Abstract
The long-term temperature response to a given change in CO2 forcing, or Earth-system sensitivity (ESS), is a key parameter quantifying our understanding about the relationship between changes in Earth’s radiative forcing and the resulting long-term Earth-system response. Current ESS estimates are subject to sizable uncertainties. Long-term carbon cycle models can provide a useful avenue to constrain ESS, but previous efforts either use rather informal statistical approaches or focus on discrete paleoevents. Here, we improve on previous ESS estimates by using a Bayesian approach to fuse deep-time CO2 and temperature data over the last 420 Myrs with a long-term carbon cycle model. Our median ESS estimate of 3.4 °C (2.6-4.7 °C; 5-95% range) shows a narrower range than previous assessments. We show that weaker chemical weathering relative to the a priori model configuration via reduced weatherable land area yields better agreement with temperature records during the Cretaceous. Research into improving the understanding about these weathering mechanisms hence provides potentially powerful avenues to further constrain this fundamental Earth-system property.
Highlights
The long-term temperature response to a given change in CO2 forcing, or Earth-system sensitivity (ESS), is a key parameter quantifying our understanding about the relationship between changes in Earth’s radiative forcing and the resulting long-term Earth-system response
We will use ΔT2x when referring to ESS within the GEOCARB model, and reserve the term “ESS” for discussion of Earth-system sensitivity more generally
We make a number of improvements relative to previous work using the GEOCARBSULFvolc model[14,19], which reduce the ESS uncertainty compared to these previous studies[14]
Summary
The long-term temperature response to a given change in CO2 forcing, or Earth-system sensitivity (ESS), is a key parameter quantifying our understanding about the relationship between changes in Earth’s radiative forcing and the resulting long-term Earth-system response. We improve on previous ESS estimates by using a Bayesian approach to fuse deep-time CO2 and temperature data over the last 420 Myrs with a long-term carbon cycle model. Based on the understanding of feedback processes, historical climate and paleoclimate records, a recent summary by Sherwood et al.[9] concluded that the most likely range (66% confidence) for the effective sensitivity (defined in terms of the 150-year temperature response to a quadrupling of CO2 forcing in the context of their general circulation model experiments) is 2.6–3.9 °C. A deep-time perspective offers insight into the “Earth-system sensitivity” (ESS)—the long-term equilibrium surface temperature response to a given CO2 forcing, including all Earth-system feedbacks[10]. The geochemical model from Royer et al.[19] uses a form of ESS that computes the overall global mean surface temperature response by explicitly accounting for forcings from changes in CO2, solar luminosity, and paleogeography. We expect that the inclusion of land-ice feedbacks leads the ESS estimate of ref. 22 (based on S[CO2, geog, solar]) to be lower than that of ref. 21 (based on S[CO2, geog, solar, land ice])
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