Carbon dioxide in the atmosphere of Earth

• 1. Introduction

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  Carbon dioxide in the atmosphere of Earth sits at 430 parts per million as of 2024, a number that sounds modest until you realize it represents a mass of 3,364 gigatonnes of gas wrapped around the planet. That figure is 54% higher than it was at the start of the Industrial Revolution. For the ten thousand years before the mid-18th century, concentrations held steady near 280 parts per million. Something broke that long equilibrium.

  The questions worth asking run in two directions: backward, into deep time, where concentrations once touched 4,000 parts per million during the Cambrian period some 500 million years ago; and forward, into a future where, according to one scientific estimate, between 20 and 35% of fossil carbon released today will persist in the atmosphere for many thousands of years. This documentary moves through both directions. It asks how a trace gas that makes up less than a tenth of a percent of the air came to govern the temperature of a planet, how scientists learned to read its levels in ancient ice, and what happens when the natural system that once kept it in balance is outpaced by human activity.

• 2. How CO2 Warms the Planet

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  Svante Arrhenius published the idea that more atmospheric carbon dioxide would raise ground temperatures in 1896, decades before anyone could measure the effect directly. His insight rested on a physical fact: carbon dioxide absorbs and re-emits infrared radiation at two specific wavelengths, 4.26 micrometres and 14.99 micrometres. Those frequencies happen to overlap with the range in which Earth's surface radiates energy back toward space.

  Light from the Sun arrives mostly in the visible spectrum, because the Sun is extremely hot. Earth, being far cooler, radiates back in the infrared. Carbon dioxide intercepts that outgoing infrared, trapping energy near the surface and warming the lower atmosphere. Less of that energy reaches the upper atmosphere, which is cooler as a result.

  Water vapor is responsible for the largest share of the total greenhouse effect, somewhere between 36 and 70 percent. But water vapor's role depends on temperature, making it an amplifier rather than a direct driver. Carbon dioxide acts as the more direct lever on the climate since the pre-industrial era. By 2013, the increase in carbon dioxide alone was estimated to account for 1.82 watts per square metre of the 2.63 watts per square metre change in radiative forcing on Earth, roughly 70% of the measured total shift. The increased forcing then drives further changes in Earth's energy balance over the longer term.

• 3. The Carbon Cycle

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  Natural processes release approximately 436 gigatonnes of carbon dioxide every year through the decay of organic material in forests, grasslands, and other land vegetation, including forest fires. New plant growth absorbs 451 gigatonnes in return, creating a slight natural surplus on the sink side. Modern volcanic activity contributes only 130 to 230 megatonnes per year, a modest addition compared to the scale of biological cycling.

  These flows belong to what scientists call the fast carbon cycle: the exchange between the atmosphere and living things. The slow carbon cycle moves carbon through oceans, soil, rocks, and volcanism over timescales of millions of years. Both cycles are linked through atmospheric carbon dioxide.

  From the pre-industrial era to around 1940, the terrestrial biosphere was a net source of carbon dioxide, driven largely by land-use changes. After that, the biosphere flipped and became a net sink as fossil carbon emissions grew. The model developed by Fortunat Joos and colleagues in Bern offers a mathematical way of tracking how long a given pulse of carbon dioxide persists in the air. Their calculations suggest that 21.7% of carbon dioxide released into the atmosphere stays there indefinitely under current conditions, absent artificial sequestration.

  The oceans absorb a significant share too. From 1850 to 2022, the ocean took up 26% of total human emissions. That absorption produces bicarbonate ions through reactions between seawater, rock, and carbon dioxide. A 2025 study published in Science Advances found that faster flow of the Antarctic Circumpolar Current at higher latitudes causes upwelling of deep waters around Antarctica, and that this process likely increases atmospheric carbon dioxide, forming a positive feedback loop for future warming.

• 4. Measuring the Air, Past and Present

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  Dave Keeling made the first reproducibly accurate measurements of atmospheric carbon dioxide from flask samples at Caltech in the 1950s. Continuous measurements at Mauna Loa Observatory have run without interruption since 1958. The daily average concentration at that site first exceeded 400 parts per million on the 10th of May 2013, a threshold that had already been crossed in the Arctic in June 2012. Data from 2013 framed that crossing as the first time in 55 years of direct measurement, and probably the first time in more than 3 million years of Earth history, that concentrations had been so high.

  Today the figures are published monthly by the National Oceanic and Atmospheric Administration. Each part per million of carbon dioxide in the atmosphere represents approximately 2.13 gigatonnes of carbon, or 7.82 gigatonnes of carbon dioxide.

  For periods before direct measurement, the most reliable record comes from air bubbles trapped in Antarctic and Greenland ice. The longest ice core record, drawn from East Antarctica, reaches back 800,000 years. During that span, carbon dioxide varied between 180 and 210 parts per million during ice ages and rose to 280-300 parts per million during warmer interglacial periods. Because the pores in ice close slowly to form bubbles deep within the firn, each measurement represents an average over up to a few centuries rather than a single year. For periods older than 800,000 years, scientists turn to proxy methods: boron and carbon isotope ratios in marine sediments, the number of stomata on fossil plant leaves, and a breakdown product of chlorophyll called phytane, which provides a continuous record with the ability to bridge gaps of more than 500 million years.

• 5. Deep Time, Extreme Swings

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  About 635 million years ago, the planet emerged from an 82-million-year period of intermittent widespread glaciation that extended all the way to the equator, a state researchers call Snowball Earth. The escape from that frozen world came when carbon dioxide released by volcanic outgassing built up to roughly 12%, or about 120,000 parts per million. The resulting greenhouse conditions were extreme: rapid deglaciation followed, along with carbonate deposition as limestone at rates possibly as fast as 40 centimetres per year.

  By the Cambrian period, roughly 500 million years ago, concentrations had fallen to around 4,000 parts per million but were still far above anything in the modern record. Concentrations during the early Palaeozoic era reached ten to fifteen times current levels. The spread of land plants during the late Devonian period, about 400 million years ago, is thought to have drawn concentrations down sharply, and plant life has since acted as both a source and a sink that provides stabilizing feedbacks.

  Around 34 million years ago, at the time of the Eocene-Oligocene extinction event, concentrations stood near 760 parts per million. The Antarctic ice sheet was taking its current form around that time, and decreasing carbon dioxide, with a tipping point estimated at 600 parts per million, was the primary driver of Antarctic glaciation. By about 20 million years ago, geochemical evidence suggests concentrations had fallen below 300 parts per million. Estimates from 2023 indicate that current concentrations may be the highest in 14 million years, though the IPCC Sixth Assessment Report placed similar levels in the mid-Pliocene warm period, roughly 3 to 3.3 million years ago.

• 6. The Human Disruption

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  From 1751 to 1900, the burning of fossil fuels released about 12 gigatonnes of carbon into the atmosphere. From 1901 to 2013, the figure was approximately 380 gigatonnes of carbon. By 2019, human extraction and burning of fossil carbon was releasing over 30 gigatonnes of carbon dioxide, equivalent to 9 billion tonnes of carbon, every year. In 2021, total human emissions since 1850 were estimated at 2,650 gigatonnes of carbon dioxide, with annual emissions running at 42 gigatonnes per year. Of that accumulated total, about 1,050 gigatonnes remained in the atmosphere after absorption by oceans and land.

  In 2010, fossil fuel combustion and cement production together released 9.14 gigatonnes of carbon worldwide, up from 6.15 gigatonnes in 1990. Land-use change contributed 0.87 gigatonnes of carbon in 2010, down from 1.45 gigatonnes in 1990. The burning of coal, petroleum, and natural gas is the leading cause; deforestation is second.

  The International Energy Agency calculated that the top 1% of emitters globally each had carbon footprints exceeding 50 tonnes of carbon dioxide in 2021, more than 1,000 times the footprint of the bottom 1%. The global average energy-related carbon footprint is around 4.7 tonnes per person.

  One consequence that receives less attention is the stratosphere's physical contraction. Carbon dioxide emissions have caused the stratosphere to contract by 400 metres since 1980, a shift with potential implications for satellite operations, GPS systems, and radio communications. Even if emissions were to stop entirely today, the climate system's response would be slow: the same ocean heat transfer that has so far moderated warming would continue to keep air temperatures elevated, and sea temperatures would keep rising, driving thermal expansion and sea-level rise.