The lifetime of nitrous oxide, the third‐most‐important human‐emitted greenhouse gas, is based to date primarily on model studies or scaling to other gases. This work calculates a semiempirical ...lifetime based on Microwave Limb Sounder satellite measurements of stratospheric profiles of nitrous oxide, ozone, and temperature; laboratory cross‐section data for ozone and molecular oxygen plus kinetics for O(1D); the observed solar spectrum; and a simple radiative transfer model. The result is 116 ± 9 years. The observed monthly‐to‐biennial variations in lifetime and tropical abundance are well matched by four independent chemistry‐transport models driven by reanalysis meteorological fields for the period of observation (2005–2010), but all these models overestimate the lifetime due to lower abundances in the critical loss region near 32 km in the tropics. These models plus a chemistry‐climate model agree on the nitrous oxide feedback factor on its own lifetime of 0.94 ± 0.01, giving N2O perturbations an effective residence time of 109 years. Combining this new empirical lifetime with model estimates of residence time and preindustrial lifetime (123 years) adjusts our best estimates of the human‐natural balance of emissions today and improves the accuracy of projected nitrous oxide increases over this century.
Key Points
Nitrous oxide lifetime is computed empirically from MLS satellite data
Empirical N2O lifetimes compared with models including interannual variability
Results improve values for present anthropogenic and preindustrial emissions
Emissions of exhaust gases and particles from oceangoing ships are a significant and growing contributor to the total emissions from the transportation sector. We present an assessment of the ...contribution of gaseous and particulate emissions from oceangoing shipping to anthropogenic emissions and air quality. We also assess the degradation in human health and climate change created by these emissions. Regulating ship emissions requires comprehensive knowledge of current fuel consumption and emissions, understanding of their impact on atmospheric composition and climate, and projections of potential future evolutions and mitigation options. Nearly 70% of ship emissions occur within 400 km of coastlines, causing air quality problems through the formation of ground-level ozone, sulphur emissions and particulate matter in coastal areas and harbours with heavy traffic. Furthermore, ozone and aerosol precursor emissions as well as their derivative species from ships may be transported in the atmosphere over several hundreds of kilometres, and thus contribute to air quality problems further inland, even though they are emitted at sea. In addition, ship emissions impact climate. Recent studies indicate that the cooling due to altered clouds far outweighs the warming effects from greenhouse gases such as carbon dioxide (CO
2) or ozone from shipping, overall causing a negative present-day radiative forcing (RF). Current efforts to reduce sulphur and other pollutants from shipping may modify this. However, given the short residence time of sulphate compared to CO
2, the climate response from sulphate is of the order decades while that of CO
2 is centuries. The climatic trade-off between positive and negative radiative forcing is still a topic of scientific research, but from what is currently known, a simple cancellation of global mean forcing components is potentially inappropriate and a more comprehensive assessment metric is required. The CO
2 equivalent emissions using the global temperature change potential (GTP) metric indicate that after 50 years the net global mean effect of current emissions is close to zero through cancellation of warming by CO
2 and cooling by sulphate and nitrogen oxides.
Observations at surface sites show an increase in global mean surface methane (CH4) of about 180 parts per billion (ppb) (above 10 %) over the period 1984–2012. Over this period there are large ...fluctuations in the annual growth rate. In this work, we investigate the atmospheric CH4 evolution over the period 1970–2012 with the Oslo CTM3 global chemical transport model (CTM) in a bottom-up approach. We thoroughly assess data from surface measurement sites in international networks and select a subset suited for comparisons with the output from the CTM. We compare model results and observations to understand causes for both long-term trends and short-term variations. Employing Oslo CTM3 we are able to reproduce the seasonal and year-to-year variations and shifts between years with consecutive growth and stagnation, both at global and regional scales. The overall CH4 trend over the period is reproduced, but for some periods the model fails to reproduce the strength of the growth. The model overestimates the observed growth after 2006 in all regions. This seems to be explained by an overly strong increase in anthropogenic emissions in Asia, having global impact. Our findings confirm other studies questioning the timing or strength of the emission changes in Asia in the EDGAR v4.2 emission inventory over recent decades. The evolution of CH4 is not only controlled by changes in sources, but also by changes in the chemical loss in the atmosphere and soil uptake. The atmospheric CH4 lifetime is an indicator of the CH4 loss. In our simulations, the atmospheric CH4 lifetime decreases by more than 8 % from 1970 to 2012, a significant reduction of the residence time of this important greenhouse gas. Changes in CO and NOx emissions, specific humidity, and ozone column drive most of this, and we provide simple prognostic equations for the relations between those and the CH4 lifetime. The reduced lifetime results in substantial growth in the chemical CH4 loss (relative to its burden) and dampens the CH4 growth.
Shipping activity has increased considerably over the last century and currently represents a significant contribution to the global emissions of pollutants and greenhouse gases. Despite this, ...information about the historical development of fuel consumption and emissions is generally limited, with little data published pre‐1950 and large deviations reported for estimates covering the last 3 decades. To better understand the historical development in ship emissions and the uncertainties associated with the estimates, we present fuel‐based CO2 and SO2 emission inventories from 1925 up to 2002 and activity‐based estimates from 1970 up to 2000. The global CO2 emissions from ships in 1925 have been estimated to 229 Tg (CO2), growing to about 634 Tg (CO2) in 2002. The corresponding SO2 emissions are about 2.5 Tg (SO2) and 8.5 Tg (SO2), respectively. Our activity‐based estimates of fuel consumption from 1970 to 2000, covering all oceangoing civil ships above or equal to 100 gross tonnage (GT), are lower compared to previous activity‐based studies. We have applied a more detailed model approach, which includes variation in the demand for sea transport, as well as operational and technological changes of the past. This study concludes that the main reason for the large deviations found in reported inventories is the applied number of days at sea. Moreover, our modeling indicates that the ship size and the degree of utilization of the fleet, combined with the shift to diesel engines, have been the major factors determining yearly fuel consumption. Interestingly, the model results from around 1973 suggest that the fleet growth is not necessarily followed by increased fuel consumption, as technical and operational characteristics have changed. Results from this study indicate that reported sales over the last 3 decades seems not to be significantly underreported as previous simplified activity‐based studies have suggested. The results confirm our previously reported modeling estimates for year 2000. Previous activity‐based studies have not considered ships less than 100 GT (e.g., today some 1.3 million fishing vessels), and we suggest that this fleet could account for an important part of the total fuel consumption (∼10%).
Emission generated by the international merchant fleet has been suggested to represent a significant contribution to the global anthropogenic emissions. To analyze the impacts of these emissions, we ...present detailed model studies of the changes in atmospheric composition of pollutants and greenhouse compounds due to emissions from cargo and passenger ships in international trade. Global emission inventories of NOx, SO2, CO, CO2, and volatile organic compounds (VOC) are developed by a bottom‐up approach combining ship‐type specific engine emission modeling, oil cargo VOC vapor modeling, alternative global distribution methods, and ship operation data. Calculated bunker fuel consumption is found in agreement with international sales statistics. The Automated Mutual‐assistance Vessel Rescue system (AMVER) data set is found to best reflect the distributions of cargo ships in international trade. A method based on the relative reporting frequency weighted by the ship size for each vessel type is recommended. We have exploited this modeled ship emissions inventory to estimate perturbations of the global distribution of ozone, methane, sulfate, and nitrogen compounds using a global 3‐D chemical transport model with interactive ozone and sulfate chemistry. Ozone perturbations are highly nonlinear, being most efficient in regions of low background pollution. Different data sets (e.g., AMVER, The Comprehensive Ocean‐Atmosphere Data Set (COADS)) lead to highly different regional perturbations. A maximum ozone perturbation of approximately 12 ppbv is obtained in the North Atlantic and in the North Pacific during summer months. Global average sulfate loading increases with 2.9%, while the increase is significantly larger over parts of western Europe (up to 8%). In contrast to the AMVER data, the COADS data give particularly large enhancements over the North Atlantic. Ship emissions reduce methane lifetime by approximately 5%. CO2 and O3 give positive radiative forcing (RF), and CH4 and sulfate give negative forcing. The total RF is small (0.01–0.02 W/m2) and connected with large uncertainties. Increase in acidification is 3–10% in certain coastal areas. The approach presented here is clearly useful for characterizing the present impact of ship emission and will be valuable for assessing the potential effect of various emission‐control options.
Global atmospheric dust is simulated using the Dust Entrainment and Deposition (DEAD) model in combination with the global‐scale Oslo chemical transport model CTM2 using meteorological data for 1996. ...Dust sources are calculated using both mean wind speeds with model resolution T63 and subgrid wind speeds. Different data sets are used to describe soil erodibility. We explain how the different assumptions about dust production affect atmospheric dust burden and deposition. Some aspects of the annual dust cycle, such as the east Asian dust emissions, are largely dependent on the data used to determine soil erodibility. Other aspects, such as the timing of the maximum in the African plume at Northern Hemisphere summer, are well modeled with all data sets applied here. We show that the daily variation in optical depth at Cape Verde on the west coast of Africa is well simulated when we assume that erodibility is correlated with surface reflectivity from Moderate‐Resolution Imaging Spetroradiometer (MODIS) satellite data. Using a subgrid probability density function of wind speed to drive the dust sources facilitates dust emissions in areas with low wind speeds. Dust concentrations in remote areas are sensitive to the parameterization of wet deposition. Our results point out the need for a detailed soil erodibility data set for global dust modeling, and they suggest that surface reflectivity is potentially valuable for producing or evaluating such data sets.
The magnitude and feedbacks of future methane release from the Arctic region are unknown. Despite limited documentation of potential future releases associated with thawing permafrost and degassing ...methane hydrates, the large potential for future methane releases calls for improved understanding of the interaction of a changing climate with processes in the Arctic and chemical feedbacks in the atmosphere. Here we apply a “state of the art” atmospheric chemistry transport model to show that large emissions of CH4 would likely have an unexpectedly large impact on the chemical composition of the atmosphere and on radiative forcing (RF). The indirect contribution to RF of additional methane emission is particularly important. It is shown that if global methane emissions were to increase by factors of 2.5 and 5.2 above current emissions, the indirect contributions to RF would be about 250% and 400%, respectively, of the RF that can be attributed to directly emitted methane alone. Assuming several hypothetical scenarios of CH4 release associated with permafrost thaw, shallow marine hydrate degassing, and submarine landslides, we find a strong positive feedback on RF through atmospheric chemistry. In particular, the impact of CH4 is enhanced through increase of its lifetime, and of atmospheric abundances of ozone, stratospheric water vapor, and CO2 as a result of atmospheric chemical processes. Despite uncertainties in emission scenarios, our results provide a better understanding of the feedbacks in the atmospheric chemistry that would amplify climate warming.
Key Points
The importance of atmospheric chemistry in response to permafrost emissions
We have complemented existing global sea transportation emission inventories with new regional emission data sets and scenarios for ship traffic and coastal activity in 2015. Emission inventories for ...2000 and 2015 are used in a global Chemical Transport Model (CTM) to quantify environmental atmospheric impacts with particular focus on the Arctic region. Although we assume that ship emissions continue to increase from 2000 to 2015, reductions are assumed for some chemical components and regions because of implementation of new regulations. Current ship traffic (2000) is estimated to contribute significantly to coastal pollution. Norwegian coastal ship traffic is responsible for more than 1/3 and 1/6 of the Norwegian NOx and SO2 emissions, respectively. For these short‐lived components the impact of Norwegian coastal emissions is regionally important. For most components the international ship transportation outside coastal waters dominates the effects. Ship emissions increase wet deposition in Scandinavia with 30–50% for nitrate and 10–25% for sulfate. In general, coastal regions with prevailing onshore winds show substantial increases in deposition of acid components. Maximum surface increase in ozone is in excess of 10 ppbv. Column ozone increases are also significant. Assuming no changes in nonshipping emissions, scenarios for shipping activities in 2015 lead to more than 20% increase in NO2 from 2000 to 2015 in some coastal areas. Ozone increases are in general small. Wet deposition of acidic species increases up to 10% in areas where current critical loads are exceeded. Regulations limiting the sulfur content in the fuel in the North Sea and English Channel will be an efficient measure to reduce sulfate deposition in nearby coastal regions. The expected oil and gas transport by ships from Norway and northwest Russia, sea transport along the Northern Sea Route and new Norwegian coastal gas power plants will have a significant regional effect by increases of acid deposition in north Scandinavia and the Kola Peninsula. Augmented levels of particles in the Arctic are calculated, and thus the contribution from ship traffic to phenomena like Arctic haze could be increasing.
A sulfur cycle chemistry scheme with dimethyl sulfide (DMS), SO2, sulfate, H2S, and methanesulfonic acid (MSA) is included in the OsloCTM2 model, and concentrations of sulfur are calculated ...interactively with the oxidant chemistry. This allows more consistent estimates of aqueous phase oxidation of SO2 to sulfate by O3, H2O2, and HO2NO2. The year 1996 is chosen as the standard, and a model run with 1996 meteorology and emissions is compared with 1996 observations. The results agree well with observations overall, although the model tends to overestimate SO2 and underestimate sulfate in Northern Hemisphere winter owing to an oxidation limitation. A global budget for 1996 quantifying the various processes is investigated. Our model results give a global lifetime (global burden) of 1 day (0.25 Tg(S)) and 3.8 days (0.5 3 Tg(S)) for SO2 and sulfate. Differences between the Southern Hemisphere, characterized by natural emissions and by loss of SO2 by O3 and H2O2 oxidation, and the Northern Hemisphere, characterized by anthropogenic emissions and by large loss by dry deposition, are revealed. Significant changes in sulfur emissions have occurred over the last decades with decrease in the Unites States and Europe and increase in Southeast Asia. U.S., European, and Chinese SO2 emissions have changed by −17.6%, −47.5%, and +93%, respectively. To study the impact of emission changes on the atmospheric composition, we have calculated distributions using the Global Emissions Inventory Activity (GEIA) 1985 inventory. The changes in sulfur emissions have significant changes on the sulfur concentrations and also some effect upon the oxidants. Increased emissions of NOx and hydrocarbons in China enhance O3, but increased sulfur inhibit the increase. The SO2 oxidation by OH, which can lead to formation of new sulfate particles, is given special attention. The model run using GEIA 1985 anthropogenic emission inventory is compared with other model studies.
Ozone and methane are chemically active climate-forcing agents affected by climate–chemistry interactions in the atmosphere. Key chemical reactions and processes affecting ozone and methane are ...presented. It is shown that climate-chemistry interactions have a significant impact on the two compounds. Ozone, which is a secondary compound in the atmosphere, produced and broken down mainly in the troposphere and stratosphre through chemical reactions involving atomic oxygen (O), NOx compounds (NO, NO2), CO, hydrogen radicals (OH, HO2), volatile organic compounds (VOC) and chlorine (Cl, ClO) and bromine (Br, BrO). Ozone is broken down through changes in the atmospheric distribution of the afore mentioned compounds. Methane is a primary compound emitted from different sources (wetlands, rice production, livestock, mining, oil and gas production and landfills).Methane is broken down by the hydroxyl radical (OH). OH is significantly affected by methane emissions, defined by the feedback factor, currently estimated to be in the range 1.3 to 1.5, and increasing with increasing methane emission. Ozone and methane changes are affected by NOx emissions. While ozone in general increase with increases in NOx emission, methane is reduced, due to increases in OH. Several processes where current and future changes have implications for climate-chemistry interactions are identified. It is also shown that climatic changes through dynamic processes could have significant impact on the atmospheric chemical distribution of ozone and methane, as we can see through the impact of Quasi Biennial Oscillation (QBO). Modeling studies indicate that increases in ozone could be more pronounced toward the end of this century. Thawing permafrost could lead to important positive feedbacks in the climate system. Large amounts of organic material are stored in the upper layers of the permafrost in the yedoma deposits in Siberia, where 2 to 5% of the deposits could be organic material. During thawing of permafrost, parts of the organic material that is deposited could be converted to methane. Furthermore, methane stored in deposits under shallow waters in the Arctic have the potential to be released in a future warmer climate with enhanced climate impact on methane, ozone and stratospheric water vapor. Studies performed by several groups show that the transport sectors have the potential for significant impacts on climate-chemistry interactions. There are large uncertainties connected to ozone and methane changes from the transport sector, and to methane release and climate impact during permafrost thawing.
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