Natural processes can and do raise carbon dioxide (CO₂) levels in the atmosphere as part of the carbon cycle; the big difference today is that human emissions now sit on top of these natural swings and dominate the long‑term increase.

Key natural CO₂ sources

  • Volcanic outgassing
    Magma contains dissolved carbon, which is released as CO₂ during eruptions and from continuous degassing at vents and mid‑ocean ridges.

Over geologic time, this volcanic CO₂ is a major input to the atmosphere–ocean system and helps set the long‑term background level of CO₂.

  • Respiration of organisms
    Plants, animals, and microbes break down organic matter for energy, releasing CO₂ in the process of respiration.

In ecosystems like tropical forests and soils rich in life, this constant “breathing out” of CO₂ can be very large, especially at night or in warm conditions.

  • Decomposition and decay
    When dead plants and animals are broken down by microbes, the carbon in their tissues is converted back to CO₂.

Heat and drought speed this up, so during warm periods or events like El Niño, global soils and forests can emit more CO₂ than usual.

  • Natural fires
    Wildfires that start from lightning or other natural causes rapidly oxidize biomass and release the carbon as CO₂.

In fire‑adapted ecosystems (savannas, some forests), this natural burning is a recurring source of atmospheric CO₂ that can spike in dry years.

  • Ocean outgassing
    The surface ocean constantly exchanges CO₂ with the air; when water is warmer or currents bring CO₂‑rich deep water to the surface, the ocean can release CO₂ to the atmosphere.

Over ice‑age cycles, changes in ocean temperature and circulation have repeatedly shifted the balance between ocean uptake and release, contributing to higher CO₂ in warmer interglacial periods.

Natural feedbacks that boost CO₂

  • Climate–carbon feedbacks
    Warming caused by orbital changes or other factors can dry out land, weaken plant growth, and speed up decomposition, all of which reduce CO₂ uptake and increase CO₂ release.

These feedbacks act like an amplifier: once warming starts, higher natural CO₂ emissions can make the warming stronger and help lock in higher atmospheric CO₂ during warm periods.

  • Glacial–interglacial cycles
    Ice‑core records show that during warm interglacial periods, natural CO₂ levels were higher, while cold glacial periods had lower CO₂, reflecting combined changes in oceans, vegetation, and soils.

In other words, natural climate shifts have always nudged the carbon cycle, causing CO₂ to rise and fall over tens of thousands of years.

How this differs from today

  • In the past, natural sources like volcanoes, respiration, decay, and ocean outgassing were balanced on average by natural “sinks” such as photosynthesis and ocean absorption, keeping CO₂ within a relatively stable range.
  • Today, human activities—especially fossil fuel burning and deforestation—add CO₂ much faster than natural sinks can remove it, so atmospheric CO₂ is rising 100–200 times faster than at the end of the last ice age, far beyond typical natural variations.

In short, nature has many ways to raise CO₂, but on human time‑scales those ups and downs used to be slow and roughly balanced; the sharp, ongoing rise now is driven mainly by human‑caused emissions layered on top of those natural processes.