Geologic constraints on the glacial amplification of Phanerozoic climate sensitivity

Abstract

The long-term carbon cycle depends on many feedbacks. Silicate weathering consumes atmospheric CO₂, but is also enhanced by the increased temperatures brought about by this important greenhouse gas. The long-term sensitivity ΔT_2x of climate to CO₂-doubling modulates the strength of this negative feedback. We update the model-experiment of Royer and others (2007) by estimating an empirical probability-density function (PDF) of ΔT_2x for the Phanerozoic by using an improved GEOCARBSULF carbon-cycle model to predict a larger, recalibrated set of proxy-CO₂ measurements from the present-day to 420 Ma. The new GEOCARBSULF parameterizes the rapid weathering of volcanic rocks, relative to plutonic rocks. Updates to the carbon-cycle model and the proxy-CO₂ data set induce opposing model responses. As a result, our experiment maintains an agreement with ΔT_2x estimates based on numerical climate models and late Cenozoic paleoclimate. For a climate sensitivity ΔT_2x that is uniform throughout the Phanerozoic, the most probable value is 3° to 4 °C. GEOCARBSULF fits the proxy-CO₂ data equally well, and with far more parameter choices, if ΔT_2x is amplified by at least a factor of two during the glacial intervals of the Paleozoic (260-340 Ma) and Cenozoic (0-40 Ma), relative to non-glacial intervals of Earth history. For glacial amplification of two, the empirical PDFs for glacial climate sensitivity predict ΔT_2x^(g)>2.0 °C with ∼99 percent probability, ΔT_2x^(g)>3.4 °C with ∼95 percent probability, and ΔT_2x^(g)>4.4 °C with ∼90 percent probability. The most probable values are ΔT_2x^(g) = 6° to 8 °C. This result supports the notion that the response of Earth's present-day surface temperature will be amplified by the millennial and longer-term waxing and waning of ice sheets.