Fact check: What were the CO2 levels during the time of the dinosaurs?

1. Big-picture: Mesozoic CO2 often far above modern levels, shaping a greenhouse world

Geochemical syntheses indicate the Mesozoic climate was dominated by elevated CO2, producing a long-lived greenhouse state relative to the preindustrial baseline. Carbon-cycle models and proxy compilations show sustained higher atmospheric CO2 across Triassic, Jurassic, and Cretaceous periods, with values generally exceeding modern concentrations by several hundred to over a thousand parts per million at times ^[1]. These elevated concentrations underpinned warmer global temperatures, altered hydrological cycles, and reduced polar ice, so the broad consensus is that the dinosaur era occurred in a high-CO2 context shaped by tectonics and long-term carbon fluxes ^{[1] [3]}.

2. Close look: The Cretaceous had strong fluctuations and distinct high-CO2 peaks

Targeted Cretaceous studies document marked variability, with the Cenomanian–Turonian interval identified as a high-CO2 peak and multiple lower-CO2 intervals that may correlate with cooler or glaciation-prone windows. Authors emphasize three intervals of comparatively low CO2 within the Cretaceous and identify the Cenomanian–Turonian as among the highest CO2 slices, implying a complex climate response rather than a monotonic trend ^[2]. These assessments rely on diverse proxy lines—carbonate and organic geochemistry, paleosols, and modeling—each contributing to a composite picture of fluctuating greenhouse conditions through the Cretaceous ^[2].

3. Mechanisms: Why CO2 rose and fell—geology, weathering, and burial

Long-term CO2 evolution during the Phanerozoic, including the Mesozoic, is explained by interacting geological processes such as volcanic degassing, continental configuration affecting weathering rates, and the burial or oxidation of organic carbon. Studies outline how silicate weathering acts as a negative feedback removing CO2, while tectonic uplift or volcanism can inject greenhouse gases, producing the multi-million-year CO2 swings that set background climates for dinosaurs ^[3]. These processes explain why CO2 trajectories show long trends and why transient highs and lows align with major tectonic or sedimentary events ^[3].

4. Wider context: Comparing other deep-time greenhouse states and the Archean contrast

Comparisons to other eras show even larger CO2 extremes existed, with Archean atmospheres hypothesized to have orders-of-magnitude higher CO2 than Phanerozoic levels, while the Eocene saw a later Cenozoic greenhouse with peak CO2 around ∼1,500 ppm. Archean reconstructions suggest extremely high CO2 (fractions of a bar in some estimates) necessary to maintain liquid water under a faint young Sun, whereas the Eocene and other Cenozoic intervals document substantial but lower peaks than many Mesozoic estimates ^{[4] [5]}. These contrasts highlight that the dinosaur-era CO2 was high but not unique in Earth history ^{[4] [5]}.

5. The evidence base: proxies disagree and produce wide uncertainty bands

Proxy methods—stomatal indices, paleosols, boron isotopes, carbonate clumped isotopes, and marine proxies—yield divergent CO2 estimates for the same intervals, and studies stress that each proxy carries specific biases linked to local environments, diagenesis, and calibration choices. This methodological diversity is why published CO2 ranges for the Mesozoic often span several hundred to over a thousand ppm; the underlying data and models yield overlapping but not identical reconstructions that must be synthesized carefully to infer likely ranges rather than precise values ^{[1] [3] [5]}. A multi-proxy, model-constrained approach is therefore preferred for robust inferences ^[1].

6. Implications for paleoclimate and dinosaur ecosystems: warmth, sea level, and habitats

Elevated CO2 during dinosaur times translated into higher global temperatures, reduced polar ice, and elevated sea levels, producing extensive shallow epicontinental seas and expanded warm habitats that influenced dinosaur distribution and ecosystem structure. Peaks in CO2 correlate with some of the warmest intervals of the Mesozoic, promoting high-latitude warmth that allowed diverse faunas beyond modern ranges. However, transient low-CO2 intervals likely produced cooler intervals and ecosystem stress, underlining that dinosaur-era climates were dynamic and regionally heterogeneous ^{[2] [1]}.

7. What remains unresolved and how future work will narrow the ranges

Key uncertainties remain in proxy calibration, spatial representativeness, and chronological resolution, so exact ppm values for many Mesozoic slices remain contested. Progress requires higher-quality, well-dated multi-proxy records integrated with Earth-system models to reconcile discrepancies and better constrain short-term events versus long-term baselines. Recent reviews and focused studies recommend coordinated proxy-model efforts to reduce uncertainty and provide more precise CO2 reconstructions for critical intervals such as the Cenomanian–Turonian and the hypothesized Cretaceous low-CO2 windows ^{[1] [2] [3]}.

8. Bottom line: high CO2 framed the age of dinosaurs, but numbers come with caveats

Synthesis of available work supports the statement that the dinosaur era was characterized by CO2 concentrations substantially above modern preindustrial levels, with peaks in the Cretaceous and meaningful fluctuations driven by geological carbon-cycle processes. Exact numerical values vary by study and proxy—some intervals likely exceeded several hundred to over a thousand ppm—so the most accurate framing is a high-CO2, dynamic climate backdrop rather than a single definitive ppm figure ^{[1] [2] [3]}.