Alien observers monitoring Earth for signs of intelligent life may well have choked on their intergalactic version of tea a few hundred thousand years ago when the first humans stumbled into focus, some 4 billion years after our planet's own emergence. It was obvious that we were a bit special: we could make fire.
As the global climate shifted at the end of the last ice age, some 10,000 years ago, humans put their fire-making skills to great use, blazing a trail across continents to clear trees for grazing and agriculture, enabling societies to develop from hunter-gatherers to rooted civilizations that produced complex technologies. We were even able to improve on our external sources of energy by expanding our range of fuels: rather than relying on what fuel grew in forests and the continual recycling of biomatter, we delved deep into the ground to extract fossil fuels made over geological timescales.
Humans proved so clever and successful that we were able to overcome almost all the environmental limitations that restricted other species to their ecological niches. The Industrial Revolution began a march towards control of the planet and its resources, which, over the past 50 years, has become truly global. Our population soared from around 10,000 individuals at the start of the Holocene, 10,000 years ago, to 7 billion today. It is estimated that it will pass a colossal 9 billion by 2050.
So, any watching aliens are now looking at a radically changed Earth, on which the land surface, oceanic and atmospheric chemistry, ecology and biology have been transformed by humans. (Human) scientists say Earth has entered the Anthropocene epoch – the Age of Man – because we have become the dominant geological force on our planet.
We have changed the composition of the atmosphere – which now contains more carbon dioxide molecules – and the oceans, which are more acidic because more of that carbon dioxide is dissolving into them. And, because carbon dioxide is a greenhouse gas, we are changing the climate by melting glaciers and raising sea levels. Our atmospheric tinkering means that scientists think we have indefinitely delayed the next Ice Age.
We have changed the covering of the planet by chopping down trees (currently we fell 130,000 sq km per year, according to the Food and Agricultural Organization of the United Nations), rerouting rivers (we manage more than half of the planet's available freshwater) and constructing highways and cities. More than half of the world’s population now lives in urban areas. At least 75% of the world's land surface has been modified by humans, according to Erle Ellis, an ecologist at the University of Maryland.
Rock and toll
We are also reshaping the planet's rocky material – mining and other excavation shifts four times the amount moved naturally by glaciers and rivers. We are changing the numbers and abundance of other living species –some believe we are at the start of the world’s sixth great extinction – and the way they are distributed around our planet, by introducing invasive species and favouring some species over others. There are now more trees on farmland than in forests, for example, and if we were to weigh all of Earth’s land vertebrates, 90% of the total would be made up of humans and the animals we have domesticated, according to Prof Vaclav Smil in his book The Earth’s Biosphere: Evolution, Dynamics, and Change.
Some of our changes are geologically profound – deforestation and the elimination and distribution of species, for example, are scarring the rocks to leave telltale evidence of our human influence for geologists to discover many thousands of years into the future. Others are immediately obvious to anyone who has looked down at our continents from the window of a aircraft and seen the patchwork of farmland monocultures where there used to be wild savannah, or the islands we have grown out of the sea off Dubai, or the mountain tops we have removed in our quest for coal. But some are more subtle and harder to see directly – the way we are changing the climate, or interfering in the nitrogen cycle, or selecting some metals but not others from the Earth's crust, for example.