A tighter constraint on Earth-system sensitivity from long-term temperature and carbon-cycle observations

Developing sound strategies to manage climate risks hinges critically on Earth-system properties, including the Earth-system sensitivity (ESS). Current ESS estimates are subject to large and deep uncertainties. Long-term carbon cycle models can provide a useful avenue to constrain ESS, but previous efforts either lack a formal data assimilation framework, or focus on discrete paleoevents. Here, we improve on ESS estimates by using a Bayesian approach to fuse deep-time paleoclimate CO2 and temperature data over the last 420 Myrs with a long-term carbon cycle model. Our best sensitivity estimate of 3.4 deg C (2.6-4.7 deg C; 5-95% range) shows a narrower range than previous assessments, implying increased learning. Our sensitivity analyses indicate that during the Cretaceous, a much weaker chemical weathering efficiency of gymnosperms and shift in the timing of gymnosperm- to angiosperm-dominated vegetation yield better agreement with temperature records. Research into improving the understanding about these plant-assisted weathering mechanisms hence provides potentially powerful avenues to further constrain this fundamental Earth-system property

Phenotopic Plasticity of Leaf Shape Along a Temperature Gradient in \u3cem\u3eAcer Rubrum\u3c/em\u3e

Both phenotypic plasticity and genetic determination can be important for understanding how plants respond to environmental change. However, little is known about the plastic response of leaf teeth and leaf dissection to temperature. This gap is critical because these leaf traits are commonly used to reconstruct paleoclimate from fossils, and such studies tacitly assume that traits measured from fossils reflect the environment at the time of their deposition, even during periods of rapid climate change. We measured leaf size and shape in Acer rubrum derived from four seed sources with a broad temperature range and grown for two years in two gardens with contrasting climates (Rhode Island and Florida). Leaves in the Rhode Island garden have more teeth and are more highly dissected than leaves in Florida from the same seed source. Plasticity in these variables accounts for at least 6–19 % of the total variance, while genetic differences among ecotypes probably account for at most 69–87 %. This study highlights the role of phenotypic plasticity in leaf-climate relationships. We suggest that variables related to tooth count and leaf dissection in A. rubrum can respond quickly to climate change, which increases confidence in paleoclimate methods that use these variables

Future climate forcing potentially without precedent in the last 420 million years

The evolution of Earth's climate on geological timescales is largely driven by variations in the magnitude of total solar irradiance (TSI) and changes in the greenhouse gas content of the atmosphere. Here we show that the slow ∼50 Wm(−2) increase in TSI over the last ∼420 million years (an increase of ∼9 Wm(−2) of radiative forcing) was almost completely negated by a long-term decline in atmospheric CO(2). This was likely due to the silicate weathering-negative feedback and the expansion of land plants that together ensured Earth's long-term habitability. Humanity's fossil-fuel use, if unabated, risks taking us, by the middle of the twenty-first century, to values of CO(2) not seen since the early Eocene (50 million years ago). If CO(2) continues to rise further into the twenty-third century, then the associated large increase in radiative forcing, and how the Earth system would respond, would likely be without geological precedent in the last half a billion years

Early Miocene Redwood Fossils from Inner Mongolia: CO2 Reconstructions and Paleoclimate Effects of a Low Mongolian Plateau

The early Miocene (~16–23 Ma) marks a critical transition in the Earth climate history from an Oligocene (~23–34 Ma) cooling trend towards the well-documented warm middle Miocene Climate Optimum at ~ 15 Ma. In eastern Asia, this transition links changes of key topographic features, such as the Tibetan plateau and the Mongolian plateau, and their impact on the reorganization of climate systems, such as the Eastern Asian summer monsoon. Yet the dynamics of the interplay among these factors remain poorly understood, precluding our understanding of future climate changes. Global temperatures during the early Miocene were warmer than the present day by 3–4 °C, which are seemingly incompatible with both the low (\u3c300 ppm) and high (\u3e800 ppm) ends of presently available reconstructions of atmospheric carbon dioxide (CO2). Here we report a rare co-occurrence of two redwoods, Metasequoia and Sequoia, from sediments of the early Miocene Hannuoba Formation in Zhuozi County in China’s Inner Mongolia at the southeastern margin of the current Mongolian plateau. The Zhuozi Metasequoia fossils possess uneven type cuticles, which dominate its living population but have rarely been reported throughout the abundant fossil record of this genus. By applying wellconstrained Franks model parameters obtained from these redwood fossils using a cleared leaf epidermis method, we estimated the early Miocene CO2 level at ~400 ppm, putting it at the lower end of the model requirement for sustaining a relatively warm global temperature during this period. Our Franks model estimates are ~100 ppm higher than that obtained using stomatal index method based on Metasequoia material, further confirming a systematic underestimation of ancient CO2 using the Metasequoia stomatal index method reported in previous analyses. We recommend a re-examination of previous CO2 reconstructions solely based upon Metasequoia’s inverse relationship between stomatal index and CO2 concentrations. Ultimately, the occurrence of these redwood fossils in Inner Mongolia is consistent with a weak or muted Eastern Asian summer monsoon in the region with the absence of an elevated Mongolian plateau during the early Miocene. A shift of moisture sources for the region accompanying the change from a Westerlies-dominated climate to the present-day monsoondominated climate system occurred after the early Miocene time

New constraints on atmospheric CO2 concentration for the Phanerozoic

Earth's atmospheric CO2 concentration (ca) for the Phanerozoic Eon is estimated from proxies and geochemical carbon cycle models. Most estimates come with large, sometimes unbounded uncertainty. Here, we calculate tightly constrained estimates of ca using a universal equation for leaf gas exchange, with key variables obtained directly from the carbon isotope composition and stomatal anatomy of fossil leaves. Our new estimates, validated against ice cores and direct measurements of ca, are less than 1000 ppm for most of the Phanerozoic, from the Devonian to the present, coincident with the appearance and global proliferation of forests. Uncertainties, obtained from Monte Carlo simulations, are typically less than for ca estimates from other approaches. These results provide critical new empirical support for the emerging view that large (~2000-3000 ppm), long-term swings in ca do not characterize the post-Devonian and that Earth's long-term climate sensitivity to ca is greater than originally thought. Key Points A novel CO2 proxy calculates past atmospheric CO2 with improved certainty CO2 is unlikely to have exceeded ~1000 ppm for extended periods post Devonian Earth's long-term climate sensitivity to CO2 is greater than originally thought

Phenotypic Plasticity of Leaf Shape along a Temperature Gradient in Acer rubrum

Both phenotypic plasticity and genetic determination can be important for understanding how plants respond to environmental change. However, little is known about the plastic response of leaf teeth and leaf dissection to temperature. This gap is critical because these leaf traits are commonly used to reconstruct paleoclimate from fossils, and such studies tacitly assume that traits measured from fossils reflect the environment at the time of their deposition, even during periods of rapid climate change. We measured leaf size and shape in Acer rubrum derived from four seed sources with a broad temperature range and grown for two years in two gardens with contrasting climates (Rhode Island and Florida). Leaves in the Rhode Island garden have more teeth and are more highly dissected than leaves in Florida from the same seed source. Plasticity in these variables accounts for at least 6–19 % of the total variance, while genetic differences among ecotypes probably account for at most 69–87 %. This study highlights the role of phenotypic plasticity in leaf-climate relationships. We suggest that variables related to tooth count and leaf dissection in A. rubrum can respond quickly to climate change, which increases confidence in paleoclimate methods that use these variables