Does CO2 keep Earth warm, and would we have an ice age without it?
📞 What Mom Said
"CO2 keeps the planet warm, and if we didn't have it we'd go back to the Ice Age."
This version is closer to true than the earlier plant claim, but it is still too simplistic. Carbon dioxide is one of the greenhouse gases that helps keep Earth warm enough for life, and without greenhouse gases the planet would be far colder. But CO2 is only one part of the greenhouse effect, alongside gases like water vapor and methane, and more CO2 is not automatically better. Scientists agree that extra human-added CO2 is strengthening the greenhouse effect and warming the planet beyond its natural balance.
Filed:
The Claim
Mom has upgraded from “plants like CO2” to a more defensible position: that CO2 is what keeps Earth warm, and without it we’d be plunged back into an ice age. This one is actually grounded in real science — but it still oversimplifies in ways that matter.
The Grain of Truth
She’s right that CO2 is a greenhouse gas and that greenhouse gases keep Earth warm. Here’s how it works:
Sunlight passes through the atmosphere and warms the Earth’s surface. The surface then radiates that energy back upward as infrared (heat) radiation. Greenhouse gases — including CO2, water vapor, and methane — absorb some of that outgoing heat and redirect it back toward the surface, keeping the planet warmer than it would otherwise be.
Without any greenhouse gases, Earth’s average surface temperature would be roughly -18°C (0°F) instead of the current +15°C (59°F). So yes — greenhouse gases, including CO2, are genuinely essential to making Earth habitable. The broad direction of Mom’s claim is correct.
Where It Gets Complicated
CO2 is not the biggest greenhouse gas. Water vapor actually accounts for roughly 50% of Earth’s greenhouse effect, with clouds contributing another 25%. CO2 contributes around 20%. It’s a significant player, but not the star of the show — and Mom’s framing implies it’s doing more work than it is.
“More CO2 = warmer” has a ceiling. While CO2 does trap heat, the relationship is logarithmic — each additional unit of CO2 has a smaller warming effect than the last. The atmosphere doesn’t just keep getting hotter indefinitely.
The ice age connection is shaky. Ice ages are primarily driven by Milankovitch cycles — slow, predictable changes in Earth’s orbit and axial tilt that affect how much solar energy reaches the surface. CO2 plays a role in amplifying these cycles, but it is not the main trigger. Saying “no CO2 = ice age” skips over a lot of orbital mechanics.
The real concern runs in the other direction. Scientists aren’t worried about too little CO2 — they’re worried about too much. Since the Industrial Revolution, human activity has raised atmospheric CO2 from about 280 parts per million to over 420 ppm. This is strengthening the greenhouse effect beyond its natural equilibrium and driving global temperatures upward faster than ecosystems and infrastructure can adapt.
The Bottom Line
Mom accidentally stumbled into a real scientific concept. CO2 does help keep Earth warm, and a planet with no greenhouse gases would be a frozen wasteland. But the claim glosses over the fact that water vapor does most of the heavy lifting, that ice ages have other causes, and — most importantly — that the actual problem isn’t too little CO2. It’s too much.