Climate Science PDFs
Climate Science PDFs Collection
- Predator-induced reduction of freshwater carbon dioxide emissions
- Predators can influence the exchange of carbon dioxide between ecosystems and the atmosphere by altering ecosys- tem processes such as decomposition and primary production, according to food web theory1,2. Empirical knowledge of such an effect in freshwater systems is limited, but it has been suggested that predators in odd-numbered food chains sup- press freshwater carbon dioxide emissions, and predators in even-numbered food chains enhance emissions2,3. Here, we report experiments in three-tier food chains in experimental ponds, streams and bromeliads in Canada and Costa Rica in the presence or absence of fish (Gasterosteus aculeatus) and invertebrate (Hesperoperla pacifica and Mecistogaster mod- esta) predators. We monitored carbon dioxide fluxes along with prey and primary producer biomass. We found substan- tially reduced carbon dioxide emissions in the presence of predators in all systems, despite differences in predator type, hydrology, climatic region, ecological zone and level of in situ primary production. We also observed lower amounts of prey biomass and higher amounts of algal and detrital biomass in the presence of predators. We conclude that predators have the potential to markedly influence carbon dioxide dynamics in freshwater systems.
- Assessing the Causes of Late Pleistocene Extinctions on the Continents
- One of the great debates about extinction is whether humans or climatic change caused the demise of the Pleistocene megafauna. Evidence from paleontology, climatology, archaeology, and ecology now supports the idea that humans contributed to extinction on some continents, but human hunting was not solely responsible for the pattern of extinction everywhere. Instead, evidence suggests that the intersection of human impacts with pronounced climatic change drove the precise timing and geography of extinction in the Northern Hemisphere. The story from the Southern Hemisphere is still unfolding. New evidence from Australia supports the view that humans helped cause extinctions there, but the correlation with climate is weak or contested. Firmer chronologies, more realistic ecological models, and regional paleoecological insights still are needed to understand details of the worldwide extinction pattern and the population dynamics of the species involved.
- Key role of symbiotic dinitrogen fixation in tropical forest secondary succession
- Forests contribute a significant portion of the land carbon sink, but their ability to sequester CO2 may be constrained by nitrogen 1–6, a major plant-limiting nutrient. Many tropical forests possess tree species capable of fixing atmospheric dinitrogen (N2)7, but it is unclear whether this functional group can supply the nitrogen needed as forests recover from disturbance or previous land use1, or expand in response to rising CO2 (refs 6, 8). Here we identify a powerful feedback mechanism in which N2 fixation can overcome ecosystem- scale deficiencies in nitrogen that emerge during periods of rapid biomass accumulation in tropical forests. Over a 300-year chronose- quence in Panama, N2-fixing tree species accumulated carbon up to nine times faster per individual than their non-fixing neighbours (greatest difference in youngest forests), and showed species-specific differences in the amount and timing of fixation. As a result of fast growth and high fixation, fixers provided a large fraction of the nitrogen needed to support net forest growth (50,000 kg carbon per hectare) in the first 12 years. A key element of ecosystem functional diversity was ensured by the presence of different N2-fixing tree species across the entire forest age sequence. These findings show that symbiotic N2 fixation can have a central role in nitrogen cycling during tropical forest stand development, with potentially important implications for the ability of tropical forests to sequester CO2.
- Biophysical controls on organic carbon fluxes in fluvial networks.pdf
- Metabolism of terrestrial organic carbon in freshwater ecosystems is responsible for a large amount of carbon dioxide outgassing to the atmosphere, in contradiction to the conventional wisdom that terrestrial organic carbon is recalcitrant and contributes little to the support of aquatic metabolism. Here, we combine recent findings from geophysics, microbial ecology and organic geochemistry to show geophysical opportunity and microbial capacity to enhance the net heterotrophy in streams, rivers and estuaries. We identify hydrological storage and retention zones that extend the residence time of organic carbon during downstream transport as geophysical opportunities for microorganisms to develop as attached biofilms or suspended aggregates, and to metabolize organic carbon for energy and growth. We consider fluvial networks as meta-ecosystems to include the acclimation of microbial communities in downstream ecosystems that enable them to exploit energy that escapes from upstream ecosystems, thereby increasing the overall energy utilization at the network level.
- Migrating Like a Herd of Cats: Climate Change and Emerging Forests in British Columbia
- We combine climate tolerances of tree species with probable changes in insect, disease, fire, and other abiotic factors to describe probable changes in distribution of tree species in British Columbia. Predicting changes in forests confronts three major sources of uncertainty: predicting weather and climate, predicting tree species’ responses, and predicting changes in factors modifying the trees’ responses (e.g., pathogens, insects, and fire). Challenges in predicting weather exist because climate projection models differ and downscaling climate is difficult, particularly where weather stations are sparse. Challenges in predicting responses of individual tree species to climate are a result of species competing under a climate regime that we have not seen before and they may not have experienced before. This challenge is aggravated by the differential response of pathogens and insects, as well as by the effects of changes in fire frequency. We first examine the responses of in- dividual species, then we consider implications for broad regional forests. Despite the uncertainty, some trends are more likely than others. We present estimates of the relative species composition of future forests in British Columbia. KEYWORDS: climate change; insects; new regional forests; pathogens; moisture stress; tree migration
- The River Discontinuum: Applying Beaver Modifications to Baseline Conditions for Restoration of Forested Headwaters
- Billions of dollars are being spent in the United States to restore rivers to a desired, yet often unknown, reference condition. In lieu of a known reference, practitioners typically assume the paradigm of a connected watercourse. Geological and ecological processes, however, create patchy and discontinuous fluvial systems. One of these processes, dam building by North American beavers (Castor canadensis), generated discontinuities throughout precolonial river systems of northern North America. Under modern conditions, beaver dams create dynamic sequences of ponds and wet meadows among free-flowing segments. One beaver impoundment alone can exceed 1000 meters along the river, flood the valley laterally, and fundamentally alter biogeochemical cycles and ecological structures. In this article, we use hierarchical patch dynamics to investigate beaver-mediated discontinuity across spatial and temporal scales. We then use this conceptual model to generate testable hypotheses addressing channel geomorphology, natural flow regime, water quality, and biota, given the importance of these factors in river restoration. Keywords: fluvial geomorphology, hierarchical patch dynamics, stream ecology, river continuum concept, river restoration
- Cross-scale Drivers of Natural Disturbances Prone to Anthropogenic Amplification: The Dynamics of Bark Beetle Eruptions
- Biome-scale disturbances by eruptive herbivores provide valuable insights into species interactions, ecosystem function, and impacts of global change. We present a conceptual framework using one system as a model, emphasizing interactions across levels of biological hierarchy and spatiotemporal scales. Bark beetles are major natural disturbance agents in western North American forests. However, recent bark beetle population eruptions have exceeded the frequencies, impacts, and ranges documented during the previous 125 years. Extensive host abundance and susceptibility, concentrated beetle density, favorable weather, optimal symbiotic associations, and escape from natural enemies must occur jointly for beetles to surpass a series of thresholds and exert widespread disturbance. Opposing feedbacks determine qualitatively distinct outcomes at junctures at the biochemical through landscape levels. Eruptions occur when key thresholds are surpassed, prior constraints cease to exert influence, and positive feedbacks amplify across scales. These dynamics are bidirectional, as landscape features influence how lower-scale processes are amplified or buffered. Climate change and reduced habitat heterogeneity increase the likelihood that key thresholds will be exceeded, and may cause fundamental regime shifts. Systems in which endogenous feedbacks can dominate after external forces foster the initial breach of thresholds appear particularly sensitive to anthropogenic perturbations. Keywords: thresholds, plant-insect interactions, landscape disturbance, forest management, anthropogenic change
- Local climatic drivers of changes in phenology at a boreal-temperate ecotone in eastern North America
- Ecosystems in biogeographical transition zones, or ecotones, tend to be highly sensitive to climate and can provide early indications of future change. To evaluate recent climatic changes and their impacts in a boreal-temperate ecotone in eastern North America, we analyzed ice phenology records (1975–2007) for five lakes in the Adirondack Mountains of northern New York State. We observed rapidly decreasing trends of up to 21 days less ice cover, mostly due to later freeze-up and partially due to earlier break-up. To evaluate the local drivers of these lake ice changes, we modeled ice phenology based on local climate data, derived climatic predictors from the models, and evaluated trends in those predictors to determine which were responsible for observed changes in lake ice. November and Decem- ber temperature and snow depth consistently predicted ice-in, and recent trends of warming and decreasing snow during these months were consistent with later ice formation. March and April temperature and snow depth consistently predicted ice-out, but the absence of trends in snow depth during these months, despite concurrent warming, resulted in much weaker trends for ice-out. Recent rates of warming in the Adirondacks are among the highest regionally, although with a different seasonality of changes (early winter > late winter) that is consistent with other lake ice records in the surrounding area. Projected future declines in snow cover could create positive feedbacks and accelerate current rates of ice loss due to warming. Climate sensitivity was greatest for the larger lakes in our study, including Wolf Lake, considered one of the most ecologically intact ‘wilderness lakes’ in eastern North America. Our study provides further evidence of climate sensitivity of the boreal-temperate ecotone of eastern North America and points to emergent conservation challenges posed by climate change in legally protected yet vulnerable landscapes like the Adirondack Park.
- Spatially and temporally consistent prediction of heavy precipitation from mean values
- Extreme precipitation can cause flooding, result in substantial damages and have detrimental effects on ecosystems1,2. Climate adaptation must therefore account for the greatest precipitation amounts that may be expected over a certain time span3. The recurrence of extreme-to-heavy precipitation is notoriously hard to predict, yet cost–benefit estimates of mitigation and successful climate adaptation will need reliable information about percentiles for daily precipitation. Here we present a new and simple formula that relates wet-day mean precipitation to heavy precipitation, providing a method for predicting and downscaling daily precipitation statistics. We examined 32,857 daily rain-gauge records from around the world and the evaluation of the method demonstrated that wet-day precipitation percentiles can be predicted with high accuracy. Evaluations against independent data demonstrated high skill in both space and time, indicating a highly robust methodology.
- Beyond Reserves and Corridors: Policy Solutions to Facilitate the Movement of Plants and Animals in a Changing Climate
- As the Earth’s climate changes, many species will have to move across human-dominated landscapes to track suitable climates and changing ecosystems. Given the magnitude of projected future climate change, expanding and connecting reserve networks—two of the most commonly recommended adaptation strategies for protecting biodiversity in a changing climate—will be necessary but insufficient for preventing climate-induced extinctions. In the present article, we explore additional policy options that could be implemented to facilitate species movements in a changing climate. We discuss both existing and new policies that have the potential to increase landscape permeability, protect species on the move, and physically move species to address climate change. Keywords: climate change, adaptation, species movement, policy
- Ecosystem Processes and Human Influences Regulate Streamflow Response to Climate Change at Long-Term Ecological Research Sites
- Analyses of long-term records at 35 headwater basins in the United States and Canada indicate that climate change effects on streamflow are not as clear as might be expected, perhaps because of ecosystem processes and human influences. Evapotranspiration was higher than was predicted by temperature in water-surplus ecosystems and lower than was predicted in water-deficit ecosystems. Streamflow was correlated with climate variability indices (e.g., the El Niño–Southern Oscillation, the Pacific Decadal Oscillation, the North Atlantic Oscillation), especially in seasons when vegetation influences are limited. Air temperature increased significantly at 17 of the 19 sites with 20- to 60-year records, but streamflow trends were directly related to climate trends (through changes in ice and snow) at only 7 sites. Past and present human and natural disturbance, vegetation succession, and human water use can mimic, exacerbate, counteract, or mask the effects of climate change on streamflow, even in reference basins. Long-term ecological research sites are ideal places to disentangle these processes.
- The Disappearing Cryosphere: Impacts and Ecosystem Responses to Rapid Cryosphere Loss
- The cryosphere—the portion of the Earth’s surface where water is in solid form for at least one month of the year—has been shrinking in response to climate warming. The extents of sea ice, snow, and glaciers, for example, have been decreasing. In response, the ecosystems within the cryosphere and those that depend on the cryosphere have been changing. We identify two principal aspects of ecosystem-level responses to cryosphere loss: (1) trophodynamic alterations resulting from the loss of habitat and species loss or replacement and (2) changes in the rates and mechanisms of biogeochemical storage and cycling of carbon and nutrients, caused by changes in physical forcings or ecological community functioning. These changes affect biota in positive or negative ways, depending on how they interact with the cryosphere. The important outcome, however, is the change and the response the human social system (infrastructure, food, water, recreation) will have to that change. Keywords: cryosphere, ecosystem response, environmental observatories
- Management practices increase the impact of roads on plant communities in forests
- The question of the interaction between management practices and road effects on forest biodiversity is of critical interest for sustainable practices and the conservation of forest communities. Forest road improvement and easier access to stand interiors via skid trails, are integral components of management. We tested whether skid trails and the use of limestone gravel for road improvement extended the road effect on plant communities further into forest habitats in a nutrient-poor environment. We analyzed how road distance and skid trail presence affect stand plant communities by examining species compo- sition, distribution of biological and ecological traits, individual species responses and environmental plant indicator values. All results showed that the road effect extended deeper into forest on skid trails, i.e. up to 20 m and even 60 m, than off skid trails, i.e. up to 10 m. Skid trails served as penetration con- duits for open-habitat species probably due to forest machinery traffic. The road effect was more damag- ing to forest species and less-competitive species on skid trails. Additionally, limestone gravel modified the acidity of adjacent poor soils, leading to a shift in species composition and to a colonization of the stand interior by basophilous species. We advocate minimizing skid trail density and using endogenous materials for roads to keep sections of forest large enough to conserve disturbance-sensitive forest species. The interaction found between road effects and management practices underlines the need to adopt a landscape-scale view and to consider multiple anthropogenic impacts in order to effectively preserve forest plant communities.
- Toward a Global Biodiversity Observing System
- Tracking biodiversity change is increasingly important in sustaining ecosystems and ultimately human well-being.
- Impacts of Climate Change on Biodiversity, Ecosystems, and Ecosystem Services Technical Input to the 2013 National Climate Assessment
- KEY FINDINGS Biodiversity and ecosystems are already more stressed than at any comparable period of human history. Climate change almost always exacerbates the problems caused by other environmental stressors including: land use change and the consequent habitat fragmentation and degradation; extraction of timber, fish, water, and other resources; biological disturbance such as the introduction of non-native invasive species, disease, and pests; and chemical, heavy metal, and nutrient pollution. As a corollary, one mechanism for reducing the negative impacts of climate change is a reduction in other stressors. Climate change is causing many species to shift their geographical ranges, distributions, and phenologies at faster rates than previously thought. Changes in terrestrial plant and animal species ranges are shifting the location and extent of biomes, and altering ecosystem structure and functioning. These rates vary considerably among species. Terrestrial species are moving up in elevation at rates 2 to 3 times greater than initial estimates. Despite faster rates of warming in terrestrial systems compared to ocean environments, the velocity of range shifts for marine taxa exceeds those reported for terrestrial species. Species and populations that are unable to shift their geographic distributions or have narrow environmental tolerances are at an increased risk of extinction. There is increasing evidence of population declines and localized extinctions that can be directly attributed to climate change. Ecological specialists and species that live at high altitudes and latitudes are particularly vulnerable to climate change. Overall, the impacts of climate change are projected to result in a net loss of global biodiversity and major shifts in the provision of ecosystem services. For example, the range and abundance of economically important marine fish are already changing due to climate change and are projected to continue changing such that some local fisheries are very likely to cease to be viable, whereas others may become more valuable if the fishing community can adapt. Range shifts will result in new community assemblages, new associations among species, and promote interactions among species that have not existed in the past. Changes in the spatial distribution and seasonal timing of flora and fauna within marine, aquatic, and terrestrial environments can result in trophic mismatches and asynchronies. Novel species assemblages can also substantially alter ecosystem structure and function and the distribution of ecosystem services. Changes in precipitation regimes and extreme events can cause ecosystem transitions, increase transport of nutrients and pollutants to downstream ecosystems, and overwhelm the ability of natural systems to mitigate harm to people from these events. Changes in extreme events affect systems differentially, because different thresholds are crossed. For example, more intense storms and increased drought coupled with warming can shift grasslands into shrublands, or facilitate domination by other grass types (for example, mixed grass to C-4 tallgrass). More heavy rainfall also increases movement of nutrients and pollutants to downstream ecosystems, restructuring processes, biota, and habitats. As a consequence, regulation of drinking water quality is very likely to be strained as high rainfall and river discharge lead to higher levels of nitrogen in rivers and greater risk of waterborne disease outbreaks. S-2 Impacts of Climate Change on Biodiversity, Ecosystems, and Ecosystem Services | Executive Summary Technical Input to the 2013 National Climate Assessment Changes in winter have big and surprising effects on ecosystems and their services. Changes in soil freezing, snow cover, and air temperature have affected carbon sequestration, decomposition, and carbon export, which influence agricultural and forest production. Seasonally snow-covered regions are especially susceptible to climate change as small changes in temperature or precipitation may result in large changes in ecosystem structure and function. Longer growing seasons and warmer winters are enhancing pest outbreaks, leading to tree mortality and more intense and extensive fires. For winter sports and recreation, future economic losses are projected to be high because of decreased or unreliable snowfall. The ecosystem services provided by coastal habitats are especially vulnerable to sea-level rise and more severe storms. The Atlantic and Gulf of Mexico coasts are most vulnerable to the loss of coastal protection services provided by wetlands and coral reefs. Along the Pacific coast long-term erosion of dunes due to increasing wave heights is projected to be an increasing problem for coastal communities. Beach recreation is also projected to suffer due to coastal erosion. Other forms of recreation are very likely to improve due to better weather, and the net effect is likely a redistribution of the industry and its economic impact, with visitors and tourism dollars shifting away from some communities in favor of others. Climate adaptation has experienced a dramatic increase in attention since the last National Climate Assessment and become a major emphasis in biodiversity conservation and natural resource policy and management. Federal and State agencies are planning for and integrating climate change research into resource management and actions to address impacts of climate change based on historical impacts, future vulnerabilities, and observations on the ground. Land managers have realized that static protected areas will not be sufficient to conserve biodiversity in a changing climate, requiring an emphasis on landscape-scale conservation, connectivity among protected habitats, and sustaining ecological functioning of working lands and waters. Agile and adaptive management approaches are increasingly under development, including monitoring, experimentation, and a capacity to evaluate and modify management actions. Risk-based framing and stakeholder-driven scenario planning will be essential in enhancing our ability to respond to the impacts of climate change. Climate change responses employed by other sectors (for example, energy, agriculture, transportation) are creating new ecosystem stresses, but also can incorporate ecosystem- based approaches to improve their efficacy. Ecosystem-based adaptation has emerged as a framework for understanding the role of ecosystem services in moderating climate impacts on people, although this concept is currently being used more on an international scale than within the United States. Ecological monitoring efforts need to be improved and better coordinated among Federal and State agencies to ensure that the impacts of climate change are adequately observed as well as to support ecological research, management, assessment, and policy. As species and ecosystem boundaries shift to keep pace with climate change, improved and better-integrated research, monitoring, and assessment efforts will be needed at national and global scales. Existing monitoring networks in the United States are not well suited for detecting and attributing the impacts of climate change to the wide range of affected species at the appropriate spatio-temporal scales.
- Water and bioenergy
- Water management expert Arjen Hoekstra, together with environmental science and energy specialists, has analysed the impact of increasing the use of biofuels in the transport sector on global water demand.
- Modelling the long-term response to positive and negative priming of soil organic carbon by black carbon
- bserved increases in the mineralization rate of labile organic carbon (LOC) in the presence of black carbon (BC) have led to speculation that corresponding decreases in non-pyrogenic (i.e. non- BC) soil organic carbon (npSOC) could significantly reduce or negate the C sequestration benefit of BC in soils. Here we show that the potential effect of an increased LOC decomposition rate on long-term npSOC stocks is negligible, even when using assump- tions that would favour large losses, potentially causing no more than 3–4 % loss of npSOC over 100 years if 50 % of above-ground crop residues were converted to BC annually. Conversely, if the BC- stimulated enhanced stabilization of npSOC that has been observed in laboratory trials is extrapolated to the long-term, it would greatly increase soil carbon stocks by 30–60 %. Annual additions of BC derived from crop residues would increase total SOC (including both BC and npSOC) by an amount five times greater than the potential increase from enhanced stabilization and an order of magnitude greater than losses of npSOC caused by annual removals of biomass to provide BC feedstock. Keywords Black carbon Soil organic carbon Terrestrial carbon cycle Fire Biochar
- Carbon-nitrogen interactions regulate climate-carbon cycle feedbacks: results from an atmosphere-ocean general circulation model.pdf
- Inclusion of fundamental ecological interactions between carbon and nitrogen cycles in the land component of an atmosphere-ocean general circulation model (AOGCM) leads to decreased carbon uptake associated with CO2 fertilization, and increased carbon uptake associated with warming of the climate system. The balance of these two opposing effects is to reduce the fraction of anthropogenic CO2 predicted to be sequestered in land ecosystems. The primary mechanism responsible for increased land carbon storage un- der radiatively forced climate change is shown to be fertilization of plant growth by increased mineralization of nitrogen directly associated with increased decomposition of soil organic matter under a warming climate, which in this particular model results in a negative gain for the climate-carbon feedback. Estimates for the land and ocean sink fractions of recent anthropogenic emissions are individually within the range of observational estimates, but the combined land plus ocean sink fractions produce an airborne fraction which is too high compared to observations. This bias is likely due in part to an underestimation of the ocean sink frac- tion. Our results show a significant growth in the airborne fraction of anthropogenic CO2 emissions over the coming century, attributable in part to a steady decline in the ocean sink fraction. Comparison to experimental studies on the fate of radio-labeled nitrogen tracers in temperate forests indicates that the model representation of competition between plants and microbes for new mineral nitrogen resources is reasonable. Our results suggest a weaker dependence of net land carbon flux on soil moisture changes in tropical regions, and a stronger positive growth response to warming in those regions, than predicted by a similar AOGCM implemented without land carbon-nitrogen interactions. We expect that the between-model uncertainty in predictions of future atmospheric CO2 concentration and associated anthropogenic climate change will be reduced as additional climate models introduce carbon-nitrogen cycle interactions in their land components.
- The evolution of growth rates on an expanding range edge
- Individuals in the vanguard of a species invasion face altered selective conditions when compared with conspecifics behind the invasion front. Assortment by dispersal ability on the expanding front, for example, drives the evolution of increased dispersal, which, in turn, leads to accel- erated rates of invasion. Here I propose an additional evolutionary mechanism to explain accelerating invasions: shifts in population growth rate (r). Because individuals in the van- guard face lower population density than those in established populations, they should (relative to individuals in established populations) experience greater r-selection. To test this possibility, I used the ongoing invasion of cane toads (Bufo marinus) across northern Australia. Life-history theory shows that the most efficient way to increase the rate of population growth is to reproduce earlier. Thus, I predict that toads on the invasion front will exhibit faster individual growth rates (and thus will reach breeding size earlier) than those from older populations. Using a common garden design, I show that this is indeed the case: both tadpoles and juvenile toads from frontal popu- lations grow around 30 per cent faster than those from older, long established populations. These results support theoretical predictions that r increases during range advance and highlight the importance of understanding the evolution of life history during range advance. Keywords: Bufo marinus; invasive species; Rhinella marina; r-selection
- On the vapour trail of an atmospheric imprint in insects
- Terrestrial arthropods, at constant risk from desiccation, are highly sensitive to atmospheric temperature and humidity. A physiological marker of these abiotic conditions could highlight phenotypic adaptations, indicate niche partitioning, and predict responses to climate change for a group representing three-quarters of the Earth’s animal species. We show that the 18O composition of insect haemolymph is such a measure, providing a dynamic and quantitatively predictable signal for respiratory gas exchange and inputs from atmospheric humidity. Using American cockroaches (Periplaneta americana) under defined experimental conditions, we show that insects respiring at low humidity demon- strate the expected enrichment in the 18O composition of haemolymph because of evapor- ation. At high humidity, however, diffusional influx of atmospheric water vapour into the animal forces haemolymph to become depleted in 18O. Additionally, using cockroaches sampled from natural habitats, we show that the haemo- lymph 18O signature is transferred to the organic material of the insect’s exoskeleton. Insect cuticle, therefore, exhibits the mean atmospheric conditions surrounding the animals prior to moulting. This discovery will help to define the climatic tolerances of species and their habitat preferences, and offers a means of quantifying the balance between niche partitioning and ‘neutral’ processes in shaping complex tropical forest communities. Keywords: stable isotopes; arthropods; niches; neutral theory; climate change
