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Long term climate implications of 2050 emission reduction targets
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A coupled atmosphere-ocean-carbon cycle model is used to examine the long term climate implications of various 2050 greenhouse gas emission reduction targets. All emission targets considered with less than 60% global reduction by 2050 break the 2.0°C threshold warming this century, a number that some have argued represents an upper bound on manageable climate warming. Even when emissions are stabilized at 90% below present levels at 2050, this 2.0°C threshold is eventually broken. Our results suggest that if a 2.0°C warming is to be avoided, direct CO2 capture from the air, together with subsequent sequestration, would eventually have to be introduced in addition to sustained 90% global carbon emissions reductions by 2050.
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Wildfire and forest harvest disturbances in the boreal forest leave different long-lasting spatial signatures
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Natural disturbances leave long-term legacies that vary among landscapes and ecosystem types, and which become integral parts of successional pro- cesses at a given location. As humans change land use, not only are immediate post-disturbance patterns altered, but the processes of recovery themselves are likely altered by the disturbance. We assessed whether short-term effects on soil and vegetation that distinguish wildfire from forest harvest persist over 60 years after disturbance in boreal black spruce forests, or post-disturbance processes of recovery promote convergence of the two disturbance types.
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Untangling the confusion around land carbon science and climate change mitigation policy
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Depletion of ecosystem carbon stocks is a significant source of atmospheric CO2 and reducing land-based emissions and maintaining land carbon stocks contributes to climate change mitigation. We summarize current understanding about human perturbation of the global carbon cycle, examine three scientific issues and consider implications for the interpretation of international climate change policy decisions, concluding that considering carbon storage on land as a means to ‘offset’ CO2 emissions from burning fossil fuels (an idea with wide currency) is scientifically flawed. The capacity of terrestrial ecosystems to store carbon is finite and the current sequestration potential primarily reflects depletion due to past land use. Avoiding emissions from land carbon stocks and refilling depleted stocks reduces atmospheric CO2 concentration, but the maximum amount of this reduction is equivalent to only a small fraction of potential fossil fuel emissions.
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Interactions between climate and habitat loss effects on biodiversity: a systematic review and meta-analysis
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Climate change and habitat loss are both key threatening processes driving the global loss in biodiversity. Yet little is known about their synergistic effects on biological populations due to the complexity underlying both processes. If the combined effects of habitat loss and climate change are greater than the effects of each threat individually, current conservation management strategies may be inefficient and at worst ineffective. Therefore, there is a pressing need to identify whether interacting effects between climate change and habitat loss exist and, if so, quantify the magnitude of their impact. In this article, we present a meta-analysis of studies that quantify the effect of habitat loss on biologi- cal populations and examine whether the magnitude of these effects depends on current climatic conditions and his- torical rates of climate change. We examined 1319 papers on habitat loss and fragmentation, identified from the past 20 years, representing a range of taxa, landscapes, land-uses, geographic locations and climatic conditions. We find that current climate and climate change are important factors determining the negative effects of habitat loss on spe- cies density and/or diversity. The most important determinant of habitat loss and fragmentation effects, averaged across species and geographic regions, was current maximum temperature, with mean precipitation change over the last 100 years of secondary importance. Habitat loss and fragmentation effects were greatest in areas with high maxi- mum temperatures. Conversely, they were lowest in areas where average rainfall has increased over time. To our knowledge, this is the first study to conduct a global terrestrial analysis of existing data to quantify and test for inter- acting effects between current climate, climatic change and habitat loss on biological populations. Understanding the synergistic effects between climate change and other threatening processes has critical implications for our ability to support and incorporate climate change adaptation measures into policy development and management response.
Keywords: climate change, habitat fragmentation, habitat loss, interactions, meta-analysis, mixed-effects logistic regression
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Effects of grazing on grassland soil carbon: a global review
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Soils of grasslands represent a large potential reservoir for storing CO2, but this potential likely depends on how grasslands are managed for large mammal grazing. Previous studies found both strong positive and negative grazing effects on soil organic carbon (SOC) but explanations for this variation are poorly developed. Expanding on previous reviews, we performed a multifactorial meta-analysis of grazer effects on SOC density on 47 independent experimen- tal contrasts from 17 studies. We explicitly tested hypotheses that grazer effects would shift from negative to positive with decreasing precipitation, increasing fineness of soil texture, transition from dominant grass species with C3 to C4 photosynthesis, and decreasing grazing intensity, after controlling for study duration and sampling depth. The six variables of soil texture, precipitation, grass type, grazing intensity, study duration, and sampling depth explained 85% of a large variation (`150 g m␣2 yr␣1) in grazing effects, and the best model included significant interactions between precipitation and soil texture (P = 0.002), grass type, and grazing intensity (P = 0.012), and study duration and soil sampling depth (P = 0.020). Specifically, an increase in mean annual precipitation of 600 mm resulted in a 24% decrease in grazer effect size on finer textured soils, while on sandy soils the same increase in precipitation pro- duced a 22% increase in grazer effect on SOC. Increasing grazing intensity increased SOC by 6–7% on C4-dominated and C4–C3 mixed grasslands, but decreased SOC by an average 18% in C3-dominated grasslands. We discovered these patterns despite a lack of studies in natural, wildlife-dominated ecosystems, and tropical grasslands. Our results, which suggest a future focus on why C3 vs. C4-dominated grasslands differ so strongly in their response of SOC to grazing, show that grazer effects on SOC are highly context-specific and imply that grazers in different regions might be managed differently to help mitigate greenhouse gas emissions.
Keywords: carbon sequestration, grasslands, grazing, grazing intensity, precipitation, soil organic carbon, soil texture
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Millennium Ecosystem Assessment: Research Needs
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The research community needs to develop analytical tools for projecting future trends and evaluating the success of interventions as well as indicators to monitor biological, physical, and social changes.
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Migration and Dispersal: Science Special Section
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INTRODUCTION: When to Go, Where to Stop
THE ABILITY TO MOVE, AT SOME STAGE IN THE LIFE CYCLE, IS FUNDAMENTAL TO SUCCESS in life. Passive drift in water columns conferred a selective advantage for early life, offering an escape from starvation and genetic uniformity. Since then, organisms have evolved many ways to disperse and migrate in response to the pressures of finding resources, escaping predators, seeking out mates and suitable breeding grounds, and distancing themselves from family. Dispersal in its broadest sense means movement away from the birthplace. Strictly speaking, migration involves travel in a periodically and geographically predictable way, whether it occurs just once or many times. In this issue, Science deals with what we know, what we need to know, and how we are going to find out more about both of these movement types.
In plants, the spore, seed, or fruit is typically the unit of dispersal. Although the many morphological adaptations for their dispersal are known, until now, researchers have been unable to determine the distances traveled or the proportion of dispersal events that lead to seedlings. In one Perspective (p. 786), Nathan describes recent developments in the modeling and measurement of the long-distance dispersal of plants. A News story by Holden (p. 779) discusses the push to come up with a theoretical framework, not just for plants, but for all moving organisms. Organisms also disperse in reaction to changing habitats and climate. The Perspective by Kokko and López-Sepulcre (p. 789) discusses the selective forces affecting this ability in animals and how dispersal translates into range expansions and contractions. Kintisch (p. 776) describes the challenges for marine scientists assessing how climate change may affect oceangoing species.
Humans have been great dispersers. Colonizing new habitat has been a hallmark of human ecology over the past million years or so. In a Review (p. 796), Mellars considers recent advances in archaeology and genetics that are illuminating the controversies over the routes taken by ancient peoples in the colonization of Asia 40,000 to 60,000 years ago. Two Perspectives consider migration: Holland et al. (p. 794) focus on migrating insects, which tend to travel in established geographical patterns across several generations rather than returning to their birthplace, and Alerstam (p. 791) discusses the accumulating and sometimes conflicting evidence about the navigational mechanisms used by animals (particularly birds) in long-distance annual migrations. In a related Report (p. 837), Muheim et al. describe the role of polarized light at dawn and sunset in calibrating the magnetic compasses of migrating birds. A News story by Morell (p. 783) describes a new model that will clarify the mix of genes and environmental responses underlying successful bird migration.
As News stories by Blackburn and Holden (p. 780) and Unger (p. 784) point out, ingenuity and persistence are beginning to pay off in new techniques for following organisms, be they fish, crabs, jellyfish, rhinos, or polar bears. Thanks to these advances, the study of the ecology and evolution of movement is charging ahead and unearthing the challenges faced by organisms in dispersing and migrating in a world undergoing anthropogenic change.
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On the Hydrologic Adjustment of Climate-Model Projections: The Potential Pitfall of Potential Evapotranspiration
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Hydrologic models often are applied to adjust projections of hydroclimatic change that come from climate models. Such adjustment includes climate-bias correction, spatial refinement (‘‘downscaling’’), and consideration of the roles of hydrologic processes that were neglected in the climate model. Described herein is a quantitative analysis of the effects of hydrologic adjustment on the projections of runoff change associated with projected twenty-first-century climate change. In a case study including three climate models and 10 river basins in the contiguous United States, the authors find that relative (i.e., fractional or percentage) runoff change computed with hydrologic adjustment more often than not was less positive (or, equivalently, more negative) than what was pro- jected by the climate models. The dominant contributor to this decrease in runoff was a ubiquitous change in runoff (median 211%) caused by the hydrologic model’s apparent amplification of the climate-model-implied growth in potential evapotranspiration. Analysis suggests that the hydrologic model, on the basis of the empirical, temperature-based modified Jensen–Haise formula, calculates a change in potential evapotranspiration that is typically 3 times the change implied by the climate models, which explicitly track surface energy budgets. In com- parison with the amplification of potential evapotranspiration, central tendencies of other contributions from hydrologic adjustment (spatial refinement, climate-bias adjustment, and process refinement) were relatively small. The authors’ findings highlight the need for caution when projecting changes in potential evapotranspiration for use in hydrologic models or drought indices to evaluate climate change impacts on water.
KEYWORDS: Hydrologic model; Climate change; Potential evapotranspi- ration
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Temperature Mediated Moose Survival in Northeastern Minnesota
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The earth is in the midst of a pronounced warming trend and temperatures in Minnesota, USA, as elsewhere, are projected to increase. Northern Minnesota represents the southern edge to the circumpolar distribution of moose (Alces alces), a species intolerant of heat. Moose increase their metabolic rate to regulate their core body temperature as temperatures rise. We hypothesized that moose survival rates would be a function of the frequency and magnitude that ambient temperatures exceeded the upper critical temperature of moose. We compared annual and seasonal moose survival in northeastern Minnesota between 2002 and 2008 with a temperature metric. We found that models based on January temperatures above the critical threshold were inversely correlated with subsequent survival and explained .78% of variability in spring, fall, and annual survival. Models based on late-spring temperatures also explained a high proportion of survival during the subsequent fall. A model based on warm-season temperatures was important in explaining survival during the subsequent winter. Our analyses suggest that temperatures may have a cumulative influence on survival. We expect that continuation or acceleration of current climate trends will result in decreased survival, a decrease in moose density, and ultimately, a retreat of moose northward from their current distribution.
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CARBON CYCLE : Fertilizing change
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Carbon cycle–climate feedbacks are expected to diminish the size of the terrestrial carbon sink over the next century. Model simulations suggest that nitrogen availability is likely to play a key role in mediating this response.
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