29: The Goldilocks Planet

Episode 29: The Goldilocks Planet 

Last episode, we met one of the fundamental paradoxes of the early Earth. Four billion years ago, our sun was much dimmer, only 75% as bright as today. We know this by looking at thousands of other stars at different stages of their lives- they all get brighter over time. Back on Earth, less sunlight means colder climates. For example, as I’m recording this episode, it’s winter in Michigan, with snow and frost on the ground. All it took was a few more hours of darkness, a few more hours in the black uncaring void of space to turn green fields into white snowdrifts.

Now imagine that the sun itself is less powerful, even at high noon. This dimmer switch is the world 4 billion years ago and by all rights, Earth should have been a frozen popsicle. And yet, the oldeset rocks tell us that the Earth was warm and had liquid water. What gives?

This idea is called the Faint Young Sun Paradox and was proposed by Carl Sagan and George Mullen in 1972. Fortunately, they didn’t just leave this paradox at our feet- they proposed a solution. Today, we’ll check Sagan’s homework and learn how scientists have tweaked his answers over fifty years, giving us a better view of the early Earth. 

We begin with a simple question: how cold would the Earth be without any sun at all?

Part 1: The Restaurant on the Moon

As you can imagine, Earth would be very cold without the sun. In the darkness of space, temperatures would quickly drop to -240 C, or -400 F. This is far colder than Antarctica, and even most laboratory freezers. Without the sun, you couldn’t read a thermometer because the mercury would be frozen. You couldn’t drink your sorrows away, because all alcohol would be frozen, even Everclear, even rubbing alcohol. You couldn’t drive anywhere because gasoline would be frozen. 

The Earth has its’ own internal heat, but this only reaches the surface near volcanoes. For most places, the blanket of rock is simply too thick and we would freeze to death.

OK, let’s turn the lights back on before we get frostbite. 

Clearly, the sun gives us the heat we need to survive. The more sunlight on the Earth, the hotter it gets. The closer a planet is to the sun like Venus or Mercury, the hotter it gets. Pretty simple.

But sunlight isn’t the only control on climate. To see why, let’s examine our neighbor, the Moon. 

The Earth and the Moon are basically the same distance from the sun. You would think that both worlds would have very similar temperatures, but this is not the case. Just ask the Apollo astronauts.

The average temperature on the moon is a bit below freezing: -20 C, 0 F. Not pleasant, but much better than a sunless world. However, during the Apollo missions, the daily highs and lows were much more extreme. Temperatures climbed above 150 F at high noon, and sank below --80 F at night, that’s 60 to -60 C for our international listeners. For reference, these daily temperatures on the moon are around the highest and lowest ever recorded on Earth by humans. And other areas of the moon get even more extreme.

In short, the Moon is generally colder than Earth, but it experiences major temperature swings, hundreds of degrees every day. In contrast, Earth’s temperature is far more stable and warmer on average. Clearly, the sun alone is not enough to control climate. 

The answer why can be found in an old dad joke: 

“Did you hear about the restaurant on the moon?” “Yeah, it has great food, but no atmosphere!”

Thank you, I’ll be here all week.

Earth’s atmosphere not only helps us breathe, it also acts like a global thermostat. Without our thin blanket of air, we would be just like the moon, baking in the day, bitter cold at night. The atmosphere is Carl Sagan’s solution for the Faint Young Sun Paradox. But before we examine the past, let’s take a literal breather. 

What’s in the modern atmosphere, and what keeps us warm at night? 

Take a deep breath in. You’ve just inhaled many different gases, but more than 99% of that breath is just three gases: nitrogen, oxygen, and argon. We’ll talk about the others in just a second, that measly fraction of a percent, but first let’s check out these Big Three. Oxygen is essential for human life- if you lower the amount just a tad, we eventually pass out or die. In contrast, nitrogen and argon are duds- they’re abundant in the air, but they don’t interact with our bodies at all. 

All right, I’ve gassed on enough, let’s return to climate. How warm is the Earth with these big three gases alone- nitrogen, oxygen, and argon? If you’ll remember, the average temperature on the moon with no atmosphere is 0 F, -20 C, pretty chilly. 

Drumroll please! With the Big Three alone, Earth’s average temperature would be… 0 F, -20 C, pretty chilly. Now hold on a second. I said that Earth’s atmosphere keeps us warm, but the Big Three Gases didn’t change the temperature at all! Not even precious life-giving oxygen!

It turns out, not all gases are created equal, and it’s finally time to talk about the remaining 1% of the air that actually controls the climate.

Part 2: Nutmeg in the Air

I like to bake in my spare time- bars, cakes, crumbles, and cookies. I learned a lot from my mother about the do’s and don’ts of the kitchen. One of the biggest don’ts I learned early involves the spice nutmeg. Many recipes we used called for nutmeg, especially around Christmastime. In all these recipes, my Mom crossed out the original amount and penciled in a half or even a quarter of the required dose. She told me that a little nutmeg goes a long way in a recipe, punching well above its’ tiny weight. A little too much could ruin a dish, and a lot too much could make you sick.

Here’s the point of this digression. Much like a cake recipe, the atmosphere contains a blend of different ingredients. Some gases like nitrogen, oxygen, and argon are much more bountiful- the flour, sugar, and butter of the air. Now flour, sugar, and butter alone can make some decent dishes, but the best ones have a little bit of spice, a touch of something different. Too little and the cake is bland, too much and it’s inedible. 

This brings us to the remaining 1% of the atmosphere, the blend of minor gases that spices up the air. Quite a few of these gases have made the news in the past few decades- the ozone hole of the 1980s, the helium shortage happening today. But by far the most infamous are the greenhouse gases like carbon dioxide, and they are the subject of today’s episode. 

In 2024, it’s hard to go a week, or even a day without hearing about climate change from greenhouse gases. So forgive me if this is all review, but understanding how a tiny amount of gas can warm the planet is not only crucial to predicting Earth’s future, but also understanding its’ past. 

Many materials act as greenhouse gases: carbon dioxide or CO2 is the most famous, but other candidates include methane, nitrous oxide, ozone, ammonia, and even water vapor. This rogues’ gallery of gas shares one trait in common: they’re all made from multiple different elements. CO2 has carbon and oxygen, H2O has hydrogen and oxygen, and so on. These large, more complex molecules are more likely to interact with heat waves than smaller, simpler gases like oxygen or nitrogen. 

To see how, let’s return to our imaginary Earth without greenhouse gases from last section. When sunlight and ultraviolet radiation hits the Earth, it warms up the ground. This heat rises back into the air- you can see this on hot pavement in the summer. Eventually that heat rises back into space, shooting past oxygen and nitrogen with no problem. The ground gets very hot in the day, but as soon as night falls, all the heat escapes back into space, and things get cold once again, just like the moon.

Now let’s examine the actual modern Earth. Just like before, sunlight reaches the surface and warms the ground. But when that heat bounces back into the air, some of it starts to hit carbon dioxide. The heat is forced back down towards the ground, keeping it here for longer. Eventually, that solar energy will escape back into space, but the longer it’s pushed around by CO2, the more this lingering heat warms Earth’s surface. So instead of a hot sunlit side and a freezing dark side, the entire planet is warmer and more stable. This is the greenhouse effect, named because greenhouse glass traps solar energy much like CO2. 

So how much does greenhouse gas warm the planet?

Let’s review our temperatures from the first section to find out. Remember, with no sun at all, Earth would be -400 F, blindingly cold. With a sun but no greenhouse gas, Earth would be 0 F with major temperature swings. For our international listeners, that’s -240 and -20 C, respectively.

Now for the Earth today. The atmosphere currently has 0.04 % carbon dioxide, which gives Earth an average temperature of… 57 F, or 14 C. To repeat, a tiny pinch of carbon dioxide raises Earth’s temperature from below freezing to livable. Imagine filling a bathtub to the brim with cold water, then adding just half a cup of a fluid that instantly heats the tub by dozens of degrees. That’s the effect of carbon dioxide on climate, that’s the nutmeg in the cake. From small things, big things grow. 

Now you can see why scientists are stressing out about rising CO2 today- if just a little CO2 turns Earth from freezing to mild, imagine what just a little more might do. Climate change is rightfully one of the largest issues of our generation, so I don’t want this next section to be taken out of context. For the rest of the episode, I’ll be talking about greenhouse gases as the savior of the ancient Earth, keeping the planet from freezing solid. Someone might misconstrue that as an endorsement of greenhouse gas or burning fossil fuels- it is not. CO2 and its’ cousins are double-edged swords- too little and we freeze, too much and we boil. Humans are pushing CO2 to levels unseen in millions of years- like the atom bomb, we are playing with tiny natural forces that have massive repercussions. 

So with that in mind, I’ll step off my stump and return us back to a simpler time, 4 billion years ago, when the sun was dimmer and Earth desperately needed a warm blanket. The question is, which greenhouse gas provided that blanket?

Part 3: The Rogues’ Gallery

Until now, we’ve focused on carbon dioxide- it’s the poster child for greenhouse gases and modern climate change. But as we’ve mentioned, it’s not the only game in town. 

In this final section, we’ll quickly run through three other candidates, three other gases that could have warmed the early Earth. It’s time to play Is This Your Gas?

Our first contestant is ammonia. Ammonia is a pungent gas that smells like urine, sweat, or window cleaner. Scientists used to think ammonia was a major part of the atmosphere. When I visited the Natural History Museum in Pretoria, South Africa, I found an old exhibit inviting you to “smell the ancient atmosphere”. Perhaps it had broken down, or more likely people did not like the odor, because there used to be ammonia in that exhibit, but not anymore. Probably for the best.

We briefly met stinky ammonia in Episode 20- it was a major ingredient in the famous Miller-Urey experiment, making organic soup out of thin air. Carl Sagan was a big fan of this experiment, showing it off in his Cosmos show. So it’s no surprise that Sagan proposed ammonia as the greenhouse gas that warmed the Early Earth. 

Is this your gas? Well, there’s a problem- ammonia is a very wimpy gas- it can take the heat, but it can’t survive ultraviolet light from the sun, the same light that gives you a nasty sunburn. This UV light shatters ammonia apart, which means no more greenhouse effect. Ammonia might have been around in Season 1 of this podcast, but after a few million years, it was toast. Thank goodness- imagine a world that smells like window cleaner.

Still, Sagan was adamant about ammonia for decades. His last major paper in 1997 argued that ammonia could have survived with a bit of help, which brings us to our second contestant: methane.

Methane is an organic gas that seeps out of wetlands and animal waste, among many other places. It’s been called some unpleasant names like “swamp gas” or “cow farts”. In both cases, methane is made by microbes in swamp water or cow guts. In fact, microbes are a major source of methane in today’s atmosphere. Methane is vanishingly small, less than a thousandth of a percent of today’s atmosphere. But don’t be fooled- as a greenhouse gas, methane is 20x more powerful than CO2. Talk about some spicy nutmeg. 

Though dangerous today, methane is an appealing greenhouse gas for the ancient Earth- you don’t need very much methane to warm the world. For bonus points, a methane-rich atmosphere could be a strong sign of life on ancient Earth, even without any fossils around.

So, is this your gas? Yes… and no. Most models of the ancient Earth need some methane- it is a part of the recipe. But you can have too much of a good thing. As you pump lots of methane into the air, something weird happens. Just like ammonia, methane and other gases are shattered apart by UV radiation. These methane leftovers mix together, forming new, more complex organic molecules, just like we saw last season. These molecules are so large, they form a visible smoggy haze in the air, a haze which blocks out the sun, reflecting light and heat back into space. So, a little methane provides a nice warm blanket, but too much forms a suffocating layer of sunscreen. The atmosphere is a weird place. 

On Earth today, methane is so low this never happens, but we can see a hazy sunscreen elsewhere in our solar system. Titan is Saturn’s largest moon. It’s larger than our own Moon, it’s even larger than the planet Mercury. But when you look at Titan through a telescope, you can’t see its’ mountains or craters, you see an impenetrable orange haze. Titan’s atmosphere is 5% methane- again, not a lot, but much more than the Earth. 

There’s a lot more I could say about Titan’s lakes of liquid methane and the possibility of life, but we’re going long already, so we’ll save that for a future episode.

Our final contestant might be the most surprising greenhouse gas- water vapor. Et tu, Brute? We need water to survive, how could you betray us like this? 

It turns out that half of Earth’s surface heat is thanks to water vapor. BUT! Hold your horses before you blame water on current climate change. When water evaporates from the ocean, the morning dew, or sweat on your skin, it enters the air as water vapor. While it’s hanging around up there, this vapor does produce a greenhouse effect. Eventually, after a week, that vapor returns to the Earth as rain, or snow, where it can’t hurt anyone anymore. 

So let’s say your villainous plan to warm the Earth is to make the world’s largest sauna, flooding the atmosphere with steam all at once. Well, things might get toasty for a few weeks, but then the Earth would stabilize, the water would eventually fall back down, and life would continue. In contrast, methane hangs around the atmosphere for a decade, and carbon dioxide remains for a century. The more CO2 we pump out, the longer it will stick around.

One last time, is this your gas? Yes and no. Water does warm the world, and it was abundant on the early Earth, but it does not drive major changes over time. Water alone isn’t enough. 

Which brings us back to our old frenemy, CO2. Love it or hate it, CO2 has been the most important greenhouse gas throughout Earth history. But it has some friends to help it along the way- water vapor and methane. Sorry, ammonia, better luck next time. Let’s wrap up and learn just how much of these gases were around on the ancient Earth.

Summary:

The sun was much dimmer 4 billion years ago. In this pale twilight, Earth’s surface was saved from frozen oblivion by greenhouse gases. The air had more than 100x today’s carbon dioxide, 1000x today’s methane, while water vapor was probably the same. 

Over time, the sun would grow brighter, and these greenhouse gases would diminish to the mild levels we know today. But that is changing as we speak. In just two hundred years, humans have increased CO2 to levels unseen in 4 million years. Not billion, but 4 million is still worrying.

If there’s only one thing you take away from this episode, here it is: you do not need a lot of CO2 or methane to change the planet’s climate. Earth needs a little warmth. We don’t need a little more, not at the rate we’re raising the thermostat.

Next episode, we return to the warm, wet Earth 4 billion years ago and move the story forward. Until now, we’ve only seen rocks forged deep underground. Next time, we’ll visit a new location and finally see rocks that formed on the bottom of Earth’s oceans.