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There is now overwhelming evidence that the impact of human activity on Earth has initiated a new geological epoch, dating from the middle of the 20th century. Long after humanity has vanished from Earth, the planet's geological record will preserve clear evidence of our existence. The following seven signs will clearly identify the Anthropocene epoch for future geologists.
1. Nuclear weapons
Our war efforts have left their mark on geology. When the first nuclear weapon was detonated on 16 July 1945 in New Mexico, it deposited radionuclides – atoms with excess nuclear energy – across a wide area. Since 1952, more explosive thermonuclear weapons have been tested, leaving a global signature of isotopes such as carbon-14 and plutonium-239.
2. Fossil fuels
The products of burning fossil fuels will also be a clear sign of the Anthropocene. Current rates of carbon emission are thought to be higher than at any time in the last 65 million years. The concentration of carbon dioxide in the atmosphere has risen sharply since 1850 and now exceeds 400 parts per million; this increase will be recorded in any Antarctic ice cores that survive global warming. Burning fossil fuels has also increased the ratio of carbon-12 to carbon-13 isotopes, which will be detectable in tree rings, limestone, and fossilised bones and shells. Our fuel consumption also spreads small, unburned particles of carbon in the air, which can become captured in sediments and glacial ice.
3. New materials
One of the biggest signs of our time will be the presence of three things we use every day: concrete, plastics and aluminium. Aluminium in its elemental form was unknown before the 19th century, but we have now produced around 500 million tonnes of it. Concrete has been around for longer – it was invented by the Romans – but in the 20th century it became our most widely used building material. We have now produced about 500 billion tonnes of it – equivalent to a kilogram for every square metre of Earth – and more than half of that was made in the last 20 years. The production of plastics, initially developed in the 1900s, has increased rapidly since the 1950s, and we now produce 500 million tonnes a year. Sediments containing any of these materials will be a clear sign of the Anthropocene.
4. Changed geology
The destruction of rainforest changes the future of Earth’s geology. So far, more than 50 per cent of Earth’s land area has been transformed by man. Deforestation, farming, drilling, mining, landfills, dam-building and coastal reclamation are all having widespread effects on sedimentary processes, disrupting how layers of rock are laid down, which will be detectable thousands of years in the future.
Our attempts to feed a rapidly increasing population will also leave clear indicators. Concentrations of nitrogen and phosphorus in soils have doubled in the last century because of our increased use of fertilisers. We produce 23.5 million tonnes of phosphorus a year, twice the rate seen during the previous epoch, the Holocene. Human activity has had perhaps the biggest impact on the nitrogen cycle for 2.5 billion years, increasing the amount of reactive nitrogen by 120 per cent compared to the Holocene.
6. Global warming
Anthropogenic climate change will be easily distinguishable in the future. Last century, Earth’s temperature rose by between 0.6 and 0.9 °C, more than the amount of natural variability seen in the Holocene, which has been calculated based on the oxygen isotopes in Greenland’s ice cores. Average global sea levels are higher than at any point in the past 115,000 years and are rising rapidly, and this change may also be detectable in the future.
7. Mass extinction
Organisms have gone extinct for as long as life has existed, but mass extinctions caused by massive global changes mark the end and beginning of several geological periods. Some estimates predict that we are on our way to the sixth mass extinction event in Earth’s history, in which with three-quarters of species will become extinct. Palaeontologists of the future will notice the sudden disappearance of many species from the fossil record as the Anthropocene gets under way.