Showing papers in "Nature Geoscience in 2016"

Reduced sediment transport in the Yellow River due to anthropogenic changes

Abstract: The erosion, transport and redeposition of sediments shape the Earth's surface, and affect the structure and function of ecosystems and society(1,2). The Yellow River was once the world's largest carrier of fluvial sediment, but its sediment load has decreased by approximately 90% over the past 60 years(3). The decline in sediment load is due to changes in water discharge and sediment concentration, which are both influenced by regional climate change and human activities. Here we use an attribution approach to analyse 60 years of runoff and sediment load observations from the traverse of the Yellow River over China's Loess Plateau - the source of nearly 90% of its sediment load. We find that landscape engineering, terracing and the construction of check dams and reservoirs were the primary factors driving reduction in sediment load from the 1970s to 1990s, but large-scale vegetation restoration projects have also reduced soil erosion from the 1990s onwards. We suggest that, as the ability of existing dams and reservoirs to trap sediments declines in the future, erosion rates on the Loess Plateau will increasingly control the Yellow River's sediment load.

Late Miocene global cooling and the rise of modern ecosystems

Abstract: A period of continental aridification and ecosystem change occurred about seven million years ago. A global sea surface temperature reconstruction identifies cooling temperatures and a strengthened meridional temperature gradient at this time.

Substantial role of macroalgae in marine carbon sequestration

Abstract: Marine macroalgae are dominant primary producers in coastal zones. A review of the published literature suggests that macroalgae may play an important role in carbon sequestration. Vegetated coastal habitats have been identified as important carbon sinks. In contrast to angiosperm-based habitats such as seagrass meadows, salt marshes and mangroves, marine macroalgae have largely been excluded from discussions of marine carbon sinks. Macroalgae are the dominant primary producers in the coastal zone, but they typically do not grow in habitats that are considered to accumulate large stocks of organic carbon. However, the presence of macroalgal carbon in the deep sea and sediments, where it is effectively sequestered from the atmosphere, has been reported. A synthesis of these data suggests that macroalgae could represent an important source of the carbon sequestered in marine sediments and the deep ocean. We propose two main modes for the transport of macroalgae to the deep ocean and sediments: macroalgal material drifting through submarine canyons, and the sinking of negatively buoyant macroalgal detritus. A rough estimate suggests that macroalgae could sequester about 173 TgC yr−1 (with a range of 61–268 TgC yr−1) globally. About 90% of this sequestration occurs through export to the deep sea, and the rest through burial in coastal sediments. This estimate exceeds that for carbon sequestered in angiosperm-based coastal habitats.

Pan-Arctic ice-wedge degradation in warming permafrost and its influence on tundra hydrology

Abstract: Ice wedges are common features of the subsurface in permafrost regions. They develop by repeated frost cracking and ice vein growth over hundreds to thousands of years. Ice-wedge formation causes the archetypal polygonal patterns seen in tundra across the Arctic landscape. Here we use field and remote sensing observations to document polygon succession due to ice-wedge degradation and trough development in ten Arctic localities over sub-decadal timescales. Initial thaw drains polygon centres and forms disconnected troughs that hold isolated ponds. Continued ice-wedge melting leads to increased trough connectivity and an overall draining of the landscape. We find that melting at the tops of ice wedges over recent decades and subsequent decimetre-scale ground subsidence is a widespread Arctic phenomenon. Although permafrost temperatures have been increasing gradually, we find that ice-wedge degradation is occurring on sub-decadal timescales. Our hydrological model simulations show that advanced ice-wedge degradation can significantly alter the water balance of lowland tundra by reducing inundation and increasing runoff, in particular due to changes in snow distribution as troughs form. We predict that ice-wedge degradation and the hydrological changes associated with the resulting differential ground subsidence will expand and amplify in rapidly warming permafrost regions. The polygonal patterns in permafrost regions are caused by the formation of ice wedges. Observations of polygon evolution reveal that rapid ice-wedge melting has occurred across the Arctic since 1950, altering tundra hydrology.

Cooling and societal change during the Late Antique Little Ice Age from 536 to around 660 AD

Abstract: Societal upheaval occurred across Eurasia in the sixth and seventh centuries. Tree-ring reconstructions suggest a period of pronounced cooling during this time associated with several volcanic eruptions. Climatic changes during the first half of the Common Era have been suggested to play a role in societal reorganizations in Europe1,2 and Asia3,4. In particular, the sixth century coincides with rising and falling civilizations1,2,3,4,5,6, pandemics7,8, human migration and political turmoil8,9,10,11,12,13. Our understanding of the magnitude and spatial extent as well as the possible causes and concurrences of climate change during this period is, however, still limited. Here we use tree-ring chronologies from the Russian Altai and European Alps to reconstruct summer temperatures over the past two millennia. We find an unprecedented, long-lasting and spatially synchronized cooling following a cluster of large volcanic eruptions in 536, 540 and 547 AD (ref. 14), which was probably sustained by ocean and sea-ice feedbacks15,16, as well as a solar minimum17. We thus identify the interval from 536 to about 660 AD as the Late Antique Little Ice Age. Spanning most of the Northern Hemisphere, we suggest that this cold phase be considered as an additional environmental factor contributing to the establishment of the Justinian plague7,8, transformation of the eastern Roman Empire and collapse of the Sasanian Empire1,2,5, movements out of the Asian steppe and Arabian Peninsula8,11,12, spread of Slavic-speaking peoples9,10 and political upheavals in China13.

Drought in the Anthropocene

Abstract: Drought management is inefficient because feedbacks between drought and people are not fully understood. In this human-influenced era, we need to rethink the concept of drought to include the human role in mitigating and enhancing drought.

Large contribution to inland water CO2 and CH4 emissions from very small ponds

Abstract: Very small ponds have been omitted from greenhouse gas budgets. Estimates of CO2 and CH4 emissions from 427 lakes and ponds show that very small ponds account for 15% of CO2 and 40% of diffusive CH4 emissions, but 8.6% of lake and pond area. Inland waters are an important component of the global carbon cycle. Although they contribute to greenhouse gas emissions1,2,3,4,5, estimates of carbon processing in these waters are uncertain. The global extent of very small ponds, with surface areas of less than 0.001 km2, is particularly difficult to map, resulting in their exclusion from greenhouse gas budget estimates. Here we combine estimates of the lake and pond global size distribution, gas exchange rates, and measurements of carbon dioxide and methane concentrations from 427 lakes and ponds ranging in surface area from 2.5 m2 to 674 km2. We estimate that non-running inland waters release 0.583 Pg C yr−1. Very small ponds comprise 8.6% of lakes and ponds by area globally, but account for 15.1% of CO2 emissions and 40.6% of diffusive CH4 emissions. In terms of CO2 equivalence, the ratio of CO2 to CH4 flux increases with surface area, from about 1.5 in very small ponds to about 19 in large lakes. The high fluxes from very small ponds probably result from shallow waters, high sediment and edge to water volume ratios, and frequent mixing. These attributes increase CO2 and CH4 supersaturation in the water and limit efficient methane oxidation. We conclude that very small ponds represent an important inland water carbon flux.

The global volume and distribution of modern groundwater

Abstract: Groundwater recharged less than 50 years ago is vulnerable to contamination and land-use changes. Data and simulations suggest that up to 6% of continental groundwater is modern—forming the largest component of the active hydrologic cycle.

Southern Ocean warming delayed by circumpolar upwelling and equatorward transport

Abstract: Unlike the Arctic, the Southern Ocean has shown little warming. An analysis of observations and numerical simulations suggests that Southern Ocean warming patterns are shaped by meridional overturning more than surface heating.

Climate-sensitive northern lakes and ponds are critical components of methane release

Abstract: Lakes and ponds represent one of the largest natural sources of the greenhouse gas methane. By surface area, almost half of these waters are located in the boreal region and northwards. A synthesis ...

High aerosol acidity despite declining atmospheric sulfate concentrations over the past 15 years

Abstract: Atmospheric sulfate levels are thought to determine the pH of small aerosol particles. Thermodynamic analysis of field aerosol data reveals that fine particles remain acidic in the southeastern United States despite large sulfate reductions.

Storm track processes and the opposing influences of climate change

Abstract: Extratropical storms contribute to precipitation, wind and temperature extremes. A synthesis of the influences of a changing climate on storm tracks reveals competing effects on meridional temperature gradients, which make projections difficult.

Groundwater quality and depletion in the Indo-Gangetic Basin mapped from in situ observations

Abstract: Groundwater abstraction from the transboundary Indo-Gangetic Basin comprises 25% of global groundwater withdrawals, sustaining agricultural productivity in Pakistan, India, Nepal and Bangladesh. Recent interpretations of satellite gravity data indicate that current abstraction is unsustainable, yet these large-scale interpretations lack the spatio-temporal resolution required to govern groundwater effectively. Here we report new evidence from high-resolution in situ records of groundwater levels, abstraction and groundwater quality, which reveal that sustainable groundwater supplies are constrained more by extensive contamination than depletion. We estimate the volume of groundwater to 200 m depth to be >20 times the combined annual flow of the Indus, Brahmaputra and Ganges, and show the water table has been stable or rising across 70% of the aquifer between 2000 and 2012. Groundwater levels are falling in the remaining 30%, amounting to a net annual depletion of 8.0 ± 3.0 km3. Within 60% of the aquifer, access to potable groundwater is restricted by excessive salinity or arsenic. Recent groundwater depletion in northern India and Pakistan has occurred within a longer history of groundwater accumulation from extensive canal leakage. This basin-wide synthesis of in situ groundwater observations provides the spatial detail essential for policy development, and the historical context to help evaluate recent satellite gravity data.

Substantial global carbon uptake by cement carbonation

Abstract: Calcination of carbonate rocks during the manufacture of cement produced 5% of global CO_2 emissions from all industrial process and fossil-fuel combustion in 2013. Considerable attention has been paid to quantifying these industrial process emissions from cement production, but the natural reversal of the process—carbonation—has received little attention in carbon cycle studies. Here, we use new and existing data on cement materials during cement service life, demolition, and secondary use of concrete waste to estimate regional and global CO_2 uptake between 1930 and 2013 using an analytical model describing carbonation chemistry. We find that carbonation of cement materials over their life cycle represents a large and growing net sink of CO_2, increasing from 0.10 GtC yr^(−1) in 1998 to 0.25 GtC yr^(−1) in 2013. In total, we estimate that a cumulative amount of 4.5 GtC has been sequestered in carbonating cement materials from 1930 to 2013, offsetting 43% of the CO_2 emissions from production of cement over the same period, not including emissions associated with fossil use during cement production. We conclude that carbonation of cement products represents a substantial carbon sink that is not currently considered in emissions inventories.

Himalayan megathrust geometry and relation to topography revealed by the Gorkha earthquake

Abstract: The Himalayan mountain range has been the locus of some of the largest continental earthquakes, including the 2015 magnitude 7.8 Gorkha earthquake. Competing hypotheses suggest that Himalayan topography is sustained and plate convergence is accommodated either predominantly on the main plate boundary fault, or more broadly across multiple smaller thrust faults. Here we use geodetic measurements of surface displacement to show that the Gorkha earthquake ruptured the Main Himalayan Thrust fault. The earthquake generated about 1 m of uplift in the Kathmandu Basin, yet caused the high Himalaya farther north to subside by about 0.6 m. We use the geodetic data, combined with geologic, geomorphological and geophysical analyses, to constrain the geometry of the Main Himalayan Thrust in the Kathmandu area. Structural analyses together with interseismic and coseismic displacements are best explained by a steep, shallow thrust fault flattening at depth between 5 and 15 km and connecting to a mid-crustal, steeper thrust. We suggest that present-day convergence across the Himalaya is mostly accommodated by this fault—no significant motion on smaller thrust faults is required. Furthermore, given that the Gorkha earthquake caused the high Himalayan mountains to subside and that our fault geometry explains measured interseismic displacements, we propose that growth of Himalayan topography may largely occur during the ongoing post-seismic phase.

Skilful predictions of the winter North Atlantic Oscillation one year ahead

Abstract: The North Atlantic Oscillation profoundly influences European and North American winter weather. Dynamical model predictions now exhibit skill in prediction of the winter North Atlantic Oscillation more than one year in advance.

Substantial nitrogen pollution embedded in international trade

Abstract: Anthropogenic emissions of reactive nitrogen have had severe environmental impacts. An analysis of reactive nitrogen emissions from the production, consumption and transport of commodities attributes roughly a quarter to international trade. Anthropogenic emissions of reactive nitrogen to the atmosphere and water bodies can damage human health and ecosystems1,2. As a measure of a nation’s contribution to this potential damage, a country’s nitrogen footprint has been defined as the quantity of reactive nitrogen emitted during the production, consumption and transportation of commodities consumed within that country, whether those commodities are produced domestically or internationally3. Here we use global emissions databases4,5, a global nitrogen cycle model6, and a global input–output database of domestic and international trade7,8 to calculate the nitrogen footprints for 188 countries as the sum of emissions of ammonia, nitrogen oxides and nitrous oxide to the atmosphere, and of nitrogen potentially exportable to water bodies. Per-capita footprints range from under 7 kg N yr−1 in some developing countries to over 100 kg N yr−1 in some wealthy nations. Consumption in China, India, the United States and Brazil is responsible for 46% of global emissions. Roughly a quarter of the global nitrogen footprint is from commodities that were traded across country borders. The main net exporters have significant agricultural, food and textile exports, and are often developing countries, whereas important net importers are almost exclusively developed economies. We conclude that substantial local nitrogen pollution is driven by demand from consumers in other countries.

Organic carbon decomposition rates controlled by water retention time across inland waters

Abstract: Organic carbon decays as it travels through inland waters from soils to the sea. Analysis of data from across the continuum of inland and marine aquatic systems reveals that the rate of organic carbon decay depends on water retention time.

Continent-sized anomalous zones with low seismic velocity at the base of Earth's mantle

Abstract: Two large zones through which seismic waves travel unusually slowly are found at the base of Earth's mantle. These zones are thermally and chemically distinct from the surrounding mantle and may be a source for mantle plumes.

Anthropogenic carbon release rate unprecedented during the past 66 million years

Abstract: Carbon release rates during the Palaeocene–Eocene Thermal Maximum are difficult to constrain. Comparing relative rates of carbon cycle and climate change at the event’s onset suggests emissions were much slower than anthropogenic emissions.

A scientific critique of the two-degree climate change target

Abstract: Many governments agreed to limit global mean temperature change to below 2 °C, yet this level has not been assessed scientifically. A synthesis of the literature suggests that temperature is the best available target quantity, but a safe level is uncertain.

Long-term accumulation and transport of anthropogenic phosphorus in three river basins

Abstract: Global food production depends on phosphorus. Phosphorus is broadly applied as fertilizer, but excess phosphorus contributes to eutrophication of surface water bodies and coastal ecosystems1. Here we present an analysis of phosphorus fluxes in three large river basins, including published data on fertilizer, harvested crops, sewage, food waste and river fluxes2, 3, 4. Our analyses reveal that the magnitude of phosphorus accumulation has varied greatly over the past 30–70 years in mixed agricultural–urban landscapes of the Thames Basin, UK, the Yangtze Basin, China, and the rural Maumee Basin, USA. Fluxes of phosphorus in fertilizer, harvested crops, food waste and sewage dominate over the river fluxes. Since the late 1990s, net exports from the Thames and Maumee Basins have exceeded inputs, suggesting net mobilization of the phosphorus pool accumulated in earlier decades. In contrast, the Yangtze Basin has consistently accumulated phosphorus since 1980. Infrastructure modifications such as sewage treatment and dams may explain more recent declines in total phosphorus fluxes from the Thames and Yangtze Rivers3, 4. We conclude that human-dominated river basins may undergo a prolonged but finite accumulation phase when phosphorus inputs exceed agricultural demand, and this accumulated phosphorus may continue to mobilize long after inputs decline.

Substantial proportion of global streamflow less than three months old

Abstract: Streamflow is a mixture of precipitation of various ages. Oxygen isotope data suggests that a third of global river discharge is sourced from rainfall within the past few months, which accounts for less than 0.1% of global groundwater.

Prebiotic chemistry and atmospheric warming of early Earth by an active young Sun

Abstract: An energetic process is needed to convert N2 into compounds essential for life Simulations show that interactions between powerful solar flares and Earth’s magnetic field could have facilitated nitrogen fixation in the early atmosphere

Twenty-five winters of unexpected Eurasian cooling unlikely due to Arctic sea-ice loss

Abstract: Winter cooling over Eurasia has been suggested to be linked to Arctic sea-ice loss. Climate model simulations reveal no evidence for such a link and instead suggest that a persistent atmospheric circulation pattern is responsible.

Intensification of landfalling typhoons over the northwest Pacific since the late 1970s

Abstract: Landfalling typhoons can cause great damage in East and Southeast Asian countries. An analysis of bias-corrected data sets reveals that the proportion of the strongest landfalling typhoons has at least doubled over the past decades.

Long-lived connection between southern Siberia and northern Laurentia in the Proterozoic

Abstract: Precambrian supercontinents Nuna-Columbia (1.7 to 1.3 billion years ago) and Rodinia (1.1 to 0.7 billion years ago) have been proposed. However, the arrangements of crustal blocks within these supercontinents are poorly known. Huge, dominantly basaltic magmatic outpourings and intrusions, covering up to millions of square kilometres, termed Large Igneous Provinces, typically accompany (super) continent breakup, or attempted breakup and offer an important tool for reconstructing supercontinents. Here we focus on the Large Igneous Province record for Siberia and Laurentia, whose relative position in Nuna-Columbia and Rodinia reconstructions is highly controversial. We present precise geochronology—nine U–Pb and six Ar–Ar ages—on dolerite dykes and sills, along with existing dates from the literature, that constrain the timing of emplacement of Large Igneous Province magmatism in southern Siberia and northern Laurentia between 1,900 and 720 million years ago. We identify four robust age matches between the continents 1,870, 1,750, 1,350 and 720 million years ago, as well as several additional approximate age correlations that indicate southern Siberia and northern Laurentia were probably near neighbours for this 1.2-billion-year interval. Our reconstructions provide a framework for evaluating the shared geological, tectonic and metallogenic histories of these continental blocks. The configurations of the ancient supercontinents are poorly known. Analysis of the ages of giant magma intrusions that affected both Siberia and Laurentia shows that the two continents were connected, possibly for as long as 1.2 billion years.

Marine methane paradox explained by bacterial degradation of dissolved organic matter

Abstract: A lot of methane is emitted from oxygenated seawater, where its production should be inhibited. Seawater incubations and organic matter characterizations reveal that bacteria aerobically produce methane from phosphonates in organic matter. Biogenic methane is widely thought to be a product of archaeal methanogenesis, an anaerobic process that is inhibited or outcompeted by the presence of oxygen and sulfate1,2,3. Yet a large fraction of marine methane delivered to the atmosphere is produced in high-sulfate, fully oxygenated surface waters that have methane concentrations above atmospheric equilibrium values, an unexplained phenomenon referred to as the marine methane paradox4,5. Here we use nuclear magnetic resonance spectroscopy to show that polysaccharide esters of three phosphonic acids are important constituents of dissolved organic matter in seawater from the North Pacific. In seawater and pure culture incubations, bacterial degradation of these dissolved organic matter phosphonates in the presence of oxygen releases methane, ethylene and propylene gas. Moreover, we found that in mutants of a methane-producing marine bacterium, Pseudomonas stutzeri, disrupted in the C–P lyase phosphonate degradation pathway, methanogenesis was also disabled, indicating that the C–P lyase pathway can catalyse methane production from marine dissolved organic matter. Finally, the carbon stable isotope ratio of methane emitted during our incubations agrees well with anomalous isotopic characteristics of seawater methane. We estimate that daily cycling of only about 0.25% of the organic matter phosphonate inventory would support the entire atmospheric methane flux at our study site. We conclude that aerobic bacterial degradation of phosphonate esters in dissolved organic matter may explain the marine methane paradox.

Emerging impact of Greenland meltwater on deepwater formation in the North Atlantic Ocean

Abstract: The Greenland ice sheet has experienced increasing mass loss since the 1990s1, 2. The enhanced freshwater flux due to both surface melt and outlet glacier discharge is assuming an increasingly important role in the changing freshwater budget of the subarctic Atlantic3. The sustained and increasing freshwater fluxes from Greenland to the surface ocean could lead to a suppression of deep winter convection in the Labrador Sea, with potential ramifications for the strength of the Atlantic meridional overturning circulation4, 5, 6. Here we assess the impact of the increases in the freshwater fluxes, reconstructed with full spatial resolution3, using a global ocean circulation model with a grid spacing fine enough to capture the small-scale, eddying transport processes in the subpolar North Atlantic. Our simulations suggest that the invasion of meltwater from the West Greenland shelf has initiated a gradual freshening trend at the surface of the Labrador Sea. Although the freshening is still smaller than the variability associated with the episodic ‘great salinity anomalies’, the accumulation of meltwater may become large enough to progressively dampen the deep winter convection in the coming years. We conclude that the freshwater anomaly has not yet had a significant impact on the Atlantic meridional overturning circulation.

Impact of decadal cloud variations on the Earth/'s energy budget

Abstract: Cloud feedbacks strongly influence the magnitude of global warming Climate model simulations show that these feedbacks vary strongly as the spatial patterns of sea surface temperatures change over time