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finding 4.4 : key-finding-4-4
Global mean atmospheric carbon dioxide (CO2) concentration has now passed 400 ppm, a level that last occurred about 3 million years ago, when global average temperature and sea level were significantly higher than today (high confidence). Continued growth in CO2 emissions over this century and beyond would lead to an atmospheric concentration not experienced in tens of millions of years (medium confidence). The present-day emissions rate of nearly 10 GtC per year suggests that there is no climate analog for this century any time in at least the last 50 million years (medium confidence).
This finding is from chapter 4 of Climate Science Special Report: The Fourth National Climate Assessment: Volume I.
Process for developing key messages: The key finding is based on a vast body of literature that summarizes the results of observations, paleoclimate analyses, and paleoclimate modeling over the past 50 years and more.
Description of evidence base: The key finding is based on a large body of research including Crowley,2d65f18b-ef81-450c-b48f-4d665f0f2407 Schneider et al.,bd8bf653-6f62-479e-964c-8d5165a274a4 Lunt et al.,97079544-53fc-496d-b8fc-871be0681b33 Otto-Bleisner et al.,6c1e37e5-22fa-4b90-951f-7757927845fb NEEM,7f28a81a-4e01-4d93-ac07-7b133a0783e2 Jouzel et al.,a5055be6-7deb-484f-b027-69c2ac661c68 Dutton et al.,c0bdfdf2-5012-4496-9d27-c8d540fd4d4b Seki et al.,4a5890ba-0829-4cdd-8dc2-e77e46a25be5 Haywood et al.,4feca1bb-a6c0-49ea-aff8-76224ffd5a82 Miller et al.,920fc97d-049b-4cf3-9702-8c14f4c3208d Royer,3be9654e-ee0f-42cf-94cd-a3ed5e32fb43 Bowen et al.,15c1814b-577b-4ba3-90ce-86948daba0e0 Kirtland Turner et al.,e236d370-b985-4c1f-9752-22830fa9e5c6 Penman et al.,edd64878-de00-482f-9672-f187bf0e7c62 Zeebe et al.,a7eaa271-3c43-4c6f-add7-305f4e0ce52e and summarized in NRCa8b2d884-2bfb-4a5a-bfa4-a28bd4d68ca9 and Masson-Delmotte et al.6f4c1264-ab24-4802-9171-ea967deecc6c
New information and remaining uncertainties: The largest uncertainty is the measurement of past sea level, given the contributions of not only changes in land ice mass, but also in solid earth, mantle, isostatic adjustments, etc. that occur on timescales of millions of years. This uncertainty increases the further back in time we go; however, the signal (and forcing) size is also much greater. There are also associated uncertainties in precise quantification of past global mean temperature and carbon dioxide levels. There is uncertainty in the age models used to determine rates of change and coincidence of response at shorter, sub-millennial timescales.
Assessment of confidence based on evidence: High confidence in the likelihood statement that past global mean temperature and sea level rise were higher with similar or higher CO2 concentrations is based on Masson-Delmotte et al.6f4c1264-ab24-4802-9171-ea967deecc6c in IPCC AR5. Medium confidence that no precise analog exists in 66 million years is based on Zeebe et al.a7eaa271-3c43-4c6f-add7-305f4e0ce52e as well as the larger body of literature summarized in Masson-Delmotte et al.6f4c1264-ab24-4802-9171-ea967deecc6c
ProvenanceThis finding was derived from figure -.2: Confidence / Likelihood
- Two massive, rapid releases of carbon during the onset of the Palaeocene-Eocene thermal maximum (15c1814b)
- Are there any satisfactory geologic analogs for a future greenhouse warming? (2d65f18b)
- Treatise on Geochemistry (Second Edition) (3be9654e)
- Alkenone and boron-based Pliocene pCO2 records (4a5890ba)
- Large-scale features of Pliocene climate: Results from the Pliocene Model Intercomparison Project (4feca1bb)
- How warm was the last interglacial? New model–data comparisons (6c1e37e5)
- chapter ipcc-ar5-wg1 chapter 5 : Information from Paleoclimate Archives (6f4c1264)
- Eemian interglacial reconstructed from a Greenland folded ice core (7f28a81a)
- High tide of the warm Pliocene: Implications of global sea level for Antarctic deglaciation (920fc97d)
- A model–data comparison for a multi-model ensemble of early Eocene atmosphere–ocean simulations: EoMIP (97079544)
- Orbital and millennial Antarctic climate variability over the past 800,000 years (a5055be6)
- Anthropogenic carbon release rate unprecedented during the past 66 million years (a7eaa271)
- Climate Stabilization Targets: Emissions, Concentrations, and Impacts over Decades to Millennia (a8b2d884)
- A reconstruction of atmospheric carbon dioxide and its stable carbon isotopic composition from the penultimate glacial maximum to the last glacial inception (bd8bf653)
- Sea-level rise due to polar ice-sheet mass loss during past warm periods (c0bdfdf2)
- Persistence of carbon release events through the peak of early Eocene global warmth (e236d370)
- Rapid and sustained surface ocean acidification during the Paleocene-Eocene Thermal Maximum (edd64878)
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