How long will the human species survive on Earth?

More than 90 per cent of all species that arose since life began on Earth 3.8 billion years ago are extinct. Modern humans arose about 200,000 years ago. We are different from all preceding species in that we are self-consciously intelligent, a property that has the potential to either hasten, retard or even prevent our eventual demise.

This question is pondered by David Grinspoon, senior scientist at the Planetary Science Institute, Arizona, in Scientific American, September 2016.

Earth’s physical history is divided into chunks of geological time. The largest chunks, called aeons, each about a billion years long, are subdivided into a series of shorter chunks, one of which is called an epoch.

So far the Earth has had four aeons and we are now living in an epoch called the Anthropocene (“new man”).

Grinspoon proposes that the Anthropocene marks the beginning of transition to a fifth aeon (Sapiezoic: “wise life”) in which the thoughts, deeds and creations of human beings will significantly shape planet Earth.

However, for an epoch to become an aeon it must last for hundreds of millions of years at least. Will humanity survive that long?

Existential threats

The survival of humanity depends on our ability to overcome many short- and long-term existential threats. Some threats are self-imposed and some natural. Grinspoon lists these threats: (1) ballooning world population growth, self-imposed, (2) global nuclear war – self-imposed; (3) human-induced global warming – self-imposed; (4) killer asteroids and comets – natural (5) cyclical long-term natural climate change – natural; and (6) eventually a much hotter sun – natural.

(1) It looks like world population numbers will not peak until beyond 2100 and then slowly decline to a level that can be reliably managed.

(2) It seems that we will take enough action on human-induced climate change to forestall disastrous consequences, but the possibility remains that we will trigger a tipping point hidden in our complex climate system that will plunge us into the abyss.

(3) The Cold War is over but international tensions remain as do huge nuclear arsenals on both sides. The possibility of an all-out disastrous nuclear war starting by accident remains.

(4) Collision of a 10km-wide asteroid with Earth 65 million years ago wiped out the dinosaurs but a future collision with an even smaller asteroid could have disastrous consequences for humanity. The dangerous Earth-crossing asteroids are now being catalogued. However, the possibility remains that a killer comet could suddenly fly at us at any time from the outer reaches of the solar system. We need to be able to detect all these asteroids/comets and either destroy or deflect them.

(5) Over tens of thousands to many millions of years, the Earth goes through natural cycles of glaciation and warming. These changes are immense compared with our current spike of human-induced warming. We are currently in an interglacial warm period, slowly heading towards the next natural glaciation. Grinspoon reckons that another Ice Age would wipe out agriculture and human civilisation along with it.

Only engineering on a global level to either heat or cool the Earth could save us from these natural killer cycles. At present our knowledge of climate control is too primitive to even think of implementing such planetary interventions, but we will have to develop this technology in order to survive in the long term.

(6) Some billions of years hence, our sun will greatly increase in luminosity, so much so that the increased heat will boil away our oceans. If we are to remain on the planet, some means will have to be employed to either cool the sun or move the Earth farther away from the sun. Otherwise humanity will have to leave Earth and colonise another planet orbiting a cooler star.

So, if we are to survive on Earth, even into the near term geological future, we have much work to do. But this is just the way things are on every level.

Even when pursuing our brief individual lives, we thrive only by working hard to solve problems.

But it is a noble adventure and the prize for success is great. Intelligent life is probably rare in our universe. We might even be unique and, if so, our obligation to survive is truly awesome.

William Reville is an emeritus professor of biochemistry at University College Cork, See undersci.ucc.ie