A commentary on: Tropical deforestation and atmospheric carbon dioxide

Silicate weathering regulates climate as a critical carbon sink, yet understanding its role in the carbon cycle is challenging because of limited knowledge about the impact of temperature and rainfall on weathering during glacial-interglacial cycles. Here we investigated the orbital scale of silicate weathering variations and their role in atmospheric carbon dioxide sequestration using reconstructions, model simulations, and modern river sediment geochemical data. Results show that silicate weathering intensity in subtropical and tropical monsoon regions follows the precession cycle and is mainly controlled by rainfall. During glacial periods, global carbon dioxide consumption by silicate weathering was lower than interglacials but remained stable at ~2.47 Teramoles per year. We propose that ice sheet expansion confined intense weathering to the subtropics and tropics during glacial times. As insolation patterns shifted with the precession cycle, rainfall belts oscillated between hemispheres, maintaining a constant weathering area and stable carbon dioxide consumption. Our study provides insights into silicate weathering’s role in the global carbon cycle, both historically and in future projections.

Mountain building results in high erosion rates and the interaction of rocks with the atmosphere, water and life. Carbon transfers that result from increased erosion could control the evolution of Earth’s long-term climate. For decades, attention has focused on the hypothesized role of mountain building in drawing down atmospheric carbon dioxide (CO2) via silicate weathering. However, it is now recognized that mountain building and erosion affect the carbon cycle in other important ways. For example, erosion mobilizes organic carbon (OC) from terrestrial vegetation, transferring it to rivers and sediments, and thereby acting to draw down atmospheric CO2 in tandem with silicate weathering. Meanwhile, exhumation of sedimentary rocks can release CO2 through the oxidation of rock OC and sulfide minerals. In this Review, we examine the mechanisms of carbon exchange between rocks and the atmosphere, and discuss the balance of CO2 sources and sinks. It is demonstrated that OC burial and oxidative weathering, not widely considered in most models, control the net CO2 budget associated with erosion. Lithology strongly influences the impact of mountain building on the global carbon cycle, with an orogeny dominated by sedimentary rocks, and thus abundant rock OC and sulfides, tending towards being a CO2 source.

Effects of fuel and forest conservation on future levels of atmospheric carbon dioxide

Effects of fuel and forest conservation on future levels of atmospheric carbon dioxide

Tropical forests affect atmospheric carbon dioxide concentrations, and hence modulate the rate of climate change - by being a source of carbon, from land-use change (deforestation), and as a sink or source of carbon in remaining intact forest. These fluxes are among the least understood and most uncertain major fluxes within the global carbon cycle. We synthesise recent research on the tropical forest biome carbon balance, suggesting that intact forests presently function as a carbon sink of approx. 1.2 Pg C a ^-1, and that deforestation emissions at the higher end of the reported 1 - 3 Pg C a^ -1 spectrum are likely. Scenarios suggest that the source from deforestation will remain high, whereas the sink in intact forest is unlikely to continue, and remaining tropical forests may become a major carbon source via one or more of (i) changing photosynthesis/respiration rates, (ii) functional/biodiversity changes within intact forest, or widespread forest collapse via (iii) drought, or (iv) fire. Each scenario risks possible positive feedbacks with the climate system suggesting that current estimates of the possible rate, magnitude and effects of global climate change over the coming decades may be conservative.

Tropical Deforestation and Climatic Change: The Conceptual Background. Guest Editorial N. Myers. Tropical Forests: Present Status and Future Outlook N. Myers. A Commentary on: Tropical Forests: Present Status and Future Outlook G.T. Prance. Palaeoecological Background: Neotropics T. van der Hammen. A Commentary on: Palaeoecological Background: Neotropics P. Colinvaux. A Synopsis of Climatic and Vegetational Change in Southeast Asia L.R. Heaney. A Commentary on: A Synopsis of Climatic and Vegetational Change in Southeast Asia P.H. Raven. History of Climate and Forests in Tropical Africa during the Last 8 Million Years A.C. Hamilton, D. Taylor. The African Rain Forest Vegetation and Palaeoenvironments during Late Quaternary J. Maley. Tropical Deforestation and Atmospheric Carbon Dioxide R.A. Houghton. A Commentary on: Tropical Deforestation and Atmospheric Carbon Dioxide L.D. Danny Harvey. Tropical Deforestation: Albedo and the Surface-Energy Balance J.H.C. Gash, W.J. Shuttleworth. A Commentary on: Tropical Deforestation: Albedo and the Surface-Energy Balance A.Henderson-Sellers. Effects of Tropical Deforestation on Global and Regional Atmospheric Chemistry M. Keller, D.J. Jacob, S.C. Wofsy, R.C. Harriss. A Commentary on: Effects of Tropical Deforestation on Global and Regional Atmospheric Chemistry P.M. Vitousek, P.A. Matson. Probable Impact of Deforestation on Hydrological Processes V.M. Meher-Homji. A Commentary on: Probable Impact of Deforestation on Hydrological Processes R.E. Dickinson. Possible Climatic Impacts of Tropical Deforestation E. Salati, C.A. Nobre. A Commentary on: Possible Climatic Impacts of Tropical Deforestation P.R. Rowntree, J. Lean. Deforestation, Climate Change and Sustainable Nutrition Security: A Case Study of India S.K. Sinha, M.S. Swaminathan. A Commentary on: Deforestation, Climate Change and Sustainable Nutrition Security: A Case Study of India N.J. Rosenberg. Tropical Forests and the Greenhouse Effect: A Management Response N. Myers, T.J. Goreau. A Commentary on: Tropical Forests and the Greenhouse Effect: A Management Response G. Marland. Buying Environmental Insurance: Prospects for Trading of Global Climate-Protection Services J.N. Swisher, G.M. Masters. A Commentary on: Buying Environmental Insurance: Prospects for Trading of Global Climate-Protection Services M. Grubb. Forests in a Warming World: A Time for New Policies G.M. Woodwell. A Commentary on: Forests in a Warming World: A Time for New Policies J. Gradwohl, R. Greenberg. The Politics of Prevention C. Schneider. A Commentary on: The Politics of Prevention J. MacNeill.

<p>The supply of fresh minerals to Earth’s surface by erosion is thought to modulate global climate by removing atmospheric carbon dioxide (CO<sub>2</sub>) through silicate weathering. In turn, weathering of accessory carbonate and sulfide minerals is a geologically-relevant CO<sub>2</sub> source, which may dampen or reverse the effect of silicate weathering on climate. Although these weathering pathways commonly operate side by side, we lack quantitative constraints on their co-evolution across erosion-rate gradients. Using stream-water chemistry across a 3 order-of-magnitude erosion-rate gradient in shales and sandstones of southern Taiwan, here, we demonstrate that silicate, sulfide, and carbonate weathering are linked: Increasing sulfide oxidation generates sulfuric acid and boosts carbonate solubility whereas silicate weathering kinetics remain constant or even decline, perhaps due to buffering of the pH by carbonates. On timescales shorter than marine sulfide compensation, CO<sub>2</sub> emission rates from weathering in rapidly-eroding terrain are more than twice the CO<sub>2</sub> sequestration rates in slow-eroding terrain. On longer timescales, CO<sub>2</sub> emissions are compensated, but CO<sub>2</sub> sequestration rates do not increase with erosion, in contrast to assumptions in carbon cycle models. We posit that these patterns are broadly applicable to many Cenozoic mountain ranges that expose dominantly siliciclastic metasediments.</p>

Atmospheric CO2 variations across glacial-interglacial cycles are tightly linked to oceanic and terrestrial carbon reservoirs, but the contribution of weathering remains poorly constrained. Using a new PCM-weathering model (Past terrestrial Carbon storage Model coupling weathering sub-model), we reconstructed global silicate and carbonate weathering fluxes over the past 120,000 years. Silicate weathering was higher during interglacials (~ 125–163 Tg C/yr) and lower at glacials (~ 119–122 Tg C/yr), tracking atmospheric CO2. In contrast, carbonate weathering increased during glacials (~ 303–320 Tg C/yr) due to expanded land exposure resulting from glacial sea-level fall and declined at interglacials (~ 168–265 Tg C/yr). On glacial-interglacial timescales, total carbon consumption by silicate and carbonate weathering far exceeded changes in oceanic and terrestrial organic carbon pools. These results highlight a dynamic, compensatory balance between silicate and carbonate weathering in modulating Quaternary carbon fluxes.

Chemical weathering processes in the Chinese Loess Plateau

<p>For over a century, geologists have vigorously debated the influence of mountains on global climate via links among rock uplift, erosion, chemical weathering, and the geological carbon cycle. For decades, the focus has been on the role of mountain building in drawing down atmospheric carbon dioxide (CO<sub>2</sub>) via silicate weathering. However, it is now recognized that mountain building and the exhumation of sedimentary rocks can release CO<sub>2</sub> through the oxidation of organic carbon in rocks (rock OC). We quantify this flux at a global scale and show that over geological timescales this source is as important as CO<sub>2</sub> emissions from volcanism.</p><p>We explore the controls of mountain erosion on CO<sub>2</sub> release to the atmosphere with a spatially explicit global simulation model that uses empirical constraints on rock OC oxidation flux. We know that erosion is a major control on this flux: rock OC oxidation increases with erosion, up to and greater than erosion rates of ~ 2 mm yr<sup>-1</sup>. This contrasts with silicate weathering, where rates are limited by reaction kinetics at high erosion rates. We here constrain the spatial distribution of high erosion rates and their overlap with OC-rich bedrock lithologies. The effect of erodibility of such lithologies means that these are predisposed to high rates of CO<sub>2</sub> release through weathering. Hence our model relies on lithological mapping to constrain the relationship between topography and exhumation rates, and global rock OC stock. We produce a probabilistic rock OC stock map by combining global lithological maps with the USGS Rock Geochemical Database, which includes over 167,000 samples for our analysis. We consider the role of erosion and chemical weathering by using a probabilistic approach that is built on catchment-scale <sup>10</sup>Be denudation rates, while rhenium-based estimates of oxidative weathering intensity and flux from river catchments around the world are used to constrain patterns in rock OC oxidation. To extrapolate the major controls on erosion and weathering we use local slopes derived from 90 m resolution digital elevation model (DEM) data and lithological maps. We combine the erosion, rock chemistry data and weathering intensity estimates to simulate global rock OC weathering rates at a 1 km grid scale via a statistical probability ensemble (Monte Carlo).</p><p>We will present the results of our model compilation, including the effect of lithology on erosion, weathering and CO<sub>2</sub> emission rates. We demonstrate that the size of the organic carbon stock in the first 1 m of bedrock is of a similar magnitude to the carbon stock of global soils, and that the emissions of CO<sub>2</sub> from this geological source are as large as the emissions from volcanic degassing. We identify regions of the Earth’s surface where rock OC could emit substantial amounts of CO<sub>2</sub> and provide new constraints on a major natural CO<sub>2</sub> flux derived from the erosion of mountains.</p>

Silicate weathering sequesters CO2 from the atmosphere and stabilizes Earth’s climate over geologic timescales. In turn, weathering of accessory carbonate and sulfide minerals is a geologically relevant CO2 source. Rock-uplift and -erosion is the primary mechanism by which fresh minerals are exposed to weathering at Earth’s surface. Therefore, the global inorganic carbon cycle is sensitive to mountain uplift and erosion. However, quantifying this sensitivity is complex, because existing data do not consider weathering of all relevant mineral phases, and because co-variation of multiple environmental factors obscures the role of erosion. Here, we analyze the sensitivity of silicate, carbonate, and sulfide weathering fluxes to erosion in four datasets of solute chemistry from small mountain streams that span well-defined erosion-rate gradients in relatively uniform metasedimentary lithologies and with limited or well-constrained variations in runoff. Across all datasets and 2-3 orders of magnitude of erosion rate, we find that silicate weathering fluxes are almost insensitive to erosion at rates >10-2 mm yr-1. In contrast, weathering fluxes from sulfide and carbonate minerals increase sub-linearly with erosion, contradicting expectations from soil data and theory. By fitting a weathering model to these data, we show that the contrasting sensitivities of silicate, carbonate, and sulfide weathering produce a distinct CO2-drawdown maximum at moderate erosion rates of ~0.1 mm/y. Below this maximum, mineral supply limits silicate weathering. Above the maximum, silicate weathering fluxes plateau and CO2 emissions from coupled sulfide oxidation and carbonate weathering increasingly dominate the carbon budget. Thus, for metasedimentary lithologies, uplift of landscapes to moderate relief and erosion rates can substantially bolster Earth’s CO2 sink whereas further uplift may decrease, rather than increase CO2 sequestration rates.

Chemical weathering characteristics and controls in the Yarlung Tsangpo River Basin: Evidence from hydrochemical composition

Silicate weathering is essential for the global carbon cycle and is a driver of climate change through the consumption of atmospheric CO 2 . Recent studies have pointed out that silicate weathering in the late Holocene was widely influenced by human activities, but the carbon sink effect of silicate weathering under anthropogenic influence remains unclear. In this study, we present continuous records of clay minerals, major elements, and terrigenous mass accumulation rates of Core 45A in the Western South China Sea to reconstruct the evolutionary history of weathering and erosion in the Red River Basin since 3800 cal yr BP. We investigate the interactions between weathering, climate, and human activities. Our results reveal that the silicate weathering intensity and erosion rate have increased significantly since ∼1500 cal yr BP, which is decoupled from the trend to a cooler and drier climate but coincides well with stronger human activities, suggesting the significance of anthropogenic influence on silicate weathering. We also reconstruct the CO 2 consumption flux induced by silicate weathering to quantitatively evaluate the impact of human activities on the carbon sink capacity of silicate weathering. The calculated CO 2 consumption fluxes contributed by anthropogenic activities on silicate weathering show an approximate 150% increase compared to natural conditions in the Red River. Thus, this study highlights that human‐enhanced silicate weathering has reduced atmospheric CO 2 and played an important role in the global carbon cycle during the late Holocene, which has never occurred in the Earth's geological past.

The global carbon cycle controls the climate change in the Earth's environment on a geological timescale and is mainly associated with greenhouse effects produced by atmospheric carbon dioxide (CO2) and methane (CH4). This paper reviews the relationship between the global carbon cycle and presumed climate events during the Cenozoic. The global carbon cycle is primarily regulated by the balance between weathering and metamorphism-volcanism. Moreover, the organic carbon subcycle involving oxidative weathering and burial is of secondary importance. The balance of these geochemical processes results in variations of atmospheric CO2. The past climate on a geological time scale is reconstructed by several geochemical and paleontological methods or proxies. For example, sea-surface and deep-water temperature are deduced from oxygen isotope ratio and Mg/Ca ratio of foraminiferal tests. Terrestrial atmospheric temperature is estimated from leaf fossil and paleovegetation. Atmospheric CO2 level is calculated from carbon isotope ratios of phytoplankton and soil carbonate, stomatal density of leaf fossil, boron isotope ratio of foraminiferal test, Ce anomaly, and global carbon cycle modeling. It is important to consider their advantages and disadvantages in order to evaluate the paleoclimate adequately. Next, we discuss climate change based on these proxies. As a general trend, the Cenozoic climate change is characterized by a transition from ice-free to ice-covered conditions across the Eocene/Oligocene boundary. The Earth's surface environment was significantly warmed from the Paleocene to the Eocene by high levels of atmospheric CO2. Thereafter, it gradually cooled towards the present, which is possibly attributed to changes in ocean currents and other marine environments accompanying continental drift. This trend has been punctuated by several short-term climate events. The Paleocene-Eocene Thermal Maximum (PETM) was a remarkable warming event at the Paleocene/Eocene boundary, possibly attributed to the release of methane from hydrates into the atmosphere. Rapid cooling occurred at the Eocene/Oligocene boundary to form extensive continental ice sheets including the Antarctica, which seems to have been caused by atmospheric CO2 and change of oceanographic circulation and marine environment. After a moderate period from the late Oligocene to the early Miocene, there was a transient but significant warming in the middle Miocene. Since then, the Earth's environment has gradually cooled towards the present accompanied by the evolution of glaciations and marine environmental changes but a causal link between cooling and global carbon cycle has recently been pointed out. Although the carbon cycle including atmospheric CO2 and CH4 cannot explain all of the global climate changes in Cenozoic, it has undoubtedly played a dominant role on the Earth's climate.

Four State of the Art (SOA) reports, ''Atmospheric Carbon Dioxide and the Global Carbon Cycle,'' ''Direct Effects of Increasing Carbon Dioxide on Vegetation,'' ''Detecting the Climatic Effects of Increasing Carbon Dioxide,'' and ''Projecting the Climatic Effects of Increasing Carbon Dioxide,'' and two companion reports, ''Characterization of Information Requirements for Studies of CO/sub 2/ Effects: Water Resources, Agriculture, Fisheries, Forests and Human Health'' and ''Glaciers, Ice Sheets, and Sea Level: Effect of a CO/sub 2/-Induced Climatic Change,'' were published by the US Department of Energy's Carbon Dioxide Research Division. Considerable information on atmospheric carbon dioxide and its possible effects on world climate is summarized in these six volumes. Each volume has its own index, but to make the information that is distributed throughout the six volumes more accessible and usable, comprehensive citation and subject indexes have been compiled. The subject indexes of the individual volumes have been edited to provide a uniformity from volume to volume and also to draw distinctions not needed in the separate volumes' indexes. Also, the comprehensive subject index has been formatted in a matrix arrangement to graphically show the distribution of subject treatment from volume to volume. Other aids include cross references between the scientific and common names of the animals and plants referred to, a glossary of special terms used, tables of data and conversion factors related to the data, and explanations of the acronyms and initialisms used in the texts of the six volumes. The executive summaries of the six volumes are collected and reproduced to allow the readers interested in the contents of one volume to rapidly gain information on the contents of the other volumes.