Carbon dioxide

Also called: CO2 · Topic: Atmosphere: Composition, Structure and Elements of Weather · NCERT: Class 11, Ch 7 "Composition and Structure of Atmosphere"; Class 11, Ch 8 "Solar Radiation, Heat Balance and Temperature"; Class 11, Ch 11 "World Climate and Climate Change"

Meaning

Carbon dioxide (CO2) is a trace gas in the air, about 0.04% of dry air by volume. It lets incoming sunlight pass through but absorbs the heat the Earth gives off, which keeps the lower air warm. NCERT calls it "meteorologically very important" because, though its share is tiny, it drives the greenhouse effect (gases in the air trapping the Earth's outgoing heat, like glass traps heat in a greenhouse). Its level is rising fast because we burn fossil fuels. It went from about 280 ppm before industry to 427.35 ppm in 2025 at Mauna Loa [3].

Explanation

What CO2 is and where it comes from

• CO2 is a gas made of one carbon atom and two oxygen atoms.
• Sources (add CO2 to the air): breathing, burning and decay.
• Sinks (take CO2 out of the air):
• Plants use it in photosynthesis (the way plants make food from sunlight, water and CO2).

• Oceans absorb it.

• Place in the air: dry air is nitrogen (about 78.08%), then oxygen (20.95%), then argon (0.93%), then CO2 (about 0.04%).

• Height limit: CO2 and water vapour are found only up to 90 km. So the greenhouse effect happens in the lower atmosphere.

How CO2 warms the Earth (the greenhouse effect)

• Step 1: sunlight comes in.
• Incoming sunlight is short-wave radiation.

• CO2 is transparent to it, so it lets it through. Sunlight reaches the ground and heats it.

• Step 2: the Earth sends heat out.

• The warm ground gives off heat as long-wave radiation. This is also called terrestrial radiation (heat sent out by the Earth).

• Step 3: CO2 traps the heat.

• CO2 is opaque to long-wave heat, so it does not let it through.

• It absorbs this heat and sends part of it back down.

• Result: the lower air stays warmer.

• Without such gases, nights would be freezing.
• The gap between day and night temperatures would also be very large.

Why CO2 keeps rising

• Main cause: burning fossil fuels (coal, oil, natural gas) adds CO2 faster than plants and oceans can absorb it.
• Unit: CO2 is measured in ppm (parts per million). This is the number of CO2 molecules in every one million molecules of air.
• The rise at Mauna Loa (annual means):
• About 280 ppm before the Industrial Revolution (before about 1750).
• 315.98 ppm in 1959 [3].
• 401.01 ppm in 2015, the first full year above 400 ppm [3].
• 424.61 ppm in 2024 and 427.35 ppm in 2025 [3].

• The monthly mean was 427.55 ppm in August 2026, up from 425.48 ppm in August 2025 [2].

• The record jump of 2024 (WMO Greenhouse Gas Bulletin No. 21, October 2025):

• Global average surface CO2 was 423.9 ppm in 2024, a record high [4].
• It rose by 3.5 ppm from 2023 to 2024. This was the largest yearly jump since modern measurements began in 1957 [4].
• Causes of the jump:
  • Human emissions continued.
  • There were more wildfires.
  • Carbon sinks (land and oceans that absorb CO2) became weaker. The 2024 El Niño and record heat cut how much CO2 they could take up [4].
• CO2 is now about 52% above pre-industrial levels [4].
• CO2 stays in the air for hundreds of years, so today's emissions will affect the climate for centuries [4].

The water vapour feedback: why small CO2 changes matter

• A positive feedback is a change that pushes itself further. Here is how it works:
• CO2 warms the air.
• For every 1 °C of warming, air can hold about 7% more water vapour [5].

• Water vapour is itself a greenhouse gas, so more of it traps more heat.

• Result: this feedback more than doubles the warming that CO2 alone would cause [5].

Where it occurs

• In the atmosphere everywhere, from the surface up to 90 km. Beyond that height it is absent.
• Mauna Loa (Hawaii, USA):
• C. David Keeling of the Scripps Institution of Oceanography began direct CO2 readings here in March 1958.
• NOAA began its own parallel readings in May 1974 [2].

• This is the world's longest unbroken record of CO2 measured directly in the air.

• Global network (WMO): the World Meteorological Organization combines readings from many sites into a global average. It gave 423.9 ppm for 2024 [4].

• Mauna Loa and WMO give slightly different numbers. Mauna Loa is one site and WMO is a global average. In an answer, always name the source and the year.
• Sinks around the world: forests and other plants, which take in CO2 through photosynthesis, and the oceans. Both took in less CO2 during the hot El Niño year of 2024 [4].
• India: India has a net-zero target for 2070. This means cutting emissions so that what India adds to the air is balanced by what is removed.

Don't confuse with

• Water vapour: this is the most abundant greenhouse gas, not CO2. It causes about half of the greenhouse effect [5]. It is also a variable gas (up to 4% in the humid tropics, under 1% in deserts). CO2 is a permanent gas with a nearly fixed share of about 0.04%.
• Argon: argon (0.93%) is the third most common gas in dry air. CO2 (about 0.04%) is fourth.
• Ozone (O3): ozone also has a big effect for a trace gas, but its main job is to filter UV rays, mostly between 10 and 50 km. CO2's main job is to trap long-wave heat in the lower air, below 90 km.
• Methane: methane is measured in ppb (parts per billion), not ppm. It was 1,942 ppb in 2024, which is 166% above pre-industrial levels, while CO2 is about 52% above [4]. So methane has risen more in percentage terms, but CO2 is far larger in amount.

Prelims Hooks

• CO2 is transparent to incoming short-wave solar radiation and opaque to outgoing long-wave terrestrial radiation. This causes the greenhouse effect.
• Order in dry air: nitrogen (78.08%) > oxygen (20.95%) > argon (0.93%) > CO2 (about 0.04%). Trap: argon, not CO2, is third.
• CO2 and water vapour are found only up to 90 km. Oxygen is almost nil by 120 km.
• Mauna Loa record: started by Keeling in 1958. The annual mean first went above 400 ppm in 2015 and reached 427.35 ppm in 2025 [2][3].
• WMO: global CO2 was 423.9 ppm in 2024, and the 3.5 ppm rise from 2023 to 2024 was the largest on record [4].
• Trap: "CO2 is the most abundant greenhouse gas" is wrong. The answer is water vapour [5].

Mains Points

• A small gas with a big effect (GS-I climatology):
• CO2 is only about 0.04% of the air, yet it controls the Earth's heat balance by trapping long-wave radiation.

• The water vapour feedback more than doubles the warming from CO2 [5]. So even small rises in CO2 matter a lot.

• Rising CO2 and why it is getting faster (GS-III climate change):

• CO2 rose from about 280 ppm before industry to 427.35 ppm in 2025 (Mauna Loa) [3].
• The 2024 jump of 3.5 ppm came from fossil fuel emissions, wildfires and weaker land and ocean sinks during El Niño [4].

• Problem: a warming climate weakens the sinks, so more CO2 stays in the air and warming speeds up.

• Policy response (GS-III environment):

• CO2 stays in the air for hundreds of years [4], so today's emissions will warm the planet for centuries.
• This is why deep emission cuts are needed, as in India's net-zero target for 2070.
• Protecting natural sinks such as forests and oceans also helps.
• More water vapour in a warmer climate means more extreme rainfall events, which links CO2 to disaster management.

Related concepts

• Atmosphere
• Composition of the atmosphere
• Water vapour

Read more

• The atmosphere and its gases