crater near the present-day Yucatan Peninsula of Mexico. Devastation at the site would have been massive, but it was the indirect, climatic effects of the impact that killed species across the globe. Most prominently, dust and aerosols kicked up by the asteroid became trapped in the atmosphere, blocking and reflecting sunlight. As well as causing a dramatic, short-term cooling, the lack of sunlight reaching the Earth inhibited photosynthesis, so many plant species became extinct. This effect was carried up the food chain, as first herbivorous, then carnivorous, species became extinct. Dinosaurs, the dominant life form during the Cretaceous Period, completely died out, while insects, early mammals, and bird-like reptiles survived, as their small size and scavenging habits made it easier to find food.
However, life on Earth has been through worse than this apocalyptic scenario. The
largest extinction in the Earth’s history, the Permian-Triassic extinction, occurred about 250 million years ago, right before the time of the dinosaurs. Up to 95% of all species on Earth were killed in this event, and life in the oceans was particularly hard-hit. It took 100 million years for the remaining species to recover from this extinction, nicknamed “The Great Dying”, and we are very lucky that life recovered at all.
So what caused the Permian-Triassic extinction? After the discovery of the K-T crater, many scientists assumed that impact events were a prerequisite for extinctions, but that probably isn’t the case. We can’t rule out the possibility that an asteroid aggravated existing conditions at the end of the Permian period. However, over the past few years, scientists have pieced together a plausible explanation for the Great Dying. It points to a trigger that is quite disturbing, given our current situation – global warming from greenhouse gases.
In the late Permian, a huge expanse of active volcanoes existed in what is now Siberia. They covered 4 million square kilometres, which is fifteen times the area of modern-day Britain (White, 2002). Over the years, these volcanoes pumped out massive quantities of carbon dioxide, increasing the average temperature of the planet. However, as the warming continued, a positive feedback kicked in: ice and permafrost melted, releasing methane that was previously safely frozen in. Methane is a far stronger greenhouse gas than carbon dioxide – over 100 years, it traps approximately 21 times more heat per molecule (IPCC AR4). Consequently, the warming became much more severe.
When the planet warms a lot in a relatively short period of time, a particularly nasty condition can develop in the oceans, known as anoxia. Since the polar regions warm more than the equator, the temperature difference between latitudes decreases. As global ocean circulation is driven by this temperature difference, ocean currents weaken significantly and the water becomes relatively stagnant. Without ocean turnover, oxygen doesn’t get mixed in – and it doesn’t help that warmer water can hold less oxygen to begin with. As a result of this oxygen depletion, bacteria in the ocean begins to produce hydrogen sulfide (H2S). That’s what makes rotten eggs smell bad, and it’s actually poisonous in large enough quantities. So if an organism wasn’t killed off by abrupt global warming, and was able to survive without much oxygen in the ocean (or didn’t live in the ocean at all), it would probably soon be poisoned by the hydrogen sulfide being formed in the oceans and eventually released into the atmosphere.
The Permian-Triassic extinction wasn’t the only time anoxia developed. It may have been a factor in the Late Triassic extinction, as well as smaller extinctions between the Big Five. Overall, it’s one reason why a warm planet tends to be less favourable to life than a cold one, as a 2008 study in the UK showed. The researchers examined 520 million years of data on fossils and temperature reconstructions, which encompasses almost the entire history of multicellular life on Earth. They found that high global temperatures were correlated with low levels of biodiversity (the number of species on Earth) and high levels of extinction, while cooler periods enjoyed high biodiversity and low extinction.
Our current situation is looking worse by the minute. Not only is the climate changing, but it’s changing in the direction which could be the least favourable to life. We don’t have volcanic activity anywhere near the scale of the Siberian Traps, but we have a source of carbon dioxide that could be just as bad: ourselves. And worst of all, we could prevent much of the coming damage if we wanted to, but political will is disturbingly low.
How bad will it get? Only time, and our decisions, will tell. A significant number of the world’s species will probably become extinct. It’s conceivable that we could cause anoxia in the oceans, if we are both irresponsible and unlucky. It wouldn’t be too hard to melt most of the world’s ice, committing ourselves to an eventual sea level rise in the tens of metres. These long-range consequences would take centuries to develop, so none of us has to worry about experiencing them. Instead, they would fall to those who come after us, who would have had no part in causing – and failing to solve – the problem.
Mayhew et al (2008). A long-term association between global temperature and biodiversity, origination and extinction in the fossil record. Proceedings of the Royal Society: Biological Sciences, 275: 47-53. Read online
Twitchett (2006). The paleoclimatology, paleoecology, and paleoenvironmental analysis of mass extinction events. Paleogeography, Paleoclimatology, Paleoecology, 234(2-4): 190-213. Read online
White (2002). Earth’s biggest “whodunnit”: unravelling the clues in the case of the end-Permian mass extinction. Philosophical Transactions of the Royal Society: Mathematical, Physical, & Engineering Sciences, 360: 2963-2985. Read online
Benton and Twitchett (2003). How to kill (almost) all life: the end-Permian extinction event. Trends in Ecology & Evolution, 18(7): 358-365. Read online