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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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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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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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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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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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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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Frequent Long-Distance Plant Colonization
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The ability of species to track their ecological niche after climate change is a major source of uncertainty in predicting their future distribution. By analyzing DNA fingerprinting (amplified fragment-length polymorphism) of nine plant species, we show that long-distance colonization of a remote arctic archipelago, Svalbard, has occurred repeatedly and from several source regions. Propagules are likely carried by wind and drifting sea ice. The genetic effect of restricted colonization was strongly correlated with the temperature requirements of the species, indicating that establishment limits distribution more than dispersal. Thus, it may be appropriate to
assume unlimited dispersal when predicting long-term range shifts in the Arctic.
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A paradigm shift in understanding and quantifying the effects of forest harvesting on floods in snow environments
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A well-established precept in forest hydrology is that any reduction of forest cover will always have a progressively smaller effect on floods with increasing return period. The underlying logic in snow environments is that during the largest snowmelt events the soils and vegetation canopy have little additional storage capacity and under these conditions much of the snowmelt will be converted to runoff regardless of the amount or type of vegetation cover. Here we show how this preconceived physical understanding, reinforced by the outcomes of numerous paired watershed studies, is indefensible because it is rationalized outside the flood frequency distribution framework. We conduct a meta-analysis of postharvest data at four catchments (3–37 km2) with moderate level of harvesting (33%–40%) to demonstrate how harvesting increases the magnitude and frequency of all floods on record (19–99 years) and how such effects can increase unchecked with increasing return period as a consequence of changes to both the mean (þ11% to þ35%) and standard deviation (12% to þ19%) of the flood frequency distribution. We illustrate how forest harvesting has substantially increased the frequency of the largest floods in all study sites regardless of record length and this also runs counter to the prevailing wisdom in hydrological science. The dominant process responsible for these newly emerging insights is the increase in net radiation associated with the conversion from longwave-dominated snowmelt beneath the canopy to shortwave-dominated snowmelt in harvested areas, further amplified or mitigated by basin characteristics such as aspect distribution, elevation range, slope gradient, amount of alpine area, canopy closure, and drainage density. Investigating first order environmental controls on flood frequency distributions, a standard research method in stochastic hydrology, represents a paradigm shift in the way harvesting effects are physically explained and quantified in forest hydrology literature.
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The Role of Livestock Production in Carbon and Nitrogen Cycles
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This review looks at the role of the livestock sector in carbon (C) and nitrogen (N) cycles from a global perspective and considers impacts at the various stages of the commodity chain. With regard to livestock, N and C cycles are closely connected to livestock’s role in land use and land-use change. Livestock’s land use includes grazing land and cropland dedicated to the production of feed crops and fodder. Considering emissions along the entire commodity chain, livestock currently contribute about 18% to the global warming effect. Live- stock contribute about 9% of total carbon dioxide (CO2) emissions, but 37% of methane (CH4), and 65% of nitrous oxide (N2O). The latter will substantially increase over the coming decades, as the pasture land is currently at maximum expanse in most regions; future expansion of the livestock sector will increasingly be crop based. The chapter also reviews mitigation options to reduce C and N emissions from livestock’s land use, production, and animal waste.
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Divergent global precipitation changes induced by natural versus anthropogenic forcing
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As a result of global warming, precipitation is likely to increase in high latitudes and the tropics and to decrease in already dry sub-tropical regions (1). The absolute magnitude and regional details of such changes, however, remain intensely debated (2,3). As is well known from El Nino studies, sea-surface-temperature gradients across the tropical Pacific Ocean can strongly influence global rainfall (4,5). Palaeoproxy evidence indicates that the difference between the warm west Pacific and the colder east Pacific increased in past periods when the Earth warmed as a result of increased solar radiation (6–9). In contrast, in most model projections of future greenhouse warming this gradient weakens (2,10,11). It has not been clear how to reconcile these two findings. Here we show in climate model simulations that the tropical Pacific sea-surface-temperature gradient increases when the warming is due to increased solar radiation and decreases when it is due to increased greenhouse-gas forcing. For the same global surface temperature increase the latter pattern produces less rainfall, notably over tropical land, which explains why in the model the late twentieth century is warmer than in the Medieval Warm Period (around AD 1000–1250) but precipitation is less. This difference is consistent with the global tropospheric energy budget (12), which requires a balance between the latent heat released in precipitation and radiative cooling. The tropospheric cooling is less for increased greenhouse gases, which add radiative absorbers to the troposphere, than for increased solar heating, which is concentrated at the Earth’s surface. Thus warming due to increased greenhouse gases produces a climate signature different from that of warming due to solar radiation changes.
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