Sudden global warming 55 million years ago provides evidence for high climate sensitivity to atmospheric CO2, but the source of the carbon remains enigmatic.
Climate sensitivity refers to the mean-annual global temperature response to CO2 doubling due to the radiative effects of CO2 and associated feedbacks. The proposed range of climate sensitivity, ~1.5 to 4.5oC, represents fast-feedback sensitivity that incorporates changes in atmospheric water vapor, sea ice, and cloud and aerosol distributions. However, other feedbacks involving changes in continental ice extent, terrestrial ecosystems, non-CO2 greenhouse gas production, and other climate system parameters, operate on longer timescales and impact the temperature of the Earth. Warming related to a doubling of CO2 including all short- and long-term feedbacks is the Earth-system climate sensitivity. For this study, we evaluate the Earth-system climate sensitivity by reconstructing middle and early Pliocene CO2 concentrations when global temperatures were ~3 to 4oC warmer than pre-industrial conditions. We demonstrate that only a minor change in CO2 was associated with substantial global warming ~4.5 million years ago, with CO2 levels in the range of ~365 to 415 ppm during peak temperatures. Given estimates of global temperatures during the Pliocene, our results support a high Earth-system climate sensitivity for at least the past ~5 million.
Paleoclimate data show that climate sensitivity is ~3oC for doubled CO2, including only fast feedback processes. Equilibrium sensitivity, including slower surface albedo feedbacks, is ~6oC for doubled CO2 for the range of climate states between glacial conditions and ice-free Antarctica. Decreasing CO2 was the main cause of a cooling trend that began 50 million years ago, the planet being nearly ice-free until CO2 fell to 450 +/-100 ppm; barring prompt policy changes, that critical level will be passed, in the opposite direction, within decades. If humanity wishes to preserve a planet similar to that on which civilization developed and to which life on Earth is adapted, paleoclimate evidence and ongoing climate change suggest that CO2 will need to be reduced from its current 385 ppm to at most 350 ppm, but likely less than that. The largest uncertainty in the target arises from possible changes of non-CO2 forcings. An initial 350 ppm CO2 target may be achievable by phasing out coal use except where CO2 is captured and adopting agricultural and forestry practices that sequester carbon. If the present overshoot of this target CO2 is not brief, there is a possibility of seeding irreversible catastrophic effects.
Earth’s modern climate, characterized by polar ice sheets and large equator-to-pole temperature gradients, is rooted in environmental changes that promoted Antarctic glaciation ~33.7 million years ago. Onset of Antarctic glaciation reflects a critical tipping point for Earth’s climate and provides a framework for investigating the role of atmospheric carbon dioxide (CO2) during major climatic change. Previously published records of alkenone-based CO2 from high- and low-latitude ocean localities suggested that CO2 increased during glaciation, in contradiction to theory. Here, we further investigate alkenone records and demonstrate that Antarctic and subantarctic data overestimate atmospheric CO2 levels, biasing long-term trends. Our results show that CO2 declined before and during Antarctic glaciation and support a substantial CO2 decrease as the primary agent forcing Antarctic glaciation, consistent with model-derived CO2 thresholds.