Carbon Dioxide — Climate Words

Carbon Dioxide

Definition coming soon!

RESEARCH
Research by Micheala Chan

20 February 2026

Carbon dioxide is a naturally occurring, colourless gas produced by living organisms and natural processes, and also by human activities like burning fossil fuels. It circulates through the carbon cycle and acts as a greenhouse gas that warms the planet. High concentrations in enclosed spaces can cause asphyxiation, and contact with dry ice can lead to frostbite or burns.
Atmospheric carbon dioxide has risen 50% in under 200 years due to human activity, far exceeding natural post-ice age increases. Atmospheric carbon is tracked in parts per million at Mauna Loa Observatory, where long‑term measurements show both a steady upward trend and a seasonal “sawtooth” cycle driven by global patterns of plant growth and decay, with levels reaching 430 ppm in June 2025.
Carbon dioxide is the most significant greenhouse gas in terms of climate change. Humans emit around 40 billion tonnes each year. Once released, it persists in the atmosphere for centuries, locking in warming long after emissions stop. Many industrial sectors also depend on carbon-intensive processes that are difficult to decarbonise.
“Carbon” and “carbon dioxide” are often conflated, but carbon is a solid element found in all living things, while CO₂ is a gaseous compound whose mass is 3.67 times that of carbon. CO₂ appears in the carbon cycle only as one form of carbon and is highlighted in climate discussions because it is the most abundant human‑produced greenhouse gas, which can make it seem like the only driver of warming.
Other greenhouse gases are compared to CO2 using “CO2-equivalents” (CO2e) based on how much heat an equal amount of CO2 would be expected to trap over the next 100 years.(3)
Scientists measure greenhouse gases mainly through spectroscopy, analysing how air samples or satellite‑beamed light absorb infrared wavelengths to identify gas concentrations. CO₂ is monitored continuously at remote sites like Mauna Loa to avoid local contamination, and these modern measurements are compared with historical records such as ice cores to track atmospheric change over time.
Carbon dioxide removal (CDR) refers to taking CO2 out of the air. While it is increasingly recognised as a crucial part of achieving climate goals, current removal (about 2 GtCO2 per year) is tiny compared to annual emissions. Almost all CDR comes from land-based methods like reforestation and soil carbon, while newer technologies receive disproportionate attention despite being early-stage. Meeting the 1.5–2 °C pathways requires hundreds of gigatonnes of CDR this century, which is far beyond current national plans. Yet public awareness remains low even as research, patents, and commercial interest rapidly expand.
Carbon offsets are credits meant to represent emissions that have been avoided, reduced, or removed. They are used by companies and governments to compensate for their ongoing emissions. Many schemes, however, exaggerate their impact and allow organisations to continue with “business as usual.” Only a small share of projects deliver real emissions reductions. Common problems include uncertain baselines, non‑permanent carbon storage, lack of additionality, leakage, and overstated benefits in projects such as forest protection or clean cookstoves, leading to widespread concerns about greenwashing despite their growing use by companies and governments.
MIT researchers found that higher concentrations of carbon dioxide can weaken ice at the microscopic level, reducing its strength and making it more prone to fracturing. CO₂ molecules interfere with the bonds between water molecules in ice crystals, displacing them and making the structure more fragile. Combined with rising global temperatures, this mechanism could accelerate ice‑cap melt beyond current projections, though it is too early to quantify by how much. The study is based on simulations, and laboratory testing is needed to confirm whether the effect holds at larger scales.
Natural carbon sinks absorb more carbon than they release and currently take up about half of human‑produced CO₂. They can’t replace emissions cuts, but they help reduce climate risks. Oceans are the largest sink, forests store huge amounts of carbon but also release it through deforestation and wildfires, soils and certain rocks hold vast carbon stocks, fungi play a major but vulnerable role in carbon storage, and large herbivores can enhance natural carbon capture by reducing wildfire fuel and pushing carbon into soils.

1
UK Health Security Agency. “Carbon Dioxide: General Information.” GOV.UK, December 4, 2024.
2
NASA. “Carbon Dioxide Concentration.” NASA, n.d.
3
Chandler, David. “Greenhouse Gases.” MIT Climate Portal, July 25, 2025.
4
Energy Education. “C vs CO2”. Energyeducation.ca, 30 April, 2018.
5
Moseman, Andrew, and Jesse Kroll. “How Are Gases in the Atmosphere Analyzed and Measured?” MIT Climate Portal, October 18, 2021.
6
Smith, Steve, Jan Minx, Greg Nemet, and Oliver Geden. “Guest Post: The State of ‘Carbon Dioxide Removal’ in Seven Charts.” Carbon Brief, January 19, 2023.
7
Gabbatiss, Josh, Daisy Dunne, Aruna Chandrasekhar, Orla Dwyer, Molly Lempriere, Yanine Quiroz, Ayesha Tandon, and Giuliana Viglione. “In‑Depth Q&A: Can ‘Carbon Offsets’ Help to Tackle Climate Change?” Carbon Brief, September 25, 2023.
8
Qin, Zhao, and Markus J. Buehler. “Carbon Dioxide Enhances Fragility of Ice Crystals.” Journal of Physics D: Applied Physics 45, no. 44 (2012): 445302.
9
Broom, Douglas. “What Are the World’s Biggest Natural Carbon Sinks?” World Economic Forum, July 26, 2023.