The integrity of oil and gas wells Jackson, Robert B.
Proceedings of the National Academy of Sciences - PNAS,
07/2014, Letnik:
111, Številka:
30
Journal Article
Hydrologic regulation of plant rooting depth Fan, Ying; Miguez-Macho, Gonzalo; Jobbágy, Esteban G. ...
Proceedings of the National Academy of Sciences - PNAS,
10/2017, Letnik:
114, Številka:
40
Journal Article
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Plant rooting depth affects ecosystem resilience to environmental stress such as drought. Deep roots connect deep soil/groundwater to the atmosphere, thus influencing the hydrologic cycle and ...climate. Deep roots enhance bedrock weathering, thus regulating the long-term carbon cycle. However, we know little about how deep roots go and why. Here, we present a global synthesis of 2,200 root observations of >1,000 species along biotic (life form, genus) and abiotic (precipitation, soil, drainage) gradients. Results reveal strong sensitivities of rooting depth to local soil water profiles determined by precipitation infiltration depth from the top (reflecting climate and soil), and groundwater table depth from below (reflecting topography-driven land drainage). In well-drained uplands, rooting depth follows infiltration depth; in waterlogged lowlands, roots stay shallow, avoiding oxygen stress below the water table; in between, high productivity and drought can send roots many meters down to the groundwater capillary fringe. This framework explains the contrasting rooting depths observed under the same climate for the same species but at distinct topographic positions. We assess the global significance of these hydrologic mechanisms by estimating root water-uptake depths using an inverse model, based on observed productivity and atmosphere, at 30″ (∼1-km) global grids to capture the topography critical to soil hydrology. The resulting patterns of plant rooting depth bear a strong topographic and hydrologic signature at landscape to global scales. They underscore a fundamental plant–water feedback pathway that may be critical to understanding plant-mediated global change.
Northern peatlands have accumulated large stocks of organic carbon (C) and nitrogen (N), but their spatial distribution and vulnerability to climate warming remain uncertain. Here, we used ...machine-learning techniques with extensive peat core data (n > 7,000) to create observation-based maps of northern peatland C and N stocks, and to assess their response to warming and permafrost thaw. We estimate that northern peatlands cover 3.7 ± 0.5 million km² and store 415 ± 150 Pg C and 10 ± 7 Pg N. Nearly half of the peatland area and peat C stocks are permafrost affected. Using modeled global warming stabilization scenarios (from 1.5 to 6 °C warming), we project that the current sink of atmospheric C (0.10 ± 0.02 Pg C·y−1) in northern peatlands will shift to a C source as 0.8 to 1.9 million km² of permafrost-affected peatlands thaw. The projected thaw would cause peatland greenhouse gas emissions equal to ∼1% of anthropogenic radiative forcing in this century. The main forcing is from methane emissions (0.7 to 3 Pg cumulative CH4-C) with smaller carbon dioxide forcing (1 to 2 Pg CO2-C) and minor nitrous oxide losses. We project that initial CO2-C losses reverse after ∼200 y, as warming strengthens peatland C-sinks. We project substantial, but highly uncertain, additional losses of peat into fluvial systems of 10 to 30 Pg C and 0.4 to 0.9 Pg N. The combined gaseous and fluvial peatland C loss estimated here adds 30 to 50% onto previous estimates of permafrost-thaw C losses, with southern permafrost regions being the most vulnerable.
Soil organic matter (SOM) anchors global terrestrial productivity and food and fiber supply. SOM retains water and soil nutrients and stores more global carbon than do plants and the atmosphere ...combined. SOM is also decomposed by microbes, returning CO
2
, a greenhouse gas, to the atmosphere. Unfortunately, soil carbon stocks have been widely lost or degraded through land use changes and unsustainable forest and agricultural practices.
To understand its structure and function and to maintain and restore SOM, we need a better appreciation of soil organic carbon (SOC) saturation capacity and the retention of above- and belowground inputs in SOM. Our analysis suggests root inputs are approximately five times more likely than an equivalent mass of aboveground litter to be stabilized as SOM. Microbes, particularly fungi and bacteria, and soil faunal food webs strongly influence SOM decomposition at shallower depths, whereas mineral associations drive stabilization at depths greater than ∼30 cm. Global uncertainties in the amounts and locations of SOM include the extent of wetland, peatland, and permafrost systems and factors that constrain soil depths, such as shallow bedrock. In consideration of these uncertainties, we estimate global SOC stocks at depths of 2 and 3 m to be between 2,270 and 2,770 Pg, respectively, but could be as much as 700 Pg smaller. Sedimentary deposits deeper than 3 m likely contain >500 Pg of additional SOC. Soils hold the largest biogeochemically active terrestrial carbon pool on Earth and are critical for stabilizing atmospheric CO
2
concentrations. Nonetheless, global pressures on soils continue from changes in land management, including the need for increasing bioenergy and food production.
Wetlands have long been drained for human use, thereby strongly affecting greenhouse gas fluxes, flood control, nutrient cycling and biodiversity
. Nevertheless, the global extent of natural wetland ...loss remains remarkably uncertain
. Here, we reconstruct the spatial distribution and timing of wetland loss through conversion to seven human land uses between 1700 and 2020, by combining national and subnational records of drainage and conversion with land-use maps and simulated wetland extents. We estimate that 3.4 million km
(confidence interval 2.9-3.8) of inland wetlands have been lost since 1700, primarily for conversion to croplands. This net loss of 21% (confidence interval 16-23%) of global wetland area is lower than that suggested previously by extrapolations of data disproportionately from high-loss regions. Wetland loss has been concentrated in Europe, the United States and China, and rapidly expanded during the mid-twentieth century. Our reconstruction elucidates the timing and land-use drivers of global wetland losses, providing an improved historical baseline to guide assessment of wetland loss impact on Earth system processes, conservation planning to protect remaining wetlands and prioritization of sites for wetland restoration
.
Horizontal drilling and hydraulic fracturing have enhanced energy production but raised concerns about drinking-water contamination and other environmental impacts. Identifying the sources and ...mechanisms of contamination can help improve the environmental and economic sustainability of shale-gas extraction. We analyzed 113 and 20 samples from drinking-water wells overlying the Marcellus and Barnett Shales, respectively, examining hydrocarbon abundance and isotopie compositions (e.g.,C₂H₆/CH₄, δ¹³C-CH₄) and providing, to our knowledge, the first comprehensive analyses of noble gases and their isotopes (e.g., ⁴He, ²⁰Ne, ³⁶Ar) in ground water near shale-gas wells. We addressed two questions. (i) Are elevated levels of hydrocarbon gases in drinking-water aquifers near gas wells natural or anthropogenic? (ii) If fugitive gas contamination exists, what mechanisms cause it? Against a backdrop of naturally occurring salt- and gas-rich groundwater, we identified eight discrete clusters of fugitive gas contamination, seven in Pennsylvania and one in Texas that showed increased contamination through time. Where fugitive gas contamination occurred, the relative proportions of thermogenic hydrocarbon gas (e.g., CH₄, ⁴He) were significantly higher (P < 0.01) and the proportions of atmospheric gases (air-saturated water e.g., N₂, ³⁶Ar) were significantly lower (P < 0.01) relative to background groundwater. Noble gas isotope and hydrocarbon data link four contamination clusters to gas leakage from intermediate-depth strata through failures of annulus cement, three to target production gases that seem to implicate faulty production casings, and one to an underground gas well failure. Noble gas data appear to rule out gas contamination by upward migration from depth through overlying geological strata triggered by horizontal drilling or hydraulic fracturing.
The rapid rise of shale gas development through horizontal drilling and high volume hydraulic fracturing has expanded the extraction of hydrocarbon resources in the U.S. The rise of shale gas ...development has triggered an intense public debate regarding the potential environmental and human health effects from hydraulic fracturing. This paper provides a critical review of the potential risks that shale gas operations pose to water resources, with an emphasis on case studies mostly from the U.S. Four potential risks for water resources are identified: (1) the contamination of shallow aquifers with fugitive hydrocarbon gases (i.e., stray gas contamination), which can also potentially lead to the salinization of shallow groundwater through leaking natural gas wells and subsurface flow; (2) the contamination of surface water and shallow groundwater from spills, leaks, and/or the disposal of inadequately treated shale gas wastewater; (3) the accumulation of toxic and radioactive elements in soil or stream sediments near disposal or spill sites; and (4) the overextraction of water resources for high-volume hydraulic fracturing that could induce water shortages or conflicts with other water users, particularly in water-scarce areas. Analysis of published data (through January 2014) reveals evidence for stray gas contamination, surface water impacts in areas of intensive shale gas development, and the accumulation of radium isotopes in some disposal and spill sites. The direct contamination of shallow groundwater from hydraulic fracturing fluids and deep formation waters by hydraulic fracturing itself, however, remains controversial.
Risks to mitigation potential of forests
Much recent attention has focused on the potential of trees and forests to mitigate ongoing climate change by acting as sinks for carbon. Anderegg
et al.
...review the growing evidence that forests' climate mitigation potential is increasingly at risk from a range of adversities that limit forest growth and health. These include physical factors such as drought and fire and biotic factors, including the depredations of insect herbivores and fungal pathogens. Full assessment and quantification of these risks, which themselves are influenced by climate, is key to achieving science-based policy outcomes for effective land and forest management.
Science
, this issue p.
eaaz7005
BACKGROUND
Forests have considerable potential to help mitigate human-caused climate change and provide society with a broad range of cobenefits. Local, national, and international efforts have developed policies and economic incentives to protect and enhance forest carbon sinks—ranging from the Bonn Challenge to restore deforested areas to the development of forest carbon offset projects around the world. However, these policies do not always account for important ecological and climate-related risks and limits to forest stability (i.e., permanence). Widespread climate-induced forest die-off has been observed in forests globally and creates a dangerous carbon cycle feedback, both by releasing large amounts of carbon stored in forest ecosystems to the atmosphere and by reducing the size of the future forest carbon sink. Climate-driven risks may fundamentally compromise forest carbon stocks and sinks in the 21st century. Understanding and quantifying climate-driven risks to forest stability are crucial components needed to forecast the integrity of forest carbon sinks and the extent to which they can contribute toward the Paris Agreement goal to limit warming well below 2°C. Thus, rigorous scientific assessment of the risks and limitations to widespread deployment of forests as natural climate solutions is urgently needed.
ADVANCES
Many forest-based natural climate solutions do not yet rely on the best available scientific information and ecological tools to assess the risks to forest stability from climate-driven forest dieback caused by fire, drought, biotic agents, and other disturbances. Crucially, many of these permanence risks are projected to increase in the 21st century because of climate change, and thus estimates based on historical data will underestimate the true risks that forests face. Forest climate policy needs to fully account for the permanence risks because they could fundamentally undermine the effectiveness of forest-based climate solutions.
Here, we synthesize current scientific understanding of the climate-driven risks to forests and highlight key issues for maximizing the effectiveness of forests as natural climate solutions. We lay out a roadmap for quantifying current and forecasting future risks to forest stability using recent advances in vegetation physiology, disturbance ecology, mechanistic vegetation modeling, large-scale ecological observation networks, and remote sensing. Finally, we review current efforts to use forests as natural climate solutions and discuss how these programs and policies presently consider and could more fully embrace physiological, climatic, and permanence uncertainty about the future of forest carbon stores and the terrestrial carbon sink.
OUTLOOK
The scientific community agrees that forests can contribute to global efforts to mitigate human-caused climate change. The community also recognizes that using forests as natural climate solutions must not distract from rapid reductions in emissions from fossil fuel combustion. Furthermore, responsibly using forests as natural climate solutions requires rigorous quantification of risks to forest stability, forests’ carbon storage potential, cobenefits for species conservation and ecosystem services, and full climate feedbacks from albedo and other effects. Combining long-term satellite records with forest plot data can provide rigorous, spatially explicit estimates of climate change–driven stresses and disturbances that decrease productivity and increase mortality. Current vegetation models also hold substantial promise to quantify forest risks and inform forest management and policies, which currently rely predominantly on historical data.
A more-holistic understanding and quantification of risks to forest stability will help policy-makers effectively use forests as natural climate solutions. Scientific advances have increased our ability to characterize risks associated with a number of biotic and abiotic factors, including risks associated with fire, drought, and biotic agent outbreaks. While the models that are used to predict disturbance risks of these types represent the cutting edge in ecology and Earth system science to date, relatively little infrastructure and few tools have been developed to interface between scientists and foresters, land managers, and policy-makers to ensure that science-based risks and opportunities are fully accounted for in policy and management contexts. To enable effective policy and management decisions, these tools must be openly accessible, transparent, modular, applicable across scales, and usable by a wide range of stakeholders. Strengthening this science-policy link is a critical next step in moving forward with leveraging forests in climate change mitigation efforts.
Effective use of forests as natural climate solutions depends on accounting for climate-driven risks, such as fire and drought.
Leveraging cutting-edge scientific tools holds great promise for improving and guiding the use of forests as natural climate solutions, both in estimating the potential of carbon storage and in estimating the risks to forest carbon storage.
ILLUSTRATION: DAVID MEIKLE
Forests have considerable potential to help mitigate human-caused climate change and provide society with many cobenefits. However, climate-driven risks may fundamentally compromise forest carbon sinks in the 21st century. Here, we synthesize the current understanding of climate-driven risks to forest stability from fire, drought, biotic agents, and other disturbances. We review how efforts to use forests as natural climate solutions presently consider and could more fully embrace current scientific knowledge to account for these climate-driven risks. Recent advances in vegetation physiology, disturbance ecology, mechanistic vegetation modeling, large-scale ecological observation networks, and remote sensing are improving current estimates and forecasts of the risks to forest stability. A more holistic understanding and quantification of such risks will help policy-makers and other stakeholders effectively use forests as natural climate solutions.
Abstract
Emission metrics, a crucial tool in setting effective exchange rates between greenhouse gases, currently require an arbitrary choice of time horizon. Here, we propose a novel framework to ...calculate the time horizon that aligns with scenarios achieving a specific temperature goal. We analyze the Intergovernmental Panel on Climate Change Special Report on Global Warming of 1.5 °C Scenario Database to find that time horizons aligning with the 1.5 °C and 2 °C global warming goals of the Paris Agreement are 24 90% prediction interval: 7, 41 and 58 90% PI: 41, 74 years, respectively. We then use these time horizons to quantify time-dependent emission metrics for methane. We find that the Global Warming Potential (GWP) values that align with the 1.5 °C and 2 °C goals are GWP
1.5 °C
= 75 90% PI: 54, 107 and GWP
2 °C
= 42 90% PI: 35, 54. For the Global Temperature change Potential (GTP) they are GTP
1.5 °C
= 41 90% PI: 16, 102 and GTP
2 °C
= 9 90% PI: 7, 16. The most commonly used time horizon, 100 years, underestimates methane’s GWP and GTP by 34% and 38%, respectively, relative to the values we calculate that align with the 2 °C goal and by 63% and 87%, respectively, relative to the 1.5 °C goal. To best align emission metrics with the Paris Agreement 1.5 °C goal, we recommend a 24 year time horizon, using 2045 as the endpoint time, with its associated GWP
1.5 °C
= 75 and GTP
1.5 °C
= 41.
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