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Greater Focus Needed on Methane Leakage From Natural Gas Infrastructure
Abstract and Figures
Natural gas is seen by many as the future of American energy: a fuel that can provide energy independence and reduce greenhouse gas emissions in the process. However, there has also been confusion about the climate implications of increased use of natural gas for electric power and transportation. We propose and illustrate the use of technology warming potentials as a robust and transparent way to compare the cumulative radiative forcing created by alternative technologies fueled by natural gas and oil or coal by using the best available estimates of greenhouse gas emissions from each fuel cycle (i.e., production, transportation and use). We find that a shift to compressed natural gas vehicles from gasoline or diesel vehicles leads to greater radiative forcing of the climate for 80 or 280 yr, respectively, before beginning to produce benefits. Compressed natural gas vehicles could produce climate benefits on all time frames if the well-to-wheels CH(4) leakage were capped at a level 45-70% below current estimates. By contrast, using natural gas instead of coal for electric power plants can reduce radiative forcing immediately, and reducing CH(4) losses from the production and transportation of natural gas would produce even greater benefits. There is a need for the natural gas industry and science community to help obtain better emissions data and for increased efforts to reduce methane leakage in order to minimize the climate footprint of natural gas.
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... Gasoline, diesel, and coal produce more CO2 per unit of energy, whereas methane produces less CO2. As a result, industrial infrastructure methane emission rates have been underestimated [1]. Current and future infrastructure emissions must be low enough to offset methane's global warming potential (GWP) to benefit from methane [1], [2]. ...
... As a result, industrial infrastructure methane emission rates have been underestimated [1]. Current and future infrastructure emissions must be low enough to offset methane's global warming potential (GWP) to benefit from methane [1], [2]. Bangladesh's energy landscape has historically been dominated by natural gas. ...
... In connection with the primary topic of this work, Fabiano studied all forms of traffic accidents, including PDO-ss (Accidents with property damage only). The authors, authors determine the frequency by geometric variables, slopes, the number and width of lanes, the kind of road segment, and traffic flow [1], [3]. ...
One of the nations with the densest populations is Bangladesh. The nation's energy use is increasing progressively. One of the greenest forms of energy is LNG. Natural gas, a liquefied gas, uses ultra-low temperatures and purification. In this study, the assessment of LNG carrier vehicles was evaluated from the perspective of Bangladesh. A cryogenic liquid tanker truck, called an LNG carrier vehicle, was found in the survey to carry liquefied natural gas in an environment with extremely low temperatures. Even though LNG is flammable, explosive, cryogenic, and has other characteristics, the safety design of an LNG carrier vehicle is crucial for the security of road transportation. Time consumption analysis and economic section analysis were conducted, and the truck was found to be the ultimate solution. LNG cryogenic liquid storage tanks, trailer running gear, piping systems, a control box, and other parts are often found in LNG carrier vehicles. Utilizing statistical tools, the validity of the survey and its findings will be evaluated by the Statistical Package for the Social Sciences (SPSS). The safety design study for these two systems was the topic of this work. A report on the compatibility of the best method for transporting LNG within all feasible safety conditions will be made possible by the research.
... DOE analyses indicate that every 10,000 U.S. homes powered with natural gas instead of coal avoids the annual emissions of 1,900 tons of NOx, 3,900 tons of SO2 and 5,200 tons of particulates [Alvarez et al, 2012]. Reduction in these emissions translate into public health benefits as these pollutants have been linked with problems such as asthma, bronchitis, lung cancer and heart disease for hundreds of thousands of Americans [California EPA, 2012]. ...
This study assessed the comparison food expenditure and consumption patterns of rural non- farm and farm households in South East Nigeria. Descriptive statistics and t-test were used to achieve the objectives. The study found that the percentage per-capita expenditure of consumptions by non-farm households for the three commodities assessed in this study, garri (29.6%) ranks first, followed by rice (24.5%) and yam (21.0%) third. The overall mean food expenditure was ₦58,053.50. On the other hand, rural farm households consume 30.70% (yam), 22.2% (garri) and 22.1% (rice) with mean food expenditure of ₦46,282.60. The statistical test of significance (t=13.3 p<0.05) indicates the existence of a statistical difference between the mean food expenditure of non-farm and farm households in the study. Legumes such as beans (65.7%), groundnut (18.9%) and melon (11.6%) ranked first, second and third respectively for both non-farm and farm households. The statistical test of comparison (t=0.212 p<0.05) shows that there is no statistical relationship between the mean food expenditure of non-farm (₦32,348.25) and farm (₦16,807.5) households. The analysis on consumption pattern for non-farm households showed that, their average consumption rate per day has fruits (2.95) as top rank, followed by confectionaries (2.88) and meat/meat product (2.56) is ranked the least. For rural farm households, tubers and cereals (2.98), fruits (2.92) and legumes (2.76) are the three top ranked commodities, consumed. In conclusion, the mean monthly food expenditure of rural farm households was significantly lesser than the mean monthly food expenditure of rural non-farm households. Fruits, legumes and tubers constituted the prominent items in the consumption basket of rural farm and non-farm households. The study recommends that food security interventions need to support households in the rural areas.
Keywords: Food expenditure, consumption pattern, food insecurity, COVID-19 pandemic
... In terms of anthropogenic methane emissions by source, emissions from natural gas systems are the highest [6]. An EDF (Environmental Defense Fund) study [7] revealed that gas emission must be controlled below 2.7%, 1.5% and 0.8%, respectively, in order to achieve climate benefit when gas is used to replace coal, petroleum and diesel. Elucidating methane emissions from natural gas systems will make great contribution to emission reduction. ...
As the second-largest contributor to historical global warming, methane emissions must be controlled to slow down temperature increase and achieve climate benefits. Due to a lack of knowledge about the specific sources and processes, a quantitative approach will lead to inaccurate estimation. In this paper, a typical local distribution company with more than 100 years of operation history was chosen. Detailed procedures of pipeline constructions and accidents were investigated, and critical steps leading to methane emissions were clarified. Then emission quantification methodologies for all processes were proposed, including a new pipeline connection, new regulator connection, emergency repair and third-party damages. As a basis for emission estimation, the distribution of parameters, such as diameter, length and pressure, was counted. Then the emission rates of all projects were calculated, and emission factors were established. The average emission rates of the new pipeline connection, new regulator connection, emergency repair, third-party damage (medium pressure) and third-party damage (low pressure) were 234 kg, 147 kg, 217 kg, 17,282 kg and 62 kg, respectively. In addition, the total methane emission in China from these sources was estimated to be about 5 × 104 t, which is large enough to attract attention. The work in this paper aims to establish a reasonable framework to evaluate venting methane emissions from the distribution process of natural gas.
... Climate change is a grand challenge that has ended up as one of the biggest environmental hazards confronting the world today. Methane is a potent contributor to climate change, which has been trapping 84 times as much heat as carbon dioxide over 20 years [1]. Among many other sources, the oil and gas industry is the largest industrial source of methane emissions [2,3]. ...
The detection of methane, a strong greenhouse gas, has increased in importance due to rising emissions, which partly originate from unreported and undetected leaks in oil and gas fields. The gas emitted by these leaks could be detected using an optical fiber-based photoacoustic sensor called PAS-WRAP. Here, we investigate the potential of silicon-based membranes as more sensitive microphones in the PAS-WRAP concept. Toward this goal, we built a setup with which the frequency response of the membranes was interrogated by an optical fiber. Multiple mounting mechanisms were tested by adapting commercial interferometry systems (OP1550, ZonaSens, Optics11 B.V.) to our case. Finally, methane detection was attempted using a silicon nitride membrane as a sensor. Our findings show a quality factor of 2.4 at 46 kHz and 33.6 at 168 kHz for a thin silicon nitride membrane. This membrane had a frequency response with a signal-to-background ratio of 1 ± 0.7 at 44 kHz when tested in a vacuum chamber with 4% methane at 0.94 bar. The signal-to-background ratio was not significant for methane detection; however, we believe that the methods and experimental procedures that we used in this work can provide a useful reference for future research into gas trace detection with optical fiber-based photoacoustic spectroscopy.
... mdpi.com/article/10.3390/atmos14020208/s1. CO:CH4 Emission Ratios [40,[93][94][95][96][97][98][99][100][101][102][103]; Figure S1. Fifth percentile CO 2 background mixing ratios (monthly averaged fifth percentile values and three-day moving average values); Figure S2. ...
Accurate accounting of the partition between anthropogenic and biogenic carbon dioxide mixing ratios (CO2Anth and CO2Bio) in urban-based CO2 measurements is key to developing effective emission reduction strategies since such measurements can provide an independent catalogue of local and regional CO2 emission inventories. In an attempt to delineate the contribution of CO2Bio to the overall urban CO2 mixing ratio enhancements, carbon monoxide (CO) was utilized as a tracer, following CO2 and CO mixing ratio measurements using a wavelength-scanned cavity ring-down spectrometer (CRDS). These measurements were performed in Cookeville, TN, (36.1628° N, 85.5016° W), a medium-sized city within the Eastern Highland Rim region of the United States. Between the years 2017 and 2019, the average seasonal wintertime CO2Bio mixing ratios varied between −0.65 ± 3.44 ppm and 0.96 ± 2.66 ppm. During the springtime, the observed CO2Bio signals were largely negative while the CO2Anth values were generally lower than the wintertime values. The contribution of CO from the isoprene oxidation reaction with the hydroxyl radical (OH) (COisoprene) to the overall CO enhancement during the growing season was estimated to be ~17–27 ppb, underscoring the importance of considering the contribution of COisoprene to untangling different CO2Anth and CO2Bio sources and sinks in high isoprene-emitting urban environments.
Recent scholarly work on sustainability transitions has highlighted the importance of the development of norms against fossil fuels, or Anti-Fossil Fuel Norms (AFFNs), for climate mitigation. However, further research is needed to understand the mechanisms and conditions by which such norms are developed and secured for successful sustainability transitions. This paper contributes to addressing this, through analysing the case of the Republic of Ireland, probing the emergence of norms against fossil fuels and notably against natural gas (also referred to here as ‘fossil gas’). Whilst Ireland is considered an overall laggard on climate action, in recent years the country has witnessed several ground-breaking supply-side moves against fossil fuels. Drawing on in-depth original research interviews and analysis of key political sources, including parliamentary debates and policy documents, the paper finds evidence of norms against fossil gas emerging in the Irish case. The analysis contributes insights into the iterative development of AFFNs at the domestic level, the role of environmental movements, and broader lessons for the development of a just transition away from fossil gas.
Key policy insights
• Initial moves against fossil fuels, even if not successful, can unleash political feedback effects which open space for more stringent policy development over time
• Delayed action on sustainability leads to more complex transition challenges, with implications for ensuring justice within transition processes
• Grassroots environmental movements are important ‘norm entrepreneurs’, providing important sources of expertise, effective communication for policy and the public on the impacts of fossil fuels and mobilisation of wider citizen support for climate policy
• AFFN discursive framings can help to build alliances, advance climate justice, and support the development of environmentally and socially just transitions.
We use 2019 TROPOMI satellite observations of atmospheric methane in an analytical inversion to quantify methane emissions from the Middle East and North Africa at up to ~25 km × 25 km resolution, using spatially allocated national UNFCCC reports as prior estimates for the fuel sector. Our resulting best estimate of anthropogenic emissions for the region is 35 % higher than the prior bottom-up estimate (+103 % for gas, +53 % for waste, +49 % for livestock, −14 % for oil) with large variability across countries. Oil and gas account for 38 % of total anthropogenic emissions in the region. TROPOMI observations can effectively optimize and separate national emissions by sector for most of the 23 countries in the region, with 6 countries accounting for most of total anthropogenic emissions including Iran (5.3 (5.0–5.5) Tg a−1; best estimate and uncertainty range), Turkmenistan (4.4 (2.8–5.1) Tg a−1), Saudi Arabia (4.3 (2.4–6.0) Tg a−1), Algeria (3.5 (2.4–4.4) Tg a−1), Egypt (3.4 (2.5–4.0) Tg a−1) , and Turkey (3.0 (2.0–4.1) Tg a−1). Most oil/gas emissions are from the production (upstream) subsector, but Iran, Turkmenistan, and Saudi Arabia have large gas emissions from transmission and distribution subsectors. We identify a high number of annual oil/gas emission hotspots in Turkmenistan, Algeria, Oman, and offshore in the Persian Gulf. We show that oil/gas methane emissions for individual countries are not related to production, invalidating a basic premise in the construction of activity-based bottom-up inventories. Instead, local infrastructure and management practices appear to be key drivers of oil/gas emissions, emphasizing the need for including top-down information from atmospheric observations in the construction of oil/gas emission inventories. We examined the methane intensity, defined as the upstream oil/gas emission per unit of methane gas produced, as a measure of the potential for decreasing emissions from the oil/gas sector, and using as reference the 0.2 % target set by industry. We find that the methane intensity in most countries is considerably higher than this target, reflecting leaky infrastructure combined with deliberate venting or incomplete flaring of gas. However, we also find that Kuwait, Saudi Arabia, and Qatar meet the industry target and thus show that the target is achievable through capture of associated gas, modern infrastructure, and concentration of operations. Decreasing methane intensities across the Middle East and North Africa to 0.2 % would achieve a 90 % decrease in oil/gas upstream emissions and a 26 % decrease of total anthropogenic methane emissions in the region, making a significant contribution toward the Global Methane Pledge.
The effects of in-cloud turbulence on the growth of cloud droplets by condensation and coalescence in low level water clouds are reviewed. It is shown that while the entrainment of dry air into clouds may explain the observed variability in the droplet spectra, and particularly in droplet concentration, doubt still exists as to whether the entrainment of subsaturated air can give rise to enhanced droplet growth by condensation, that is growth in excess of that expected in an adiabatic parcel. It is also shown that the increase in the length of the path of some of the droplets in stratocumulus due to turbulent updraughts can significantly increase the rate of production of drizzle sized droplets by coalescence. The effects of turbulence are generally believed to increase the collection kernels of cloud droplets, although the magnitude of the increase is in doubt. The impact of such changes on the growth of a population of cloud droplets by coalescence is sensitive to their magnitude. In particular, such changes may be capable of partially overcoming the barrier to growth which occurs between the condensation and coalescence processes in still air at a droplet radius of around 20 μm, although further detailed studies are required.
China and India are the two largest anthropogenic aerosol generating countries in the world. In this study, we develop a new inventory of sulfur dioxide (SO2) and primary carbonaceous aerosol (i.e., black and organic carbon, BC and OC) emissions from these two countries for the period 1996-2010, using a technology-based methodology. Emissions from major anthropogenic sources and open biomass burning are included, and time-dependent trends in activity rates and emission factors are incorporated in the calculation. Year-specific monthly temporal distributions for major sectors and gridded emissions at a resolution of 0.1°×0.1° distributed by multiple year-by-year spatial proxies are also developed. In China, the interaction between economic development and environmental protection causes large temporal variations in the emission trends. From 1996 to 2000, emissions of all three species showed a decreasing trend (by 9 %-17 %) due to a slowdown in economic growth, a decline in coal use in non-power sectors, and the implementation of air pollution control measures. With the economic boom after 2000, emissions from China changed dramatically. BC and OC emissions increased by 46 % and 33 % to 1.85 Tg and 4.03 Tg in 2010. SO2 emissions first increased by 61 % to 34.0 Tg in 2006, and then decreased by 9.2 % to 30.8 Tg in 2010 due to the wide application of flue-gas desulfurization (FGD) equipment in power plants. Driven by the remarkable energy consumption growth and relatively lax emission controls, emissions from India increased by 70 %, 41 %, and 35 % to 8.81 Tg, 1.02 Tg, and 2.74 Tg in 2010 for SO2, BC, and OC, respectively. Monte Carlo simulations are used to quantify the emission uncertainties. The average 95 % confidence intervals (CIs) of SO2, BC, and OC emissions are estimated to be -16 %-17 %, -43 %-93 %, and -43 %-80 % for China, and -15 %-16 %, -41 %-87 %, and -44 %-92 % for India, respectively. Sulfur content, fuel use, and sulfur retention of hard coal and the actual FGD removal efficiency are the main contributors to the uncertainties of SO2 emissions. Biofuel combustion related parameters (i.e., technology divisions, fuel use, and emission factor determinants) are the largest source of OC uncertainties, whereas BC emissions are also sensitive to the parameters of coal combustion in the residential and industrial sectors and the coke-making process. Comparing our results with satellite observations, we find that the trends of estimated emissions in both China and India are in good agreement with the trends of aerosol optical depth (AOD) and SO2 retrievals obtained from different satellites.
Natural gas is widely considered to be an environmentally cleaner fuel than coal because it does not produce detrimental by-products
such as sulfur, mercury, ash and particulates and because it provides twice the energy per unit of weight with half the carbon
footprint during combustion. These points are not in dispute. However, in their recent publication in Climatic Change Letters,
Howarth et al. (2011) report that their life-cycle evaluation of shale gas drilling suggests that shale gas has a larger GHG footprint than coal
and that this larger footprint “undercuts the logic of its use as a bridging fuel over the coming decades”. We argue here
that their analysis is seriously flawed in that they significantly overestimate the fugitive emissions associated with unconventional
gas extraction, undervalue the contribution of “green technologies” to reducing those emissions to a level approaching that
of conventional gas, base their comparison between gas and coal on heat rather than electricity generation (almost the sole
use of coal), and assume a time interval over which to compute the relative climate impact of gas compared to coal that does
not capture the contrast between the long residence time of CO2 and the short residence time of methane in the atmosphere. High leakage rates, a short methane GWP, and comparison in terms
of heat content are the inappropriate bases upon which Howarth et al. ground their claim that gas could be twice as bad as
coal in its greenhouse impact. Using more reasonable leakage rates and bases of comparison, shale gas has a GHG footprint
that is half and perhaps a third that of coal.
This paper provides estimates of emissions of two important but often not well-characterized greenhouse gas (GHG) emissions related to transportation energy use: methane (CH4) and nitrous oxide (N2O). The paper focuses on emissions of CH4 and N2O from motor vehicles because unlike emissions of CO2, which are relatively easy to estimate, emissions of CH4 and N2O are a function of many complex aspects of combustion dynamics and of the type of emission control systems used. They therefore cannot be derived easily and instead must be determined through the use of published emission factors for each combination of fuel, end-use technology, combustion conditions, and emission control system. Furthermore, emissions of CH4 and N2O may be particularly important with regard to the relative CO2-equivalent GHG emissions of the use of alternative transportation fuels, in comparison with the use of conventional fuels. By analyzing a database of emission estimates, we develop emission factors for N2O and CH4 from conventional vehicles, in order to supplement recent EPA and IPCC estimates, and we estimate relative emissions of N2O and CH4 from different alternative fuel passenger cars, light-duty trucks, and heavy-duty vehicles.
We evaluate the greenhouse gas footprint of natural gas obtained by high-volume hydraulic fracturing from shale formations,
focusing on methane emissions. Natural gas is composed largely of methane, and 3.6% to 7.9% of the methane from shale-gas
production escapes to the atmosphere in venting and leaks over the life-time of a well. These methane emissions are at least
30% more than and perhaps more than twice as great as those from conventional gas. The higher emissions from shale gas occur
at the time wells are hydraulically fractured—as methane escapes from flow-back return fluids—and during drill out following
the fracturing. Methane is a powerful greenhouse gas, with a global warming potential that is far greater than that of carbon
dioxide, particularly over the time horizon of the first few decades following emission. Methane contributes substantially
to the greenhouse gas footprint of shale gas on shorter time scales, dominating it on a 20-year time horizon. The footprint
for shale gas is greater than that for conventional gas or oil when viewed on any time horizon, but particularly so over 20years.
Compared to coal, the footprint of shale gas is at least 20% greater and perhaps more than twice as great on the 20-year horizon
and is comparable when compared over 100years.
KeywordsMethane–Greenhouse gases–Global warming–Natural gas–Shale gas–Unconventional gas–Fugitive emissions–Lifecycle analysis–LCA–Bridge fuel–Transitional fuel–Global warming potential–GWP
Carbon dioxide (CO2) emissions from fossil fuel combustion may be reduced by using natural gas rather than coal to produce energy. Gas produces
approximately half the amount of CO2 per unit of primary energy compared with coal. Here we consider a scenario where a fraction of coal usage is replaced by
natural gas (i.e., methane, CH4) over a given time period, and where a percentage of the gas production is assumed to leak into the atmosphere. The additional
CH4 from leakage adds to the radiative forcing of the climate system, offsetting the reduction in CO2 forcing that accompanies the transition from coal to gas. We also consider the effects of: methane leakage from coal mining;
changes in radiative forcing due to changes in the emissions of sulfur dioxide and carbonaceous aerosols; and differences
in the efficiency of electricity production between coal- and gas-fired power generation. On balance, these factors more than
offset the reduction in warming due to reduced CO2 emissions. When gas replaces coal there is additional warming out to 2,050 with an assumed leakage rate of 0%, and out to
2,140 if the leakage rate is as high as 10%. The overall effects on global-mean temperature over the 21st century, however,
are small.
Substitution of natural gas for coal is one means of reducing carbon dioxide (CO2) emissions. However, natural gas and coal use also results in emissions of other radiatively active substances including methane (CH4), sulfur dioxide (SO2), a sulfate aerosolprecursor, and black carbon (BC) particles. Will switching from coal to gas reduce the net impact of fossil fuel use on global climate? Using the electric utility sector as an example, changes in emissions of CO2, CH4,SO2 and BC resulting from the replacement of coal by natural gas are evaluated, and their modeled net effect on global mean-annual temperature calculated. Coal-to-gas substitution initially produces higher temperatures relative to continued coal use. This warming is due to reduced SO2 emissionsand possible increases in CH4 emissions, and can last from 1 to 30years, depending on the sulfur controls assumed. This is followed by a net decrease in temperature relative to continued coal use, resulting from lower emissions of CO2 and BC. The length of this period and the extent of the warming or cooling expected from coal-to-gas substitution is found to depend on key uncertainties and characteristics of the substitutions, especially those related to: (1) SO2 emissions and consequentsulphate aerosol forcing; and (2) the relative efficiencies of the power plantsinvolved in the switch.
A model of the U.S. automobile market is used to test the role that natural gas vehicles (NGVs) might play in reducing greenhouse-gas emissions. Since natural gas (primarily methane) emits less CO2 per unit of energy than petroleum products, NGVs are an obvious pathway to lower CO2 emissions. High-and low-demand scenarios are used to forecast the emissions from unrestricted growth and a modest program of conservation, respectively. Based on these scenarios, a reference scenario is developed that projects a possible future path of automobile use and efficiency. It is found that without a dramatic increase in automobile use, fuel consumption and greenhouse-gas emissions from automobiles in the United States will probably decrease in the future, provided that efficiency continues to improve at modest rates. In theory, NGVs can help shift emissions even further down.
A second objective is to quantify the role that leaking methane might play in offsetting some of the greenhouse advantages of NGVs. To do this, a simple atmospheric chemistry model is applied to the reference scenario; several leak rates and feedback factors are used to test the sensitivity of the projected green-house forcing from now until 2050. Committed warming beyond 2050 is not included, and the results should be interpreted with that in mind.
It is highly unlikely that switching automobiles from gasoline to natural gas will appreciably lower future greenhouse forcing. Constraints on vehicle miles travelled as well as continued improvements in vehicle efficiency will make a much larger contribution towards controlling global warming.
The on-road transportation (ORT) and power generation (PG) sectors are major contributors to carbon dioxide (CO2) emissions and a host of short-lived radiatively-active air pollutants, including tropospheric ozone and fine aerosol particles, that exert complex influences on global climate. Effective mitigation of global climate change necessitates action in these sectors for which technology change options exist or are being developed. Most assessments of possible energy change options to date have neglected non-CO2 air pollutant impacts on radiative forcing (RF). In a multi-pollutant approach, we apply a global atmospheric composition-climate model to quantify the total RF from the global and United States (U.S.) ORT and PG sectors. We assess the RF for 2 time horizons: 20- and 100-year that are relevant for understanding near-term and longer-term impacts of climate change, respectively. ORT is a key target sector to mitigate global climate change because the net non-CO2 RF is positive and acts to enhance considerably the CO2 warming impacts. We perform further sensitivity studies to assess the RF impacts of a potential major technology shift that would reduce ORT emissions by 50% with the replacement energy supplied either by a clean zero-emissions source (S1) or by the PG sector, which results in an estimated 20% penalty increase in emissions from this sector (S2). We examine cases where the technology shift is applied globally and in the U.S. only. The resultant RF relative to the present day control is negative (cooling) in all cases for both S1 and S2 scenarios, global and U.S. emissions, and 20- and 100-year time horizons. The net non-CO2 RF is always important relative to the CO2 RF and outweighs the CO2 RF response in the S2 scenario for both time horizons. Assessment of the full impacts of technology and policy strategies designed to mitigate global climate change must consider the climate effects of ozone and fine aerosol particles.
Transport affects climate directly and indirectly through mechanisms that operate on very different timescales and cause both warming and cooling. We calculate contributions to the historical development in global mean temperature for the main transport sectors (road transport, aviation, shipping and rail) based on estimates of historical emissions and by applying knowledge about the various forcing mechanisms from detailed studies. We also calculate the development in future global mean temperature for four transport scenarios consistent with the IPCC SRES scenarios, one mitigation scenario and one sensitivity test scenario. There are large differences between the transport sectors in terms of sign and magnitude of temperature effects and with respect to the contributions from the long- and short-lived components. Since pre-industrial times, we calculate that transport in total has contributed 9% of total net man-made warming in the year 2000. The dominating contributor to warming is CO2, followed by tropospheric O3. By sector, road transport is the largest contributor; 11% of the warming in 2000 is due to this sector. Likewise, aviation has contributed 4% and rail ∼1%. Shipping, on the other hand, has caused a net cooling up to year 2000, with a contribution of −7%, due to the effects of SO2 and NOx emissions. The total net contribution from the transport sectors to total man-made warming is ∼15% in 2050, and reaches 20% in 2100 in the A1 and B1 scenarios. For all scenarios and throughout the century, road transport is the dominating contributor to warming. Due to the anticipated reduction in sulphur content of fuels, the net effect of shipping changes from cooling to warming by the end of the century. Significant uncertainties are related to the estimates of historical and future net warming mainly due to cirrus, contrails and aerosol effects, as well as uncertainty in climate sensitivity.
