Interannual variability in the oxygen isotopes of atmospheric CO2 driven by El Niño

The stable isotope ratios of atmospheric CO(2) ((18)O/(16)O and (13)C/(12)C) have been monitored since 1977 to improve our understanding of the global carbon cycle, because biosphere-atmosphere exchange fluxes affect the different atomic masses in a measurable way. Interpreting the (18)O/(16)O variability has proved difficult, however, because oxygen isotopes in CO(2) are influenced by both the carbon cycle and the water cycle. Previous attention focused on the decreasing (18)O/(16)O ratio in the 1990s, observed by the global Cooperative Air Sampling Network of the US National Oceanic and Atmospheric Administration Earth System Research Laboratory. This decrease was attributed variously to a number of processes including an increase in Northern Hemisphere soil respiration; a global increase in C(4) crops at the expense of C(3) forests; and environmental conditions, such as atmospheric turbulence and solar radiation, that affect CO(2) exchange between leaves and the atmosphere. Here we present 30 years' worth of data on (18)O/(16)O in CO(2) from the Scripps Institution of Oceanography global flask network and show that the interannual variability is strongly related to the El Ni?o/Southern Oscillation. We suggest that the redistribution of moisture and rainfall in the tropics during an El Ni?o increases the (18)O/(16)O ratio of precipitation and plant water, and that this signal is then passed on to atmospheric CO(2) by biosphere-atmosphere gas exchange. We show how the decay time of the El Ni?o anomaly in this data set can be useful in constraining global gross primary production. Our analysis shows a rapid recovery from El Ni?o events, implying a shorter cycling time of CO(2) with respect to the terrestrial biosphere and oceans than previously estimated. Our analysis suggests that current estimates of global gross primary production, of 120 petagrams of carbon per year, may be too low, and that a best guess of 150-175 petagrams of carbon per year better reflects the observed rapid cycling of CO(2). Although still tentative, such a revision would present a new benchmark by which to evaluate global biospheric carbon cycling models.     

The amount of carbon released from peat and forest fires in Indonesia during 1997

Tropical peatlands are one of the largest near-surface reserves of terrestrial organic carbon, and hence their stability has important implications for climate change. In their natural state, lowland tropical peatlands support a luxuriant growth of peat swamp forest overlying peat deposits up to 20 metres thick. Persistent environmental change-in particular, drainage and forest clearing-threatens their stability, and makes them susceptible to fire. This was demonstrated by the occurrence of widespread fires throughout the forested peatlands of Indonesia during the 1997 El Ni?o event. Here, using satellite images of a 2.5 million hectare study area in Central Kalimantan, Borneo, from before and after the 1997 fires, we calculate that 32% (0.79 Mha) of the area had burned, of which peatland accounted for 91.5% (0.73 Mha). Using ground measurements of the burn depth of peat, we estimate that 0.19-0.23 gigatonnes (Gt) of carbon were released to the atmosphere through peat combustion, with a further 0.05 Gt released from burning of the overlying vegetation. Extrapolating these estimates to Indonesia as a whole, we estimate that between 0.81 and 2.57 Gt of carbon were released to the atmosphere in 1997 as a result of burning peat and vegetation in Indonesia. This is equivalent to 13-40% of the mean annual global carbon emissions from fossil fuels, and contributed greatly to the largest annual increase in atmospheric CO(2) concentration detected since records began in 1957 (ref. 1).     

Annual fluxes of carbon from deforestation and regrowth in the Brazilian Amazon

The distribution of sources and sinks of carbon among the world's ecosystems is uncertain. Some analyses show northern mid-latitude lands to be a large sink, whereas the tropics are a net source; other analyses show the tropics to be nearly neutral, whereas northern mid-latitudes are a small sink. Here we show that the annual flux of carbon from deforestation and abandonment of agricultural lands in the Brazilian Amazon was a source of about 0.2 Pg Cyr(-1) over the period 1989-1998 (1 Pg is 10(15) g). This estimate is based on annual rates of deforestation and spatially detailed estimates of deforestation, regrowing forests and biomass. Logging may add another 5-10% to this estimate, and fires may double the magnitude of the source in years following a drought. The annual source of carbon from land-use change and fire approximately offsets the sink calculated for natural ecosystems in the region. Thus this large area of tropical forest is nearly balanced with respect to carbon, but has an interannual variability of +/- 0.2 PgC yr(-1).     

Low host specificity of herbivorous insects in a tropical forest

Two decades of research have not established whether tropical insect herbivores are dominated by specialists or generalists. This impedes our understanding of species coexistence in diverse rainforest communities. Host specificity and species richness of tropical insects are also key parameters in mapping global patterns of biodiversity. Here we analyse data for over 900 herbivorous species feeding on 51 plant species in New Guinea and show that most herbivorous species feed on several closely related plant species. Because species-rich genera are dominant in tropical floras, monophagous herbivores are probably rare in tropical forests. Furthermore, even between phylogenetically distant hosts, herbivore communities typically shared a third of their species. These results do not support the classical view that the coexistence of herbivorous species in the tropics is a consequence of finely divided plant resources; non-equilibrium models of tropical diversity should instead be considered. Low host specificity of tropical herbivores reduces global estimates of arthropod diversity from 31 million (ref. 1) to 4 6 million species. This finding agrees with estimates based on taxonomic collections, reconciling an order of magnitude discrepancy between extrapolations of global diversity based on ecological samples of tropical communities with those based on sampling regional faunas.     

Old-growth forests as global carbon sinks

Old-growth forests remove carbon dioxide from the atmosphere at rates that vary with climate and nitrogen deposition. The sequestered carbon dioxide is stored in live woody tissues and slowly decomposing organic matter in litter and soil. Old-growth forests therefore serve as a global carbon dioxide sink, but they are not protected by international treaties, because it is generally thought that ageing forests cease to accumulate carbon. Here we report a search of literature and databases for forest carbon-flux estimates. We find that in forests between 15 and 800 years of age, net ecosystem productivity (the net carbon balance of the forest including soils) is usually positive. Our results demonstrate that old-growth forests can continue to accumulate carbon, contrary to the long-standing view that they are carbon neutral. Over 30 per cent of the global forest area is unmanaged primary forest, and this area contains the remaining old-growth forests. Half of the primary forests (6 x 10(8) hectares) are located in the boreal and temperate regions of the Northern Hemisphere. On the basis of our analysis, these forests alone sequester about 1.3 +/- 0.5 gigatonnes of carbon per year. Thus, our findings suggest that 15 per cent of the global forest area, which is currently not considered when offsetting increasing atmospheric carbon dioxide concentrations, provides at least 10 per cent of the global net ecosystem productivity. Old-growth forests accumulate carbon for centuries and contain large quantities of it. We expect, however, that much of this carbon, even soil carbon, will move back to the atmosphere if these forests are disturbed.     

A unifying framework for dinitrogen fixation in the terrestrial biosphere

Dinitrogen (N(2)) fixation is widely recognized as an important process in controlling ecosystem responses to global environmental change, both today and in the past; however, significant discrepancies exist between theory and observations of patterns of N(2) fixation across major sectors of the land biosphere. A question remains as to why symbiotic N(2)-fixing plants are more abundant in vast areas of the tropics than in many of the mature forests that seem to be nitrogen-limited in the temperate and boreal zones. Here we present a unifying framework for terrestrial N(2) fixation that can explain the geographic occurrence of N(2) fixers across diverse biomes and at the global scale. By examining trade-offs inherent in plant carbon, nitrogen and phosphorus capture, we find a clear advantage to symbiotic N(2) fixers in phosphorus-limited tropical savannas and lowland tropical forests. The ability of N(2) fixers to invest nitrogen into phosphorus acquisition seems vital to sustained N(2) fixation in phosphorus-limited tropical ecosystems. In contrast, modern-day temperatures seem to constrain N(2) fixation rates and N(2)-fixing species from mature forests in the high latitudes. We propose that an analysis that couples biogeochemical cycling and biophysical mechanisms is sufficient to explain the principal geographical patterns of symbiotic N(2) fixation on land, thus providing a basis for predicting the response of nutrient-limited ecosystems to climate change and increasing atmospheric CO(2).     

Respiration as the main determinant of carbon balance in European forests

Carbon exchange between the terrestrial biosphere and the atmosphere is one of the key processes that need to be assessed in the context of the Kyoto Protocol. Several studies suggest that the terrestrial biosphere is gaining carbon, but these estimates are obtained primarily by indirect methods, and the factors that control terrestrial carbon exchange, its magnitude and primary locations, are under debate. Here we present data of net ecosystem carbon exchange, collected between 1996 and 1998 from 15 European forests, which confirm that many European forest ecosystems act as carbon sinks. The annual carbon balances range from an uptake of 6.6 tonnes of carbon per hectare per year to a release of nearly 1 t C ha(-1) yr(-1), with a large variability between forests. The data show a significant increase of carbon uptake with decreasing latitude, whereas the gross primary production seems to be largely independent of latitude. Our observations indicate that, in general, ecosystem respiration determines net ecosystem carbon exchange. Also, for an accurate assessment of the carbon balance in a particular forest ecosystem, remote sensing of the normalized difference vegetation index or estimates based on forest inventories may not be sufficient.     

Young organic matter as a source of carbon dioxide outgassing from Amazonian rivers

Rivers are generally supersaturated with respect to carbon dioxide, resulting in large gas evasion fluxes that can be a significant component of regional net carbon budgets. Amazonian rivers were recently shown to outgas more than ten times the amount of carbon exported to the ocean in the form of total organic carbon or dissolved inorganic carbon. High carbon dioxide concentrations in rivers originate largely from in situ respiration of organic carbon, but little agreement exists about the sources or turnover times of this carbon. Here we present results of an extensive survey of the carbon isotope composition (13C and 14C) of dissolved inorganic carbon and three size-fractions of organic carbon across the Amazonian river system. We find that respiration of contemporary organic matter (less than five years old) originating on land and near rivers is the dominant source of excess carbon dioxide that drives outgassing in medium to large rivers, although we find that bulk organic carbon fractions transported by these rivers range from tens to thousands of years in age. We therefore suggest that a small, rapidly cycling pool of organic carbon is responsible for the large carbon fluxes from land to water to atmosphere in the humid tropics.     

Tectonic contraction across Los Angeles after removal of groundwater pumping effects

After the 1987 Whittier Narrows and 1994 Northridge earthquakes revealed that blind thrust faults represent a significant threat to metropolitan Los Angeles, a network of 250 continuously recording global positioning system (GPS) stations was deployed to monitor displacements associated with deep slip on both blind and surface faults. Here we augment this GPS data with interferometric synthetic aperture radar imagery to take into account the deformation associated with groundwater pumping and strike-slip faulting. After removing these non-tectonic signals, we are left with 4.4 mm yr-1 of uniaxial contraction across the Los Angeles basin, oriented N 36 degrees E (perpendicular to the major strike-slip faults in the area). This indicates that the contraction is primarily accommodated on thrust faults rather than on the northeast-trending strike-slip faults. We have found that widespread groundwater and oil pumping obscures and in some cases mimics the tectonic signals expected from the blind thrust faults. In the 40-km-long Santa Ana basin, groundwater withdrawal and re-injection produces 12 mm yr-1 of long-term subsidence, accompanied by an unprecedented seasonal oscillation of 55 mm in the vertical direction and 7 mm horizontally.     

江西省森林碳储量与碳密度研究

为探索和估算我国森林碳汇功能提供准确和可靠的基础数据,利用江西省森林资源二类清查资料,运用材积源生物量法对江西省森林碳储量和碳密度进行了研究。森林植被乔木层碳密度的特征为,全省不同森林类型碳密度由大到小依次为硬阔林、针阔混交林、毛竹林、国外松林、杉木林、软阔林、灌木林、马尾松林和经济林;乔木层碳密度随着林龄的增加而增大,随着人口密度的增加而降低。不同森林碳储量由大到小依次为杉木林、马尾松林、硬阔林、灌木林、经济林、毛竹林、针阔混交林、国外松林和软阔林,全省南部和中西部要高于中东部和北部。江西省森林总碳储量为1.5G t C,占全国森林总碳储量的5.33%。     

中国亚热带森林土壤呼吸的基本特点

中国亚热带森林在全球森林生态系统中具有独特的植被类型及结构,长期受人类活动干扰(毁林、造林等),森林土壤碳呼吸特征不明确,较难准确估算土壤碳的分配情况.分析了我国亚热带森林土壤呼吸在不同森林类型、不同退化和恢复演替阶段,以及造林再造林等的影响条件下的变化规律.结果表明:我国亚热带区域中的天然林的土壤呼吸通常表现为针叶林(如马尾松林)<松阔或针阔混交林<季风常绿阔林;马尾松与杉木人工林广布中国亚热带地区,两种林分的CO2年通量相当,中龄林较之幼龄林,土壤有机碳库呈现减少趋势;不同研究区域相同林分的土壤呼吸效率初步表现为南强北弱;我国亚热带森林的土壤呼吸通量与世界其他地区的热带森林土壤呼吸通量相差无己,其释放的碳量对全球碳通量的影响应加以重视.     

Soil fertility limits carbon sequestration by forest ecosystems in a CO2-enriched atmosphere

Northern mid-latitude forests are a large terrestrial carbon sink. Ignoring nutrient limitations, large increases in carbon sequestration from carbon dioxide (CO2) fertilization are expected in these forests. Yet, forests are usually relegated to sites of moderate to poor fertility, where tree growth is often limited by nutrient supply, in particular nitrogen. Here we present evidence that estimates of increases in carbon sequestration of forests, which is expected to partially compensate for increasing CO2 in the atmosphere, are unduly optimistic. In two forest experiments on maturing pines exposed to elevated atmospheric CO2, the CO2-induced biomass carbon increment without added nutrients was undetectable at a nutritionally poor site, and the stimulation at a nutritionally moderate site was transient, stabilizing at a marginal gain after three years. However, a large synergistic gain from higher CO2 and nutrients was detected with nutrients added. This gain was even larger at the poor site (threefold higher than the expected additive effect) than at the moderate site (twofold higher). Thus, fertility can restrain the response of wood carbon sequestration to increased atmospheric CO2. Assessment of future carbon sequestration should consider the limitations imposed by soil fertility, as well as interactions with nitrogen deposition.     

中国杉木林生态系统碳储量研究

 利用中国4次森林资源清查资料以及中国森林生态系统定位观测研究站（CFERN）的实测数据,估算了中国1977-2003年4个时期杉木林生态系统的碳储量,分析了其年龄结构特征、垂直分配结构特征、时空动态格局和贮碳潜力。总碳储量研究结果 1977-1981为 1.09 Gt,1984-1988为1.496 Gt,1994-1998为2.446 Gt,1999-2003为2.866 Gt。江西、湖南、浙江、福建、云南、广西和广东7省的杉木林碳储量约占84%～86%。幼、中龄杉木林碳储量在79%~83.90%之间,随着杉木林的演替成熟,我国杉木林生态系统是一个潜在的碳库。在垂直分布上,乔木层碳储量占9.38%~10.63%,林下植被占0.6%~0.7%,土壤占87.99%~89.02%,枯落物占0.68%~0.78%,不同时期杉木林生态系统碳储量的垂直分配结构基本相似。1999-2003期间中国杉木林生态系统碳素现存量为2.866 Gt,一个龄级期（10 a）后碳储量将达到3.772 Gt,并以90.63 Mt·a^-1的平均积累速率递增,是一个贮存潜力大,增长速率快的碳库。     

林业碳汇提升的主要原理和途径

降低大气CO₂含量、缓解气候变暖,已成为当今科学界和国际社会广泛关注的前沿热点问题。林业碳汇作为基于自然解决方案实现“碳达峰、碳中和”的一个重要途径,在应对全球气候变化方面发挥着基础性、战略性、独特的作用。林业碳汇不仅是森林碳汇,林产品碳汇也起着不可忽视的重要作用。林业碳汇潜力提升是一个森林生态系统净碳收支平衡和全产业链林产品碳汇的调控过程,主要包括无机碳的植物固定(光合过程、净生产力等)、土壤有机碳的周转与固定(动植物和微生物残体分解与黏土固定)、林产品碳的固持(林产品产量、木材转换效率、种类和使用寿命等)等3方面的调控原理。笔者从森林碳汇和林产品碳汇两个维度阐述了提升林业碳汇的主要原理、方法或途径。提升林业碳汇潜力的主要途径包括:①通过适地适树、适钙适树人工造林,以增加森林面积;②以完善森林经营措施来增加森林净生产力;③利用矿质黏土对有机碳的保护来增加森林土壤碳汇;④提升林产品产量和改进林产品用途以增加其寿命。在全球尺度上,增加森林面积或提高森林净生产力3.4%,或用可再生能源替换薪炭木材,再将薪炭木材用于制造锯材和人造板,都可以连续30 a每年增加1 Pg的碳汇量。减少全球森林火灾面积1/4或增加森林土壤有机碳含量0.23%,也可以增加碳汇1 Pg。此外,林业固碳还有巨大潜力可以挖掘。     

Acceleration of global warming due to carbon-cycle feedbacks in a coupled climate model

The continued increase in the atmospheric concentration of carbon dioxide due to anthropogenic emissions is predicted to lead to significant changes in climate. About half of the current emissions are being absorbed by the ocean and by land ecosystems, but this absorption is sensitive to climate as well as to atmospheric carbon dioxide concentrations, creating a feedback loop. General circulation models have generally excluded the feedback between climate and the biosphere, using static vegetation distributions and CO2 concentrations from simple carbon-cycle models that do not include climate change. Here we present results from a fully coupled, three-dimensional carbon-climate model, indicating that carbon-cycle feedbacks could significantly accelerate climate change over the twenty-first century. We find that under a 'business as usual' scenario, the terrestrial biosphere acts as an overall carbon sink until about 2050, but turns into a source thereafter. By 2100, the ocean uptake rate of 5 Gt C yr(-1) is balanced by the terrestrial carbon source, and atmospheric CO2 concentrations are 250 p.p.m.v. higher in our fully coupled simulation than in uncoupled carbon models, resulting in a global-mean warming of 5.5 K, as compared to 4 K without the carbon-cycle feedback.     

300-Myr-old magmatic CO2 in natural gas reservoirs of the west Texas Permian basin

Except in regions of recent crustal extension, the dominant origin of carbon dioxide in fluids in sedimentary basins has been assumed to be from crustal organic matter or mineral reactions. Here we show, by contrast, that Rayleigh fractionation caused by partial degassing of a magma body can explain the CO2/3He ratios and delta13C(CO2) values observed in CO2-rich natural gases in the west Texas Val Verde basin and also the mantle 3He/22Ne ratios observed in other basin systems. Regional changes in CO2/3He and CO2/CH4 ratios can be explained if the CO2 input pre-dates methane generation in the basin, which occurred about 280 Myr ago. Uplift to the north of the Val Verde basin between 310 and 280 Myr ago appears to be the only tectonic event with appropriate timing and location to be the source of the magmatic CO2. Our identification of magmatic CO2 in a foreland basin indicates that the origin of CO2 in other mid-continent basin systems should be re-evaluated. Also, the inferred closed-system preservation of natural gas in a trapping structure for approximately 300 Myr is far longer than the residence time predicted by diffusion models.     

Towards robust regional estimates of CO2 sources and sinks using atmospheric transport models

Information about regional carbon sources and sinks can be derived from variations in observed atmospheric CO2 concentrations via inverse modelling with atmospheric tracer transport models. A consensus has not yet been reached regarding the size and distribution of regional carbon fluxes obtained using this approach, partly owing to the use of several different atmospheric transport models. Here we report estimates of surface-atmosphere CO2 fluxes from an intercomparison of atmospheric CO2 inversion models (the TransCom 3 project), which includes 16 transport models and model variants. We find an uptake of CO2 in the southern extratropical ocean less than that estimated from ocean measurements, a result that is not sensitive to transport models or methodological approaches. We also find a northern land carbon sink that is distributed relatively evenly among the continents of the Northern Hemisphere, but these results show some sensitivity to transport differences among models, especially in how they respond to seasonal terrestrial exchange of CO2. Overall, carbon fluxes integrated over latitudinal zones are strongly constrained by observations in the middle to high latitudes. Further significant constraints to our understanding of regional carbon fluxes will therefore require improvements in transport models and expansion of the CO2 observation network within the tropics.     

Pervasive alteration of tree communities in undisturbed Amazonian forests

Amazonian rainforests are some of the most species-rich tree communities on earth. Here we show that, over the past two decades, forests in a central Amazonian landscape have experienced highly nonrandom changes in dynamics and composition. Our analyses are based on a network of 18 permanent plots unaffected by any detectable disturbance. Within these plots, rates of tree mortality, recruitment and growth have increased over time. Of 115 relatively abundant tree genera, 27 changed significantly in population density or basal area--a value nearly 14 times greater than that expected by chance. An independent, eight-year study in nearby forests corroborates these shifts in composition. Contrary to recent predictions, we observed no increase in pioneer trees. However, genera of faster-growing trees, including many canopy and emergent species, are increasing in dominance or density, whereas genera of slower-growing trees, including many subcanopy species, are declining. Rising atmospheric CO2 concentrations may explain these changes, although the effects of this and other large-scale environmental alterations remain uncertain. These compositional changes could have important impacts on the carbon storage, dynamics and biota of Amazonian forests.     

Recent contributions of glaciers and ice caps to sea level rise

Glaciers and ice caps (GICs) are important contributors to present-day global mean sea level rise. Most previous global mass balance estimates for GICs rely on extrapolation of sparse mass balance measurements representing only a small fraction of the GIC area, leaving their overall contribution to sea level rise unclear. Here we show that GICs, excluding the Greenland and Antarctic peripheral GICs, lost mass at a rate of 148?±?30?Gt?yr(-1) from January 2003 to December 2010, contributing 0.41?±?0.08?mm?yr(-1) to sea level rise. Our results are based on a global, simultaneous inversion of monthly GRACE-derived satellite gravity fields, from which we calculate the mass change over all ice-covered regions greater in area than 100?km(2). The GIC rate for 2003-2010 is about 30 per cent smaller than the previous mass balance estimate that most closely matches our study period. The high mountains of Asia, in particular, show a mass loss of only 4?±?20?Gt?yr(-1) for 2003-2010, compared with 47-55?Gt?yr(-1) in previously published estimates. For completeness, we also estimate that the Greenland and Antarctic ice sheets, including their peripheral GICs, contributed 1.06?±?0.19?mm?yr(-1) to sea level rise over the same time period. The total contribution to sea level rise from all ice-covered regions is thus 1.48?±?0.26?mm?(-1), which agrees well with independent estimates of sea level rise originating from land ice loss and other terrestrial sources.     

宝天曼不同海拔典型栎类林植被与土壤碳贮量及其分配格局

森林是陆地最大的碳库,主要包括植被碳库和土壤碳库两部分.地带性森林植被与土壤碳贮量核算及其分配格局成为全球变化背景下的研究热点.采用样地调查法,对河南省宝天曼不同海拔高度的3种典型栎类林植被与土壤碳贮量及其分配格局进行了对比研究.结果表明3种林型地上和地下植被碳贮量都随海拔升高而降低,但是地上/地下植被碳贮量比率却随海拔升高而增加.0¹0 cm、1010 cm、10²0 cm和2020 cm和20³0 cm土壤有机碳(SOC)贮量都随海拔升高表现出先升高后降低的趋势,该波动主要由底层土引起.森林总碳贮量并没有随海拔变化而表现出显著差异(p>0.05),由此推断植被和土壤碳贮量间存在着补偿效应.