The Earth is ancient. Billions of years
old. A timescale so vast that it's difficult for us to
conceptualize. Yet despite, this the geological scars, imparted by the eons of
time, allow us to piece together the story of our world. We can't travel back
into the past, but we can still reconstruct what happened. But what about the future? How will the world change? What will happen to this cradle of life
spinning through the dark? How does the story end? Our ability to predict the future falls
short when it comes to human society. Be it a technological revolution, a stock
market crash, or the collapse of a civilization. We have to accept that the
future of our society is unknown and, ultimately, in our own hands. But the
Earth is different - because over billions of years the Earth is governed by
physical principles, like atmospheric chemistry, orbital mechanics, and
radiative forcing. Things we do understand and things we can even make
predictions for. So today, I invite you to join us for a voyage through time
together - from the very beginning until the bitter end.
A journey to the end of the world. To facilitate this journey, let's imagine
that we somehow have the ability to travel through time - from Earth's genesis
right up until its demise. You will essentially be a chrononaut witnessing
the eons play out firsthand. In total, we have about 12 billion years of history
to cover. A useful device to contextualise this vast ocean of time is
to compress each billion years into a month. So the history of Earth spans
precisely one calendar year, forming on January 1st and dying on December
31st. At this scale, each day on the calendar then represents 33 million
years and each second is 384 years. Bracing for your voyage, you step into
the time machine - it begins to warm up, and soon blinding lights fill your
vision, nausea sweeps across you, and in the blink of an eye we wake up at the
beginning. Earth's first day. Stepping out, it looks like you're on an
alien planet. This is the proto-Earth, 4.53 billion
years ago. The surface is fractured, having only recently formed, and it's
constantly changing as new meteorites come hailing down
smashing apart the young surface - adding to the proto Earth's growing mass. You
look down at two watches on your wrists. Your left shows that just three minutes
of compressed time have passed, but your right shows that tens of thousands of
years have flitted by here on Earth. Looking back up, you see a dark sky
smothered in ash and dust with a faint flickering Sun barely perceptible
through the clouds. The primordial atmosphere at this time is hydrogen and
helium, the same materials from which the Sun has also recently formed.
You watch for hours as the proto-Earth accumulates more mass, yet actually
shrinks in size. As the planets mass and gravity increases, it compresses the rock
beneath you. You look at your left watch to see that it's just gone 7 a.m.
whereas your right reads almost 10 million years. Up to now, the composition
of the proto-Earth has been fairly homogeneous - rock mixed with metals - but around this time something changes. The compression of the embryonic globe has caused it to heat up. Not only this, but much of the accreted material is
radioactive, which in turn heats the planet up further. The surface and
interior literally melt, forming a giant and deep ocean of magma. Within this
fluid heavy materials, like iron and nickel, sink under gravity to the very
center of the proto-Earth forming her core. As they move down, they rub against
the mantle generating friction and heat. The planet warms further, melting more
rock, and allowing yet more iron to sink. A runaway heating effect takes place
inside the planet. You have just borne witness to the "iron
catastrophe" - an event geologists long hypothesized about. Looking up at the sky,
you see an aurora borealis for the first time, as this newly forged iron
core has established a magnetic field around the planet -
protecting it from the young Sun's more harmful radiation. As the day comes to an
end, this young world seems finally at peace, after millions of years of
tumultuous formation, internal hemorrhaging, and tortured resculpting.
But a shadow looms. For Earth's suffering is not yet complete. It's about to face
an event of unimaginable force and power. One that will reshape the planet forever. On the second day, you look up at the sky
and see another small bright disk besides the Sun bob into view sometimes,
like it's coming closer and then receding away - a harbinger of doom. You
realize then that it's Theia - a nearby Mars-sized world that's also recently
formed. But unlike the real Mars this body has become gravitationally trapped
in Earth's Lagrange point L4, but it's too big, too unstable, to stay there being
tugged around by the gravity of not just the proto-Earth but Venus and the other
planets. It wobbles around precariously menacingly - getting ever closer. It's at
7:02 a.m. that the sky turns dark, and is consumed by Theia, as it begins its final
deadly plunge into the proto-Earth. Because of its smaller size, this is a
suicide run for Theia, a battle it cannot win. Yet it promises to cause as much
devastation as it can in this sacrificial act. Worlds collide - vaporizing much of their and tearing off the Earth's newly formed
terrestrial flesh into space. So violent, so powerful is this collision that it
would be detectable from hundreds of light-years away. Despite Theia's
best-effort, Earth somehow clings on to its existence. Deformed, unrecognizable,
but still standing. Much of what was the planet's material now orbits around it
as debris. Some of it rains back down punishing the Earth once again, but much
of it coalesces via gravitation forming rings, then clumps, then moonlets. As
midnight approaches you bear witness to the first Moon-rise. It's like nothing
you've ever seen. The Moon appears 30 times bigger than you're used to, being
30 times closer than the modern day. The Earth has become a parent, as its
daughter world now circles around its mother like a newborn fawn. Finally, the
Earth can cool, can solidify, can move forward into her future. It's day five. Over the last couple of
days comets have peppered the Earth. Each one has brought with it a supply of
water, volatiles, and even organic molecule. There is now so much water,
that today you see oceans forming across the planet. The site of a cooler Earth
covered in bodies of water calms your mind. For the first time, you see
something that resembles the planet that you know and love. It's spartan, desolate,
devoid of any life, but for the first time, it looks like the Earth. What
excites you now is that life could surely start at any moment and that you
might bear witness to that first spark of emergence. January 14th. It's been nine
days since the oceans formed by your clock, but 300 million years have passed
here in local time. The last week or so has been surreal. The Earth has a special
beauty in this untouched barren state. Wandering the surface, you find oceans,
rivers, shorelines - each one a possible site for life's first moment. Organic
chemistry is everywhere. Acetone, amides, nitriles, carbonyls and even amino acids. Today you explore a collection of hot springs up against the
shoreline - land that will one day become part of Western Australia. It's here that
you detect a small body of water with an elevated ratio of carbon-12 to its
heavier carbon 13 isotropic counterpart. You remember that life prefers to take
up carbon-12 over carbon-13, due to the lower energy cost of working with
lighter molecules, and so this hints that life might be active here. You find the
warm pool of pale water along the shore. It's loaded with organic molecules,
alcohols and sugars. Excitedly looking at your scanner, you detect proteins,
self-replicating chemicals, RNA, and protocells. It's delicate, primitive, but
unmistakably alive. You doubt that this pool is really the first instance of
life but it's so basic that what you're looking at is surely nascent. You pause
in profound epiphany as you look down upon what is essentially the origins of
you. Your 4.1 billion year old ancestor. Clinging to his newfound existence,
barely able to survive - even in ideal conditions - these tiny cells will one day
shape the world. As the days and weeks pass by for you,
you watch millions of years flick by. Life has percolated now across the
waters of the world. Despite being limited to simple microbial forms,
macroscopic mats of these organisms called stromatolites betray life's
expanding presence. The young Sun is almost a third less luminous than the one
that you know. But the Earth's atmosphere is rich in heat-trapping carbon dioxide,
keeping it warm enough for life to thrive. As the ambient supply of sugars
dwindles, with fewer cometary deliveries each day, life begins to adapt. On
February the 1st, you find the first signs of photosynthesis within some
cyanobacteria. These organisms pull in carbon dioxide and water molecules and
use sunlight to forge their own sugars, producing oxygen as a waste product.
At first the oxygen levels are barely noticeable, but these photosynthetic life
forms are so successful that they become ever more abundant. Beginning March 6th,
you detect a rapid rise in oxygen levels, now creeping above 1%, 2%, 3%... But life on Earth is not adapted to an oxygen-rich atmosphere. This highly reactive molecule
actually poisons much of Earth's life. You watch in despair as mass extinctions
sweep across the planet, killing about half of everything that lives. In this
era of change, the first eukaryote cells evolve. Sophisticated cells with
membranes around their organelles, larger volumes difficult for other cells to attack, and
more efficient metabolism. Just two weeks later, on March 30th, you find the first
colonies of these cells working together to form the first multicellular
organism. Some of the cells in these organisms begin to specialize to
specific functions, enabling yet more complex life to develop. It's on April
29th that simple animals first appear, like jellyfish and sponges. Up to now
life has largely been aquatic, leaving the land untouched, but on May 1st, Earth
enters the Cambrian period and with it an explosion of life evolves in a
diversity of new and wonderful directions, taking over the land and
changing the oceans. You've now been on your journey for four months but four
billion years have passed locally. It's staggering to think about the fact that
so much time has passed here. Walking the Earth during this time is spectacular - a
paleontologists dream. Life is even more diverse than imagined. So many bizarre
failed evolutionary experiments, but through trial and error life adapts,
competes and evolves into fascinating and complex forms. It's just over a week
later, on May 10th, that the first dinosaurs evolve during the Triassic
period. Watching these giants wander the Earth is a childhood fantasy. Creatures
like the Allosaurus, the Stegosaurus, the Triceratops, and of course the
Tyrannosaurus Rex. Each one simultaneously terrifying and
captivating. You relish studying these giants, but just five days later an
enormous meteorite more than ten miles across enters the atmosphere. The impact
is truly decimating - raising a cloud of dust into the atmosphere that blots out
the Sun, darkening the skies, and cooling the world. You mournfully watch as the era of the dinosaurs abruptly ends, marking the
transition between the Cretaceous and Paleogene periods.
It is now on May 15 that small mammals start to fill the ecological niche left
open your direct ancestors. You check your watch as history fulfills itself. In
two days, on May 17th, you'll pass the date that you left the Earth. Thus far,
you've witnessed the past play out broadly familiar events inferred from the
geological record, but now - now you are about to move into the future. You watch
as the mammals advance over the Earth, growing in size and complexity - great
apes, hominids, Homo sapiens - this is it you're about to see her own fate, the
future of the human race. At 3:48 p.m. modern humans emerge. Just thirteen
minutes later they begin farming and erecting buildings as you pass the
Neolithic Revolution. Less than one minute after that point, twenty thousand
years by local time, it's over. Humanity is gone. The tape played so fast you
couldn't see what happened to us. Did we move to another planet? Did we switch to
artificial intelligence? Did we simply extinguish ourselves? In a flash, our
cities and monuments crumble decay and disintegrate into dust, blowing off into
the wind. Civilization doesn't rise again for us,
or for any of the species, for the Earth will not sustain these ideal conditions
which we enjoyed during our brief episode of thought, self-awareness and reflection.
The Earth's biosphere will only diminish from this point on. May 20th. It's been three days since
humanity disappeared. For the umpteenth time the continents have converged into
a giant supercontinent. You sit there depressed, watching the eons flick by,
the future unfold. The world is changing. When you first arrived, the Earth's
atmosphere was rich in carbon dioxide, keeping the planet warm
despite the Sun's faint output. But over the past four and a half billion years,
the Sun has grown more luminous as its core gradually contracts. It's now 30%
more luminous, bathing the Earth in ever greater heat. As the insolation upon
the Earth rises, evaporation cycles speed up, precipitation increases. As these
rains fall through the atmosphere the droplets soak up molecules of carbon
dioxide, forming a weak carbonic acid. In human lifetimes
you'd never noticed this, but now you see it. The rain dissolving away the rocks
through weathering. The carbonic acid reacts with the silicon materials in the
rock forming carbonates that wash down to the ocean beds. The warmer the Sun
becomes, the faster this weathering occurs, pulling out ever greater amounts
of carbon dioxide from the atmosphere, faster than volcanism or life's
respiration can replenish it. The Earth is becoming starved of carbon dioxide.
This is the way it's always been carbon dioxide and temperature in a constant
back-and-forth keeping the atmosphere more or less the same temperature
throughout the ages - a global thermostat operating on the timescale of hundreds
of thousands of years. Because CO2 is a heat-trapping gas, its decreasing levels
help keep the planet cool in the face of a warming Sun. But carbon dioxide and
only go so low... By May 27th, carbon dioxide levels have
decreased so much that the bio productivity of the planet is one half
of that of the era that you came from. Carbon dioxide is the fuel of
photosynthesis, and without it the food chain dwindles. It's ironic - the very
chemical the humanity fought so hard to remove from the Earth's atmosphere is
now the very chemical the biosphere so desperately lacks. Over the next couple
of days, you watch the sped-up Earth rapidly change. Forests become less
common, animals fewer in number, and the planet becomes less green with each
passing hour. It's on June 1st that CO2 drops to dangerous levels, below 150
parts per million, leading to the mass extinction of all organisms that rely on
the C3 photosynthesis reaction. But long ago, life evolved a more sophisticated
alternative called C4 that can cope with CO2 levels as low as 10 parts per
million. In the blink of an eye, these C4 plants and microbes fill the ecological
hole opened by their demised cousins. But ultimately, this just delays the
inevitable, as the Sun just keeps warming, and so the CO2 just keep tumbling. How
long can this go on for? It's on June 13th that you get your
answer. Your right hand watch tells you at the year is 900 million AD. It's now
that CO2 levels finally fall below 10 ppm - a critical value and the point where
photosynthesis fails. Life cannot adapt. C4 plants like millet, maize and
sugarcane are the first to die, followed by photosynthetic microbes. The surface
turns gray and brown. Within the space of 3 million years, or just over 2 hours by
your watch, you are now forced to bear witness to
the most heartbreaking spectacle. Without photosynthesis, oxygen levels rapidly
decline across the world. Large herbivores are the first to perish, their
food supply gone. Large placental mammals go next with their relatively high
oxygen requirements. Next, you watch as small mammals die in their burrows, birds
fall to the ground, dead fish fill the oceans. By the end
of this terrible two hours, even invertebrates like the insects are gone.
The Earth's oxygen has been depleted - the reign of multicellular life is over. You
reflect back then it was a good run, for two and a half billion years they roamed
the Earth, about 20% of the planet's lifetime. But now you look out at an
Earth that has gone into shock, returning to his earlier microbial era. It's still
a living world but one very different from the one that you once knew. With the carbon dioxide depleted,
there's no buffering left for the Earth's temperature - the planet begins to
rapidly warm. Not only does the temperature rise but
surface water becomes more saline, salty, as water evaporates away. The remaining
life departs the equatorial zones and finds some solace in the polar regions
and high-altitude lakes, where the temperatures remain coo. Life might be
restricted to microbial forms, as it was in the earliest days on Earth, but this
life is far more sophisticated. For unlike those primitive simple life forms,
the microbes living now are the product of five billion years of evolution -
highly competitive Darwinian natural selection honing their genomes. In
particular, extremophiles adapted for high-temperature and saline environments
resist extinction and persist as the Earth warms. Watching this play out gives
you some comfort to see these advanced cells stare down the Earth's warming
climate. At least they still hold the torch of life going. Towards the end of
June, over a billion years into our future, the Earth's daughter - the Moon - has gradually receded away so far that its gravitation no longer acts to stabilize
the Earth's obliquity. This causes the planet to axially precess, leading to
much greater seasonal variations. Much of the Earth becomes uninhabitable, even to
extremophiles. By July, what little life is left is forced to hide away from the
surface, either deep underground or take refuge in cold trap caves formed by
collapsed lava tubes. On July 25th, the Earth's core, which has been gradually
cooling since its inception, finally solidifies; leading to a total collapse of the
Earth's magnetic field. By now, the global mean temperature is over 100
degrees Celsius and the oceans rapidly evaporate. The water content of the
atmosphere, and even the stratosphere, rises tipping the Earth into a "moist
greenhouse" State. This high-altitude water is split by the Sun's ultraviolet
radiation into its constituent hydrogen and oxygen atoms. As the oxygen sinks
back down, the lighter hydrogen escapes into space. Through this process of
photodissociation, the Earth begins to rapidly lose its water content, much like
what happened to its sister planet Venus eons ago, By the end of July, 2.5 billion
AD, the oceans are gone and the 3arth is a dry barren world. This is the end for
life. Without water, nothing can survive. The future from here is sterile, plate
tectonics shut down, and the mountains weather away. The world looks alien, more
like Venus than your Earth. The Sun continues to grow ever more luminous
over the billions of years that pass scorching the Earth's inanimate surface.
As the Sun's core converts hydrogen to helium, it contracts raising the internal
temperature and pressure - which has largely driven the changes you've
observed throughout the Sun's history thus far. But, by October 30th, five and a
half billion years into our future, the core is now so hot that the surrounding
hydrogen begins to fuse, causing the Sun to now finally leave the so called
"main sequence" phase of its life. As this shell of hydrogen burns around
the core, it deposits energy directly into the surrounding envelope, causing
the Sun to expand. At first this expansion is gradual, but it heralds the
demise of the Earth. It's December 31st - the Earth's last day.
You know that the Sun is bracing to enter its giant phase, an episode of
rapid and deadly expansion. As that final evening breaks, you watch the Sun grow
in size in your red sky. It morphs in color, reaching beautiful deep orange
hues, growing ever larger. By 9:15 p.m. the planet Mercury has been swallowed by
the Sun, yet the Sun's appetite is not yet satisfied as it continues to swell.
At 11:16 p.m. you watch in awe as Venus is engulfed. The Sun is terrifyingly,
unimaginably large - a red giant filling nearly one half of your sky. With each
and every minute, it expands further. But perhaps there is some hope, for as the
Sun has evolved is lost about a quarter of its mass causing the Earth's orbit to
move slightly outwards. Like a game of cat and mouse, you wonder if this
movement is far enough to escape engulfment. Maybe the Earth will be spared
being swallowed whole. But as the Sun closes in to its prize, in that final
terrible hour, the Earth literally drags through the stellar chromosphere
and now starts to spiral in towards the Sun, not away.
As the Earth reverses course, you accept that it's over. Here, at the end of all
things, your sky is filled with fire as the Earth finally rests, returning to its maker. In Earth's 12 billion year old story,
life blossomed for just over half of its age, and multicellular life makes up just
one-fifth of that story, and humanity? Well, we may be just the blink of an eye.
We recognize that *where* we live is special, but so too is *when*. For we live
in Earth's glory days, a world of temperate conditions, stabilized obliquity, active
geology and diverse flourishing life. An era where not just multi-cellular life is
possible, but animals, agriculture, and even civilizations. Ultimately the fate
of ourselves is up to us. We can choose to accelerate our demise, or, we
can look after our world, its life, and each other. The candle doesn't have to go
out here - new stars, new worlds are forming right
now, The Universe offers infinite potential to those who dare, and so, where
we go from here is a choice that I leave to you. Feynman: "The truth is so remarkable, so amazing. That's a much more exciting story to many people than the tales which other people
used to make up, who were worried about the Universe, that we were living on the back
of a turtle or something like that. They were wonderful stories, but the truth is
so much more remarkable.
