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Comment:Nuclear winter is a real and present danger
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Models show that even a ‘small’ nuclear war would cause catastrophic climate change. Such findings must inform policy, says Alan Robock.
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COMMENTARY: Overshoot, adapt and recover
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We will probably overshoot our current climate targets, so policies of adaptation and recovery need much more attention, say Martin Parry, Jason Lowe and Clair Hanson. FROM THE TEXT: “We should be planning to adapt
to at least 4°C of warming.”
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commentary: the case for mandatory sequestration
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the fact that cumulative carbon dioxide emissions are more important than annual emission rates calls for a fresh approach to climate change mitigation. one option would be a mandatory link between carbon sequestration and fossil fuel extraction. FROM THE TEXT: With current emissions around 10 billion tonnes of carbon per year, and over three trillon tonnes still available in fossil fuel reserves (4,11), emissions need to fall,on average, by over 2% per year from now on to avoid releasing the trillionth tonne.The longer emissions are allowed to rise, the faster they will have to fall thereafter to stay within the same cumulative total.
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Commentary: The climate policy narrative for a dangerously warming world
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It is time to acknowledge that global average temperatures are likely to rise above the 2 °C policy target and consider how that deeply troubling prospect should affect priorities for communicating and managing the risks of a dangerously warming climate.
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Committed terrestrial ecosystem changes due to climate change
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Targets for stabilizing climate change are often based on considerations of the impacts of different levels of global warming, usually assessing the time of reaching a particular level of warming. However, some aspects of the Earth system, such as global mean temperatures1 and sea level rise due to thermal expansion2 or the melting of large ice sheets3 , continue to respond long after the stabilization of radiative forcing. Here we use a coupled climate–vegetation model to show that in turn the terrestrial biosphere shows significant inertia in its response to climate change. We demonstrate that the global terrestrial biosphere can continue to change for decades after climate stabilization. We suggest that ecosystems can be committed to long-term change long before any response is observable: for example, we find that the risk of significant loss of forest cover in Amazonia rises rapidly for a global mean temperature rise above 2 ◦ C. We conclude that such committed ecosystem changes must be considered in the definition of dangerous climate change, and subsequent policy development to avoid it.
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Comparative Drought Responses of Quercus ilex L. and Pinus sylvestris L. in a Montane Forest Undergoing a Vegetation Shift
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Different functional and structural strategies to cope with water shortage exist both within and across plant communities. The current trend towards increasing drought in many regions could drive some species to their physiological limits of drought tolerance, potentially leading to mortality episodes and vegetation shifts. In this paper, we study the drought responses of Quercus ilex and Pinus sylvestris in a montane Mediterranean forest where the former species is replacing the latter in association with recent episodes of drought-induced mortality. Our aim was to compare the physiological responses to variations in soil water content (SWC) and vapor pressure deficit (VPD) of the two species when living together in a mixed stand or separately in pure stands, where the canopies of both species are completely exposed to high radiation and VPD. P. sylvestris showed typical isohydric behavior, with greater losses of stomatal conductance with declining SWC and greater reductions of stored non-structural carbohydrates during drought, consistent with carbon starvation being an important factor in the mortality of this species. On the other hand, Q. ilex trees showed a more anisohydric behavior, experiencing more negative water potentials and higher levels of xylem embolism under extreme drought, presumably putting them at higher risk of hydraulic failure. In addition, our results show relatively small changes in the physiological responses of Q. ilex in mixed vs. pure stands, suggesting that the current replacement of P. sylvestris by Q. ilex will continue.
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Comparing carbon sequestration in temperate freshwater wetland communities
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High productivity and waterlogged conditions make many freshwater wetlands significant carbon sinks. Most wet- land carbon studies focus on boreal peatlands, however, with less attention paid to other climates and to the effects of hydrogeomorphic settings and the importance of wetland vegetation communities on carbon sequestration. This study compares six temperate wetland communities in Ohio that belong to two distinct hydrogeomorphic types: an isolated depressional wetland site connected to the groundwater table, and a riverine flow-through wetland site that receives water from an agricultural watershed. Three cores were extracted in each community and analyzed for total carbon content to determine the soil carbon pool. Sequestration rates were determined by radiometric dating with 137Cs and 210Pb on a set of composite cores extracted in each of the six communities. Cores were also extracted in uplands adjacent to the wetlands at each site. Wetland communities had accretion rates ranging from 3.0 to 6.2 mm yr␣1. The depressional wetland sites had higher (P < 0.001) organic content (146 ± 4.2 gC kg␣1) and lower (P < 0.001) bulk density (0.55 ± 0.01 Mg m␣3) than the riverine ones (50.1 ± 6.9 gC kg␣1 and 0.74 ± 0.06 Mg m␣3). The soil carbon was 98–99% organic in the isolated depressional wetland communities and 85–98% organic in the riv- erine ones. The depressional wetland communities sequestered 317 ± 93 gC m␣2 yr␣1, more (P < 0.01) than the river- ine communities that sequestered 140 ± 16 gC m␣2 yr␣1. The highest sequestration rate was found in the Quercus palustris forested wetland community (473 gC m␣2 yr␣1), while the wetland community dominated by water lotus (Nelumbo lutea) was the most efficient of the riverine communities, sequestering 160 gC m␣2 yr␣1. These differences in sequestration suggest the importance of addressing wetland types and communities in more detail when assessing the role of wetlands as carbon sequestering systems in global carbon budgets.
Keywords: 137Cs, 210Pb, carbon accumulation, Gahanna Woods, Nelumbo lutea, Old Woman Creek, Phragmites australis, Quercus palustris, wetland community, wetland hydrgeomorphology
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Competitive and demographic leverage points of community shifts under climate warming
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Accelerating rates of climate change and a paucity of whole-community studies of climate impacts limit our ability to forecast shifts in ecosystem structure and dynamics, particularly because climate change can lead to idiosyncratic responses via both demographic effects and altered species interactions. We used a multispecies model to predict which processes and species’ responses are likely to drive shifts in the composition of a space- limited benthic marine community. Our model was parametrized from experimental manipulations of the community. Model simulations indicated shifts in species dominance patterns as temperatures increase, with projected shifts in composition primarily owing to the temperature dependence of growth, mortality and competition for three critical species. By contrast, warming impacts on two other species (rendering them weaker competitors for space) and recruitment rates of all species were of lesser importance in determining projected community changes. Our analysis reveals the impor- tance of temperature-dependent competitive interactions for predicting effects of changing climate on such communities. Furthermore, by identify- ing processes and species that could disproportionately leverage shifts in community composition, our results contribute to a mechanistic understand- ing of climate change impacts, thereby allowing more insightful predictions of future biodiversity patterns.
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Complexity of Coupled Human and Natural Systems
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Integrated studies of coupled human and natural systems reveal new and complex patterns and processes not evident when studied by social or natural scientists separately. Synthesis of six case studies from around the world shows that couplings between human and natural systems vary across space, time, and organizational units. They also exhibit nonlinear dynamics with thresholds, reciprocal feedback loops, time lags, resilience, heterogeneity, and surprises. Furthermore, past couplings have legacy effects on present conditions and future possibilities.
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Conifer regeneration following stand-replacing wildfire varies along an elevation gradient in a ponderosa pine forest, Oregon, USA
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Climate change is expected to increase disturbances such as stand-replacing wildfire in many ecosystems, which have the potential to drive rapid turnover in ecological communities. Ecosystem recovery, and therefore maintenance of critical structures and functions (resilience), is likely to vary across environmental gradients such as moisture availability, but has received little study. We examined conifer regeneration a decade following complete stand-replacing wildfire in dry coniferous forests spanning a 700 m elevation gradient where low elevation sites had relatively high moisture stress due to the combination of high temperature and low precipitation. Conifer regeneration varied strongly across the elevation gradient, with little tree regeneration at warm and dry low elevation sites. Logistic regression models predicted rapid increases in regeneration across the elevation gradient for both seedlings of all conifer species and ponderosa pine seedlings individually. This pattern was especially pronounced for well-established seedlings (P38 cm in height). Graminoids dominated lower elevation sites following wildfire, which may have added to moisture stress for seedlings due to competition for water. These results suggest moisture stress can be a critical factor limiting conifer regeneration following stand- replacing wildfire in dry coniferous forests, with predicted increases in temperature and drought in the coming century likely to increase the importance of moisture stress. Strongly moisture limited forested sites may fail to regenerate for extended periods after stand-replacing disturbance, suggesting these sites are high priorities for management intervention where maintaining forests is a priority.
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