Earth System Climate Sensitivity

A 12-Million-Year Temperature History of the Tropical Pacific Ocean

Yi Ge Zhang, Mark Pagani, Zhonghui Liu

The appearance of permanent El Ninolike conditions prior to 3 million years ago is founded on sea-surface temperature (SST) reconstructions that show invariant Pacific warm pool temperatures and negligible equatorial zonal temperature gradients. However, only a few SST records are available, and these are potentially compromised by changes in seawater chemistry, diagenesis, and calibration limitations. For this study, we establish new biomarker-SST records and show that the Pacific warm pool was ~4°C warmer 12 million years ago. Both the warm pool and cold tongue slowly cooled toward modern conditions while maintaining a zonal temperature gradient of ~3°C in the late Miocene, which increased during the Plio-PleistoceneOur results contrast with previous temperature reconstructions that support the supposition of a permanent El NiĖo–like state.

An Ancient Carbon Mystery

Mark Pagani, Ken Caldeira, David Archer, James C. Zachos

Sudden global warming 55 million years ago provides evidence for high climate sensitivity to atmospheric CO2, but the source of the carbon remains enigmatic.

High Earth-System Climate Sensitivity determined from Pliocene CO2 Concentrations

Mark Pagani, Zhonghui Liu, Jonathan LaRiviere, Ana Christina Ravelo

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-COgreenhouse 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 COconcentrations 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 COlevels 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.

Target Atmospheric CO2: Where Should Humanity Aim?

James Hansen, Makiko Sato, Pushker Kharecha, David Beerling, Robert Berner,Valerie Masson-Delmotte, Mark Pagani, Maureen Raymo, Dana L. Royer, James C. Zachos

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.

The Role of Carbon Dioxide During the Onset of Antarctic Glaciation

Mark Pagani, Matthew Huber, Zhonghui Liu, Steven M. Bohaty, Jorijntje Henderiks, Willem Sijp, Srinath Krishnan, Robert M. DeConto

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.