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Anthropogenic influence on multidecadal changes in reconstructed global evapotranspiration
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Global warming is expected to intensify the global hydrological cycle1, with an increase of both evapotranspiration (EVT) and precipitation. Yet, the magnitude and spatial distribution of this global and annual mean response remains highly uncertain2. Better constraining land EVT in twenty-first-century climate scenarios is critical for predicting changes in surface climate, including heatwaves3 and droughts4, evaluating impacts on ecosystems and water resources5, and designing adaptation policies. Continental scale EVT changes may already be underway6,7, but have never been attributed to anthropogenic emissions of greenhouse gases and sulphate aerosols. Here we provide global gridded estimates of annual EVT and demonstrate that the latitudinal and decadal differentiation of recent EVT variations cannot be understood without invoking the anthropogenic radiative forcings. In the mid-latitudes, the emerging picture of enhanced EVT confirms the end of the dimming decades 8 and highlights the possible threat posed by increasing drought frequency to managing water resources and achieving food security in a changing climate.
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Enhanced poleward moisture transport and amplified northern high-latitude wetting trend
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Observations and climate change projections forced by greenhouse gas emissions have indicated a wetting trend in northern high latitudes, evidenced by increasing Eurasian Arctic river discharges (1–3). The increase in river discharge has accelerated in the latest decade and an unprecedented, record high discharge occurred in 2007 along with an extreme loss of Arctic summer sea-ice cover (4–6). Studies have ascribed this increasing discharge to various factors attributable to local global warming effects, including intensifying precip- itation minus evaporation, thawing permafrost, increasing greenness and reduced plant transpiration7–11. However, no agreement has been reached and causal physical processes remain unclear. Here we show that enhancement of poleward atmospheric moisture transport (AMT) decisively contributes to increased Eurasian Arctic river discharges. Net AMT into the Eurasian Arctic river basins captures 98% of the gauged climatological river discharges. The trend of 2.6% net AMT increase per decade accounts well for the 1.8% per decade increase in gauged discharges and also suggests an increase in underlying soil moisture. A radical shift of the atmospheric circulation pattern induced an unusually large AMT and warm surface in 2006–2007 over Eurasia, resulting in the record high discharge.
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Analysing fossil-fuel displacement
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It is commonly assumed that fossil fuels can be replaced by alternative forms of energy. Now research challenges this assumption, and highlights the role of non-technological solutions to reduce fossil-fuel consumption.
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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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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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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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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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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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Ecohydrologic separation of water between trees and streams in a Mediterranean climate
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Water movement in upland humid watersheds from the soil surface to the stream is often described using the concept of translatory flow (1,2), which assumes that water entering the soil as precipitation displaces the water that was present previously, pushing it deeper into the soil and eventually into the stream (2). Within this framework, water at any soil depth is well mixed and plants extract the same water that eventually enters the stream. Here we present water-isotope data from various pools throughout a small watershed in the Cascade Mountains, Oregon, USA. Our data imply that a pool of tightly bound water that is retained in the soil and used by trees does not participate in translatory flow, mix with mobile water or enter the stream. Instead, water from initial rainfall events after rainless summers is locked into small pores with low matric potential until transpiration empties these pores during following dry summers. Winter rainfall does not displace this tightly bound water. As transpiration and stormflow are out of phase in the Mediterranean climate of our study site, two separate sets of water bodies with different isotopic characteristics exist in trees and streams. We conclude that complete mixing of water within the soil cannot be assumed for similar hydroclimatic regimes as has been done in the past (3,4) .
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A reality check on the shale revolution
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The production of shale gas and oil in the United States is overhyped and the costs are underestimated, says J. David Hughes.
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