What “parts per million” means — and why the number keeps going up
You’ve probably seen numbers like “420 ppm CO₂” or heard that CO₂ levels are “the highest in human history.” But what does that actually mean? What is “ppm”? And why does this number matter so much for the future of life on Earth?
This page explains what CO₂ ppm means, how we know it used to be about 280 ppm, why it’s now over 420 ppm, and why that change is a big deal for everyone — not just scientists.
“ppm” stands for parts per million. It’s a way of saying “out of a million pieces, how many are this one thing?” In this case, it means: out of a million air molecules, how many are CO₂.
So if CO₂ is 280 ppm, that means 280 out of every 1,000,000 air molecules are CO₂. If it’s 420 ppm, that means 420 out of every 1,000,000. CO₂ is still a tiny fraction of the air — but small changes in this tiny fraction have huge effects on Earth’s temperature.
We use ppm instead of percent because CO₂ is a trace gas. Percent would hide the important changes. ppm makes them visible.
CO₂ (carbon dioxide) is a gas made of one carbon atom and two oxygen atoms. It’s a natural part of Earth’s atmosphere. You exhale CO₂ every time you breathe out. Plants use CO₂ to grow.
CO₂ comes from natural sources like breathing, wildfires, and volcanoes. But the extra CO₂ driving climate change comes mainly from burning coal, oil, and gas, cutting down forests, and certain industrial processes.
CO₂ is especially important because it acts like a thermostat for the planet. When there’s more CO₂ in the air, Earth’s temperature goes up. When there’s less, it goes down.
To understand why CO₂ ppm matters, it helps to picture Earth wrapped in a thermal blanket.
Earth’s thermal blanket is the invisible layer of greenhouse gases around the planet that lets sunlight in but slows heat from escaping back into space. It functions like a heat‑trapping blanket.
This blanket is made of invisible greenhouse gases — like CO₂, methane, and water vapor — that let most sunlight in but slow some of the heat from escaping back into space. That trapped heat keeps Earth warm enough for life.
Your blanket at home stays roughly the same thickness over time. Earth’s thermal blanket does not. It’s dynamic. When we add more greenhouse gases, we make the blanket thicker. A thicker blanket means more trapped heat and a warmer planet.
CO₂ ppm is one of the clearest ways to measure how thick that blanket has become. To learn more about how this heat-trapping works, see our Greenhouse Effect Explained page.
CO₂ makes up less than 0.05% of the air, but that tiny amount has a big impact on temperature. If we used percent, the change from 0.028% (280 ppm) to 0.042% (420 ppm) would look small and easy to ignore.
ppm shows the change more clearly. Going from 280 ppm to over 420 ppm is a jump of about 50%. For Earth’s thermal blanket, that’s huge.
Before the industrial era, for thousands of years, CO₂ in the atmosphere was about 280 ppm. It wobbled a little with natural cycles, but it stayed in a narrow range.
Since we began burning large amounts of coal, oil, and gas, CO₂ has climbed rapidly. Today, it’s over 420 ppm and still rising. That means Earth’s thermal blanket is much thicker than it used to be.
The key point is not just the number itself, but the speed of the change. CO₂ is rising faster than at any time in human history. Our climate system is being pushed harder and faster than it can naturally adjust.
Understanding what different CO₂ levels mean helps us see how Earth’s heat‑trapping blanket changes over time.
| CO₂ ppm | Time Period | What Earth Was / Will Be Like | What It Means for Earth’s Heat‑Trapping Blanket |
|---|---|---|---|
| 280 ppm | Pre‑industrial | Stable climate with predictable seasons and slow natural changes. | Thin, balanced blanket that kept Earth’s temperature steady. |
| 420+ ppm | Today | More heatwaves, rising seas, stronger storms, and shifting weather patterns. | Much thicker blanket that traps more heat than at any time in human history. |
| 450 ppm | ~2030s | Higher risk of extreme events, more coastal flooding, and increased climate stress. | Approaching dangerous thickness with more heat locked into the system. |
| 500 ppm | ~2050s | Severe heat, major strain on food and water systems, and widespread adaptation challenges. | Very thick blanket with large amounts of warming locked in for generations. |
| 550–600 ppm | Late 21st century | Major ecosystem shifts and multi‑meter sea‑level rise unfolding over time. | Extremely thick blanket, committing Earth to long‑term warming for centuries. |
| 650–700+ ppm | Early 22nd century | Profound climate disruptions and major habitability challenges in some regions. | Exceptionally thick blanket with warming that persists for many generations. |
You might be wondering: How do we know CO₂ used to be 280 ppm? How do we know it’s over 420 ppm now?
For the past, scientists use ice cores — long cylinders of ice drilled from Antarctica and Greenland. As snow fell and compacted into ice over hundreds of thousands of years, tiny bubbles of air were trapped inside. Those bubbles are samples of Earth’s ancient atmosphere.
By measuring the CO₂ in those bubbles, scientists can see how CO₂ changed over time. The record shows that for thousands of years before the industrial era, CO₂ stayed around 280 ppm.
For the present, we measure CO₂ directly from the air. Since 1958, instruments at places like Mauna Loa Observatory in Hawaii have been recording CO₂ levels every day. This record is known as the Keeling Curve.
These measurements show CO₂ climbing from about 315 ppm in 1958 to over 420 ppm today. The ice cores and modern measurements fit together like two pieces of the same story: stable CO₂ in the past, rapid rise in the last century.
CO₂ is a greenhouse gas. That means it helps trap heat in Earth’s thermal blanket. When we add more CO₂ to the atmosphere, we thicken that blanket.
More CO₂ → thicker blanket → more trapped heat → a warmer planet.
CO₂ is especially powerful because it stays in the atmosphere for a long time — often hundreds of years. That means the CO₂ we emit today will keep influencing the climate for generations.
CO₂ keeps rising because we are adding it to the atmosphere faster than Earth can remove it. Oceans, forests, and soils do absorb some CO₂, but they can’t keep up with the amount we’re emitting.
As long as emissions stay high, CO₂ ppm will continue to climb, the thermal blanket will keep getting thicker, and the planet will keep warming.
Higher CO₂ ppm is not just a scientific detail. It shows up in everyday life as:
As CO₂ ppm climbs, these impacts become more frequent and more severe. That’s why scientists are concerned about letting CO₂ rise much further without major changes in how we produce energy, grow food, and use land.
CO₂ ppm is one of the clearest signals of how much we are thickening Earth’s thermal blanket. Before industrial times, that blanket was thinner — about 280 ppm. Today, it’s over 420 ppm and rising.
The more CO₂ we add, the thicker the blanket gets, and the more heat is trapped. That has consequences for every part of life on Earth. Understanding CO₂ ppm isn’t about memorizing a number. It’s about recognizing what that number tells us: how much we’ve changed the atmosphere, and how important it is to change course.