The greenhouse effect
Sunlight is mostly visible light, which passes easily through the atmosphere and warms Earth’s surface. The warm surface then gives off energy as infrared radiation (heat). Gases such as water vapour, carbon dioxide and methane absorb some of this infrared radiation and re-emit it in all directions, including back down. This natural greenhouse effect keeps Earth about 33 °C warmer than it would otherwise be — essential for life. Adding more greenhouse gases strengthens the effect.
Rising carbon dioxide
Bubbles of ancient air trapped in Antarctic ice show that CO₂ stayed below about 300 ppm (parts per million) for at least 800,000 years. Direct measurements at Mauna Loa, Hawaii, began in 1958 at about 315 ppm; by 2024 the yearly average had passed 420 ppm. The rise comes mainly from burning fossil fuels, with a contribution from deforestation.
Earth’s energy balance
Earth’s temperature settles where incoming solar energy equals outgoing infrared energy. Extra CO₂ reduces outgoing energy, creating an imbalance called radiative forcing, measured in watts per square metre. Earth then warms until it radiates enough extra heat to restore the balance. In the diagram, the number of heat arrows trapped and sent back down grows with the forcing.
Why warming lags behind
The oceans can store enormous amounts of heat. The upper ocean warms over years to decades, while the deep ocean takes centuries. As a result, surface warming lags behind rising CO₂, and some warming is “in the pipeline” even if concentrations stop rising. Try the “frozen at today’s level” scenario and watch temperature keep climbing for decades before levelling off.
Climate sensitivity
Equilibrium climate sensitivity is the eventual warming for a doubling of CO₂. It includes feedbacks: warming adds water vapour (itself a greenhouse gas) and melts reflective ice, which amplify the initial effect. The IPCC’s Sixth Assessment Report gives a best estimate of 3 °C with a likely range of 2.5–4 °C.
The model’s equations
The simulator uses a two-layer energy-balance model, a standard teaching and research tool:
CO₂ forcing: F = 5.35 · ln(CO₂ / CO₂ in 1850) W/m²Surface & upper ocean: C · dT/dt = F − λ·T − γ·(T − T_deep)Deep ocean: C_deep · dT_deep/dt = γ·(T − T_deep)λ is the climate feedback parameter (3.71 W/m² divided by the climate sensitivity), C and C_deep are heat capacities, and γ controls heat flow into the deep ocean. Eventual warming is F / λ.
Key terms
- ppm (parts per million)
- Number of CO₂ molecules per million molecules of dry air.
- Radiative forcing
- A change in Earth’s energy balance, in W/m², caused by a factor such as greenhouse gases or aerosols.
- Feedback
- A process that amplifies (positive) or reduces (negative) an initial change, such as melting ice exposing darker ocean.
- Net zero
- A state where human greenhouse gas emissions are balanced by removals from the atmosphere.
Impacts and responses
- Impacts: more intense heatwaves and heavy rainfall, rising sea levels from melting ice and warming water, shrinking Arctic sea ice, and stress on ecosystems such as coral reefs.
- Mitigation: cutting emissions by switching to low-carbon energy, improving efficiency, protecting forests and removing CO₂.
- Adaptation: preparing for changes already under way, such as stronger flood defences and heat-resilient cities.
- Policy: the 2015 Paris Agreement aims to keep warming well below 2 °C and to pursue efforts to limit it to 1.5 °C.
Common misconceptions
- “Climate change is caused by the ozone hole.” They are separate problems; ozone loss affects ultraviolet radiation.
- “If emissions stop, temperatures will drop right away.” CO₂ stays in the air for centuries, so temperatures stay high for a long time.
- “A cold winter disproves global warming.” Weather varies day to day; climate is the long-term average.
- “CO₂ is a tiny fraction of the air, so it can’t matter.” Small amounts of strong absorbers have large effects, just as a drop of ink colours a glass of water.
What the model simplifies
This is a teaching model, not a forecast. It includes only CO₂; in reality methane and other gases add warming while air-pollution particles (aerosols) offset part of it — historically these have roughly cancelled. It treats the whole planet as a single average and ignores natural variability such as El Niño, volcanic eruptions and the carbon cycle’s response to warming. The future scenarios are simplified paths, not official IPCC scenarios. Real projections come from comprehensive climate models; see the IPCC report for authoritative numbers.
Key takeaways
- Greenhouse gases trap outgoing infrared heat, warming the surface.
- CO₂ has risen from about 280 ppm to over 420 ppm, mainly from fossil fuels.
- Oceans slow the response, so some warming continues even after CO₂ stops rising.
- The amount of future warming depends mainly on future emissions and on climate sensitivity.