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Global Cooling by Grassland Soils of the Geological Past and Near Future
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Keywords
grass, mammal, coevolution, paleosol, paleoclimate, carbon sequestration, albedo
Abstract
Major innovations in the evolution of vegetation such as the Devonian ori- gin of forests created new weathering regimes and soils (Alfisols, Histosols) that increased carbon consumption and sequestration and ushered in the Permian-Carboniferous Ice Age. Similarly, global expansion of grasslands and their newly evolved, carbon-rich soils (Mollisols) over the past 40 mil- lion years may have induced global cooling and ushered in Pleistocene glacia- tion. Grassland evolution has been considered a consequence of mountain uplift and tectonic reorganization of ocean currents, but it can also be viewed as a biological force for global change through coevolution of grasses and grazers. Organisms in such coevolutionary trajectories adapt to each other rather than to their environment, and so can be forces for global change. Some past farming practices have aided greenhouse gas release. However, modern grassland agroecosystems are a potential carbon sink already under intensive human management, and carbon farming techniques may be useful in curbing anthropogenic global warming.
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Global declines in oceanic nitrification rates as a consequence of ocean acidification
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Ocean acidification produced by dissolution of anthropogenic carbon dioxide (CO2) emissions in seawater has profound conse- quences for marine ecology and biogeochemistry. The oceans have absorbed one-third of CO2 emissions over the past two centuries, altering ocean chemistry, reducing seawater pH, and affecting marine animals and phytoplankton in multiple ways. Microbially mediated ocean biogeochemical processes will be pivotal in deter- mining how the earth system responds to global environmental change; however, how they may be altered by ocean acidification is largely unknown. We show here that microbial nitrification rates decreased in every instance when pH was experimentally reduced (by 0.05–0.14) at multiple locations in the Atlantic and Pacific Oceans. Nitrification is a central process in the nitrogen cycle that produces both the greenhouse gas nitrous oxide and oxidized forms of nitrogen used by phytoplankton and other microorgan- isms in the sea; at the Bermuda Atlantic Time Series and Hawaii Ocean Time-series sites, experimental acidification decreased am- monia oxidation rates by 38% and 36%. Ammonia oxidation rates were also strongly and inversely correlated with pH along a gradi- ent produced in the oligotrophic Sargasso Sea (r2 = 0.87, P < 0.05). Across all experiments, rates declined by 8–38% in low pH treat- ments, and the greatest absolute decrease occurred where rates were highest off the California coast. Collectively our results sug- gest that ocean acidification could reduce nitrification rates by 3–44% within the next few decades, affecting oceanic nitrous oxide production, reducing supplies of oxidized nitrogen in the upper layers of the ocean, and fundamentally altering nitrogen cycling in the sea.
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Global diversity of drought tolerance and grassland climate-change resilience
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Drought reduces plant productivity, induces widespread plant mortality and limits the geographic distribution of plant species1–7. As climates warm and precipitation patterns shift in the future8,9, understanding the distribution of the diversity of plant drought tolerance is central to predicting future ecosystem function and resilience to climate change10–12 . These questions are especially pressing for the world’s 11,000 grass species13, which dominate a large fraction of the terrestrial biosphere14, yet are poorly characterized with respect to re- sponses to drought. Here, we show that physiological drought tolerance, which varied tenfold among 426 grass species, is well distributed both climatically and phylogenetically, sug- gesting most native grasslands are likely to contain a high diversity of drought tolerance. Consequently, local species may help maintain ecosystem functioning in response to changing drought regimes without requiring long-distance migrations of grass species. Furthermore, physiologically drought-tolerant species had higher rates of water and carbon dioxide exchange than intolerant species, indicating that severe droughts may generate legacies for ecosystem functioning. In all, our findings suggest that diverse grasslands throughout the globe have the potential to be resilient to drought in the face of climate change through the local expansion of drought-tolerant species.
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Global evidence that deforestation amplifies flood risk and severity in the developing world
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With the wide acceptance of forest-protection policies in the developing world comes a requirement for clear demonstrations of how deforestation may erode human well-being and economies. For centuries, it has been believed that forests provide protection against flooding. However, such claims have given rise to a heated polemic, and broad-scale quantitative evidence of the possible role of forests in flood protection has not been forthcoming. Using data collected from 1990 to 2000 from 56 developing countries, we show using generalized linear and mixed-effects models contrasted with information- theoretic measures of parsimony that flood frequency is negatively correlated with the amount of remaining natural forest and positively correlated with natural forest area loss (after controlling for rainfall, slope and degraded landscape area). The most parsimo- nious models accounted for over 65% of the variation in flood frequency, of which nearly 14% was due to forest cover variables alone. During the decade investigated, nearly 100 000 people were killed and 320 million people were displaced by floods, with total reported economic damages exceeding US$1151 billion. Extracted measures of flood severity (flood duration, people killed and displaced, and total damage) showed some weaker, albeit detectable correlations to natural forest cover and loss. Based on an arbitrary decrease in natural forest area of 10%, the model-averaged prediction of flood frequency increased between 4% and 28% among the countries modeled. Using the same hypothetical decline in natural forest area resulted in a 4–8% increase in total flood duration. These correlations suggest that global-scale patterns in mean forest trends across countries are meaningful with respect to flood dynamics. Unabated loss of forests may increase or exacerbate the number of flood-related disasters, negatively impact millions of poor people, and inflict trillions of dollars in damage in disadvantaged economies over the coming decades. This first global-scale empirical demonstration that forests are correlated with flood risk and severity in developing countries reinforces the imperative for large-scale forest protection to protect human welfare, and suggests that reforestation may help to reduce the frequency and severity of flood-related catastrophes.
Keywords: conservation, damage, flooding events, forest loss, generalized linear mixed-effects models, generalized linear models, human displacement, projected costs, rainfall
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Global imprint of climate change on marine life
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Past meta-analyses of the response of marine organisms to climate change have examined a limited range of locations1,2, taxonomic groups2–4 and/or biological responses5,6. This has precluded a robust overview of the effect of climate change in the global ocean. Here, we synthesized all available studies of the consistency of marine ecological observations with expectations under climate change. This yielded a meta- database of 1,735 marine biological responses for which either regional or global climate change was considered as a driver. Included were instances of marine taxa responding as expected, in a manner inconsistent with expectations, and taxa demonstrating no response. From this database, 81–83% of all observations for distribution, phenology, community composition, abundance, demography and calcification across taxa and ocean basins were consistent with the expected impacts of climate change. Of the species responding to climate change, rates of distribution shifts were, on average, consistent with those required to track ocean surface temperature changes. Conversely, we did not find a relationship between regional shifts in spring phenology and the seasonality of temperature. Rates of observed shifts in species’ distributions and phenology are comparable to, or greater, than those for terrestrial systems.
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Global land use change, economic globalization, and the looming land scarcity
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A central challenge for sustainability is how to preserve forest ecosystems and the services that they provide us while enhancing food production. This challenge for developing countries confronts the force of economic globalization, which seeks cropland that is shrinking in availability and triggers deforestation. Four mechanisms— the displacement, rebound, cascade, and remittance effects—that are amplified by economic globalization accelerate land conversion. A few developing countries have managed a land use transition over the recent decades that simultaneously increased their forest cover and agricultural production. These countries have relied on various mixes of agricultural intensification, land use zoning, forest protection, increased reliance on imported food and wood products, the creation of off-farm jobs, foreign capital investments, and remittances. Sound policies and innovations can therefore rec- oncile forest preservation with food production. Globalization can be harnessed to increase land use efficiency rather than leading to uncontrolled land use expansion. To do so, land systems should be understood and modeled as open systems with large flows of goods, people, and capital that connect local land use with global- scale factors.
land change | forest transition food sustainability
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Global protected area expansion is compromised by projected land-use and parochialism
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Protected areas are one of the main tools for halting the continuing global biodiversity crisis1–4 caused by habitat loss, fragmentation and other anthropogenic pressures5–8. According to the Aichi Biodiversity Target 11 adopted by the Convention on Biological Diversity, the protected area network should be expanded to at least 17% of the terrestrial world by 2020 (http://www.cbd.int/sp/targets). To max- imize conservation outcomes, it is crucial to identify the best expan- sion areas. Here we show that there is a very high potential to increase protection of ecoregions and vertebrate species by expanding the pro- tected area network, but also identify considerable risk of ineffective outcomes due to land-use change and uncoordinated actions between countries. We use distribution data for 24,757 terrestrial vertebrates assessed under the International Union for the Conservation of Nature (IUCN) ‘red list of threatened species’9, and terrestrial eco- regions10 (827), modified by land-use models for the present and 2040, and introduce techniques for global and balanced spatial con- servation prioritization. First, we show that with a coordinated global protected area network expansion to 17% of terrestrial land, average protection of species ranges and ecoregions could triple. Second, if projected land-use change by 2040 (ref. 11) takes place, it becomes infeasible to reach the currently possible protection levels, and over 1,000 threatened species would lose more than 50% of their present effective ranges worldwide. Third, we demonstrate a major efficiency gap between national and global conservation priorities. Strong evi- dence is shown that further biodiversity loss is unavoidable unless international action is quickly taken to balance land-use and biodiversity conservation. The approach used here can serve as a framework for repeatable and quantitative assessment of efficiency, gaps and expansion of the global protected area network globally, regionally and nationally, considering current and projected land-use pressures.
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Global Resilience of Tropical Forest and Savanna to Critical Transitions
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It has been suggested that tropical forest and savanna could represent alternative stable states,
implying critical transitions at tipping points in response to altered climate or other drivers.
So far, evidence for this idea has remained elusive, and integrated climate models assume smooth
vegetation responses. We analyzed data on the distribution of tree cover in Africa, Australia,
and South America to reveal strong evidence for the existence of three distinct attractors:
forest, savanna, and a treeless state. Empirical reconstruction of the basins of attraction indicates
that the resilience of the states varies in a universal way with precipitation. These results allow
the identification of regions where forest or savanna may most easily tip into an alternative
state, and they pave the way to a new generation of coupled climate models.
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Global shifts towards positive species interactions with increasing environmental stress
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The study of positive species interactions is a rapidly evolving field in ecology. Despite decades of research, controversy has emerged as to whether positive and negative interactions predictably shift with increasing environmental stress as hypothesised by the stress-gradient hypothesis (SGH). Here, we provide a synthesis of 727 tests of the SGH in plant communities across the globe to examine its generality across a variety of ecological factors. Our results show that plant interactions change with stress through an outright shift to facilitation (survival) or a reduction in competition (growth and reproduction). In a limited number of cases, plant interactions do not respond to stress, but they never shift towards competition with stress. These findings are consistent across stress types, plant growth forms, life histories, origins (invasive vs. native), climates, ecosystems and methodologies, though the magnitude of the shifts towards facilitation with stress is dependent on these factors. We suggest that future studies should employ standardised defini- tions and protocols to test the SGH, take a multi-factorial approach that considers variables such as plant traits in addition to stress, and apply the SGH to better understand how species and communities will respond to environmental change.
Keywords
Biotic interactions, community ecology, ecosystems and climates, environmental stress, facilitation, invasive species, meta-analysis, plant traits, the stress-gradient hypothesis.
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Global temperature change
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We conclude that global warming of more than 1°C, relative to 2000, will constitute ‘‘dangerous’’ climate change as judged from likely effects on sea level and extermination of species.
climate change El Niños global warming sea level species extinctions
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