Imagine walking through a forest where dragonflies are the size of hawks and millipedes stretch longer than your leg. Sounds like science fiction — but these creatures were very real over 300 million years ago, in Earth’s Carboniferous period. What made this era so different from today isn’t just the presence of enormous bugs — it’s the air itself.
So here’s the big question: Was the volume of our atmosphere larger back then? And was the air pressure at sea level actually higher than it is today?
The secret sauce in the Carboniferous: Oxygen overload
The Carboniferous period was a golden age for lush plant life, from massive lycopod trees to swampy, moss-covered bogs. As these forests photosynthesized day in and day out, they pumped an enormous amount of oxygen into the atmosphere, driving levels from around 20% (where we sit today) up to a whopping 30–35%.
That’s a massive jump — enough to spark wildfires more easily and, most fascinatingly, to support much larger life forms. But what does that mean for the overall atmosphere?
Was Earth’s atmospheric volume bigger?
Here’s the twist: while the oxygen percentage skyrocketed, the dominant gas in Earth’s atmosphere (nitrogen) likely stayed about the same. This means the total volume of the atmosphere didn’t dramatically increase, but don’t stop reading there.
Yes, the volume only grew modestly. But the mass — and thus the air pressure at sea level — definitely increased. Why? Because a higher concentration of oxygen adds to the total weight of the air pressing down on us. More oxygen = denser, heavier air = higher surface pressure.
Let’s break it down:
- Carboniferous oxygen levels: 30–35%
- Modern oxygen levels: ~21%
- Nitrogen: stayed roughly constant
- Atmospheric volume: modest increase at most
- Air pressure at sea level: significantly higher, thanks to denser air
How we know: experiments and evidence
Researchers have actually tested how increased oxygen affects living creatures. In a lab experiment from 2010, entomologist Dr. John VandenBrooks raised dragonflies in an atmosphere of 35% oxygen — and they grew much larger than their modern-day counterparts. No surprise there: more oxygen means insects, which rely on diffusion through their exoskeletons to breathe, get a power boost. Their evolutionary ceiling — quite literally — rises.
As for the air pressure? Even though we can’t bottle ancient air, scientists use indirect evidence, such as fossilized bubbles in amber and models of gas composition based on ancient rock chemistry. These tell us that the higher oxygen levels back then weren’t just hype — they actually thickened the air.
But wait… why did it all change?
Eventually, all that plant material buried during the Carboniferous turned into the coal we burn today. As Earth shifted into the Permian period, volcanic activity released CO₂, and oxygen levels dropped. The giants disappeared. The air thinned. And that strange high-pressure era faded away.
The takeaway: size matters — and so does air
So if you’ve ever wondered how giant bugs once ruled the skies, the answer lies in Earth’s ancient atmosphere. More oxygen didn’t necessarily inflate the total volume of air, but it made each breath denser and more powerful. That extra pressure wasn’t just a number — it was a life force.
It’s a reminder that even invisible things — such as the air around us — shape our history, biology, and future in ways we’re only beginning to understand.
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