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Activation of old carbon by erosion of coastal and subsea permafrost in Arctic Siberia
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The future trajectory of greenhouse gas concentrations depends on interactions between climate and the biogeosphere1,2. Thawing of Arctic permafrost could release significant amounts of carbon into the atmosphere in this century3. Ancient Ice Complex deposits outcropping along the 7,000-kilometre-long coastline of the East Siberian Arctic Shelf (ESAS)4,5, and associated shallow subsea permafrost6,7, are two large pools of permafrost carbon8, yet their vulnerabilities towards thawing and decomposition are largely unknown9–11. Recent Arctic warming is stronger than has been predicted by several degrees, and is particularly pronounced over the coastal ESAS region12,13. There is thus a pressing need to improve our understanding of the links between permafrost carbon and climate in this relatively inaccessible region. Here we show that extensive release of carbon from these Ice Complex deposits dominates (57 6 2 per cent) the sedimentary carbon budget of the ESAS, the world’s largest continental shelf, over- whelming the marine and topsoil terrestrial components. Inverse modelling of the dual-carbon isotope composition of organic carbon accumulating in ESAS surface sediments, using Monte Carlo simulations to account for uncertainties, suggests that 44 6 10 teragrams of old carbon is activated annually from Ice Complex permafrost, an order of magnitude more than has been suggested by previous studies14. We estimate that about two-thirds (66 6 16 per cent) of this old carbon escapes to the atmosphere as carbon dioxide, with the remainder being re-buried in shelf sediments. Thermal collapse and erosion of these carbon-rich Pleistocene coastline and seafloor deposits may accelerate with Arctic amplification of climate warming 2,13.
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Has the Earth’s sixth mass extinction already arrived?
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Palaeontologists characterize mass extinctions as times when the Earth loses more than three-quarters of its species in a geologically short interval, as has happened only five times in the past 540 million years or so. Biologists now suggest that a sixth mass extinction may be under way, given the known species losses over the past few centuries and millennia. Here we review how differences between fossil and modern data and the addition of recently available palaeontological information influence our understanding of the current extinction crisis. Our results confirm that current extinction rates are higher than would be expected from the fossil record, highlighting the need for effective conservation measures.
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Natural and anthropogenic variations in methane sources during the past two millennia
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Methane is an important greenhouse gas that is emitted from multiple natural and anthropogenic sources. Atmospheric methane concentrations have varied on a number of timescales in the past, but what has caused these variations is not always well understood1–8. The different sources and sinks of methane have specific isotopic signatures, and the isotopic composition of methane can therefore help to identify the environmental drivers of variations in atmo- spheric methane concentrations9. Here we present high-resolution carbon isotope data (d13C content) for methane from two ice cores from Greenland for the past two millennia. We find that the d13C content underwent pronounced centennial-scale variations between 100 BC and AD 1600. With the help of two-box model calculations, we show that the centennial-scale variations in isotope ratios can be attributed to changes in pyrogenic and biogenic sources. We find correlations between these source changes and both natural climate variability—such as the Medieval Climate Anomaly and the Little Ice Age—and changes in human population and land use, such as the decline of the Roman empire and the Han dynasty, and the population expansion during the medieval period.
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Comment: Time to Model all Life on Earth
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To help transform our understanding of the biosphere, ecologists — like climate scientists — should simulate whole ecosystems, argue Drew Purves and colleagues. FROM THE TEXT: General circulation models, which simulatethe physics and chemistry of Earth’s land, ocean and atmosphere, embody scientists’ best understanding of how the climate system works and are crucial to making predictions and shaping policies. We think that analogous general ecosystem models (GEMs) could radically improve understanding of the biosphere and inform policy decisions about biodiversity and conservation.
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Comment: Don’t judge species on their origins
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SUMMARY: Conservationists should assess organisms on environmental impact rather than on whether they are natives, argue Mark Davis and 18 other ecologists. FROM THE TEXT: Nativeness is not a sign of evolutionary fitness or of a species having positive effects.The insect currently suspected to be killing
more trees than any other in North Americais the native mountain pine beetle Dendroctonus ponderosae. Classifying biota according to their adherence to cultural standards of belonging, citizenship, fair play and
morality does not advance our understanding of ecology. Over the past few decades, this perspective has led many conservation and restoration efforts down paths that make little ecological or economic sense
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Shifts in Season
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Is the rising heat forcing change on the seasons? To find out, observed data may be superior to model projections.
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Atmospheric CO2 forces abrupt vegetation shifts locally, but not globally
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It is possible that anthropogenic climate change will drive the Earth system into a qualitatively different state1. Although different types of uncertainty limit our capacity to assess this risk 2, Earth system scientists are particularly concerned about tipping elements, large-scale components of the Earth system that can be switched into qualitatively different states by small perturbations. Despite growing evidence that tipping elements exist in the climate system1,3, whether large-scale vegetation systems can tip into alternative states is poorly understood4. Here we show that tropical grassland, savanna and forest ecosystems, areas large enough to have powerful impacts on the Earth system, are likely to shift to alternative states. Specifically, we show that increasing atmospheric CO2 concentration will force transitions to vegetation states characterized by higher biomass and/or woody-plant dominance. The timing of these critical transitions varies as a result of between-site variance in the rate of temperature increase, as well as a dependence on stochastic variation in fire severity and rainfall. We further show that the locations of bistable vegetation zones (zones where alternative vegetation states can exist) will shift as climate changes. We conclude that even though large-scale directional regime shifts in terrestrial ecosystems are likely, asynchrony in the timing of these shifts may serve to dampen, but not nullify, the shock that these changes may represent to the Earth system.
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Increased soil emissions of potent greenhouse gases under increased atmospheric CO2
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Increasing concentrations of atmospheric carbon dioxide (CO2) can affect biotic and abiotic conditions in soil, such as microbial activity and water content 1,2. In turn, these changes might be expected to alter the production and consumption of the important greenhouse gases nitrous oxide (N2O) and methane (CH4) (refs 2, 3). However, studies on fluxes of N2O and CH4 from soil under increased atmo- spheric CO2 have not been quantitatively synthesized. Here we show, using meta-analysis, that increased CO2 (ranging from 463 to 780 parts per million by volume) stimulates both N2O emissions from upland soils and CH4 emissions from rice paddies and natural wetlands. Because enhanced greenhouse-gas emissions add to the radiative forcing of terrestrial ecosystems, these emissions are expected to negate at least 16.6 per cent of the climate change mitigation potential previously predicted from an increase in the terrest- rial carbon sink under increased atmospheric CO2 concentrations4. Our results therefore suggest that the capacity of land ecosystems to slow climate warming has been overestimated.
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Asymmetric effects of economic growth and decline on CO2 emissions
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Letter to Editor: Excerpt: "Why does economic decline not have an effect on CO2 emissions that is symmetrical with the effect of economic growth? There are various reasons that this may occur, but the asymmetry is probably due to the fact that economic growth produces durable goods, such as cars and energy-intensive homes, and infrastructure, such as manufacturing facilities and transportation networks, that are not removed by economic decline and that continue to contribute to CO2 emissions even after growth is curtailed."
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Global diversity of drought tolerance and grassland climate-change resilience
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Drought reduces plant productivity, induces widespread plant mortality and limits the geographic distribution of plant species1–7. As climates warm and precipitation patterns shift in the future8,9, understanding the distribution of the diversity of plant drought tolerance is central to predicting future ecosystem function and resilience to climate change10–12 . These questions are especially pressing for the world’s 11,000 grass species13, which dominate a large fraction of the terrestrial biosphere14, yet are poorly characterized with respect to re- sponses to drought. Here, we show that physiological drought tolerance, which varied tenfold among 426 grass species, is well distributed both climatically and phylogenetically, sug- gesting most native grasslands are likely to contain a high diversity of drought tolerance. Consequently, local species may help maintain ecosystem functioning in response to changing drought regimes without requiring long-distance migrations of grass species. Furthermore, physiologically drought-tolerant species had higher rates of water and carbon dioxide exchange than intolerant species, indicating that severe droughts may generate legacies for ecosystem functioning. In all, our findings suggest that diverse grasslands throughout the globe have the potential to be resilient to drought in the face of climate change through the local expansion of drought-tolerant species.
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