Climate Sensitivity Calculator

Climate sensitivity — formally called Equilibrium Climate Sensitivity (ECS) — is a measure of how much the global average temperature is expected to rise if the concentration of CO₂ in the atmosphere doubles. The IPCC's best central estimate is approximately 3 °C of warming per doubling, though the likely range spans 2.5 °C to 4 °C based on current evidence.

The standard preindustrial baseline is 280 ppmv (parts per million by volume), representing atmospheric CO₂ around the year 1850 before large-scale industrialization. This is the reference point used by the IPCC and most climate scientists when measuring human-caused warming.

The relationship between CO₂ and temperature is logarithmic, not linear. The formula is: ΔT = ECS × log₂(CO₂ / baseline CO₂). This means each successive doubling of CO₂ produces the same fixed temperature increment defined by the ECS value, regardless of the starting concentration.

Climate sensitivity is uncertain because Earth's climate system involves complex feedback mechanisms — including water vapor, clouds, ice-albedo, and ocean heat uptake — that are difficult to fully quantify. Different models, observational datasets, and paleoclimate records give overlapping but not identical estimates, which is why scientists report a range rather than a single number.

As of 2024, atmospheric CO₂ has exceeded 420 ppmv, compared to the preindustrial level of 280 ppmv. This represents an increase of about 50%, or roughly 0.58 doublings, which corresponds to approximately 1.0–1.2 °C of observed warming — consistent with the sensitivity estimates used in this calculator.

A low sensitivity of 1.5 °C means the climate reacts less strongly to CO₂ increases, giving society more time and carbon budget to avoid dangerous warming thresholds. A high sensitivity of 4.5 °C means the same CO₂ emissions produce much larger warming, potentially crossing critical tipping points sooner. The difference between these scenarios has enormous implications for climate policy and mitigation targets.

No. Equilibrium Climate Sensitivity represents the long-term temperature change once the climate system fully adjusts to a CO₂ doubling, which can take centuries due to ocean thermal inertia. The Transient Climate Response (TCR) — warming measured at the moment of doubling — is typically lower, around 1.8 °C for the IPCC central estimate.

According to current CO₂ trajectories and IPCC assessments, limiting warming to 1.5 °C above preindustrial levels requires reaching net-zero CO₂ emissions globally by around 2050 and rapidly reducing all greenhouse gas emissions. At a central climate sensitivity of 3 °C, staying below 1.5 °C requires keeping CO₂ concentrations well below 420 ppmv on a sustained basis — a target that becomes increasingly difficult with each passing year of emissions.