Silicate weathering as a feedback and forcing in Earth's climate and carbon cycle

Current understanding of the long-term carbon cycle posits that Earth's climate is stabilized by a negative feedback involving CO2 consumption by chemical weathering of silicate minerals. This theory holds that silicate weathering responds to climate: when atmospheric pCO2 and surface temperatures rise, chemical weathering accelerates, consuming more atmospheric CO2 and cooling global climate; when pCO2 falls, weathering fluxes decrease, permitting buildup of CO2 and consequent warming. However, the functional dependence of global weathering rates on atmospheric pCO2 (Earth's “weathering curve”) remains highly uncertain, with a variety of mathematical formulations proposed in the literature. We explore the factors influencing this relationship, and how they may have changed over Earth history. We then revisit classic carbon cycle model experiments to demonstrate how the choice of weathering curve has dramatic consequences for the response of the Earth system to several types of climatic and carbon-cycle perturbations. First, the slope of the weathering curve determines the timescale of recovery and the “long tail” of elevated pCO2 following carbon release events. Second, the nature of Earth's weathering curve determines the response of pCO2 to changing volcanic CO2 degassing, which has varied significantly over geologic timescales. Finally, we demonstrate how changes to Earth's weathering curve over time driven by, for example, tectonic or evolutionary processes, can act as a forcing, in addition to a feedback, in the carbon cycle and climate. These examples highlight the importance of constraining Earth's weathering curve, both for improving our understanding of past carbon cycle perturbations and predicting the future impact of anthropogenic carbon release on long timescales

There is no Neogene denudation conundrum

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Response to Comment on "Revised paleoaltimetry data show low Tibetan Plateau elevation during the Eocene"

International audienceValdes et al. contest our results, suggesting failings in our modeling approach as well as in our comparison with data. Although their comment points to interesting ideas of improvement, we find that their critique reflects an incomplete understanding of our methods and is not supported by the material they provide

Reconstruction of continental temperatures and oxygen isotope compositions of precipitation based on clumped and oxygen isotope analysis of pedogenic siderites

Earth's climate sensitivity – defined as the temperature increase for a doubling of pCO2 – and the mechanisms responsible for amplification of high latitude warming remain controversial. The latest Paleocene/earliest Eocene (LPEE; 57-55 million years ago) is a time when pCO2 peaked between 1400 and 4000 ppm, which allows us to evaluate the climatic response to high pCO2. Here, we present a reconstruction of continental temperatures and oxygen isotope compositions of precipitation – reflective of specific humidity – based on clumped and oxygen isotope analysis of pedogenic siderites. We show that continental mean annual temperature reached 41 °C in the equatorial tropics, and summer temperatures reached 23 °C in the Arctic. The oxygen isotope compositions of precipitation reveal that compared to the present-day the hot LPEE climate was characterized by an increase in specific humidity and the average residence time of atmospheric moisture and by a decrease in the subtropical-to-polar specific humidity gradient. The global increase in specific humidity reflects the fact that atmospheric vapor content is more sensitive to changes in pCO2 than evaporation and precipitation, resulting in an increase in the residence time of moisture in the atmosphere. Pedogenic siderite data from other super-greenhouse periods support the evidence that the spatial patterns of specific humidity and warmth are related providing new means to evaluate Earth's climate sensitivity

Lake area constraints on past hydroclimate in the western United States: Application to Pleistocene Lake Bonneville

Lake shoreline remnants found in basins of the western United States reflect wetter conditions during Pleistocene glacial periods. The size distribution of paleolakes, such as Lake Bonneville, provide a first-order constraint on the competition between regional precipitation delivery and evaporative demand. In this contribution we downscale previous work using lake mass balance equations and Budyko framework constraints to determine past hydroclimate change for the Bonneville and Provo shoreline extents of Lake Bonneville during the last glacial cycle. For the Bonneville basin we derive new relationships between temperature depression and precipitation factor change relative to modern. These scaling relationships are combined with rebound-corrected estimates of lake area and volume and macrofossil-derived surface temperatures to make quantitative estimates of precipitation and water residence times for the lake. For the Bonneville shoreline (~1552 m) we calculate that, prior to spillover to the Snake River drainage, precipitation rates were ~1.37 times modern, with a water residence time of ~185 years. For the Provo shoreline (1444 m), during the period of steady-state spillover, we calculate that precipitation rates were at least 1.26 times modern, with a residence time of ~102 years. These calculations suggest minimal difference in the hydrologic regime between the Bonneville shoreline highstand and the Provo shoreline stillstand during the last glacial termination. These estimates of hydroclimate scaling relationships differ in sensitivity with previous hydrologic modeling for Lake Bonneville and are complementary to those recently derived from glacier mass balance modeling from the Wasatch Mountains