Carbon dioxide in the atmosphere of Earth

Based on Wikipedia: Carbon dioxide in the atmosphere of Earth

The air you're breathing right now contains more carbon dioxide than any human being has ever inhaled in the entire 300,000-year history of our species. In fact, you'd have to go back 14 million years—long before humans existed, before our ape ancestors had even diverged from the lineage that would become orangutans—to find atmospheric conditions like today's.

That's not hyperbole. It's measurement.

The Number That Changed Everything

In 2024, the concentration of carbon dioxide in Earth's atmosphere hit 427 parts per million. That sounds abstract, so let me translate it: for every million molecules floating around you, 427 of them are carbon dioxide. Before the Industrial Revolution—that transformative period starting in the mid-1700s when humans began burning coal at scale—that number held steady at around 280 parts per million. It had stayed there, remarkably stable, for ten thousand years.

We've increased it by 50 percent in roughly 250 years.

To put that in concrete terms: the atmosphere currently holds about 3,341 gigatonnes of carbon dioxide. A gigatonne is a billion metric tons. Since 1850, humans have pumped roughly 2,650 gigatonnes of carbon dioxide into the sky, mostly by digging up ancient carbon in the form of coal, oil, and natural gas, then setting it on fire. About 1,050 gigatonnes of that remains overhead—the rest has been absorbed by the oceans and by plants, creating its own cascade of consequences we'll get to shortly.

Why Carbon Dioxide Matters: The Blanket Effect

Carbon dioxide is what scientists call a greenhouse gas, but that term can be misleading. Your backyard greenhouse works by trapping warm air inside glass walls. The atmospheric greenhouse effect works through an entirely different mechanism involving the invisible dance of light and molecules.

Here's what actually happens. The sun bombards Earth with light, most of it in the visible spectrum—the wavelengths our eyes evolved to detect. This light passes through the atmosphere largely unimpeded, strikes the ground, and warms it. The warmed surface then radiates energy back toward space, but at different wavelengths. Because the ground is much cooler than the sun, it emits infrared radiation—light our eyes can't see, but that our skin can feel as heat.

Carbon dioxide molecules have a peculiar property: they're transparent to visible light but absorb infrared radiation at specific wavelengths. Picture it like a bouncer at a club who lets visible light walk right in but stops infrared light at the door. When a carbon dioxide molecule absorbs an infrared photon, it vibrates—either through an antisymmetric stretching motion or a bending motion—then re-emits the energy in a random direction. Some of that re-emitted energy heads back toward Earth's surface.

The result? Energy that was trying to escape to space gets redirected back down, warming the lower atmosphere while cooling the upper atmosphere. More carbon dioxide means more infrared light gets intercepted. More interception means more warming.

This isn't speculation or theory. It's basic physics, first described by the Swedish chemist Svante Arrhenius in 1896. He calculated, with remarkable accuracy for his era, that doubling atmospheric carbon dioxide would raise global temperatures by several degrees Celsius.

Earth's Deep History of Carbon

To understand where we are, it helps to know where we've been. Earth's atmosphere hasn't always looked like it does today—not remotely.

About 500 million years ago, during the Cambrian period, carbon dioxide concentrations reached roughly 4,000 parts per million—nearly ten times current levels. The planet was warmer then, with no ice caps at the poles. Life was confined entirely to the oceans, and the first complex animals were just emerging: trilobites, early mollusks, the ancestors of everything that would eventually crawl onto land.

Concentrations spiked again around 400 million years ago during the Devonian period, hitting approximately 2,000 parts per million. Another peak came during the Triassic, between 220 and 200 million years ago—the era when dinosaurs first appeared. In contrast, during the glacial periods of the last two million years, carbon dioxide dropped as low as 180 parts per million, coinciding with massive ice sheets covering much of North America and Europe.

The pattern is clear: more carbon dioxide correlates with warmer climates. Less carbon dioxide correlates with ice ages. This isn't coincidence. It's causation, running in both directions through feedback loops we're still working to fully understand.

But here's what makes our current moment unprecedented: the speed. Previous swings in carbon dioxide took thousands to millions of years, driven by volcanic activity, the slow weathering of rocks, the burial of organic matter that would become fossil fuels. We're accomplishing comparable changes in decades. The current rate of increase—about 2 parts per million per year and accelerating—hasn't been seen in at least 800,000 years of ice core records, and probably far longer.

The Keeling Curve: Watching the Planet Breathe

We know all this because of a single meticulous scientist and a remote volcano in Hawaii.

In the 1950s, a geochemist named Charles David Keeling developed techniques for measuring atmospheric carbon dioxide with unprecedented precision. In 1958, he began continuous measurements at the Mauna Loa Observatory, perched 11,000 feet above sea level on Hawaii's Big Island, far from local pollution sources. He kept measuring until his death in 2005, and the observations continue today under the National Oceanic and Atmospheric Administration (NOAA).

The resulting graph—known as the Keeling Curve—is one of the most important scientific datasets ever collected. It shows an unmistakable upward trend, from around 315 parts per million when measurements began to over 420 parts per million today. But within that relentless rise, something beautiful emerges: a yearly oscillation, like the planet breathing.

Every spring and summer in the Northern Hemisphere, the curve dips by about 6 or 7 parts per million as plants across North America, Europe, and Asia burst into leaf and pull carbon dioxide from the air through photosynthesis. Every autumn and winter, as those plants go dormant and their fallen leaves decay, the curve rises again by 8 or 9 parts per million. The Northern Hemisphere dominates this rhythm because it contains far more land—and therefore far more vegetation—than the Southern Hemisphere at comparable latitudes.

It's a kind of planetary pulse, visible only because we bothered to look.

Following the Carbon: How We Know It's Us

Some skeptics have asked: how do we know the rising carbon dioxide comes from human activity? Maybe it's volcanic. Maybe it's some natural cycle we don't understand.

Multiple lines of evidence converge on the same answer.

First, there's simple accounting. We know how much fossil fuel humanity burns each year—currently about 9 billion tonnes of carbon, releasing over 30 gigatonnes of carbon dioxide annually. We know how much carbon dioxide is accumulating in the atmosphere. The numbers match, with the difference accounted for by absorption into oceans and vegetation.

Second, there's isotopic fingerprinting. Carbon comes in different isotopes—atoms with the same number of protons but different numbers of neutrons. Plants preferentially absorb the lighter isotope, carbon-12, over the heavier carbon-13. Fossil fuels, being ancient plant matter, are enriched in carbon-12. As we burn them, the atmosphere's ratio of carbon-12 to carbon-13 shifts in exactly the pattern you'd expect if the new carbon is coming from fossil sources. It is.

Third, atmospheric oxygen is declining. Burning fossil fuels requires oxygen—one molecule of oxygen for each atom of carbon. As carbon dioxide rises, oxygen falls, at the precisely predicted ratio. If the carbon dioxide were coming from volcanic sources, this wouldn't happen.

Fourth, there's geography. Carbon dioxide concentrations are higher in the Northern Hemisphere, where most people live and most emissions originate. This gradient has grown more pronounced as emissions have increased—exactly what you'd expect from localized human sources, not diffuse natural ones.

The Ocean's Burden

Not all the carbon dioxide we emit stays in the atmosphere. Since 1850, the oceans have absorbed about 26 percent of our total emissions. On one level, this has been fortunate—it's slowed atmospheric warming. On another level, it's creating a crisis of its own.

When carbon dioxide dissolves in seawater, it forms carbonic acid, which dissociates into hydrogen ions and bicarbonate. The hydrogen ions make the water more acidic. Since the Industrial Revolution, ocean acidity has increased by about 30 percent. This might not sound dramatic until you consider what it means for the creatures that live there.

Many marine organisms—corals, oysters, mussels, certain plankton—build their shells and skeletons from calcium carbonate. In more acidic water, calcium carbonate dissolves more readily, making it harder for these organisms to build and maintain their structures. Some research suggests that ocean acidification could disrupt the base of marine food webs, with cascading effects all the way up to the fish we eat.

The ocean also redistributes carbon dioxide through its global circulation. A study published in early 2025 found something concerning: the Antarctic Circumpolar Current—the massive ring of water flowing around Antarctica—is speeding up. Faster currents are causing more upwelling of deep water, which releases dissolved carbon dioxide back into the atmosphere. This represents a positive feedback loop: warming accelerates ocean circulation, which releases more carbon dioxide, which causes more warming.

The Persistence Problem

Here's perhaps the most sobering fact about carbon dioxide: it doesn't go away quickly.

Scientists have developed models to track how long emitted carbon dioxide persists in the atmosphere. The Bern model, developed by Fortunat Joos and colleagues, provides one estimate. According to their calculations, roughly 22 percent of carbon dioxide released into the atmosphere remains there essentially forever—or at least for timescales far beyond human planning horizons. The rest gets absorbed by oceans and vegetation, but even that absorption takes decades to centuries.

What this means in practice is that carbon dioxide emissions are largely irreversible on any timescale that matters for human civilization. Even if we stopped all emissions tomorrow, concentrations would remain elevated for thousands of years. The warming already "baked in" by current concentrations will continue to unfold for centuries.

Between 20 and 35 percent of all the fossil carbon we've already transferred to the atmosphere will persist as elevated carbon dioxide for many thousands of years. We're not just affecting our children's climate. We're affecting our descendants a hundred generations hence.

Effects Beyond Temperature

Rising carbon dioxide does more than warm the planet. Some effects are obvious; others are surprisingly subtle.

Plants grow faster when carbon dioxide is more abundant—a phenomenon called the carbon dioxide fertilization effect. In some contexts, this is beneficial: crop yields have increased partly because of elevated carbon dioxide. But the effect isn't uniformly positive. Faster-growing plants often produce less nutritious food, with lower concentrations of protein and essential minerals. Weeds benefit from fertilization just as crops do, potentially making agriculture harder, not easier.

Higher up in the atmosphere, rising carbon dioxide is causing the stratosphere to contract. Since 1980, the stratospheric layer has shrunk by about 400 meters. The upper atmosphere cools as the lower atmosphere warms—remember, carbon dioxide traps heat near the surface—and cooler air is denser, hence more compact. This contraction could affect satellite operations, GPS accuracy, and radio communications in ways we're only beginning to understand.

And then there's the simple fact of direct radiative forcing—the extra energy carbon dioxide is trapping near Earth's surface. As of 2013, increased carbon dioxide was responsible for about 1.82 watts per square meter of additional warming, out of a total change of 2.63 watts per square meter from all greenhouse gases. That's roughly 70 percent of the total human-caused greenhouse effect coming from this one molecule.

The Fast Cycle and the Slow Cycle

To truly understand carbon dioxide's role in Earth's systems, you need to grasp the two carbon cycles that govern our planet.

The fast carbon cycle moves carbon between the atmosphere and living things on timescales of days to centuries. Plants absorb carbon dioxide through photosynthesis, incorporating carbon into their tissues. Animals eat plants, burning that carbon for energy and exhaling carbon dioxide. When plants and animals die, decomposers break down their remains, releasing carbon back to the atmosphere. Forest fires accelerate this release. New growth reabsorbs it.

The numbers are staggering. Every year, the decay of organic material in forests and grasslands releases about 436 gigatonnes of carbon dioxide. Meanwhile, new plant growth absorbs about 451 gigatonnes. Under natural conditions, these flows roughly balance.

The slow carbon cycle operates over millions of years. Volcanic eruptions release carbon dioxide stored deep in the Earth—but only about 130 to 230 megatonnes per year, a tiny fraction of human emissions. Meanwhile, the weathering of rocks slowly absorbs carbon dioxide, and the burial of organic matter locks carbon away in sediments that may eventually become fossil fuels. Over geological time, these processes have regulated atmospheric carbon dioxide, keeping it within bounds compatible with liquid water and life.

What we've done is reach deep into the slow cycle—extracting carbon that took millions of years to accumulate—and inject it into the fast cycle at unprecedented rates. The natural processes that regulate atmospheric carbon dioxide simply cannot keep pace.

The Inequality of Emissions

Not everyone contributes equally to this problem. The International Energy Agency estimates that in 2021, the top 1 percent of global emitters each had carbon footprints exceeding 50 tonnes of carbon dioxide—more than a thousand times greater than the footprints of the bottom 1 percent. The global average for energy-related emissions is about 4.7 tonnes per person, but that average masks enormous disparities.

The United States, Europe, and other wealthy regions have contributed the majority of historical emissions, having industrialized earliest and most intensively. China is now the largest annual emitter, but on a per-capita basis, Americans still produce far more carbon dioxide than Chinese citizens. The nations most vulnerable to climate change—low-lying island states, drought-prone regions of Africa—have contributed least to the problem.

This disparity raises profound questions of justice. The carbon already in the atmosphere will affect everyone, but it was put there primarily by the richest societies on Earth. How to allocate responsibility for addressing the consequences remains one of the defining political challenges of our era.

Where We Stand

On May 10, 2013, daily average carbon dioxide concentration at Mauna Loa exceeded 400 parts per million for the first time in the 55-year measurement record—and, as researchers noted at the time, probably for the first time in more than 3 million years of Earth history. By 2018, concentrations had reached 410 parts per million. By October 2023, seasonally adjusted concentrations hit 422.17 parts per million.

The trend shows no sign of bending. Annual emissions continue at about 42 gigatonnes of carbon dioxide per year. Despite international agreements and growing renewable energy capacity, the atmospheric accumulation continues.

We've built a civilization on fossil carbon. Unwinding that dependency will take decades, and even if we succeed, the carbon dioxide we've already emitted will linger for millennia. We're running an uncontrolled experiment on the only planet we have, with results that will unfold long after everyone alive today is gone.

The measurements continue at Mauna Loa, as they have since 1958. Every month, NOAA publishes updated figures. The Keeling Curve keeps climbing, a jagged line recording our impact on Earth's atmosphere, one part per million at a time.