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Activation of old carbon by erosion of coastal and subsea permafrost in Arctic Siberia: 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.

Ecohydrologic separation of water between trees and streams in a Mediterranean climate: 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) .

A reality check on the shale revolution: The production of shale gas and oil in the United States is overhyped and the costs are underestimated, says J. David Hughes.

Civil conflicts are associated with the global climate: It has been proposed that changes in global climate have been responsible for episodes of widespread violence and even the collapse of civilizations 1,2. Yet previous studies have not shown that violence can be attributed to the global climate, only that random weather events might be correlated with conflict in some cases 3–7. Here we directly associate planetary-scale climate changes with global patterns of civil conflict by examining the dominant inter- annual mode of the modern climate 8–10, the El Nino/Southern Oscillation (ENSO). Historians have argued that ENSO may have driven global patterns of civil conflict in the distant past11–13, a hypothesis that we extend to the modern era and test quantitatively. Using data from 1950 to 2004, we show that the probability of new civil conflicts arising throughout the tropics doubles during El Nino years relative to La Nina years. This result, which indicates that ENSO may have had a role in 21% of all civil conflicts since 1950, is the first demonstration that the stability of modern societies relates strongly to the global climate.

Comment: The end of cheap coal: New forecasts suggest that coal reserves will run out faster than many believe. Energy policies relying on cheap coal have no future, say Richard Heinberg and David Fridley.

The rebound effect is overplayed: Increasing energy efficiency brings emissions savings. Claims that it backfires are a distraction, say Kenneth Gillingham and colleagues.

The Next Dust Bowl: Drought is the most pressing problem caused by climate change. It receives too little attention, says Joseph Romm.

Observed increase in local cooling effect of deforestation at higher latitudes: Deforestation in mid- to high latitudes is hypothesized to have the potential to cool the Earth’s surface by altering biophysical processes1–3. In climate models of continental-scale land clearing, the cooling is triggered by increases in surface albedo and is reinforced by a land albedo–sea ice feedback 4,5. This feedback is crucial in the model predictions; without it other biophysical processes may overwhelm the albedo effect to generate warming instead5. Ongoing land-use activities, such as land management for climate mitigation, are occurring at local scales (hectares) presumably too small to generate the feedback, and it is not known whether the intrinsic biophysical mechanism on its own can change the surface temperature in a consistent manner6,7. Nor has the effect of deforestation on climate been demonstrated over large areas from direct observations. Here we show that surface air temper- ature is lower in open land than in nearby forested land. The effect is 0.85 6 0.44 K (mean 6 one standard deviation) northwards of 456N and 0.2160.53K southwards. Below 356N there is weak evidence that deforestation leads to warming. Results are based on comparisons of temperature at forested eddy covariance towers in the USA and Canada and, as a proxy for small areas of cleared land, nearby surface weather stations. Night-time temperature changes unrelated to changes in surface albedo are an important contributor to the overall cooling effect. The observed latitudinal dependence is consistent with theoretical expectation of changes in energy loss from convection and radiation across latitudes in both the daytime and night-time phase of the diurnal cycle, the latter of which remains uncertain in climate models8.

Genealogy of nature conservation: a political perspective: Modern nature conservation is a product of post-Enlightenment modernity; I explore the heterogeneity of its conceptual and ideological background. The 19th century legacy comprises concern over human-caused extinctions; protests against excessive hunting and cruelty toward animals; utilitarian care for natural resources; and romantic sensibility concerning the value of nature for human health and spirituality. The 20th century added into conservation thinking increasing consciousness about human biospheric dependence; efforts to identify appropriate conservation targets; and most recently concern over the loss of biodiversity. The politics of nature conservation has taken shape within the framework of politics of nature, that is, choices vis-á-vis nature that have been made either as deliberate decisions on resource use or as side-effects of subsistence practices of various types. Because of tensions and conflicts with alternative ways of using nature, formulating realistic conservation policies has been a complicated task. Problems and uncertainties emerge: pursuing material aspirations of the current world society will necessarily bring about damage to ecological systems of the Earth. The way forward is to identify feasible alternatives in the midst of the tensions and ambiguities that arise, and to open up space for carrying through conservation initiatives. Keywords conservation thought, conservation policy, conservation governance, utilitarian conservation, romanticism, genealogy, framing, normativity, normative order, action space

Forests fuel fish growth in freshwater deltas: Aquatic ecosystems are fuelled by biogeochemical inputs from surrounding lands and within- lake primary production. Disturbances that change these inputs may affect how aquatic ecosystems function and deliver services vital to humans. Here we test, using a forest cover gradient across eight separate catchments, whether disturbances that remove terrestrial biomass lower organic matter inputs into freshwater lakes, thereby reducing food web productivity. We focus on deltas formed at the stream-lake interface where terrestrial-derived particulate material is deposited. We find that organic matter export increases from more forested catchments, enhancing bacterial biomass. This transfers energy upwards through communities of heavier zooplankton, leading to a fourfold increase in weights of plankti- vorous young-of-the-year fish. At least 34% of fish biomass is supported by terrestrial primary production, increasing to 66% with greater forest cover. Habitat tracers confirm fish were closely associated with individual catchments, demonstrating that watershed protection and restoration increase biomass in critical life-stages of fish.

Delayed phenology and reduced fitness associated with climate change in a wild hibernator: The most commonly reported ecological effects of climate change are shifts in phenologies, in particular of warmer spring temperatures leading to earlier timing of key events 1,2. Among animals, however, these reports have been heavily biased towards avian phenologies, whereas we still know comparatively little about other seasonal adaptations, such as mammalian hibernation. Here we show a significant delay (0.47 days per year, over a 20-year period) in the hibernation emergence date of adult females in a wild population of Columbian ground squirrels in Alberta, Canada. This finding was related to the climatic conditions at our study location: owing to within-individual phenotypic plasticity, females emerged later during years of lower spring temperature and delayed snowmelt. Although there has not been a significant annual trend in spring temperature, the date of snowmelt has become progressively later owing to an increasing prevalence of late-season snowstorms. Importantly, years of later emergence were also associated with decreased individual fitness. There has consequently been a decline in mean fitness (that is, population growth rate) across the past two decades. Our results show that plastic responses to climate change may be driven by climatic trends other than increasing temperature, and may be associated with declines in individual fitness and, hence, population viability.

Coastal habitats shield people and property from sea-level rise and storms: Extreme weather, sea-level rise and degraded coastal ecosystems are placing people and property at greater risk of damage from coastal hazards 1–5. The likelihood and magnitude of losses may be reduced by intact reefs and coastal vegetation 1, especially when those habitats fringe vulnerable communities and infrastructure. Using five sea-level-rise scenarios, we calculate a hazard index for every 1 km2 of the United States coastline. We use this index to identify the most vulnerable people and property as indicated by being in the upper quartile of hazard for the nation’s coastline. The number of people, poor families, elderly and total value of residential property that are most exposed to hazards can be reduced by half if existing coastal habitats remain fully intact. Coastal habitats defend the greatest number of people and total property value in Florida, New York and California. Our analyses deliver the first national map of risk reduction owing to natural habitats and indicates where conservation and restoration of reefs and vegetation have the greatest potential to protect coastal communities.

Carbon loss from an unprecedented Arctic tundra wildfire: Arctic tundra soils store large amounts of carbon (C) in organic soil layers hundreds to thousands of years old that insulate, and in some cases maintain, permafrost soils1,2. Fire has been largely absent from most of this biome since the early Holocene epoch3, but its frequency and extent are increasing, probably in response to climate warming4. The effect of fires on the C balance of tundra landscapes, however, remains largely unknown. The Anaktuvuk River fire in 2007 burned 1,039 square kilometres of Alaska’s Arctic slope, making it the largest fire on record for the tundra biome and doubling the cumulative area burned since 1950 (ref. 5). Here we report that tundra ecosystems lost 2,016 6 435 g C m22 in the fire, an amount two orders of magnitude larger than annual net C exchange in undisturbed tundra6. Sixty per cent of this C loss was from soil organic matter, and radiocarbon dating of residual soil layers revealed that the maximum age of soil C lost was 50 years. Scaled to the entire burned area, the fire released approximately 2.1 teragrams of C to the atmosphere, an amount similar in magnitude to the annual net C sink for the entire Arctic tundra biome averaged over the last quarter of the twentieth century7. The mag- nitude of ecosystem C lost by fire, relative to both ecosystem and biome-scale fluxes, demonstrates that a climate-driven increase in tundra fire disturbance may represent a positive feedback, potentially offsetting Arctic greening 8 and influencing the net C balance of the tundra biome.

A large source of low-volatility secondary organic aerosol: Forests emit large quantities of volatile organic compounds (VOCs) to the atmosphere. Their condensable oxidation products can form secondary organic aerosol, a significant and ubiquitous component of atmospheric aerosol 1,2, which is known to affect the Earth’s radiation balance by scattering solar radiation and by acting as cloud condensation nuclei 3. The quantitative assessment of such climate effects remains hampered by a number of factors, including an incom- plete understanding of how biogenic VOCs contribute to the formation of atmospheric secondary organic aerosol. The growth of newly formed particles from sizes of less than three nanometres up to the sizes of cloud condensation nuclei (about one hundred nanometres) in many continental ecosystems requires abundant, essentially non- volatile organic vapours4–6, but the sources and compositions of such vapours remain unknown. Here we investigate the oxidation of VOCs, in particular the terpene a-pinene, under atmospherically relevant conditions in chamber experiments. We find that a direct pathway leads from several biogenic VOCs, such as monoterpenes, to the for- mation of large amounts of extremely low-volatility vapours. These vapours form at significant mass yield in the gas phase and condense irreversibly onto aerosol surfaces to produce secondary organic aero- sol, helping to explain the discrepancy between the observed atmo- spheric burden of secondary organic aerosol and that reported by many model studies2. We further demonstrate how these low-volatility vapours can enhance, or even dominate, the formation and growth of aerosol particles over forested regions, providing a missing link between biogenic VOCs and their conversion to aerosol particles. Our findings could help to improve assessments of biosphere–aerosol– climate feedback mechanisms 6–8, and the air quality and climate effects of biogenic emissions generally.

Terrestrial water fluxes dominated by transpiration: Renewable fresh water over continents has input from precipitation and losses to the atmosphere through evaporation and transpiration. Global-scale estimates of transpiration from climate models are poorly constrained owing to large uncertainties in stomatal conductance and the lack of catchment-scale measurements required for model calibration, resulting in a range of predictions spanning 20 to 65 per cent of total terrestrial evapotranspiration (14,000 to 41,000 km3 per year) (refs 1–5). Here we use the distinct isotope effects of transpiration and evaporation to show that transpiration is by far the largest water flux from Earth’s continents, representing 80 to 90 per cent of terrestrial evapotranspiration. On the basis of our analysis of a global data set of large lakes and rivers, we conclude that transpiration recycles 62,000 6 8,000 km3 of water per year to the atmosphere, using half of all solar energy absorbed by land surfaces in the process. We also calculate CO2 uptake by terrestrial vegetation by connecting transpiration losses to carbon assimilation using water-use efficiency ratios of plants, and show the global gross primary productivity to be 129 6 32 giga- tonnes of carbon per year, which agrees, within the uncertainty, with previous estimates6. The dominance of transpiration water fluxes in continental evapotranspiration suggests that, from the point of view of water resource forecasting, climate model development should prioritize improvements in simulations of biological fluxes rather than physical (evaporation) fluxes.

Probabilistic cost estimates for climate change mitigation: For more than a decade, the target of keeping global warming below 2 6C has been a key focus of the international climate debate1. In response, the scientific community has published a number of scenario studies that estimate the costs of achieving such a target 2–5. Producing these estimates remains a challenge, particularly because of relatively well known, but poorly quantified, uncertainties, and owing to limited integration of scientific knowledge across disciplines6. The integrated assessment community, on the one hand, has extensively assessed the influence of technological and socio-economic uncertainties on low-carbon scenarios and asso- ciated costs2–4,7. The climate modelling community, on the other hand, has spent years improving its understanding of the geo- physical response of the Earth system to emissions of greenhouse gases8–12. This geophysical response remains a key uncertainty in the cost of mitigation scenarios but has been integrated with assess- ments of other uncertainties in only a rudimentary manner, that is, for equilibrium conditions6,13. Here we bridge this gap between the two research communities by generating distributions of the costs associated with limiting transient global temperature increase to below specific values, taking into account uncertainties in four factors: geophysical, technological, social and political. We find that political choices that delay mitigation have the largest effect on the cost–risk distribution, followed by geophysical uncertainties, social factors influencing future energy demand and, lastly, technological uncertainties surrounding the availability of greenhouse gas miti- gation options. Our information on temperature risk and mitigation costs provides crucial information for policy-making, because it clarifies the relative importance of mitigation costs, energy demand and the timing of global action in reducing the risk of exceeding a global temperature increase of 2 6C, or other limits such as 3 6C or 1.5 6C, across a wide range of scenarios.

Biodiversity loss and its impact on humanity: The most unique feature of Earth is the existence of life, and the most extraordinary feature of life is its diversity. Approximately 9 million types of plants, animals, protists and fungi inhabit the Earth. So, too, do 7 billion people. Two decades ago, at the first Earth Summit, the vast majority of the world’s nations declared that human actions were dismantling the Earth’s ecosystems, eliminating genes, species and biological traits at an alarming rate. This observation led to the question of how such loss of biological diversity will alter the functioning of ecosystems and their ability to provide society with the goods and services needed to prosper.

International trade drives biodiversity threats in developing nations: Human activities are causing Earth’s sixth major extinction event1— an accelerating decline of the world’s stocks of biological diversity at rates 100 to 1,000 times pre-human levels2. Historically, low-impact intrusion into species habitats arose from local demands for food, fuel and living space3. However, in today’s increasingly globalized economy, international trade chains accelerate habitat degradation far removed from the place of consumption. Although adverse effects of economic prosperity and economic inequality have been confirmed4,5, the importance of international trade as a driver of threats to species is poorly understood. Here we show that a signifi- cant number of species are threatened as a result of international trade along complex routes, and that, in particular, consumers in developed countries cause threats to species through their demand of commodities that are ultimately produced in developing countries. We linked 25,000 Animalia species threat records from the International Union for Conservation of Nature Red List to more than 15,000 commodities produced in 187 countries and evaluated more than 5billion supply chains in terms of their biodiversity impacts. Excluding invasive species, we found that 30% of global species threats are due to international trade. In many developed countries, the consumption of imported coffee, tea, sugar, textiles, fish and other manufactured items causes a biodiversity footprint that is larger abroad than at home. Our results emphasize the importance of examining biodiversity loss as a global systemic phe- nomenon, instead of looking at the degrading or polluting producers in isolation. We anticipate that our findings will facilitate better regulation, sustainable supply-chain certification and consumer product labelling.

A global synthesis reveals biodiversity loss as a major driver of ecosystem change: Evidence is mounting that extinctions are altering key processes important to the productivity and sustainability of Earth’s ecosystems (1–4). Further species loss will accelerate change in ecosystem processes (5–8), but it is unclear how these effects compare to the direct effects of other forms of environmental change that are both driving diversity loss and altering ecosystem function. Here we use a suite of meta-analyses of published data to show that the effects of species loss on productivity and decomposition—two processes important in all ecosystems—are of comparable magnitude to the effects of many other global environmental changes. In experiments, intermediate levels of species loss (21–40%) reduced plant production by 5–10%, comparable to previously documented effects of ultraviolet radiation and climate warming. Higher levels of extinction (41–60%) had effects rivalling those of ozone, acidification, elevated CO2 and nutrient pollution. At intermediate levels, species loss generally had equal or greater effects on decomposition than did elevated CO2 and nitrogen addition. The identity of species lost also had a large effect on changes in productivity and decomposition, generating a wide range of plausible outcomes for extinction. Despite the need for more studies on interactive effects of diversity loss and environmental changes, our analyses clearly show that the ecosystem consequences of local species loss are as quantitatively significant as the direct effects of several global change stressors that have mobilized major international concern and remediation efforts (9).

Approaching a state shift in Earth’s biosphere: Localized ecological systems are known to shift abruptly and irreversibly from one state to another when they are forced across critical thresholds. Here we review evidence that the global ecosystem as a whole can react in the same way and is approaching a planetary-scale critical transition as a result of human influence. The plausibility of a planetary-scale ‘tipping point’ highlights the need to improve biological forecasting by detecting early warning signs of critical transitions on global as well as local scales, and by detecting feedbacks that promote such transitions. It is also necessary to address root causes of how humans are forcing biological changes.

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