Evidence for higher Earth-system sensitivity from long-term carbon-cycle observations
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Projections of future temperature are critical for developing sound strategies to address climate risks, but depend on deeply uncertain Earth system properties, including the Earth-system sensitivity (ESS). The ESS is the long-term (e.g., millennial-scale and longer) global warming resulting from a doubling of atmospheric carbon dioxide (CO_2) levels, relative to pre-industrial conditions. Long-term carbon cycle models provide a common approach to estimate ESS, but previous efforts either lack a formal data assimilation framework, or focus on paleo periods with the most available data. Here, we improve on ESS estimates by using a Bayesian approach to fuse deep-time paleoclimate data with a long-term carbon cycle model. Our updated ESS estimate of 5.1 ^∘C (3.8-6.6 ^∘C; 5-95 higher and narrower range than previous assessments, implying increased long-term future temperatures and risks. Our sensitivity analysis reveals that chemical and plant-assisted weathering parameters interact strongly with ESS in affecting the simulated atmospheric CO_2. Research into improving the understanding about these weathering processes hence provides potentially powerful avenues for further constraining this fundamental and policy-relevant Earth-system property.
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