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

Provenance

This finding was derived from figure -.2: Confidence / Likelihood