Atmospheric scientists predict that in about a billion years’ time oxygen levels in the Earth’s atmosphere will drop catastrophically, leading to the extinction of all multicellular life. The reason for this is that carbon dioxide levels will drop to such low levels, partly because our sun will have got hotter, and this will result in photosynthesis (the predominant source of atmospheric oxygen) no longer being possible.
As the Earth’s atmosphere has changed, so too has its average global temperature. There have been five spells when the Earth has been cold, so-called Icehouse Earth epochs. These chills lasted millions of years, repeatedly causing major extinctions. Yet some of these icehouse spells also led to periods of evolutionary innovation. The Sturtian glaciation lasted for nearly 60 million years in a period known as snowball Earth, ending about 660 million years ago. It may well have been a catalyst for the evolution and subsequent spread of multicellular life. During some icehouse Earth periods glaciers advanced and retreated, but areas of ice on the planet are a consistent feature of our planet when it is in this state. We are currently in an icehouse earth epoch (there is ice at the poles and on mountains), but we are between ice ages, meaning there is not as much ice as there was 15,000 years ago or likely will be 50,000 years hence. Whether this ice age happens or the planet leaves the current icehouse spell may well depend on humanity’s production of carbon dioxide and other greenhouse gases in the coming decades and centuries. Exactly what triggers major icehouse Earth events is unclear, but it could be variations in the Earth’s orbit, changes in the composition of the atmosphere, or even changes in the amount of volcanic activity.
Our planet has also experienced periods of sweltering heat, with the Earth being in its greenhouse state for about 85 per cent of its existence. Today, the average global temperature is 13.9 degrees Celsius, but during greenhouse periods the average temperature has hit 30 degrees. When you hear these numbers, you may wonder what all the fuss about anthropogenic climate change is about. Life will undoubtedly survive even a rapid change in climate of a few degrees, but many species will not. Humans might survive, but we would doubtless suffer, as our civilizations are fragile and susceptible to collapse.
In much of the past, the climate changed relatively slowly, over hundreds of thousands or millions of years. But occasionally there would be much more rapid change caused by asteroid impacts or major volcanic eruptions. The Siberian Traps are evidence of a period of massive volcanic activity that lasted for 2 million years, about 250 million years ago. Temperatures can change quite quickly when volcanoes start erupting.
The word ‘trap’ is a geological term used to describe a type of rock formation. Nearly 3 million square miles of modern-day Siberia, an area about the size of Australia, is covered with basaltic rock that was formed from ancient volcanic eruptions. Huge amounts of lava were ejected from a massive build-up of magna under the Earth’s surface. Not only did these eruptions cover a vast tract of land with molten rock, they would also have spewed huge quantities of carbon dioxide and other greenhouse gases into the atmosphere, along with ash and other debris. The ash and debris in the atmosphere would have temporarily reduced sunlight, lowering the temperature on Earth. Once the ash and debris settled, the carbon dioxide would have remained, being accumulated in plants, sediments and the ocean relatively slowly, acting as a greenhouse gas that warmed the planet. Estimates suggest that the Siberian Traps produced between 12,000 and 100,000 gigatons of carbon dioxide, where a gigaton is a billion tonnes. The carbon dioxide that was released increased the concentration of carbon dioxide in the atmosphere by four times, which was enough to raise the temperature of the Permian atmosphere by 10–15 degrees Celsius and the temperature of tropical oceans to 40 degrees Celsius.
The Siberian Traps continued to erupt for approximately 2 million years, although most of the activity may have occurred in about a quarter of that time. These eruptions resulted in four out of five ocean-living species being driven extinct, and 70 per cent of terrestrial species being lost forever. Tetrapods, the animal group to which humans belong, were very nearly driven extinct and are thought to have survived on only a small part of the ancient continent Pangea.
Volcanic eruptions can cause home-grown mass extinctions, but the cause of the best known of the mass extinctions came from the heavens. If you gaze at the night sky on a cloudless evening in the absence of artificial light, you can see shooting stars. The best places to see these are in areas such as deserts during colder nights, where there is little haze in the atmosphere. The first time I saw the night sky on a clear, cold night from my father-in-law’s farm in rural Queensland was remarkable. The Milky Way traversed the sky, with countless stars. Through binoculars, I could see another galaxy. Meteors in the higher reaches of our atmosphere caused shooting stars every few minutes, and I was transfixed. If I had grown up in a place like this, I might well have become a space scientist, but by the time Sonya and I got together I was already a professor of zoology.
Most of the rocks on a collision course with Earth are small, and they burn up in our atmosphere as meteors I watch at the farm. Larger ones can make it further into our atmosphere. On 30 June 1908, a 100,000-kilogram rock travelling at over 60,000 mph entered Earth’s atmosphere, causing what Earth scientists refer to as the Tunguska event. Much of the 50–60-metre-wide rock burnt up as it came through the atmosphere, but at a height of 5 miles above the Earth it shattered into small pieces, causing a meteor airburst. As the meteor collided with our thicker atmosphere it caused a large explosion, which in the case of the Tunguska event flattened 80 million trees in Siberia over an area of 2,150 square kilometres. In this case, humanity escaped significant tragedy. Had the meteor impacted the Earth’s lower atmosphere over a built-up area, millions could have died.
Some meteoroids do not burn up but make it all the way to the planet’s surface. Their fate depends upon their size, their angle of approach to Earth and their composition. In October 1992, a meteorite hit Earth in Peekskill, New York, destroying a car. It partly disintegrated during its transit and the core that hit the surface of the Earth weighed 890 grams. It had been travelling through our solar system for 4.4 billion years, ending its journey by hitting the boot of a twelve-year-old Chevy Malibu. It really is the journey and not the destination that matters. I was unable to find out whether the seventeen-year-old owner of the car, Michelle Knapp, was able to claim against her insurance, but I suspect not, given the behaviour of the insurance companies I have had dealings with over much more earthly accidents. I do not feel too sorry for Michelle though: Wikipedia reports that she sold the meteorite for $50,000, and the damaged car, which had cost her $300, for $25,000. I imagine she has been dining out on the story ever since. I certainly would.