Showing papers in "Nature Geoscience in 2011"

Mangroves among the most carbon-rich forests in the tropics

Abstract: Mangrove forests occur along ocean coastlines throughout the tropics, and support numerous ecosystem services, including fisheries production and nutrient cycling. However, the areal extent of mangrove forests has declined by 30-50% over the past half century as a result of coastal development, aquaculture expansion and over-harvesting1, 2, 3, 4. Carbon emissions resulting from mangrove loss are uncertain, owing in part to a lack of broad-scale data on the amount of carbon stored in these ecosystems, particularly below ground5. Here, we quantified whole-ecosystem carbon storage by measuring tree and dead wood biomass, soil carbon content, and soil depth in 25 mangrove forests across a broad area of the Indo-Pacific region—spanning 30° of latitude and 73° of longitude—where mangrove area and diversity are greatest4, 6. These data indicate that mangroves are among the most carbon-rich forests in the tropics, containing on average 1,023 Mg carbon per hectare.

Acidification of subsurface coastal waters enhanced by eutrophication

Abstract: Human inputs of nutrients to coastal waters can lead to the excessive production of algae, a process known as eutrophication. Microbial consumption of this organic matter lowers oxygen levels in the water 1‐3 . In addition, the carbon dioxide produced during microbial respiration increases acidity. The dissolution of atmospheric carbon dioxide in ocean waters also raises acidity, a process known as ocean acidification. Here, we assess the combined impact of eutrophication and ocean acidification on acidity in the coastal ocean, using data collected in the northern Gulf of Mexico and the East China Sea—two regions heavily influenced by nutrient‐laden rivers. We show that eutrophication in these waters is associated with the development of hypoxia and the acidification of subsurface waters, as expected. Model simulations, using data collected from the northern Gulf of Mexico, however, suggest that the drop in pH since pre-industrial times is greater than that expected from eutrophication and ocean acidification alone. We attribute the additional drop in pH— of 0.05 units—to a reduction in the ability of these carbon dioxide-rich waters to buffer changes in pH. We suggest that eutrophication could increase the susceptibility of coastal

Spatially variable response of Himalayan glaciers to climate change affected by debris cover

Abstract: The present state and future evolution of Himalayan glaciers has been controversial. An analysis of remotely sensed frontal changes and surface velocities from glaciers in the greater Himalaya between 2000 and 2008 shows large regional variability in the responses of Himalayan glaciers to climate change.

Signatures of the Antarctic ozone hole in Southern Hemisphere surface climate change

Abstract: Roughly 90% of atmospheric ozone is found in the lower stratosphere in the ozone layer Since about the 1970s, anthropogenic emissions of ozone-depleting gases have led to depletion of ~3–4% of the total overhead ozone averaged over the globe 1 The strongest depletion is found over Antarctica during spring, when photochemical processes combine with a unique set of meteorological conditions to greatly increase the effectiveness of ozone-depleting gases, and more than half of the total overhead ozone is destroyed Characteristics of the resulting Antarctic ozone hole are reviewed in refs 1 and 2, and the identification and attribution of the phenomenon was recently celebrated in a special edition of Nature (http://wwwnaturecom/nature/focus/ ozonehole/) The Antarctic ozone hole is evident in ozone observations taken every spring since about the early 1980s 1 Its annual onset coincides with the return of sunlight to the cold polar stratosphere during September/October, and its decay with the collapse of the stratospheric vortex during November/December 1,2 The most obvious surface impact is an increase in ultraviolet radiation reaching the surface 1 Over the past decade, however, it has become clear that the ozone hole is also associated with widespread changes in the Southern Hemisphere tropospheric circulation and surface climate Our purpose here is to review the evidence that suggests that the Antarctic ozone hole has had a demonstrable effect on the surface climate of the Southern Hemisphere The ozone hole and Southern Hemisphere circulation Ozone absorbs incoming solar radiation Hence the depletion of ozone over Antarctica leads to cooling of the polar stratosphere 2,3

Significant efflux of carbon dioxide from streams and rivers in the United States

Abstract: Current estimates of carbon dioxide evasion from inland waters are based on incomplete spatial coverage. Streams and rivers in the United States release 97 Tg of carbon to the atmosphere each year, according to an analysis of chemical and morphological data.

Observational evidence for soil-moisture impact on hot extremes in southeastern Europe

Abstract: Modelling studies have postulated a possible impact of soil-moisture deficit and drought on hot extremes. An analysis of observational indices from central and southeastern Europe confirms that summer hot extremes are linked to soil-moisture deficits in southeastern Europe but does not detect a similar effect in central Europe.

Carbon emission from hydroelectric reservoirs linked to reservoir age and latitude

Abstract: Hydroelectric reservoirs cover an area of 3.4 x 10(5) km(2) and comprise about 20% of all reservoirs. In addition, they contain large stores of formerly terrestrial organic carbon. Significant amou ...

Stronger ocean circulation and increased melting under Pine Island Glacier ice shelf

Abstract: The ice shelf buttressing Antarctica’s Pine Island Glacier has been melting rapidly. Observations taken between 1994 and 2009 show that meltwater production has increased by about 50% since 1994, as a result of a stronger circulation below the ice shelf.

Convergent Cenozoic CO2 history

Abstract: Reconstructions of atmospheric carbon dioxide concentrations over the past 65 million years are heading towards consensus. It is time for systematic testing of the proxies, against measurements and against each other.

Chinese stalagmite δ 18 O controlled by changes in the Indian monsoon during a simulated Heinrich event

Abstract: Carbonate cave deposits in India and China are assumed to record the intensity of monsoon precipitation, because the 18 O of the carbonate tracks the isotopic signature of precipitation. These records show spatially coherent variability throughout the last ice age and suggest that monsoon strength was altered during the millennial-scale climate variations known as Dansgaard‐Oeschger events and during the Heinrich cooling events. Here we use a numerical climate model with an embedded oxygen-isotope model to assess what caused the shifts in the oxygen-isotope signature of precipitation during a climate perturbation designed to mimic a Heinrich event. Our simulations show that a sudden increase in North Atlantic sea-ice extent during the last glacial period leads to cooling in the Northern Hemisphere, reduced precipitation over the Indian basin and weakening of the Indian monsoon. The precipitation is isotopically heavier over India and the water vapour exported to China is isotopically enriched. Our model broadly reproduces the enrichment of 18 O over Northern India and East Asia evident in speleothem records during Heinrich events. We therefore conclude that changes in the 18 O of cave carbonates associated with Heinrich events reflect changes in the intensity of Indian rather than East Asian monsoon precipitation.

Drought-induced carbon loss in peatlands

Abstract: Peatlands store vast amounts of organic carbon, owing to anoxic conditions, which prevent decay. Laboratory and field experiments suggest that drought-induced increases in oxygen stimulate microbial growth and the breakdown of peatland carbon.

Long-term impacts of aerosols on the vertical development of clouds and precipitation

Abstract: Aerosols alter cloud density and the radiative balance of the atmosphere. This leads to changes in cloud microphysics and atmospheric stability, which can either suppress or foster the development of clouds and precipitation. The net effect is largely unknown, but depends on meteorological conditions and aerosol properties. Here, we examine the long-term impact of aerosols on the vertical development of clouds and rainfall frequencies, using a 10-year dataset of aerosol, cloud and meteorological variables collected in the Southern Great Plains in the United States. We show that cloud-top height and thickness increase with aerosol concentration measured near the ground in mixed-phase clouds—which contain both liquid water and ice—that have a warm, low base. We attribute the effect, which is most significant in summer, to an aerosol-induced invigoration of upward winds. In contrast, we find no change in cloud-top height and precipitation with aerosol concentration in clouds with no ice or cool bases. We further show that precipitation frequency and rain rate are altered by aerosols. Rain increases with aerosol concentration in deep clouds that have a high liquid-water content, but declines in clouds that have a low liquid-water content. Simulations using a cloud-resolving model confirm these observations. Our findings provide unprecedented insights of the long-term net impacts of aerosols on clouds and precipitation.

Recent acceleration of biomass burning and carbon losses in Alaskan forests and peatlands

Abstract: Climate change has increased the area affected by forest fires in boreal North America. An analysis of the depth of burning in forests and peatlands in Alaska indicates that ground-layer combustion has accelerated regional carbon losses.

Positive feedback and momentum growth during debris-flow entrainment of wet bed sediment

Abstract: Debris flows typically occur when intense rainfall or snowmelt triggers landslides or extensive erosion on steep, debris-mantled slopes. The flows can then grow dramatically in size and speed as they entrain material from their beds and banks, but the mechanism of this growth is unclear. Indeed, momentum conservation implies that entrainment of static material should retard the motion of the flows if friction remains unchanged. Here we use data from large-scale experiments to assess the entrainment of bed material by debris flows. We find that entrainment is accompanied by increased flow momentum and speed only if large positive pore pressures develop in wet bed sediments as the sediments are overridden by debris flows. The increased pore pressure facilitates progressive scour of the bed, reduces basal friction and instigates positive feedback that causes flow speed, mass and momentum to increase. If dryer bed sediment is entrained, however, the feedback becomes negative and flow momentum declines. We infer that analogous feedbacks could operate in other types of gravity-driven mass flow that interact with erodible beds. The mechanisms by which debris flows acquire mass and momentum as they entrain material are unclear. Large-scale experiments suggest that the pore pressure of wet bed sediment increases as the flow moves over the bed, leading to reduced friction and progressive scouring of the base.

Regionally differentiated contribution of mountain glaciers and ice caps to future sea-level rise

Abstract: The contribution to sea-level rise from mountain glaciers and ice caps has grown over the past decades. A projection of their melting during the twenty-first century based on temperature and precipitation projections from ten climate models suggests that by 2100 these glaciers will lose about 21% of their total global volume. The contribution to sea-level rise from mountain glaciers and ice caps has grown over the past decades. They are expected to remain an important component of eustatic sea-level rise for at least another century1,2, despite indications of accelerated wastage of the ice sheets3,4,5. However, it is difficult to project the future contribution of these small-scale glaciers to sea-level rise on a global scale. Here, we project their volume changes due to melt in response to transient, spatially differentiated twenty-first century projections of temperature and precipitation from ten global climate models. We conduct the simulations directly on the more than 120,000 glaciers now available in the World Glacier Inventory6, and upscale the changes to 19 regions that contain all mountain glaciers and ice caps in the world (excluding the Greenland and Antarctic ice sheets). According to our multi-model mean, sea-level rise from glacier wastage by 2100 will amount to 0.124±0.037 m, with the largest contribution from glaciers in Arctic Canada, Alaska and Antarctica. Total glacier volume will be reduced by 21±6%, but some regions are projected to lose up to 75% of their present ice volume. Ice losses on such a scale may have substantial impacts on regional hydrology and water availability7.

Mass wasting triggered by the 2008 Wenchuan earthquake is greater than orogenic growth

Abstract: Shallow earthquakes lead to the uplift of mountain ranges, but also trigger landslides that remove mass. An analysis of the 2008 Wenshuan earthquake suggests that more material was removed from the orogen by widespread landslides than was added by coseismic uplift.

Deep-sea mud in the Pacific Ocean as a potential resource for rare-earth elements

Abstract: World demand for rare-earth elements and the metal yttrium is rapidly increasing. An analysis of more than 2,000 seafloor sediment samples suggests that deep-sea mud constitutes a highly promising giant resource for these elements.

Radiative forcing and albedo feedback from the Northern Hemisphere cryosphere between 1979 and 2008

Abstract: The extent of snow cover and sea ice in the Northern Hemisphere has declined since 1979, suggesting a positive feedback of surface reflectivity on climate. A synthesis of a variety of remote sensing and field measurements suggests that this albedo feedback from the Northern Hemisphere cryosphere falls between 0.3 and 1.1 W m−2 K−1. The extent of snow cover1 and sea ice2 in the Northern Hemispherehas declined since 1979, coincident with hemispheric warming and indicative of a positive feedback of surface reflectivity on climate. This albedo feedback of snow on land has been quantified from observations at seasonal timescales3,4,5,6, and century-scale feedback has been assessed using climate models7,8,9,10. However, the total impact of the cryosphere on radiative forcing and albedo feedback has yet to be determined from measurements. Here we assess the influence of the Northern Hemisphere cryosphere on Earth’s radiation budget at the top of the atmosphere—termed cryosphere radiative forcing—by synthesizing a variety of remote sensing and field measurements. We estimate mean Northern Hemisphere forcing at −4.6 to −2.2 W m−2, with a peak in May of −9.0±2.7 W m−2. We find that cyrospheric cooling declined by 0.45 W m−2 from 1979 to 2008, with nearly equal contributions from changes in land snow cover and sea ice. On the basis of these observations, we conclude that the albedo feedback from the Northern Hemisphere cryosphere falls between 0.3 and 1.1 W m−2 K−1, substantially larger than comparable estimates obtained from 18 climate models.

Winter warming in West Antarctica caused by central tropical Pacific warming

Abstract: The Pacific sector of Antarctica has experienced substantial warming in the past 30 years. Observations of global surface temperatures and atmospheric circulation data show that the warming in continental West Antarctica is linked to sea surface temperature changes in the tropical Pacific Ocean.

Solar forcing of winter climate variability in the Northern Hemisphere

Abstract: An influence of solar irradiance variations on Earth’s surface climate has been repeatedly suggested. Simulations with a climate model driven by satellite measurements of solar ultraviolet irradiance show an atmospheric response to the solar minimum that resembles the negative phase of the North Atlantic Oscillation.

Microfossils of sulphur-metabolizing cells in 3.4-billion-year-old rocks of Western Australia

Abstract: Sulphur isotope data from early Archaean rocks suggest that microbes with metabolisms based on sulphur existed almost 3.5 billion years ago, leading to suggestions that the earliest microbial ecosystems were sulphur-based 1‐5 . However, morphological evidence for these sulphur-metabolizing bacteria has been elusive. Here we report the presence of microstructures from the 3.4-billion-year-old Strelley Pool Formation in Western Australia that are associated with micrometre-sized pyrite crystals. The microstructures we identify exhibit indicators of biological affinity, including hollow cell lumens, carbonaceous cell walls enriched in nitrogen, taphonomic degradation, organization into chains and clusters, and 13 C values of 33

Eddy-induced reduction of biological production in eastern boundary upwelling systems

Abstract: Eddies and other mesoscale oceanic processes, such as fronts, can enhance biological production in the ocean, according to several open-ocean studies. The effect is thought to be particularly pronounced in low-nutrient environments, where mesoscale processes increase the net upward flux of limiting nutrients. However, eddies have been suggested to suppress production in the highly productive eastern boundary upwelling systems. Here, we examine the relationship between satellite-derived estimates of net primary production, of upwelling strength, and of eddy-kinetic energy--a measure of the intensity of mesoscale activity--in the four most productive eastern boundary upwelling systems. We show that high levels of eddy activity tend to be associated with low levels of biological production, indicative of a suppressive effect. Simulations using eddy-resolving models of two of these upwelling systems support the suggestion that eddies suppress production, and show that the downward export of organic matter is also reduced. According to these simulations, the reduction in production and export results from an eddy-induced transport of nutrients from the nearshore environment to the open ocean. Eddies might have a similar effect on marine productivity in other oceanic systems that are characterized by intense eddy activity, such as the Southern Ocean.

Enigmatic origin of the largest-known carbon isotope excursion in Earth's history

Abstract: Carbonate rocks of Middle Ediacaran age record the largest excursion in carbon isotopic compositions in Earth history. A review of the data offers two intriguing explanations: an extraordinary perturbation of the carbon cycle, or post-depositional alteration that is global, rather than local. Carbonate rocks from the Middle Ediacaran period in locations all over the globe record the largest excursion in carbon isotopic compositions in Earth history. This finding suggests a dramatic reorganization of Earth's carbon cycle. Named the Shuram excursion for its original discovery in the Shuram Formation, Oman, the anomaly closely precedes impressive events in evolution, including the rise of large metazoans and the origin of biomineralization in animals. Instead of a true record of the carbon cycle at the time of sedimentation, the carbon isotope signature recorded in the Shuram excursion could be caused by alteration following deposition of the carbonate sediments, a scenario supported by several geochemical indicators. However, such secondary processes are intrinsically local, which makes it difficult to explain the coincident occurrence of carbon isotope anomalies in numerous records around the globe. Both possibilities are intriguing: if the Shuram excursion preserves a genuine record of ancient seawater chemistry, it reflects a perturbation to the carbon cycle that is stronger than any known perturbations of the modern Earth. If, however, it represents secondary alteration during burial of sediments, then marine sediments must have been globally preconditioned in a unique way, to allow ordinary and local processes to produce an extraordinary and widespread response.

Carbonate dissolution during subduction revealed by diamond-bearing rocks from the Alps

Abstract: .Carbon should therefore be retained in the slab and transportedto great depths in the mantle, rather than supply the arcvolcanoes. Here we identify diamonds in ultrahigh-pressurerocks from the Italian western Alps that have an oceanicorigin. We assess the geochemistry of diamond-bearing fluidinclusions and find that they contain bicarbonate, carbonateand sulphate ions, silica monomers, and crystals of carbonateand silicate. This fluid geochemistry indicates that carbon wasreleased from the slab at relatively shallow depths throughdissolution, not decarbonation. We conclude that dissolution,driven by fluids released from the subducted slab, is animportant mechanism for the transfer of carbon into themantle and ultimately back into the atmosphere, helping tobalance the carbon flux.

Two contrasting Phanerozoic orogenic systems revealed by hafnium isotope data

Abstract: Two types of mountain-forming systems exist on Earth — external orogens form during oceanic–continental plate collisions and internal orogens form where continental plates collide. Hafnium isotope data from orogens worldwide show that each has produced a distinctive signature over the past 550 Myr, reflecting the contrasting subduction geometry in each setting.

Diapirs as the source of the sediment signature in arc lavas

Abstract: Lavas erupted above subduction zones commonly show evidence for recycling of subducted sediments. Geochemical analyses of sedimentary rocks that experienced subduction indicate that trace elements in the sediments can be efficiently recycled, because metamorphosed sediments rise buoyantly from the subducting plate and undergo partial melting at high temperatures in the overlying mantle wedge.

Stability of the West Antarctic ice sheet in a warming world

Abstract: (equivalent to 0.28 to 0.56 mm yr −1 sea-level rise), with the rate growing over at least the past two decades 1–4 . These observations intensify concerns about the ice sheet’s stability that were first raised more than four decades ago 5

Formation of manganese oxides by bacterially generated superoxide

Abstract: Manganese oxide minerals control numerous environmental processes, including the fate of contaminants. Laboratory experiments with a common species of marine bacteria reveal that bacterially generated superoxide can oxidize manganese ions, generating manganese oxides.

Limited overlap between the seismic gap and coseismic slip of the great 2010 Chile earthquake

Abstract: The Mw 8.8 Chile earthquake on 27 February 2010 occurred in a broad region where tectonic loading has been accumulating since the great 1835 earthquake. A joint inversion of tsunami and geodetic data suggests that the 2010 earthquake rupture only partly overlapped with the zone of preseismic locking.

Metal flux from hydrothermal vents increased by organic complexation

Abstract: Hydrothermal vents in the sea floor release large volumes of hot, metal-rich fluids into the deep ocean. Mounting evidence suggests that organic compounds bind to and stabilize metals in hydrothermal fluids, thereby increasing metal flux to the open ocean.