2 00:00:03,270 --> 00:00:05,490 MATT O'DOWD: Thanks to brilliant.org for supporting PBS Digital Studios.

Feeling a little chilly this winter?

Never fear.

In several hundred million years, the brightening sun will turn the poles into tropical paradises.

And then evaporate the oceans and extinguish all life.

11 00:00:25,180 --> 00:00:27,759 The sun is slowly burning through its fuel.

Hydrogen is fused into helium in the sun's core, producing energy that keeps it shining and keeps the earth warm and hospitable to life.

But that fuel will run out, after which the sun will swell into a red giant and flash fry the earth.

In fact, that frying, well slow roasting, will begin much earlier.

See, the sun is getting brighter even now.

This has complex, and for the most part terrible, implications for life.

The end of the world will come sooner than you think.

All stars brighten as they age and they deplete their fuel.

Now that sounds counterintuitive.

Why does less fuel mean a brighter star?

Well, the sun is able to remain as a giant ball of fiery hydrogen due to a delicate balance between the outer flow of energy produced by fusion in the core and the gravitational crush of its immense mass.

The rate of fusion of hydrogen into helium increases with the temperature and density of the core.

So when the sun was born, it's initial gravitational collapse from a giant gas cloud was halted as soon as the core became dense enough, dense enough for fusion to produce exactly the energy to balance that gravitational crush.

At that point, the sun had plenty of hydrogen fuel.

But over time that hydrogen has been diluted.

As the sun's fuel gets diluted, you might expect fusion to slow down.

But as soon as that starts to happen, the drop in energy production disrupts the delicate balance between outward pressure and gravity.

The core collapses slightly under the immense weight above it.

That increases the density and temperature of the core which increases fusion rate, once again re-establishing equilibrium between gravitational crush and energy production.

But with the same mass packed into a smaller volume, the core is under more gravitational pressure than before.

That means it needs to increase its energy output to resist the increased crush.

So over time, the sun's core shrinks and heats up, brightening the entire star.

This increased energy output actually causes its outer layers to expand slightly even as the core shrinks.

This all takes billions of years.

It's been calculated that the sun should currently be increasing in energy output by close to 1% every 100 million years.

But that rate will increase.

Now that's way too small to have any effect on human time scales.

But it's huge on geological time scales.

Now the sun was around 30% dimmer and 10% smaller when it first formed, but it will be about 2/3 again as bright and a third again as large before it finishes burning it store of hydrogen fuel in around 5 billion years.

But how much more energy will be too much?

And what does this mean for the Earth?

Well, after a brief boom in the Antarctic beach front property market, the oceans will evaporate.

But that's not the first disaster to result from the sun's brightening.

In around 600 million years, most photosynthesis will shut down, killing the majority of plant life.

And this is due to CO2 leaching from the atmosphere.

Normally we associate higher temperatures with increased CO2.

CO2 warms the surface due to its greenhouse effect and warm oceans are less able to absorb carbon so more CO2 is released.

This dangerous feedback cycle dominates on the time scales of the glaciation cycle and of anthropogenic climate change.

However, over hundreds of millions of years, CO2 will actually be lost due to rising temperature.

See, the rate of weathering of silicate rocks increases with temperature and that process sucks in CO2 to produce carbonate minerals.

In around 600 million years, CO2 levels will be at 50 parts per million compared to the current 400 parts per million.

And this will be too low for photosynthesis by C3 carbon fixation.

The vast majority of plant species rely on C3 photosynthesis including all trees.

So these will all die.

Plants using the more efficient C4 carbon fixation, including many of the grasses, will hold out for a bit but not for long with the ever increasing temperatures dropping CO2 and the evaporating oceans.

About them oceans, currently Earth is comfortably inside the solar system's Goldilocks or habitable zone.

This is the distance from the sun where sustained liquid water is possible.

The brightening of the sun means the habitable zone has shifted outwards since the formation of the solar system.

In fact, in the beginning when the sun was at 70% of its current brightness, Earth would have been outside the outer edge of the habitable zone, at least given its current atmosphere.

Its entire surface should've been frozen solid.

Yet the geological record tells us that there was plenty of water back then and life appeared quickly.

Earth was highly habitable.

This discrepancy between the astrophysical prediction and the geological evidence is called the faint young sun paradox, and was pointed out by Carl Sagan and George Mullen in the early '70s.

But planet surface temperature and the location of the habitable zone depends on the planet's atmosphere as well as the star's brighteners.

If the early Earth had a much stronger greenhouse effect due to high carbon dioxide or methane content, then the increased heat retention may have compensated for the fainter sun.

There are a few plausible scenarios that give the right levels of greenhouse gases.

However, there is not yet a consensus on the exact solution to this paradox.

Whatever the solution, life has found a way to exist on Earth for at least 3 and 1/2 billion years.

But life doesn't have such a long stretch ahead of it.

The Goldilocks zone will expand beyond Earth's orbit.

And depending on how our atmosphere evolves, our beautiful blue orb may become a desert planet relatively soon.

Now the oceans don't actually need to boil for this to happen.

As atmospheric water vapor increases H2O molecules in the stratosphere will be broken into hydrogen and oxygen and the light hydrogen atoms will be lost to space.

This is happening already but will massively increase as solar radiation rises.

Exactly when Earth loses all the oceans depends on complex climate models.

For example, the increase in the greenhouse effect due to increased atmospheric water vapor will speed things up.

But we may delay the inevitable by offsetting that greenhouse rise with the loss of CO2 or the decrease in atmospheric pressure due to nitrogen loss.

The increase in cloud cover may reflect some of the extra radiation.

However, most models agree that runaway ocean loss begins around a billion years from now or perhaps a little earlier.

Whenever it happens, the end result is the same.

The surface of the earth becomes Arrakis, a single vast desert.

That doesn't necessarily mean the end of life.

Currently there are several oceans worth of water locked in minerals within Earth's mantle.

And this will continue to be released to the crust.

Life will surely adapt and cling to these last wet patches on or below the surface.

But ultimately life will vanish in the reverse order that it arrived.

After the initial extinction wave from the loss of much of Earth's plant life, other complex multicellular organisms will succumb to heat, perhaps even before the oceans have gone.

Single celled extremophiles will last a while longer being much more able to adapt as surface temperature approaches the boiling point of water and atmospheric CO2 plummets.

Complex eukaryotic cells will give out next.

But simple prokaryotic life, the very first life to emerge, will hold out until CO2 flatlines, perhaps in less than 2 billion years time.

Nice work, prokaryotes.

You were the first to arrive and you will be the last to leave.

Don't let the door smack you in the flagellum on your way out.

Right.

So that's all very cheery, isn't it?

So how out some bright sides to distract us from the inevitable end of all life?

Well, Mars will warm up a bit.

It's currently on the outer edge of the habitable zone.

And so its ice reserves should thaw as the sun brightens.

Unfortunately, it has neither the mass nor the atmosphere needed to retain the released water.

It'll be lost to space just like its initial atmosphere was.

But perhaps we'll have advanced to the point of being able to replenish the Martian atmosphere by then.

But if we're talking about large scale geoengineering then I suppose we could also try to save the Earth.

Now we're not going to stop the sun from brightening but we might try to block some of the extra light.

Giant beach umbrella?

Actually seeding the upper atmosphere with reflective nanoparticles might do the trick, assuming humans or our descendants are around in another several hundred million years, this tick should be well within our capability.

Then perhaps we can hold out until the sun eventually exhausts its fuel and becomes a red giant expanding to consume the only home we've ever known in all of space time.