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What are the projections for atmospheric CO2 concentration by 2100 under different emission scenarios?
Executive Summary
By 2100 atmospheric CO2 concentrations diverge dramatically across scenario families: low-emission pathways target concentrations near 420–440 ppm, mid-range scenarios reach ~540–670 ppm, and high-emission futures climb toward ~940–1,260 ppm or higher depending on assumptions about fossil fuel use and carbon sinks. The scientific literature and scenario databases (RCPs, SSPs, IPCC AR6 and related studies) present overlapping but not identical ranges; the choice of scenario family and assumptions about carbon-cycle feedbacks, land-use emissions, and policy implementation determine the outcome [1] [2] [3] [4].
1. Extracting the central claims — a quick read of the numbers that matter
The assembled analyses state that projected CO2 by 2100 spans a very wide band driven by emissions policy choices and model differences. One compilation reports a broad range of 490–1,260 ppm (with a narrower subset of 540–970 ppm under certain SRES-like scenarios), noting fossil-fuel emissions dominate 21st-century concentration outcomes and that land-use fluxes could only reduce concentrations by about 40–70 ppm unless fossil fuel burning declines substantially [3]. Representative Concentration Pathways (RCPs) and Shared Socioeconomic Pathways (SSPs) provide scenario-specific numbers: RCP2.6 ~420–440 ppm by 2100, RCP4.5 ~540 ppm, RCP6 ~660 ppm, RCP8.5 ~940 ppm [1]. A separate dataset aligned with current pledges produces ~670 ppm by 2100 under then-current commitments, highlighting that pledged policy is insufficient to stabilize concentrations [2]. These claims converge on a clear point: policy choices determine whether CO2 stabilizes near pre-industrial multiples of ~1.5–2× or rises to 3× or more.
2. Recent scenario families — SSPs, RCPs and model-driven extremes
Recent modeled scenario families expand earlier RCP ranges and introduce the SSP framework with updated socio-economic assumptions. The SSP-based dataset provided for CMIP6 and MAGICC7 shows that SSPs span outcomes from more ambitious mitigation (SSP1-2.6) up to very high-emission worlds exceeding 2,000 ppm by 2150 in extreme cases, implying 2100 concentrations well above 1,000 ppm for high-end SSPs [4]. The IPCC-linked analyses and AR6-aligned reports present percentile bands across hundreds to thousands of pathways, with policy-implemented pathways clustering at higher concentrations and mitigation-aligned pathways clustering at lower concentrations; the reports emphasize that pathways compatible with 1.5°C require rapid reductions and net-zero around mid-century to keep CO2 near the low hundreds of ppm above pre-industrial levels [5] [6] [7].
3. What recent policy-tracking analyses add — the “current pledges” story
Analyses that translate current nationally determined contributions and stated strategies into concentration outcomes find intermediate-high outcomes. One policy-tracking projection from Climate Interactive cited here estimates atmospheric CO2 near ~670 ppm by 2100 if countries fully implement pledges available in 2015–2016 datasets; that projection warns that even full implementation of stated mid-century strategies then would not stabilize greenhouse gases or global temperature by 2100 [2]. The AR6 synthesis similarly shows a red band of pathways reflecting policies implemented through 2020 and NDCs that result in medium-to-high concentrations and temperature outcomes, underscoring the gap between pledges and mitigation consistent with Paris goals [7] [6].
4. Why scenario differences are so large — carbon cycle, assumptions, and model choices
Scenario divergence arises principally from three sources: emissions trajectories (how much fossil fuel is burned), modelled carbon sinks and feedbacks (how much additional CO2 the land and oceans absorb), and treatment of negative-emission technologies and land-use change. Earlier SRES-era compilations and newer SSPs differ in their socioeconomic storylines and technological assumptions, producing ranges from modest stabilization to runaway concentrations exceeding 1,000 ppm [3] [4]. The IPCC emphasizes percentile spreads across models and pathways because small differences in cumulative emissions translate into large concentration and warming differences by 2100, and because feedbacks (e.g., permafrost or biosphere responses) introduce further upward uncertainty [5] [7].
5. Bottom line and remaining uncertainties — what decisionmakers should take from this
The consistent, evidence-based takeaway is that CO2 concentration by 2100 is not a single forecast but a conditional outcome: low hundreds of ppm if aggressive mitigation and net-zero are achieved, mid-to-high hundreds under current policy trajectories, and near or above 1,000 ppm under high-fossil-fuel scenarios. Policymakers must therefore treat projected concentrations as scenario-dependent risk bounds rather than precise predictions; the literature documents the numerical ranges (RCP/SSP and synthesis reports) and links them explicitly to policy assumptions and modeled carbon-cycle responses [1] [3] [6]. Key uncertainties that would materially shift the ranges are changes in global mitigation ambition, deployment of negative-emissions technologies at scale, and carbon-cycle feedbacks that could reduce natural sink capacity, any of which would alter where on the concentration spectrum the world lands by 2100 [3] [4] [7].