Showing papers in "Nature Geoscience in 2014"

Recent Arctic amplification and extreme mid-latitude weather

Abstract: The Arctic region has warmed more than twice as fast as the global average — a phenomenon known as Arctic amplification. The rapid Arctic warming has contributed to dramatic melting of Arctic sea ice and spring snow cover, at a pace greater than that simulated by climate models. These profound changes to the Arctic system have coincided with a period of ostensibly more frequent extreme weather events across the Northern Hemisphere mid-latitudes, including severe winters. The possibility of a link between Arctic change and mid-latitude weather has spurred research activities that reveal three potential dynamical pathways linking Arctic amplification to mid-latitude weather: changes in storm tracks, the jet stream, and planetary waves and their associated energy propagation. Through changes in these key atmospheric features, it is possible, in principle, for sea ice and snow cover to jointly influence mid-latitude weather. However, because of incomplete knowledge of how high-latitude climate change influences these phenomena, combined with sparse and short data records, and imperfect models, large uncer - tainties regarding the magnitude of such an influence remain. We conclude that improved process understanding, sustained and additional Arctic observations, and better coordinated modelling studies will be needed to advance our understanding of the influences on mid-latitude weather and extreme events.

Arctic amplification dominated by temperature feedbacks in contemporary climate models

Abstract: Changes in climate are amplified in the Arctic region. An analysis of the CMIP5 state-of-the-art climate models reveals that temperature feedbacks are the dominant factor in this amplification, whereas the change in reflectivity of the Earth’s surface as sea ice and snow melt makes only a secondary contribution. Climate change is amplified in the Arctic region. Arctic amplification has been found in past warm1 and glacial2 periods, as well as in historical observations3,4 and climate model experiments5,6. Feedback effects associated with temperature, water vapour and clouds have been suggested to contribute to amplified warming in the Arctic, but the surface albedo feedback—the increase in surface absorption of solar radiation when snow and ice retreat—is often cited as the main contributor7,8,9,10. However, Arctic amplification is also found in models without changes in snow and ice cover11,12. Here we analyse climate model simulations from the Coupled Model Intercomparison Project Phase 5 archive to quantify the contributions of the various feedbacks. We find that in the simulations, the largest contribution to Arctic amplification comes from a temperature feedbacks: as the surface warms, more energy is radiated back to space in low latitudes, compared with the Arctic. This effect can be attributed to both the different vertical structure of the warming in high and low latitudes, and a smaller increase in emitted blackbody radiation per unit warming at colder temperatures. We find that the surface albedo feedback is the second main contributor to Arctic amplification and that other contributions are substantially smaller or even opposeArctic amplification.

Atmospheric circulation as a source of uncertainty in climate change projections

Abstract: Scientific confidence in climate change effects is much higher for aspects related to global patterns of surface temperature, than for circulation. Circulation will remain hard to predict, necessitating a risk-based approach to decision making.

Mega-heatwave temperatures due to combined soil desiccation and atmospheric heat accumulation

Abstract: Extreme heatwave events are expected to become increasingly common as a consequence of climate change. Analyses of the 2003 and 2010 mega-heatwaves in Europe suggest that atmospheric boundary-layer dynamics and feedbacks with the drying land surface lead to the build-up of heat in the atmosphere and extremely hot temperatures.

Robust Arctic sea-ice influence on the frequent Eurasian cold winters in past decades

Abstract: Severe winters have occurred frequently in mid-latitude Eurasia during the past decade. Simulations with a 100-member ensemble of an atmospheric model detect an influence of declining Arctic sea-ice cover.

Wave attenuation over coastal salt marshes under storm surge conditions

Abstract: Salt marshes protect coastlines against waves. Wave flume experiments show that marsh vegetation causes substantial wave dissipation and prevents erosion of the underlying surface, even during extreme storm surge conditions.

Global assessment of trends in wetting and drying over land

Abstract: Past continental dryness trends are difficult to assess. A comprehensive analysis of hundreds of combinations of data sets suggests that only 24.6% of the global land area have been exposed to robust dryness changes since 1948.

Persistent growth of CO2 emissions and implications for reaching climate targets

Abstract: Efforts to limit climate change below a given temperature level require that global emissions of CO2 cumulated over time remain below a limited quota. This quota varies depending on the temperature level, the desired probability of staying below this level and the contributions of other gases. In spite of this restriction, global emissions of CO2 from fossil fuel combustion and cement production have continued to grow by 2.5% per year on average over the past decade. Two thirds of the CO2 emission quota consistent with a 2 °C temperature limit has already been used, and the total quota will likely be exhausted in a further 30 years at the 2014 emissions rates. We show that CO2 emissions track the high end of the latest generation of emissions scenarios, due to lower than anticipated carbon intensity improvements of emerging economies and higher global gross domestic product growth. In the absence of more stringent mitigation, these trends are set to continue and further reduce the remaining quota until the onset of a potential new climate agreement in 2020. Breaking current emission trends in the short term is key to retaining credible climate targets within a rapidly diminishing emission quota.

Evidence for oxygenic photosynthesis half a billion years before the Great Oxidation Event

Abstract: The early Earth was characterized by the absence of oxygen in the ocean-atmosphere system, in contrast to the well-oxygenated conditions that prevail today. Atmospheric concentrations first rose to appreciable levels during the Great Oxidation Event, roughly 2.5-2.3 Gyr ago. The evolution of oxygenic photosynthesis is generally accepted to have been the ultimate cause of this rise, but it has proved difficult to constrain the timing of this evolutionary innovation1, 2. The oxidation of manganese in the water column requires substantial free oxygen concentrations, and thus any indication that Mn oxides were present in ancient environments would imply that oxygenic photosynthesis was ongoing. Mn oxides are not commonly preserved in ancient rocks, but there is a large fractionation of molybdenum isotopes associated with the sorption of Mo onto the Mn oxides that would be retained. Here we report Mo isotopes from rocks of the Sinqeni Formation, Pongola Supergroup, South Africa. These rocks formed no less than 2.95 Gyr ago3 in a nearshore setting. The Mo isotopic signature is consistent with interaction with Mn oxides. We therefore infer that oxygen produced through oxygenic photosynthesis began to accumulate in shallow marine settings at least half a billion years before the accumulation of significant levels of atmospheric oxygen.

Brownness of organics in aerosols from biomass burning linked to their black carbon content

Abstract: Atmospheric aerosols can exert an important influence on Earth’s climate. Combustion chamber experiments reveal that the absorption properties of brown carbon aerosols from biomass burning are linked to their black carbon content.

Humic substances as fully regenerable electron acceptors in recurrently anoxic environments

Abstract: Humic substances make up a significant fraction of the natural organic matter in terrestrial and aquatic environments. Laboratory experiments suggest that humic substances serve as fully regenerable electron acceptors in recurrently anoxic environments such as peatlands.

Hadean age for a post-magma-ocean zircon confirmed by atom-probe tomography

Abstract: The only physical evidence from the earliest phases of Earth’s evolution comes from zircons, ancient mineral grains that can be dated using the U‐Th‐Pb geochronometer 1 . Oxygen isotope ratios from such zircons have been used to infer when the hydrosphere and conditions habitable to life were established 2,3 . Chemical homogenization of Earth’s crust and the existence of a magma ocean have not been dated directly, but must have occurred earlier 4 . However, the accuracy of the U‐Pbzirconagescanplausiblybebiasedbypoorlyunderstood processes of intracrystalline Pb mobility 5‐7 . Here we use atomprobe tomography 8 to identify and map individual atoms in the oldest concordant grain from Earth, a 4.4-Gyr-old Hadean zircon with a high-temperature overgrowth that formed about 1Gyrafterthemineral’score.Isolatednanoclusters,measuring about 10nm and spaced 10‐50nm apart, are enriched in incompatible elements including radiogenic Pb with unusually high 207 Pb/ 206 Pbratios.Wedemonstratethatthelengthscales of these clusters make U‐Pb age biasing impossible, and that they formed during the later reheating event. Our tomography data thereby confirm that any mixing event of the silicate Earth must have occurred before 4.4Gyr ago, consistent with

Delamination and recycling of Archaean crust caused by gravitational instabilities

Abstract: The volume of Archaean crust preserved at Earth’s surface today is low. Thermodynamic calculations and geodynamic modelling show that the thick, primary crust that would have formed on a much hotter Archaean Earth was denser than the underlying mantle, and would have therefore been recycled back into the mantle as drips.

Volcanic contribution to decadal changes in tropospheric temperature

Abstract: Global mean surface and tropospheric temperatures have shown slower warming since 1998 than found in climate model simulations. A detailed analysis of observations and climate model simulations suggests that the observed influence of volcanic eruptions on tropospheric temperature has been significant, and that the discrepancy between climate simulations and observations is reduced by up to 15% when twenty-first century volcanic eruptions are accounted for in the models. Despite continued growth in atmospheric levels of greenhouse gases, global mean surface and tropospheric temperatures have shown slower warming since 1998 than previously1,2,3,4,5. Possible explanations for the slow-down include internal climate variability3,4,6,7, external cooling influences1,2,4,8,9,10,11 and observational errors12,13. Several recent modelling studies have examined the contribution of early twenty-first-century volcanic eruptions1,2,4,8 to the muted surface warming. Here we present a detailed analysis of the impact of recent volcanic forcing on tropospheric temperature, based on observations as well as climate model simulations. We identify statistically significant correlations between observations of stratospheric aerosol optical depth and satellite-based estimates of both tropospheric temperature and short-wave fluxes at the top of the atmosphere. We show that climate model simulations without the effects of early twenty-first-century volcanic eruptions overestimate the tropospheric warming observed since 1998. In two simulations with more realistic volcanic influences following the 1991 Pinatubo eruption, differences between simulated and observed tropospheric temperature trends over the period 1998 to 2012 are up to 15% smaller, with large uncertainties in the magnitude of the effect. To reduce these uncertainties, better observations of eruption-specific properties of volcanic aerosols are needed, as well as improved representation of these eruption-specific properties in climate model simulations.

Co-evolution of eukaryotes and ocean oxygenation in the Neoproterozoic era

Abstract: The oxygenation of the Earth's deep oceans is often thought to have triggered the evolution of simple animals. A review article proposes that instead, the evolution of animal life set off a series of biogeochemical feedbacks that promoted the oxygenation of the deep sea. The Neoproterozoic era (about 1,000 to 542 million years ago) was a time of turbulent environmental change. Large fluctuations in the carbon cycle were associated with at least two severe — possible Snowball Earth — glaciations. There were also massive changes in the redox state of the oceans, culminating in the oxygenation of much of the deep oceans. Amid this environmental change, increasingly complex life forms evolved. The traditional view is that a rise in atmospheric oxygen concentrations led to the oxygenation of the ocean, thus triggering the evolution of animals. We argue instead that the evolution of increasingly complex eukaryotes, including the first animals, could have oxygenated the ocean without requiring an increase in atmospheric oxygen. We propose that large eukaryotic particles sank quickly through the water column and reduced the consumption of oxygen in the surface waters. Combined with the advent of benthic filter feeding, this shifted oxygen demand away from the surface to greater depths and into sediments, allowing oxygen to reach deeper waters. The decline in bottom-water anoxia would hinder the release of phosphorus from sediments, potentially triggering a potent positive feedback: phosphorus removal from the ocean reduced global productivity and ocean-wide oxygen demand, resulting in oxygenation of the deep ocean. That, in turn, would have further reinforced eukaryote evolution, phosphorus removal and ocean oxygenation.

Ebullition and storm-induced methane release from the East Siberian Arctic Shelf

Abstract: Vast quantities of carbon are stored in shallow Arctic reservoirs, such as submarine and terrestrial permafrost. Submarine permafrost on the East Siberian Arctic Shelf started warming in the early Holocene, several thousand years ago. However, the present state of the permafrost in this region is uncertain. Here, we present data on the temperature of submarine permafrost on the East Siberian Arctic Shelf using measurements collected from a sediment core, together with sonar-derived observations of bubble flux and measurements of seawater methane levels taken from the same region. The temperature of the sediment core ranged from -1.8 to 0 degrees C. Although the surface layer exhibited the lowest temperatures, it was entirely unfrozen, owing to significant concentrations of salt. On the basis of the sonar data, we estimate that bubbles escaping the partially thawed permafrost inject 100-630 mg methane m(-2) d(-1) into the overlying water column. We further show that water-column methane levels had dropped significantly following the passage of two storms. We suggest that significant quantities of methane are escaping the East Siberian Shelf as a result of the degradation of submarine permafrost over thousands of years. We suggest that bubbles and storms facilitate the flux of this methane to the overlying ocean and atmosphere, respectively.

Short-term modulation of Indian summer monsoon rainfall by West Asian dust

Abstract: The Indian summer monsoon is influenced by numerous factors, including aerosol-induced changes to clouds, surface and atmospheric heating, and atmospheric circulation. An analysis of satellite data and global climate model simulations suggests that dust aerosol levels over the Arabian Sea, West Asia and the Arabian Peninsula are positively correlated with the intensity of the Indian summer monsoon.

Eastward expansion of the Tibetan Plateau by crustal flow and strain partitioning across faults

Abstract: The Tibetan Plateau is expanding eastwards, but the modes of deformation are poorly understood. High-resolution seismic images from the region identify localized zones of weak crustal rocks as well as deep faults, implying that deformation occurs through a combination of crustal flow and movement of rigid blocks of crust.

Carbon dioxide released from subduction zones by fluid-mediated reactions

Abstract: The balance between carbonate subduction into the deep Earth and CO2 release through degassing at volcanoes is critical for the carbon cycle. Geochemical analyses of an exhumed subduction zone complex in Greece show that fluid-mediated reactions could liberate significant amounts of carbon from the subducting slab for later release at arc volcanoes.

Copper enrichment in arc magmas controlled by overriding plate thickness

Abstract: The characteristics of magmas typically associated with porphyry copper deposits are thought to be imparted in the mantle. Statistical assessment of over 40,000 geochemical analyses of magmatic rocks formed in subduction zones worldwide, however, shows that the characteristics of these magmatic rocks are systematically controlled by the thickness of the arc crust.

Warming early Mars with CO2 and H2

Abstract: Ancient valleys suggest a warm early Mars where liquid water flowed, but a greenhouse effect strong enough to offset a dim early Sun has been difficult to explain. Climate simulations suggest that sufficient concentrations of the greenhouse gases CO2 and H2 — outgassed during volcanic eruptions — could have warmed Mars above water’s freezing point.

Deeply incised submarine glacial valleys beneath the Greenland ice sheet

Abstract: The bed topography beneath the Greenland ice sheet controls the flow of ice and its discharge into the ocean. A combination of sparse radar soundings of ice thickness and high-resolution ice motion data suggest that many submarine ice-covered valleys extend significantly deeper below sea level and farther inland than thought.

Recurring slope lineae in equatorial regions of Mars

Abstract: The presence of liquid water is a requirement of habitability on a planet. Possible indicators of liquid surface water on Mars include intermittent flow-like features observed on sloping terrains. These recurring slope lineae are narrow, dark markings on steep slopes that appear and incrementally lengthen during warm seasons on low-albedo surfaces. The lineae fade in cooler seasons and recur over multiple Mars years. Recurring slope lineae were initially reported to appear and lengthen at mid-latitudes in the late southern spring and summer and are more common on equator-facing slopes where and when the peak surface temperatures are higher. Here we report extensive activity of recurring slope lineae in equatorial regions of Mars, particularly in the deep canyons of Valles Marineris, from analysis of data acquired by the Mars Reconnaissance Orbiter. We observe the lineae to be most active in seasons when the slopes often face the sun. Expected peak temperatures suggest that activity may not depend solely on temperature. Although the origin of the recurring slope lineae remains an open question, our observations are consistent with intermittent flow of briny water. Such an origin suggests surprisingly abundant liquid water in some near-surface equatorial regions of Mars. Dark streaks that appear on the surface of Mars during warm seasons have been observed at the mid-latitudes and tentatively attributed to the flow of briny water. Imagery from the Mars Reconnaissance Orbiter over multiple Mars years suggests that these seasonally active features are also present in equatorial regions, where liquid surface water is not expected.

The Indonesian seas and their role in the coupled ocean–climate system

Abstract: The Indonesian seas provide the only connection between ocean basins in the tropics. A review of observational data and model results concludes that vertical mixing determines the physical properties of water in the Indonesian throughflow.

Reconciling warming trends

Abstract: Any divergence between real-world climate phenomena and prior expectations poses interesting science questions. The case of the apparent slow-down of warming since the record El Nino event in 1997/1998 is no exception. The global mean surface temperature trend was smaller 1 between 1997 and 2013 (0.07±0.08 °C per decade) than over the last 50 years (0.16 ± 0.02 °C per decade), highlighting questions about the mechanisms that regulate decadal variability in the Earth’s temperature. In addition, the warming trend in the most recent 15-year period is near the lower edge of the 5–95% range of projections from a collection of climate models that were part of the Coupled Model Intercomparison Project Phase 5 (CMIP5). Why most of the model simulations suggest more warming than has been observed is a second question that deserves further exploration. Short-term fluctuations in global mean surface temperature anomalies have been a perennial focus of public discussions related to climate change. We should expect to see climate changes, by definition, only in the long-term trends that average over stochastic weather and year-to-year fluctuations such as those associated with the El Nino/Southern Oscillation (ENSO), which favoured a cool La Nina phase in the past few years. On decadal timescales, global mean surface temperatures are expected to vary, too. One cause might be the chaotic internal variability of the coupled system of oceans and atmosphere, for example in the tropical Pacific Ocean 2

Changbaishan volcanism in northeast China linked to subduction-induced mantle upwelling

Abstract: The Changbaishan volcanic complex in China cannot be easily explained as the consequence of a mantle plume. Seismic images from the region identify buoyant mantle material that may have been entrained and dragged downwards by the subducting Pacific Plate, but is now escaping upwards through a gap in the plate and producing the intraplate volcanism.

Sediment supply as a driver of river meandering and floodplain evolution in the Amazon Basin

Abstract: The role of externally imposed sediment supplies on the evolution of meandering rivers and their floodplains is poorly understood, despite analytical advances in our physical understanding of river meandering1, 2. The Amazon river basin hosts tributaries that are largely unaffected by engineering controls and hold a range of sediment loads, allowing us to explore the influence that sediment supply has on river evolution. Here we calculate average annual rates of meander migration within 20 reaches in the Amazon Basin from Landsat imagery spanning 1985–2013. We find that rivers with high sediment loads experience annual migration rates that are higher than those of rivers with lower sediment loads. Meander cutoff also occurs more frequently along rivers with higher sediment loads. Differences in meander migration and cutoff rates between the study reaches are not explained by differences in channel slope or river discharge. Because faster meander migration and higher cutoff rates lead to increased sediment-storage space in the resulting oxbows, we suggest that sediment supply modulates the reshaping of floodplain environments by meandering rivers. We conclude that imposed sediment loads influence planform changes in lowland rivers across the Amazon.

Plant growth enhancement by elevated CO2 eliminated by joint water and nitrogen limitation

Abstract: Rising atmospheric CO2 concentrations can fertilize plant growth. The resulting increased plant uptake of CO2 could, in turn, slow increases in atmospheric CO2 levels and associated climate warming. CO2 fertilization effects may be enhanced when water availability is low, because elevated CO2 also leads to improved plant water-use efficiency. However, CO2 fertilization effects may be weaker when plant growth is limited by nutrient availability. How variation in soil nutrients and water may act together to influence CO2 fertilization is unresolved. Here we report plant biomass levels from a five-year, open-air experiment in a perennial grassland under two contrasting levels of atmospheric CO2, soil nitrogen and summer rainfall, respectively. We find that the presence of a CO2 fertilization effect depends on the amount of available nitrogen and water. Specifically, elevated CO2 levels led to an increase in plant biomass of more than 33% when summer rainfall, nitrogen supply, or both were at the higher levels (ambient for rainfall and elevated for soil nitrogen). But elevated CO2 concentrations did not increase plant biomass when both rainfall and nitrogen were at their lower level. We conclude that given widespread, simultaneous limitation by water and nutrients, large stimulation of biomass by rising atmospheric CO2 concentrations may not be ubiquitous. Elevated CO2 is known to fertilize plant growth, resulting in greater uptake of atmospheric CO2 by plants. However, CO2 fertilization in a perennial grassland is absent when plants are jointly limited by both water and nitrogen.

Widespread methane leakage from the sea floor on the northern US Atlantic margin

Abstract: Methane emissions from the sea floor affect ocean chemistry and can reach the atmosphere. Observations from the northern US Atlantic margin reveal about 570 gas plumes at water depths between 50 and 1,700 m.

Snowfall less sensitive to warming in Karakoram than in Himalayas due to a unique seasonal cycle

Abstract: Glaciers in the Karakoram mountains have been stable in mass, whereas in nearby regions, mass loss has prevailed. Climate model simulations reveal a unique seasonal cycle in Karakoram snowfall that contributes to this pattern.