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Duality in climate science
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Delivery of palatable 2 °C mitigation scenarios depends on speculative negative emissions or changing the past. Scientists must make their assumptions transparent and defensible, however politically uncomfortable the conclusions.
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Medieval warming initiated exceptionally large wildfire outbreaks in the Rocky Mountains
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Many of the largest wildfires in US history burned in recent decades, and climate change explains much of the increase in area burned. The frequency of extreme wildfire weather will increase with continued warming, but many uncertainties still exist about future fire regimes, including how the risk of large fires will persist as vegetation changes. Past fire-climate relationships provide an opportunity to constrain the related uncertainties, and reveal widespread burn- ing across large regions of western North America during past warm intervals. Whether such episodes also burned large portions of individual landscapes has been difficult to determine, however, because uncertainties with the ages of past fires and limited spatial resolution often prohibit specific estimates of past area burned. Accounting for these challenges in a subalpine landscape in Colorado, we estimated century-scale fire synchroneity across 12 lake- sediment charcoal records spanning the past 2,000 y. The percent- age of sites burned only deviated from the historic range of vari- ability during the Medieval Climate Anomaly (MCA) between 1,200 and 850 y B.P., when temperatures were similar to recent decades. Between 1,130 and 1,030 y B.P., 83% (median estimate) of our sites burned when temperatures increased ∼0.5 °C relative to the preceding centuries. Lake-based fire rotation during the MCA decreased to an estimated 120 y, representing a 260% higher rate of burning than during the period of dendroecological sampling (360 to −60 y B.P.). Increased burning, however, did not persist throughout the MCA. Burning declined abruptly before temperatures cooled, indicating possible fuel limitations to continued burning.
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Human mining activity across the ages determines the genetic structure of modern brown trout (Salmo trutta L.) populations
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Humans have exploited the earth’s metal resources for thousands of years leaving behind a legacy of toxic metal contamination and poor water quality. The southwest of England provides a well-defined example, with a rich history of metal mining dating to the Bronze Age. Mine water washout continues to negatively impact water quality across the region where brown trout (Salmo trutta L.) populations exist in both metal-impacted and relatively clean rivers. We used micro- satellites to assess the genetic impact of mining practices on trout populations in this region. Our analyses demonstrated that metal-impacted trout populations have low genetic diversity and have experienced severe population declines. Metal-river trout populations are genetically distinct from clean-river populations, and also from one another, despite being geographically proximate. Using approximate Bayesian computation (ABC), we dated the origins of these genetic patterns to periods of intensive mining activity. The historical split of contemporary metal-impacted populations from clean-river fish dated to the Medieval period. Moreover, we observed two distinct genetic populations of trout within a single catchment and dated their divergence to the Industrial Revolution. Our investigation thus provides an evaluation of contemporary population genetics in showing how human-altered landscapes can change the genetic makeup of a species.
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THE COST OF INACTION: RECOGNISING THE VALUE AT RISK FROM CLIMATE CHANGE
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The asset management industry—and thus the wider community of investors of all sizes— is facing the prospect of significant losses from the effects of climate change. Assets can be directly damaged by floods, droughts and severe storms, but portfolios can also be harmed indirectly, through weaker growth and lower asset returns. Climate change is a long-term, probably irreversible problem beset by substantial uncertainty. Crucially, however, climate change is a problem of extreme risk: this means that the average losses to be expected are not the only source of concern; on the contrary, the outliers, the particularly extreme scenarios, may matter most of all.
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Global change and the groundwater management challenge
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With rivers in critical regions already exploited to capacity throughout the world and ground- water overdraft as well as large-scale contamination occurring in many areas, we have entered an era in which multiple simultaneous stresses will drive water management. Increasingly, groundwater resources are taking a more prominent role in providing freshwater supplies. We discuss the competing fresh ground- water needs for human consumption, food production, energy, and the environment, as well as physical hazards, and conflicts due to transboundary overexploitation. During the past 50 years, groundwater man- agement modeling has focused on combining simulation with optimization methods to inspect important problems ranging from contaminant remediation to agricultural irrigation management. The compound challenges now faced by water planners require a new generation of aquifer management models that address the broad impacts of global change on aquifer storage and depletion trajectory management, land subsidence, groundwater-dependent ecosystems, seawater intrusion, anthropogenic and geogenic contamination, supply vulnerability, and long-term sustainability. The scope of research efforts is only beginning to address complex interactions using multiagent system models that are not readily formulated as optimization problems and that consider a suite of human behavioral responses.
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Evaporative cooling over the Tibetan Plateau induced by vegetation growth
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Here, we show that in contrast to arctic regions, increased growing season vegetation activity over the TP may have attenuated surface warming. This negative feedback on growing season vegetation temperature is attributed to enhanced evapotranspiration (ET). The extra energy available at the surface, which results from lower albedo, is efficiently dissipated by evaporative cooling. The net effect is a decrease in daily maximum temperature and the diurnal temperature range, which is supported by statistical analyses of in situ observations and by decomposition of the surface energy budget. A daytime cooling effect from increased vegetation activity is also modeled from a set of regional weather research and forecasting (WRF) mesoscale model simulations, but with a magnitude smaller than observed, likely because the WRF model simulates a weaker ET enhancement. Our results suggest that actions to restore native grasslands in degraded areas, roughly one-third of the plateau, will both facilitate a sustainable ecological development in this region and have local climate cobenefits. More accurate simulations of the biophysical coupling between the land surface and the atmosphere are needed to help understand regional climate change over the TP, and possible larger scale feedbacks between climate in the TP and the Asian monsoon system.
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Climate change-associated tree mortality increases without decreasing water availability
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Here, we reveal temporally increasing tree mortality across all study species over the last three decades in the central boreal forests of Canada, where long-term water availability has increased without apparent climate change-associated drought. Our results suggest that the consequences of climate change on tree mortality are more profound than previously thought.
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On underestimation of global vulnerability to tree mortality and forest die-off from hotter drought in the Anthropocene
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Despite recent observational, experimental, and modeling studies suggesting increased vulnerability of trees to hotter drought and associated pests and pathogens, substantial debate remains among research, management and policy-making communities regarding future tree mortality risks. We summarize key mortality- relevant findings, differentiating between those implying lesser versus greater levels of vulnerability. Evidence suggesting lesser vulnerability includes forest benefits of elevated [CO2] and increased water-use efficiency; observed and modeled increases in forest growth and canopy greening; widespread increases in woody-plant biomass, density, and extent; compensatory physiological, morphological, and genetic mechanisms; dampening ecological feedbacks; and potential mitigation by forest management. In contrast, recent studies document more rapid mortality under hotter drought due to negative tree physiological responses and accelerated biotic attacks. Additional evidence suggesting greater vulnerability includes rising background mortality rates; projected increases in drought frequency, intensity, and duration; limitations of vegetation models such as inadequately represented mortality processes; warming feedbacks from die-off; and wildfire synergies. Grouping these findings we identify ten contrasting perspectives that shape the vulnerability debate but have not been discussed collectively. We also present a set of global vulnerability drivers that are known with high confidence: (1) droughts eventually occur everywhere; (2) warming produces hotter droughts; (3) atmospheric moisture demand increases nonlinearly with temperature during drought; (4) mortality can occur faster in hotter drought, consistent with fundamental physiology; (5) shorter droughts occur more frequently than longer droughts and can become lethal under warming, increasing the frequency of lethal drought nonlinearly; and (6) mortality happens rapidly relative to growth intervals needed for forest recovery. These high-confidence drivers, in concert with research supporting greater vulnerability perspectives, support an overall viewpoint of greater forest vulnerability globally. We surmise that mortality vulnerability is being discounted in part due to difficulties in predicting threshold responses to extreme climate events. Given the profound ecological and societal implications of underestimating global vulnerability to hotter drought, we highlight urgent challenges for research, management, and policy-making communities.
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Forecasting the response of Earth’s surface to future climatic and land use changes: A review of methods and research needs
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In the future, Earth will be warmer, precipitation events will be more extreme, global mean
sea level will rise, and many arid and semiarid regions will be drier. Human modifications of landscapes
will also occur at an accelerated rate as developed areas increase in size and population density. We now
have gridded global forecasts, being continually improved, of the climatic and land use changes (C&LUC)
that are likely to occur in the coming decades. However, besides a few exceptions, consensus forecasts do
not exist for how these C&LUC will likely impact Earth-surface processes and hazards. In some cases, we
have the tools to forecast the geomorphic responses to likely future C&LUC. Fully exploiting these models
and utilizing these tools will require close collaboration among Earth-surface scientists and Earth-system
modelers. This paper assesses the state-of-the-art tools and data that are being used or could be used to
forecast changes in the state of Earth’s surface as a result of likely future C&LUC. We also propose strategies
for filling key knowledge gaps, emphasizing where additional basic research and/or collaboration
across disciplines are necessary. The main body of the paper addresses cross-cutting issues, including the
importance of nonlinear/threshold-dominated interactions among topography, vegetation, and sediment
transport, as well as the importance of alternate stable states and extreme, rare events for understanding
and forecasting Earth-surface response to C&LUC. Five supplements delve into different scales or process
zones (global-scale assessments and fluvial, aeolian, glacial/periglacial, and coastal process zones) in
detail.
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Sour Streams in Appalachia: Mapping Nature’s Buffer Against Sulfur Deposition
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Sulfur emissions are regulated by the Environmental Protection Agency, but sulfuric acid that has
leached into soil and streams can linger in the environment and harm vegetation and aquatic life. Some
watersheds are better able to buffer streams against acidification than others; scientists learned why in
southern Appalachia.
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