2Keywords: methane; shale gas; unconventional gas; greenhouse gas; fracking; hydraulic fracturing... more 2Keywords: methane; shale gas; unconventional gas; greenhouse gas; fracking; hydraulic fracturing In April 2011, we published the first comprehensive analysis of greenhouse gas (GHG) emissions from shale gas obtained by hydraulic fracturing, with a focus on methane emissions. Our analysis was challenged by Cathles et al. (2012). Here, we respond to those criticisms. We stand by our approach and findings. The latest EPA estimate for methane emissions from shale gas falls within the range of our estimates but not those of Cathles et al, which are substantially lower. Cathles et al. believe the focus should be just on electricity generation, and the global warming potential of methane should be considered only on a 100-year time scale. Our analysis covered both electricity (30 % of US usage) and heat generation (the largest usage), and we evaluated both 20- and 100-year integrated time frames for methane. Both time frames are important, but the decadal scale is critical, given the urge...
In April 2011, we published the first comprehensive analysis of greenhouse gas (GHG) emissions fr... more In April 2011, we published the first comprehensive analysis of greenhouse gas (GHG) emissions from shale gas obtained by hydraulic fracturing, with a focus on methane emissions. Our analysis was challenged by Cathles et al. (2012). Here, we respond to those criticisms. We stand by our approach and findings. The latest EPA estimate for methane emissions from shale gas falls within the range of our estimates but not those of Cathles et al. which are substantially lower. Cathles et al. believe the focus should be just on electricity generation, and the global warming potential of methane should be considered only on a 100-year time scale. Our analysis covered both electricity (30 % of US usage) and heat generation (the largest usage), and we evaluated both 20- and 100-year integrated time frames for methane. Both time frames are important, but the decadal scale is critical, given the urgent need to avoid climate-system tipping points. Using all available information and the latest cli...
In April 2011, we published the first comprehensive analysis of greenhouse gas (GHG) emissions fr... more In April 2011, we published the first comprehensive analysis of greenhouse gas (GHG) emissions from shale gas obtained by hydraulic fracturing, with a focus on methane emissions. Our analysis was challenged by Cathles et al. (2012). Here, we respond to those criticisms. We stand by our approach and findings. The latest EPA estimate for methane emissions from shale gas falls within the range of our estimates but not those of Cathles et al. which are substantially lower. Cathles et al. believe the focus should be just on electricity generation, and the global warming potential of methane should be considered only on a 100-year time scale. Our analysis covered both electricity (30% of US usage) and heat generation (the largest usage), and we evaluated both 20and 100-year integrated time frames for methane. Both time frames are important, but the decadal scale is critical, given the urgent need to avoid climate-system tipping points. Using all available information and the latest climat...
The International SCOPE Biofuels Project gratefully acknowledges support from the United Nations ... more The International SCOPE Biofuels Project gratefully acknowledges support from the United Nations Foundation, Deutsche Forschungsgemeinschaft, the David & Lucile Packard Foundation, UNEP, the Cornell Center for a Sustainable Future, the Biogeochemistry & Biocomplexity Initiative at Cornell University, an endowment provided to Cornell University by David R. Atkinson, and the Wuppertal Institute for Climate, Environment, and Energy.
Methane is the second largest contributor to human-caused global warming after carbon dioxide. Ha... more Methane is the second largest contributor to human-caused global warming after carbon dioxide. Hansen and Sato (2004) and Hansen et al. (2007) suggested that a warming of the Earth to 1.8o C above the 1890-1910 baseline may trigger a large and rapid increase in the release of methane from the arctic due to melting of permafrost. While there is a wide range in both the magnitude and timing of projected carbon release from thawing permafrost in the literature (e.g. Schaefer et al., 2011), warming consistently leads to greater release. This release will therefore in turn cause a positive feedback of accelerated global warming (Zimov et al. 2006).
In April 2011, we published the first comprehensive analysis of greenhouse gas (GHG) emissions fr... more In April 2011, we published the first comprehensive analysis of greenhouse gas (GHG) emissions from shale gas obtained by hydraulic fracturing, with a focus on methane emissions. Our analysis was challenged by Cathles et al. (2012). Here, we respond to those criticisms. We stand by our approach and findings. The latest EPA estimate for methane emissions from shale gas falls within the range of our estimates but not those of Cathles et al. which are substantially lower. Cathles et al. believe the focus should be just on electricity generation, and the global warming potential of methane should be considered only on a 100-year time scale. Our analysis covered both electricity (30% of US usage) and heat generation (the largest usage), and we evaluated both 20-and 100-year integrated time frames for methane. Both time frames are important, but the decadal scale is critical, given the urgent need to avoid climate-system tipping points. Using all available information and the latest climate science, we conclude that for most uses, the GHG footprint of shale gas is greater than that of other fossil fuels on time scales of up to 100 years. When used to generate electricity, the shale-gas footprint is still significantly greater than that of coal at decadal time scales but is less at the century scale. We reiterate our conclusion from our April 2011 paper that shale gas is not a suitable bridge fuel for the 21st Century. 1 Introduction Promoters view shale gas as a bridge fuel that allows continued reliance on fossil fuels while reducing greenhouse gas (GHG) emissions. Our April 2011 paper in Climatic Change challenged this view (Howarth et al. 2011). In the first comprehensive analysis of the GHG emissions from shale gas, we concluded that methane emissions lead to a large Climatic Change
The number of operators has decreased substantially over the 5-year reporting period for all well... more The number of operators has decreased substantially over the 5-year reporting period for all well types except unconventional, Table S1. Table S1. Number of operators reporting on each type of well.
... Dennis P. Swaney ??? Renee L. Santoro ??? Robert W. Howarth ??? Bongghi Hong ??? Kieran P. Do... more ... Dennis P. Swaney ??? Renee L. Santoro ??? Robert W. Howarth ??? Bongghi Hong ??? Kieran P. Donaghy ... supply system is a subject of recurring interest and has been discussed previously by many authors, including Wegmann (1896), van Burkalow (1959), Weidner (1974), Gandy ...
The number of operators has decreased substantially over the 5-year reporting period for all well... more The number of operators has decreased substantially over the 5-year reporting period for all well types except unconventional, Table S1. Table S1. Number of operators reporting on each type of well.
The contamination of surface water and ground water by human activities, such as fossil fuel extr... more The contamination of surface water and ground water by human activities, such as fossil fuel extraction and agriculture, can be difficult to assess due to incomplete knowledge of the chemicals...
BACKGROUND: Higher risk of exposure to environmental health hazards near oil and gas wells has sp... more BACKGROUND: Higher risk of exposure to environmental health hazards near oil and gas wells has spurred interest in quantifying populations that live in proximity to oil and gas development. The available studies on this topic lack consistent methodology and ignore aspects of oil and gas development of value to public health-relevant assessment and decision-making. OBJECTIVES: We aim to present a methodological framework for oil and gas development proximity studies grounded in an understanding of hydrocarbon geology and development techniques. METHODS: We geospatially overlay locations of active oil and gas wells in the conterminous United States and Census data to estimate the population living in proximity to hydrocarbon development at the national and state levels. We compare our methods and findings with existing proximity studies. RESULTS: Nationally, we estimate that 17.6 million people live within 1,600 m (∼ 1 mi) of at least one active oil and/or gas well. Three of the eight studies overestimate populations at risk from actively producing oil and gas wells by including wells without evidence of production or drilling completion and/or using inappropriate population allocation methods. The remaining five studies, by omitting conventional wells in regions dominated by historical conventional development, significantly underestimate populations at risk. CONCLUSIONS: The well inventory guidelines we present provide an improved methodology for hydrocarbon proximity studies by acknowledging the importance of both conventional and unconventional well counts as well as the relative exposure risks associated with different primary production categories (e.g., oil, wet gas, dry gas) and developmental stages of wells.
ABSTRACT Background / Purpose: Natural gas is widely promoted as a bridge fuel which allows socie... more ABSTRACT Background / Purpose: Natural gas is widely promoted as a bridge fuel which allows society to continue to use fossil fuels while reducing greenhouse gas emissions. Here, we review recent research on the greenhouse gas footprint of natural gas (conventional gas and shale gas), focusing on methane emissions. Main conclusion: When methane emissions are included, the greenhouse gas footprint of natural gas is larger than that of other fossil fuels, particularly when methane and carbon dioxide are compared over the decadal time scale. Shale gas has higher emissions than conventional gas.
ABSTRACT Background/Question/Methods Natural gas is widely promoted as a “clean fuel” that has a ... more ABSTRACT Background/Question/Methods Natural gas is widely promoted as a “clean fuel” that has a lower greenhouse gas footprint that other fossil fuels. Is this true? And as conventional natural gas is increasingly replaced by unconventional gas from shale formations, tight sands, and coal beds, what is the consequence on greenhouse gas emissions? In this talk, we will evaluate all available information on greenhouse gas emissions from both conventional and unconventional gas, focusing on methane emissions. Results/Conclusions Most estimates for methane emissions from conventional gas systems, considering the total life cycle from well drilling to delivery of gas to consumers, estimate that between 1.6 and 3.9 of the lifetime production of gas is emitted to the atmosphere as methane. For unconventional gas, most studies estimate that methane emissions are 40% to 60% greater than for unconventional gas. Several lines of evidence indicate that most of the emission estimates are low, for both conventional and unconventional gas. Nonetheless, when viewed on the time scale of an integrated 20-year period following emission, methane dominates the greenhouse gas footprint of both conventional gas and unconventional gas, using all reasonable estimates of emissions. At present, when viewed at this 20-year time scale, methane emissions make up almost half of the entire greenhouse gas inventory of the US. At longer time scales, the importance of methane is less, since the residence time of methane in the atmosphere is less than for carbon dioxide.
ABSTRACT Background/Question/Methods Natural gas is widely promoted as a “clean fuel” that has a ... more ABSTRACT Background/Question/Methods Natural gas is widely promoted as a “clean fuel” that has a lower greenhouse gas footprint that other fossil fuels. Is this true? And as conventional natural gas is increasingly replaced by unconventional gas from shale formations, tight sands, and coal beds, what is the consequence on greenhouse gas emissions? In this talk, we will evaluate all available information on greenhouse gas emissions from both conventional and unconventional gas, focusing on methane emissions. Results/Conclusions Most estimates for methane emissions from conventional gas systems, considering the total life cycle from well drilling to delivery of gas to consumers, estimate that between 1.6 and 3.9 of the lifetime production of gas is emitted to the atmosphere as methane. For unconventional gas, most studies estimate that methane emissions are 40% to 60% greater than for unconventional gas. Several lines of evidence indicate that most of the emission estimates are low, for both conventional and unconventional gas. Nonetheless, when viewed on the time scale of an integrated 20-year period following emission, methane dominates the greenhouse gas footprint of both conventional gas and unconventional gas, using all reasonable estimates of emissions. At present, when viewed at this 20-year time scale, methane emissions make up almost half of the entire greenhouse gas inventory of the US. At longer time scales, the importance of methane is less, since the residence time of methane in the atmosphere is less than for carbon dioxide.
ABSTRACT Background / Purpose: Natural gas is widely promoted as a bridge fuel which allows socie... more ABSTRACT Background / Purpose: Natural gas is widely promoted as a bridge fuel which allows society to continue to use fossil fuels while reducing greenhouse gas emissions. Here, we review recent research on the greenhouse gas footprint of natural gas (conventional gas and shale gas), focusing on methane emissions. Main conclusion: When methane emissions are included, the greenhouse gas footprint of natural gas is larger than that of other fossil fuels, particularly when methane and carbon dioxide are compared over the decadal time scale. Shale gas has higher emissions than conventional gas.
Proceedings of the National Academy of Sciences of the United States of America, Jan 29, 2014
Casing and cement impairment in oil and gas wells can lead to methane migration into the atmosphe... more Casing and cement impairment in oil and gas wells can lead to methane migration into the atmosphere and/or into underground sources of drinking water. An analysis of 75,505 compliance reports for 41,381 conventional and unconventional oil and gas wells in Pennsylvania drilled from January 1, 2000-December 31, 2012, was performed with the objective of determining complete and accurate statistics of casing and cement impairment. Statewide data show a sixfold higher incidence of cement and/or casing issues for shale gas wells relative to conventional wells. The Cox proportional hazards model was used to estimate risk of impairment based on existing data. The model identified both temporal and geographic differences in risk. For post-2009 drilled wells, risk of a cement/casing impairment is 1.57-fold [95% confidence interval (CI) (1.45, 1.67); P < 0.0001] higher in an unconventional gas well relative to a conventional well drilled within the same time period. Temporal differences bet...
2Keywords: methane; shale gas; unconventional gas; greenhouse gas; fracking; hydraulic fracturing... more 2Keywords: methane; shale gas; unconventional gas; greenhouse gas; fracking; hydraulic fracturing In April 2011, we published the first comprehensive analysis of greenhouse gas (GHG) emissions from shale gas obtained by hydraulic fracturing, with a focus on methane emissions. Our analysis was challenged by Cathles et al. (2012). Here, we respond to those criticisms. We stand by our approach and findings. The latest EPA estimate for methane emissions from shale gas falls within the range of our estimates but not those of Cathles et al, which are substantially lower. Cathles et al. believe the focus should be just on electricity generation, and the global warming potential of methane should be considered only on a 100-year time scale. Our analysis covered both electricity (30 % of US usage) and heat generation (the largest usage), and we evaluated both 20- and 100-year integrated time frames for methane. Both time frames are important, but the decadal scale is critical, given the urge...
In April 2011, we published the first comprehensive analysis of greenhouse gas (GHG) emissions fr... more In April 2011, we published the first comprehensive analysis of greenhouse gas (GHG) emissions from shale gas obtained by hydraulic fracturing, with a focus on methane emissions. Our analysis was challenged by Cathles et al. (2012). Here, we respond to those criticisms. We stand by our approach and findings. The latest EPA estimate for methane emissions from shale gas falls within the range of our estimates but not those of Cathles et al. which are substantially lower. Cathles et al. believe the focus should be just on electricity generation, and the global warming potential of methane should be considered only on a 100-year time scale. Our analysis covered both electricity (30 % of US usage) and heat generation (the largest usage), and we evaluated both 20- and 100-year integrated time frames for methane. Both time frames are important, but the decadal scale is critical, given the urgent need to avoid climate-system tipping points. Using all available information and the latest cli...
In April 2011, we published the first comprehensive analysis of greenhouse gas (GHG) emissions fr... more In April 2011, we published the first comprehensive analysis of greenhouse gas (GHG) emissions from shale gas obtained by hydraulic fracturing, with a focus on methane emissions. Our analysis was challenged by Cathles et al. (2012). Here, we respond to those criticisms. We stand by our approach and findings. The latest EPA estimate for methane emissions from shale gas falls within the range of our estimates but not those of Cathles et al. which are substantially lower. Cathles et al. believe the focus should be just on electricity generation, and the global warming potential of methane should be considered only on a 100-year time scale. Our analysis covered both electricity (30% of US usage) and heat generation (the largest usage), and we evaluated both 20and 100-year integrated time frames for methane. Both time frames are important, but the decadal scale is critical, given the urgent need to avoid climate-system tipping points. Using all available information and the latest climat...
The International SCOPE Biofuels Project gratefully acknowledges support from the United Nations ... more The International SCOPE Biofuels Project gratefully acknowledges support from the United Nations Foundation, Deutsche Forschungsgemeinschaft, the David & Lucile Packard Foundation, UNEP, the Cornell Center for a Sustainable Future, the Biogeochemistry & Biocomplexity Initiative at Cornell University, an endowment provided to Cornell University by David R. Atkinson, and the Wuppertal Institute for Climate, Environment, and Energy.
Methane is the second largest contributor to human-caused global warming after carbon dioxide. Ha... more Methane is the second largest contributor to human-caused global warming after carbon dioxide. Hansen and Sato (2004) and Hansen et al. (2007) suggested that a warming of the Earth to 1.8o C above the 1890-1910 baseline may trigger a large and rapid increase in the release of methane from the arctic due to melting of permafrost. While there is a wide range in both the magnitude and timing of projected carbon release from thawing permafrost in the literature (e.g. Schaefer et al., 2011), warming consistently leads to greater release. This release will therefore in turn cause a positive feedback of accelerated global warming (Zimov et al. 2006).
In April 2011, we published the first comprehensive analysis of greenhouse gas (GHG) emissions fr... more In April 2011, we published the first comprehensive analysis of greenhouse gas (GHG) emissions from shale gas obtained by hydraulic fracturing, with a focus on methane emissions. Our analysis was challenged by Cathles et al. (2012). Here, we respond to those criticisms. We stand by our approach and findings. The latest EPA estimate for methane emissions from shale gas falls within the range of our estimates but not those of Cathles et al. which are substantially lower. Cathles et al. believe the focus should be just on electricity generation, and the global warming potential of methane should be considered only on a 100-year time scale. Our analysis covered both electricity (30% of US usage) and heat generation (the largest usage), and we evaluated both 20-and 100-year integrated time frames for methane. Both time frames are important, but the decadal scale is critical, given the urgent need to avoid climate-system tipping points. Using all available information and the latest climate science, we conclude that for most uses, the GHG footprint of shale gas is greater than that of other fossil fuels on time scales of up to 100 years. When used to generate electricity, the shale-gas footprint is still significantly greater than that of coal at decadal time scales but is less at the century scale. We reiterate our conclusion from our April 2011 paper that shale gas is not a suitable bridge fuel for the 21st Century. 1 Introduction Promoters view shale gas as a bridge fuel that allows continued reliance on fossil fuels while reducing greenhouse gas (GHG) emissions. Our April 2011 paper in Climatic Change challenged this view (Howarth et al. 2011). In the first comprehensive analysis of the GHG emissions from shale gas, we concluded that methane emissions lead to a large Climatic Change
The number of operators has decreased substantially over the 5-year reporting period for all well... more The number of operators has decreased substantially over the 5-year reporting period for all well types except unconventional, Table S1. Table S1. Number of operators reporting on each type of well.
... Dennis P. Swaney ??? Renee L. Santoro ??? Robert W. Howarth ??? Bongghi Hong ??? Kieran P. Do... more ... Dennis P. Swaney ??? Renee L. Santoro ??? Robert W. Howarth ??? Bongghi Hong ??? Kieran P. Donaghy ... supply system is a subject of recurring interest and has been discussed previously by many authors, including Wegmann (1896), van Burkalow (1959), Weidner (1974), Gandy ...
The number of operators has decreased substantially over the 5-year reporting period for all well... more The number of operators has decreased substantially over the 5-year reporting period for all well types except unconventional, Table S1. Table S1. Number of operators reporting on each type of well.
The contamination of surface water and ground water by human activities, such as fossil fuel extr... more The contamination of surface water and ground water by human activities, such as fossil fuel extraction and agriculture, can be difficult to assess due to incomplete knowledge of the chemicals...
BACKGROUND: Higher risk of exposure to environmental health hazards near oil and gas wells has sp... more BACKGROUND: Higher risk of exposure to environmental health hazards near oil and gas wells has spurred interest in quantifying populations that live in proximity to oil and gas development. The available studies on this topic lack consistent methodology and ignore aspects of oil and gas development of value to public health-relevant assessment and decision-making. OBJECTIVES: We aim to present a methodological framework for oil and gas development proximity studies grounded in an understanding of hydrocarbon geology and development techniques. METHODS: We geospatially overlay locations of active oil and gas wells in the conterminous United States and Census data to estimate the population living in proximity to hydrocarbon development at the national and state levels. We compare our methods and findings with existing proximity studies. RESULTS: Nationally, we estimate that 17.6 million people live within 1,600 m (∼ 1 mi) of at least one active oil and/or gas well. Three of the eight studies overestimate populations at risk from actively producing oil and gas wells by including wells without evidence of production or drilling completion and/or using inappropriate population allocation methods. The remaining five studies, by omitting conventional wells in regions dominated by historical conventional development, significantly underestimate populations at risk. CONCLUSIONS: The well inventory guidelines we present provide an improved methodology for hydrocarbon proximity studies by acknowledging the importance of both conventional and unconventional well counts as well as the relative exposure risks associated with different primary production categories (e.g., oil, wet gas, dry gas) and developmental stages of wells.
ABSTRACT Background / Purpose: Natural gas is widely promoted as a bridge fuel which allows socie... more ABSTRACT Background / Purpose: Natural gas is widely promoted as a bridge fuel which allows society to continue to use fossil fuels while reducing greenhouse gas emissions. Here, we review recent research on the greenhouse gas footprint of natural gas (conventional gas and shale gas), focusing on methane emissions. Main conclusion: When methane emissions are included, the greenhouse gas footprint of natural gas is larger than that of other fossil fuels, particularly when methane and carbon dioxide are compared over the decadal time scale. Shale gas has higher emissions than conventional gas.
ABSTRACT Background/Question/Methods Natural gas is widely promoted as a “clean fuel” that has a ... more ABSTRACT Background/Question/Methods Natural gas is widely promoted as a “clean fuel” that has a lower greenhouse gas footprint that other fossil fuels. Is this true? And as conventional natural gas is increasingly replaced by unconventional gas from shale formations, tight sands, and coal beds, what is the consequence on greenhouse gas emissions? In this talk, we will evaluate all available information on greenhouse gas emissions from both conventional and unconventional gas, focusing on methane emissions. Results/Conclusions Most estimates for methane emissions from conventional gas systems, considering the total life cycle from well drilling to delivery of gas to consumers, estimate that between 1.6 and 3.9 of the lifetime production of gas is emitted to the atmosphere as methane. For unconventional gas, most studies estimate that methane emissions are 40% to 60% greater than for unconventional gas. Several lines of evidence indicate that most of the emission estimates are low, for both conventional and unconventional gas. Nonetheless, when viewed on the time scale of an integrated 20-year period following emission, methane dominates the greenhouse gas footprint of both conventional gas and unconventional gas, using all reasonable estimates of emissions. At present, when viewed at this 20-year time scale, methane emissions make up almost half of the entire greenhouse gas inventory of the US. At longer time scales, the importance of methane is less, since the residence time of methane in the atmosphere is less than for carbon dioxide.
ABSTRACT Background/Question/Methods Natural gas is widely promoted as a “clean fuel” that has a ... more ABSTRACT Background/Question/Methods Natural gas is widely promoted as a “clean fuel” that has a lower greenhouse gas footprint that other fossil fuels. Is this true? And as conventional natural gas is increasingly replaced by unconventional gas from shale formations, tight sands, and coal beds, what is the consequence on greenhouse gas emissions? In this talk, we will evaluate all available information on greenhouse gas emissions from both conventional and unconventional gas, focusing on methane emissions. Results/Conclusions Most estimates for methane emissions from conventional gas systems, considering the total life cycle from well drilling to delivery of gas to consumers, estimate that between 1.6 and 3.9 of the lifetime production of gas is emitted to the atmosphere as methane. For unconventional gas, most studies estimate that methane emissions are 40% to 60% greater than for unconventional gas. Several lines of evidence indicate that most of the emission estimates are low, for both conventional and unconventional gas. Nonetheless, when viewed on the time scale of an integrated 20-year period following emission, methane dominates the greenhouse gas footprint of both conventional gas and unconventional gas, using all reasonable estimates of emissions. At present, when viewed at this 20-year time scale, methane emissions make up almost half of the entire greenhouse gas inventory of the US. At longer time scales, the importance of methane is less, since the residence time of methane in the atmosphere is less than for carbon dioxide.
ABSTRACT Background / Purpose: Natural gas is widely promoted as a bridge fuel which allows socie... more ABSTRACT Background / Purpose: Natural gas is widely promoted as a bridge fuel which allows society to continue to use fossil fuels while reducing greenhouse gas emissions. Here, we review recent research on the greenhouse gas footprint of natural gas (conventional gas and shale gas), focusing on methane emissions. Main conclusion: When methane emissions are included, the greenhouse gas footprint of natural gas is larger than that of other fossil fuels, particularly when methane and carbon dioxide are compared over the decadal time scale. Shale gas has higher emissions than conventional gas.
Proceedings of the National Academy of Sciences of the United States of America, Jan 29, 2014
Casing and cement impairment in oil and gas wells can lead to methane migration into the atmosphe... more Casing and cement impairment in oil and gas wells can lead to methane migration into the atmosphere and/or into underground sources of drinking water. An analysis of 75,505 compliance reports for 41,381 conventional and unconventional oil and gas wells in Pennsylvania drilled from January 1, 2000-December 31, 2012, was performed with the objective of determining complete and accurate statistics of casing and cement impairment. Statewide data show a sixfold higher incidence of cement and/or casing issues for shale gas wells relative to conventional wells. The Cox proportional hazards model was used to estimate risk of impairment based on existing data. The model identified both temporal and geographic differences in risk. For post-2009 drilled wells, risk of a cement/casing impairment is 1.57-fold [95% confidence interval (CI) (1.45, 1.67); P < 0.0001] higher in an unconventional gas well relative to a conventional well drilled within the same time period. Temporal differences bet...
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