What temperature rise is expected with rise in CO2

1. Why scientists say “it depends”: equilibrium versus transient numbers that matter

Climate scientists use different definitions when they answer “how much warming for a given CO2 rise,” and these definitions produce systematically different numbers. Equilibrium Climate Sensitivity (ECS) measures the long-term global temperature change after the system, including slow ocean warming and ice-sheet feedbacks, reaches a new steady state following a doubling of CO2; ECS estimates in recent literature commonly span roughly 2 to 5 K but studies vary on the tails ^{[1] [5]}. By contrast, Transient Climate Response (TCR) and the Transient Climate Response to cumulative Emissions (TCRE) describe near‑to‑mid‑century warming and the roughly linear warming per cumulative carbon emitted; these transient metrics are smaller because the ocean temporarily sequesters heat, and are therefore more directly linked to policy timescales and carbon budgets ^{[5] [6]}.

2. Recent studies — convergence on mid-range values, disagreement on extremes

Multiple recent analyses move toward converging mid-range estimates but disagree on the probability of very low or very high sensitivity. A 2024–2025 wave of work finds likely ECS values concentrated in the 2–4.5 K window while tightening probability away from very low values and sometimes raising the lower bound for policy-relevant risk assessments ^{[2] [1]}. A paleoclimate reanalysis reduced one worst‑case doubling estimate from 5°C to about 4°C, leaving the central estimate near 3°C and the best‑case near 2°C, signaling that extreme high-end warming may be less likely than some prior reconstructions suggested ^[7]. Yet other ensemble and data‑assimilation approaches warn that very high ECS values can remain difficult to rule out definitively even with decades of additional observations, preserving nontrivial tail risks ^[4].

3. The carbon-budget view — linearity and the TCRE practical rule

For many policy questions, the useful relation is between cumulative CO2 emissions and global mean warming. The IPCC and newer syntheses provide a TCRE that is approximately linear over relevant emission ranges, with recent narrowed estimates centering around 1.4–2.2°C per trillion tonnes of carbon emitted, down from earlier broader ranges of 0.8–2.5°C. That linear relationship underpins the carbon‑budget framework that translates remaining allowable emissions into temperature outcomes and probabilities for 1.5°C or 2°C limits; regional responses and non‑CO2 forcings complicate the picture, but the global scaling rule remains a robust planning tool ^{[3] [8] [6]}.

4. Transient projections — what to expect by mid‑ and end‑century

Transient metrics that incorporate ocean heat uptake and current inertia yield more actionable end‑of‑century ranges than ECS alone. Several studies indicate that end‑century warming under high emissions is consistent with multi‑degree Celsius increases and that TCR values are typically smaller than ECS—commonly in the 1.0–2.5 K range—reflecting slower realized warming ^[5]. Model ensembles and observation‑constrained analyses published between 2024 and 2025 provide middle‑of‑distribution TCR and end‑century projections that policymakers can use to assess near‑term risks, but they also emphasize remaining uncertainty, particularly from cloud feedbacks and long‑term ocean responses that could push outcomes higher if feedback strengths are underestimated ^{[2] [4]}.

5. What’s missing and why uncertainty persists despite better data

Despite better palaeoclimate reconstructions and improved data assimilation, key feedbacks remain imperfectly constrained, especially cloud and some ocean processes, so uncertainty—notably the possibility of higher ECS—persists. Some methods suggest we will narrow low ECS values by century’s end, but high ECS values may remain elusive to decisively constrain even with many decades of additional observations, leaving risk managers to account for residual tails when setting policy ^[4]. The practical upshot is that while transient responses and carbon‑budget metrics are now more useful for near‑term planning, long‑term equilibrium outcomes retain enough uncertainty to warrant precaution in emissions trajectories ^{[4] [3]}.