What are the most likely climate scenarios for global temperature and sea level by 2100?

1. The scenarios framework: emissions drive the spread

Climate science does not single out one “most likely” 2100 climate; instead the AR6 scenarios (SSP1‑1.9, SSP1‑2.6, SSP2‑4.5, SSP3‑7.0, SSP5‑8.5) map plausible futures tied to emissions and policy choices, and the IPCC stresses that warming and sea level outcomes depend on those pathways rather than on a deterministic prediction ^{[1] [4]}.

2. Temperature by 2100: best, middle and worst cases

Under the most ambitious mitigation pathway, global warming can be kept close to the Paris aims — roughly 1.5°C to below 2°C by 2100 — with some assessments saying the “best‑case” scenario yields about 1.8°C in 2100, while intermediate scenarios put warming in the 2–3°C range and a high‑emissions trajectory (SSP5‑8.5) leads to multi‑degree warming—IPCC summaries and stakeholder analyses cite a high‑end central estimate near 4.4°C for the worst pathway ^{[5] [2] [4]}. The IPCC also notes it is likely the world will reach or exceed 1.5°C within decades under current trends, making lower‑warming outcomes contingent on rapid, sustained emissions cuts ^[2].

3. Sea level by 2100: expected rise, ranges and extremes

Global mean sea level is projected to rise in all scenarios through 2100; the mainstream AR6‑based assessments place plausible 2100 GMSL rises commonly between roughly 0.3 m and 1.0 m for most medium and low emissions scenarios, with higher values possible under continued high emissions or if rapid ice‑sheet processes materialize—U.S. summaries cite a plausible overall envelope of ≈0.3–2.0 m by 2100 when low‑probability rapid ice loss is included ^{[1] [6] [3]}. For the 1.5°C world the IPCC gives an indicative GMSL range of about 0.26–0.77 m relative to 1986–2005, and AR6 presents intermediate ranges (e.g., SSP2‑4.5 and SSP5‑8.5 show substantially larger likely ranges, and a low‑confidence SSP5‑8.5 variant including rapid ice loss centers near 0.88 m with a 0.63–1.60 m range) ^{[7] [6] [3]}.

4. Uncertainties and “tail risks”: ice sheets, rates and regional differences

The largest near‑term uncertainty for century‑scale sea level is ice‑sheet dynamics—Antarctic and Greenland instabilities could substantially increase 2100 sea level but are classed as low‑to‑medium confidence in timing and magnitude; AR6 therefore reports medium confidence ranges for standard projections while also providing low‑confidence, high‑impact possibilities extending the upper bound toward meters by 2100 and beyond ^{[3] [7]}. Observations show accelerating sea level rise (e.g., a doubling of rate between 1993 and 2023 in satellite records), which informs upward revisions of projections and highlights regional variation driven by land subsidence and ocean circulation ^{[8] [6]}.

5. Judging the “most likely” outcome and what that means

Given current policies and emissions trends, many expert syntheses indicate a central expectation of warming above 1.5°C and likely between ~2–4°C by 2100 unless rapid mitigation occurs, with corresponding sea level rise most likely in the several‑decimeter to around one‑meter range under mid‑to‑high emissions pathways; the IPCC explicitly declines to label a single scenario “most likely,” instead tying likelihood to real‑world emissions choices and noting persistent risks even under strong mitigation ^{[2] [1] [4]}. Alternative, outlying analyses that report much larger temperature rises by 2100 (for example some institutional projections cited outside IPCC synthesis) exist but reflect different assumptions about feedbacks and emissions trajectories and are not the consensual mid‑range of AR6 assessments ^[9]. Policymakers and planners should therefore treat a band—roughly a few tenths to about a meter as a practical planning range for 2100 sea level, and 1.5–4°C for temperature depending on mitigation—as the operative, evidence‑based set of scenarios while recognizing lower‑probability tails that could be far more severe ^{[3] [7] [1]}.