Net Radiative Forcing Research Articles

Future impact of non-land based traffic emissions on atmospheric ozone and OH – an optimistic scenario and a possible mitigation strategy

Abstract. The impact of future emissions from aviation and shipping on the atmospheric chemical composition has been estimated using an ensemble of six different atmospheric chemistry models. This study considers an optimistic emission scenario (B1) taking into account e.g. rapid introduction of clean and resource-efficient technologies, and a mitigation option for the aircraft sector (B1 ACARE), assuming further technological improvements. Results from sensitivity simulations, where emissions from each of the transport sectors were reduced by 5%, show that emissions from both aircraft and shipping will have a larger impact on atmospheric ozone and OH in near future (2025; B1) and for longer time horizons (2050; B1) compared to recent time (2000). However, the ozone and OH impact from aircraft can be reduced substantially in 2050 if the technological improvements considered in the B1 ACARE will be achieved. Shipping emissions have the largest impact in the marine boundary layer and their ozone contribution may exceed 4 ppbv (when scaling the response of the 5% emission perturbation to 100% by applying a factor 20) over the North Atlantic Ocean in the future (2050; B1) during northern summer (July). In the zonal mean, ship-induced ozone relative to the background levels may exceed 12% near the surface. Corresponding numbers for OH are 6.0 × 105 molecules cm−3 and 30%, respectively. This large impact on OH from shipping leads to a relative methane lifetime reduction of 3.92 (±0.48) on the global average in 2050 B1 (ensemble mean CH4 lifetime is 8.0 (±1.0) yr), compared to 3.68 (±0.47)% in 2000. Aircraft emissions have about 4 times higher ozone enhancement efficiency (ozone molecules enhanced relative to NOx molecules emitted) than shipping emissions, and the maximum impact is found in the UTLS region. Zonal mean aircraft-induced ozone could reach up to 5 ppbv at northern mid- and high latitudes during future summer (July 2050; B1), while the relative impact peaks during northern winter (January) with a contribution of 4.2%. Although the aviation-induced impact on OH is lower than for shipping, it still causes a reduction in the relative methane lifetime of 1.68 (±0.38)% in 2050 B1. However, for B1 ACARE the perturbation is reduced to 1.17 (±0.28)%, which is lower than the year 2000 estimate of 1.30 (±0.30)%. Based on the fully scaled perturbations we calculate net radiative forcings from the six models taking into account ozone, methane (including stratospheric water vapour), and methane-induced ozone changes. For the B1 scenario, shipping leads to a net cooling with radiative forcings of −28.0 (±5.1) and −30.8 (±4.8) mW m−2 in 2025 and 2050, respectively, due to the large impact on OH and, thereby, methane lifetime reductions. Corresponding values for the aviation sector shows a net warming effect with 3.8 (±6.1) and 1.9 (±6.3) mW m−2, respectively, but with a small net cooling of -0.6 (±4.6) mW m−2 for B1 ACARE in 2050.

Simulations of Contrail Optical Properties and Radiative Forcing for Various Crystal Shapes

AbstractThe aim of this study is to investigate the sensitivity of radiative-forcing computations to various contrail crystal shape models. Contrail optical properties in the shortwave and longwave ranges are derived using a ray-tracing geometric method and the discrete dipole approximation method, respectively. Both methods present good correspondence of the single-scattering albedo and the asymmetry parameter in a transition range (3–8 μm). There are substantial differences in single-scattering properties among 10 crystal models investigated here (e.g., hexagonal columns and plates with different aspect ratios, and spherical particles). The single-scattering albedo and the asymmetry parameter both vary by up to 0.1 among various crystal shapes. The computed single-scattering properties are incorporated in the moderate-resolution atmospheric radiance and transmittance model (MODTRAN) radiative transfer code to simulate solar and infrared fluxes at the top of the atmosphere. Particle shapes have a strong impact on the contrail radiative forcing in both the shortwave and longwave ranges. The differences in the net radiative forcing among optical models reach 50% with respect to the mean model value. The hexagonal-column and hexagonal-plate particles show the smallest net radiative forcing, and the largest forcing is obtained for the spheres. The balance between the shortwave forcing and longwave forcing is highly sensitive with respect to the assumed crystal shape and may even change the sign of the net forcing. The optical depth at which the mean diurnal radiative forcing changes sign from positive to negative varies from 4.5 to 10 for a surface albedo of 0.2 and from 2 to 6.5 for a surface albedo of 0.05. Contrails are probably never that optically thick (except for some aged contrail cirrus), however, and so will not have a cooling effect on climate.

Role of sea surface temperature responses in simulation of the climatic effect of mineral dust aerosol

Abstract. Mineral dust aerosol can be transported over the nearby oceans and influence the energy balance at the sea surface. The role of dust-induced sea surface temperature (SST) responses in simulations of the climatic effect of dust is examined by using a general circulation model with online simulation of mineral dust and a coupled mixed-layer ocean model. Both the longwave and shortwave radiative effects of mineral dust aerosol are considered in climate simulations. The SST responses are found to be very influential on simulated dust-induced climate change, especially when climate simulations consider the two-way dust-climate coupling to account for the feedbacks. With prescribed SSTs and dust concentrations, we obtain an increase of 0.02 K in the global and annual mean surface air temperature (SAT) in response to dust radiative effects. In contrast, when SSTs are allowed to respond to radiative forcing of dust in the presence of the dust cycle-climate interactions, we obtain a global and annual mean cooling of 0.09 K in SAT by dust. The extra cooling simulated with the SST responses can be attributed to the following two factors: (1) The negative net (shortwave plus longwave) radiative forcing of dust at the surface reduces SST, which decreases latent heat fluxes and upward transport of water vapor, resulting in less warming in the atmosphere; (2) The positive feedback between SST responses and dust cycle. The dust-induced reductions in SST lead to reductions in precipitation (or wet deposition of dust) and hence increase the global burden of small dust particles. These small particles have strong scattering effects, which enhance the dust cooling at the surface and further reduce SSTs.

Uncertainties in climate assessment for the case of aviation NO.

Nitrogen oxides emitted from aircraft engines alter the chemistry of the atmosphere, perturbing the greenhouse gases methane (CH(4)) and ozone (O(3)). We quantify uncertainties in radiative forcing (RF) due to short-lived increases in O(3), long-lived decreases in CH(4) and O(3), and their net effect, using the ensemble of published models and a factor decomposition of each forcing. The decomposition captures major features of the ensemble, and also shows which processes drive the total uncertainty in several climate metrics. Aviation-specific factors drive most of the uncertainty for the short-lived O(3) and long-lived CH(4) RFs, but a nonaviation factor dominates for long-lived O(3). The model ensemble shows strong anticorrelation between the short-lived and long-lived RF perturbations (R(2)=0.87). Uncertainty in the net RF is highly sensitive to this correlation. We reproduce the correlation and ensemble spread in one model, showing that processes controlling the background tropospheric abundance of nitrogen oxides are likely responsible for the modeling uncertainty in climate impacts from aviation.

Photoacoustic Measurements of Amplification of the Absorption Cross Section for Coated Soot Aerosols

A quantitative understanding of the absorption and scattering properties of mixed soot and aerosol particles is necessary for evaluating the Earth's energy balance. Uncertainty in the net radiative forcing of atmospheric aerosols is relatively large and may be limited by oversimplified models that fail to predict these properties for bare and externally mixed soot particles. In this laboratory study of flame-generated soot, we combine photoacoustic spectroscopy, particle counting techniques, and differential mobility analysis to obtain high-precision measurements of the size-dependent absorption cross section of uncoated and coated soot particles. We investigate how the coating of soot by nonabsorbing films of dibutyl phthalate (chosen as a surrogate for sulfuric acid) affects the particles’ morphology and optical properties. Absorption measurements were made with photoacoustic spectroscopy using a laser at λ = 405 nm. We report measurements and model calculations of the absolute cross section, mass absor...

Global modeling of mixed-phase clouds: The albedo and lifetime effects of aerosols

[1] We present a global modeling study of mixed‐phase clouds and have performed sensitivity simulations to explore the ways in which aerosol particles can affect this type of cloud. This study extends previous similar studies in that it takes into account not only the so‐called aerosol lifetime effects on mixed‐phase clouds but also aerosol effects on their albedo. Our findings generally agree with previous studies in that an increase in ice‐nucleating aerosol particles (IN) leads to a decreased cloud lifetime and therefore a warming of the Earth‐atmosphere system. However, an increase in IN will also generally decrease ice crystal sizes, thereby increasing the cloud albedo, which is analogous to the well‐established Twomey effect on liquid clouds. This decrease in ice crystal effective radii leads to an increase in cloud albedo and hence to a cooling that counteracts the lifetime effect of mixed‐phase clouds. Taking both the albedo and lifetime effects of mixed‐phase clouds into account, we find the net radiative forcing effect of an IN increase to be positive but small, which is in contrast to a much stronger warming that is found if the albedo effect is not taken into account. The latter has been the common approach in global studies of aerosol effects on mixed‐phase clouds so far. Results were found to be extremely sensitive to the choice of heterogeneous freezing parameterization and the maximum fraction of black carbon particles available for ice nucleation.

A global survey of aerosol-liquid water cloud overlap based on four years of CALIPSO-CALIOP data

Abstract. Simulating the radiative impacts of aerosols located above liquid water clouds presents a significant challenge. In particular, absorbing aerosols, such as smoke, may have significant impact in such situations and even change the sign of net radiative forcing. It is not possible to reliably obtain information on such overlap events from existing passive satellite sensors. However, the CALIOP instrument onboard NASA's CALIPSO satellite allows us to examine these events with unprecedented accuracy. Using four years of collocated CALIPSO 5 km Aerosol and Cloud Layer Version 3 Products (June 2006–May 2010), we quantify, for the first time, the characteristics of overlapping aerosol and water cloud layers globally. We investigate seasonal variability in these characteristics over six latitude bands to understand the hemispheric differences when all aerosol types are included in the analysis (the AAO case). We also investigate frequency of smoke aerosol-cloud overlap (the SAO case). Globally, the frequency is highest during the JJA months in the AAO case, while for the SAO case, it is highest in the SON months. The seasonal mean overlap frequency can regionally exceed 20% in the AAO case and 10% in the SAO case. In about 5–10% cases the vertical distance between aerosol and cloud layers is less than 100 m, while about in 45–60% cases it less than a kilometer in the annual means for different latitudinal bands. In about 70–80% cases, aerosol layers are less than a kilometer thick, while in about 18–22% cases they are 1–2 km thick. The frequency of aerosol layers 2–3 km thick is about 4–5% in the tropical belts during overlap events. Over the regions where high aerosol loadings are present, the overlap frequency can be up to 50% higher when quality criteria on aerosol/cloud feature detection are relaxed. Over the polar regions, more than 50% of the overlapping aerosol layers have optical thickness less than 0.02, but the contribution from the relatively optically thicker aerosol layers increases towards the equatorial regions in both hemispheres. The results suggest that the frequency of occurrence of overlap events is far from being negligible globally.

Boreal lichen woodlands: A possible negative feedback to climate change in eastern North America

Because of successive forest fires, closed-canopy black spruce forests are susceptible to a shift towards open lichen–spruce woodlands in parts of the boreal forest of eastern North America. The shift from dark black spruce canopies to pale lichen ground cover offers a dramatic contrast in reflectance that may compensate for the CO 2 emissions from forest fires in terms of radiative forcing. We have therefore looked at the climate change feedback that would result from the generation of lichen woodlands through changes in albedo and in stored carbon. Using albedo estimates based on MODIS imagery and incoming solar radiation for the period between 2000 and 2008 along with forest biomass estimates for eastern Canada, we have estimated that net radiative forcing for the conversion from closed-canopy coniferous forests to open lichen woodlands would be about −0.12 nW m −2 ha −1, and would therefore generate a cooling effect in the atmosphere. Based on current estimates of area in open lichen woodlands within the closed-canopy black spruce–moss forests of eastern Canada, we estimate that a current net forcing of −0.094 mW m −2 has already arisen from such conversions. As projections of future climate have been linked to increased probability of forest fires, the generation of open lichen woodlands provides a possible negative feedback to climate change. Results also suggest that carbon sequestration through the afforestation of boreal lichen woodlands may not provide a climate change mitigation benefit.

Sensitivity of contrail coverage and contrail radiative forcing to selected key parameters

Abstract Estimates of global mean radiative forcing of line-shaped contrails are associated with a high level of uncertainty. Recent estimates for present day air traffic range from 5.4 mWm−2 to 25.6 mWm−2. The aim of this research paper is to systematically study the sensitivity of contrail radiative forcing to selected key parameters and to highlight the most important factors for this large uncertainty range, while employing an improved version of the ECHAM climate model. The dominating parameters on contrail radiative forcing are found to be the detection threshold used for calibrating contrail coverage to observations, and the mean optical depth. Assuming a detection threshold of 0.05 instead of 0.02 yields an increase of the total coverage, resulting in a 146% increase of global mean contrail radiative forcing. Employing a globally constant optical depth of up to 0.3, increases the net radiative forcing by 140% over the reference case which has a mean optical depth of 0.08. An upgraded parameterisation of potential contrail coverage yields a significantly larger amount of tropical contrails, increasing the contrail radiative forcing by 53%. The calibration to an alternative observation region along with the assumption of a higher visibility threshold yields an increase of the radiative forcing by 46%. Moderate sensitivity of global contrail radiative forcing (∼15%) is found for an improvement of model climate and for changes in particle shape. The air traffic inventory, air traffic density parameter, and the diurnal variation of air traffic have only a small effect on global and annual mean contrail radiative forcing, but their influence on regional and seasonal contrail radiative forcing may nevertheless be important.

Radiative forcing over the conterminous United States due to contemporary land cover land use change and sensitivity to snow and interannual albedo variability

Satellite‐derived land cover land use (LCLU), snow and albedo data, and incoming surface solar radiation reanalysis data were used to study the impact of LCLU change from 1973 to 2000 on surface albedo and radiative forcing for 58 ecoregions covering 69% of the conterminous United States. A net positive surface radiative forcing (i.e., warming) of 0.029 Wm−2 due to LCLU albedo change from 1973 to 2000 was estimated. The forcings for individual ecoregions were similar in magnitude to current global forcing estimates, with the most negative forcing (as low as −0.367 Wm−2) due to the transition to forest and the most positive forcing (up to 0.337 Wm−2) due to the conversion to grass/shrub. Snow exacerbated both negative and positive forcing for LCLU transitions between snow‐hiding and snow‐revealing LCLU classes. The surface radiative forcing estimates were highly sensitive to snow‐free interannual albedo variability that had a percent average monthly variation from 1.6% to 4.3% across the ecoregions. The results described in this paper enhance our understanding of contemporary LCLU change on surface radiative forcing and suggest that future forcing estimates should model snow and interannual albedo variation.

Importance of representing optical depth variability for estimates of global line-shaped contrail radiative forcing.

Estimates of the global radiative forcing by line-shaped contrails differ mainly due to the large uncertainty in contrail optical depth. Most contrails are optically thin so that their radiative forcing is roughly proportional to their optical depth and increases with contrail coverage. In recent assessments, the best estimate of mean contrail radiative forcing was significantly reduced, because global climate model simulations pointed at lower optical depth values than earlier studies. We revise these estimates by comparing the probability distribution of contrail optical depth diagnosed with a climate model with the distribution derived from a microphysical, cloud-scale model constrained by satellite observations over the United States. By assuming that the optical depth distribution from the cloud model is more realistic than that from the climate model, and by taking the difference between the observed and simulated optical depth over the United States as globally representative, we quantify uncertainties in the climate model's diagnostic contrail parameterization. Revising the climate model results accordingly increases the global mean radiative forcing estimate for line-shaped contrails by a factor of 3.3, from 3.5mW/m(2) to 11.6mW/m(2) for the year 1992. Furthermore, the satellite observations and the cloud model point at higher global mean optical depth of detectable contrails than often assumed in radiative transfer (off-line) studies. Therefore, we correct estimates of contrail radiative forcing from off-line studies as well. We suggest that the global net radiative forcing of line-shaped persistent contrails is in the range 8-20mW/m(2) for the air traffic in the year 2000.

Radiative forcing due to changes in ozone and methane caused by the transport sector

The year 2000 radiative forcing (RF) due to changes in O3 and CH4 (and the CH4-induced stratospheric water vapour) as a result of emissions of short-lived gases (oxides of nitrogen (NOx), carbon monoxide and non-methane hydrocarbons) from three transport sectors (ROAD, maritime SHIPping and AIRcraft) are calculated using results from five global atmospheric chemistry models. Using results from these models plus other published data, we quantify the uncertainties. The RF due to short-term O3 changes (i.e. as an immediate response to the emissions without allowing for the long-term CH4 changes) is positive and highest for ROAD transport (31 mW m �2 ) compared to SHIP (24 mW m �2 ) and AIR (17 mW m �2 ) sectors in four of the models. All five models calculate negative RF from the CH4 perturbations, with a larger impact from the SHIP sector than for ROAD and AIR. The net RF of O3 and CH4 combined (i.e. including the impact of CH4 on ozone and stratospheric water vapour) is positive for ROAD (þ16(� 13) (one standard deviation) mW m �2 ) and AIR (þ6(� 5) mW m �2 ) traffic sectors and is negative for SHIP (� 18(� 10) mW m �2 ) sector in all five models. Global Warming Potentials (GWP) and Global Temperature change Potentials (GTP) are presented for AIR NOx emissions; there is a wide spread in the results from the 5 chemistry models, and it is shown that differences in the methane response relative to the O3 response drive much of the spread.

Forestation of boreal peatlands: Impacts of changing albedo and greenhouse gas fluxes on radiative forcing

We estimated the magnitude of the radiative forcing (RF) due to changes in albedo following the forestation of peatlands, and calculated the net RF by taking into account the changes in both the albedo and the greenhouse gas (GHG) fluxes during one forest rotation. Data on radiation, tree biomass, and soil GHG fluxes were combined with models for canopy cover, tree carbon accumulation, and the RF due to increased atmospheric GHG concentrations for four typical site cases in Finland covering two soil nutrient levels in the south and north of the country. We also studied the observed long‐term surface temperatures to detect any indications of drainage‐induced effects. The magnitude of the albedo‐induced RF was similar to that caused by the carbon sequestration of the growing trees. At three site cases out of four the drainage induced a cooling or negative RF, the tendency for cooling being higher at sites with a higher nutrient level. The differences in albedo‐induced RF mainly arose from the spring season due to (1) the different snow cover duration in the south versus the north, and (2) the different albedos of drained and undrained snow covered peatlands. An increase in the maximum daily temperatures was observed in April in southern Finland, where the most intensive drainage practices have taken place, suggesting that forestry drainage has potentially affected the local climate. Our results show that the decreasing albedo resulting from peatland forestation contributes significantly to the RF, balancing out or even exceeding the cooling effect due to the changing GHG fluxes.

Direct radiative effect of aerosols emitted by transport: from road, shipping and aviation

Abstract. Aerosols and their precursors are emitted abundantly by transport activities. Transportation constitutes one of the fastest growing activities and its growth is predicted to increase significantly in the future. Previous studies have estimated the aerosol direct radiative forcing from one transport sub-sector, but only one study to our knowledge estimated the range of radiative forcing from the main aerosol components (sulphate, black carbon (BC) and organic carbon) for the whole transportation sector. In this study, we compare results from two different chemical transport models and three radiation codes under different hypothesis of mixing: internal and external mixing using emission inventories for the year 2000. The main results from this study consist of a positive direct radiative forcing for aerosols emitted by road traffic of +20±11 mW m−2 for an externally mixed aerosol, and of +32±13 mW m−2 when BC is internally mixed. These direct radiative forcings are much higher than the previously published estimate of +3±11 mW m−2. For transport activities from shipping, the net direct aerosol radiative forcing is negative. This forcing is dominated by the contribution of the sulphate. For both an external and an internal mixture, the radiative forcing from shipping is estimated at −26±4 mW m−2. These estimates are in very good agreement with the range of a previously published one (from −46 to −13 mW m−2) but with a much narrower range. By contrast, the direct aerosol forcing from aviation is estimated to be small, and in the range −0.9 to +0.3 mW m−2.

Elemental carbon and sulfate aerosols over a rural mountain site in the northeastern United States: Regional emissions and implications for climate change

The effect of elemental carbon (EC) on global as well as regional climate forcing is potentially very important. However, the EC data for northeastern U.S. is sparse. Daily EC concentrations, [EC], and [SO 4 ] were measured in the northeastern U.S. at a regionally representative rural site, Whiteface Mountain (WFM; 44.366°N, 73.903°W, 1.5 km amsl, above mean sea level), New York (NY), for 1997. The air mass origin was determined using 6-h backward in time air trajectories obtained from the Hybrid Single-Particle Lagrangian Integrated Trajectory Model (HYSPLIT 4). [EC] and [SO 4 ] were highly variable and influenced by synoptic–scale meteorology (rainy vs dry periods). The maximum daily [EC] and [SO 4 ] were 364 ± 55 and 28,800 ± 3000 ng m −3 , respectively. [EC] and [SO 4 ] also showed seasonal variations at WFM. Occurrences of high daily [EC] were mainly in spring months, while peak daily [SO 4 ] concentrations occurred in summer months. This behavior of aerosols is due to the fact that the sources of EC and SO 4 are not the same and also due to the enhanced photochemical activity during summer months that increased the production of SO 4 from SO 2 . High [EC] and [SO 4 ] values were associated with westerly air flow from the industrialized Midwestern U.S. Sector analysis using HYSPLIT 4 air trajectories showed that regions lying between the southwest and northwest of the WFM contributed 81% and 83% of the [EC] and [SO 4 ], respectively. The monthly net direct radiative forcing for shortwave (SW) due to EC and SO 4 aerosols at the top of the atmosphere (TOA) varied from −0.05 to −0.50 W m −2 , with an annual average of −0.20 ± 0.15 W m −2 that gives a net cooling effect. Average net radiative forcing at WFM for clear sky is lower than the global average radiative forcing reported by IPCC ( Foster and Ramaswamy, 2007 ).

Radiative impacts of clouds in the tropical tropopause layer

We quantify the seasonal and spatial variations of cloud radiative impacts in the tropical tropopause layer (TTL) by using cloud retrievals from Cloud‐Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO), International Satellite Cloud Climatology Project (ISCCP) and CloudSat. Over the convective regions including Western Pacific, Africa, South America, and South Asia, we find pronounced solar heating and infrared cooling in the lower part of the TTL (<∼16 km). The solar heating weakens above 16 km and nearly diminishes at 18 km, whereas the infrared cooling extends vertically throughout the TTL. The net cloud radiative forcing, which is the summation of cloud solar and infrared radiative forcing, has heating below ∼16 km and turns to mostly cooling above 17 km. The net cloud radiative heating over the convective regions is mainly contributed from solar radiation, whereas the weak net cloud radiative heating surrounding these regions is due to infrared heating. To further examine the impacts of different cloud types in the TTL, we classified TTL clouds in terms of cloud optical depths (τ) as thin cirrus (τ < 0.3), thick cirrus (0.3 ≤ τ < 3), and opaque clouds (τ ≥ 3). In the solar part, thin and thick cirrus play a relatively small role and the impact of cloud‐free air above clouds is negligible. The solar heating is dominantly contributed from the solar absorption near the top of opaque clouds. In the infrared part, the thick cirrus heating is mainly confined over the convective regions in the lower part of TTL while the thin cirrus infrared heating is more prevalent both vertically and horizontally in the TTL, which is the dominant infrared heating source. The infrared cooling in cloud‐free air above clouds is dominant above 17 km, whereas the infrared cooling near the top of opaque clouds is dominant below. Despite the infrared heating effects of thin and thick cirrus clouds, the infrared cooling from the opaque cloud top and cloud‐free air above clouds outweighs the heating effects so that the ensemble mean cloud infrared radiative forcing is mostly cooling except outside the convective regions.

Annual carbon gas budget for a subarctic peatland, Northern Sweden

Abstract. Temperatures in the Arctic regions are rising, thawing permafrost and exposing previously stable soil organic carbon (OC) to decomposition. This can result in northern latitude soils, which have accumulated large amounts of OC potentially shifting from atmospheric C sinks to C sources with positive feedback on climate warming. In this paper, we estimate the annual net C gas balance (NCB) of the subarctic mire Stordalen, based on automatic chamber measurements of CO2 and total hydrocarbon (THC; CH4 and NMVOCs) exchange. We studied the dominant vegetation communities with different moisture and permafrost characteristics; a dry Palsa underlain by permafrost, an intermediate thaw site with Sphagnum spp. and a wet site with Eriophorum spp. where the soil thaws completely. Whole year accumulated fluxes of CO2 were estimated to 29.7, −35.3 and −34.9 gC m−2 respectively for the Palsa, Sphagnum and Eriophorum sites (positive flux indicates an addition of C to the atmospheric pool). The corresponding annual THC emissions were 0.5, 6.2 and 31.8 gC m−2 for the same sites. Therefore, the NCB for each of the sites was 30.2, −29.1 and −3.1 gC m−2 respectively for the Palsa, Sphagnum and Eriophorum site. On average, the whole mire was a CO2 sink of 2.6 gC m−2 and a THC source of 6.4 gC m−2 over a year. Consequently, the mire was a net source of C to the atmosphere by 3.9 gC m−2 (based on area weighted estimates for each of the three plant communities). Early and late snow season efflux of CO2 and THC emphasize the importance of winter measurements for complete annual C budgets. Decadal vegetation changes at Stordalen indicate that both the productivity and the THC emissions increased between 1970 and 2000. Considering the GWP100 of CH4, the net radiative forcing on climate increased 21% over the same time. In conclusion, reduced C compounds in these environments have high importance for both the annual C balance and climate.

Intercomparison of radiative forcing calculations of stratospheric water vapour and contrails

Seven groups have participated in an intercomparison study of calculations of radiative forcing (RF) due to stratospheric water vapour (SWV) and contrails. A combination of detailed radiative transfer schemes and codes for global-scale calculations have been used, as well as a combination of idealized simulations and more realistic global-scale changes in stratospheric water vapour and contrails. Detailed line-by-line codes agree within about 15 % for longwave (LW) and shortwave (SW) RF, except in one case where the difference is 30 %. Since the LW and SW RF due to contrails and SWV changes are of opposite sign, the differences between the models seen in the individual LW and SW components can be either compensated or strengthened in the net RF, and thus in relative terms uncertainties are much larger for the net RF. Some of the models used for global-scale simulations of changes in SWV and contrails differ substantially in RF from the more detailed radiative transfer schemes. For the global-scale calculations we use a method of weighting the results to calculate a best estimate based on their performance compared to the more detailed radiative transfer schemes in the idealized simulations.

Shipping Emissions: From Cooling to Warming of Climate—and Reducing Impacts on Health

International shipping has been a fast growing sector of the global economy and its share of total anthropogenic emissions is significant, having effects on climate, air quality, and human health. The nature of the contribution to climate change is complex: In addition to warming by CO2 emissions, ship emissions of sulfur dioxide (SO2) cause cooling through effects on atmospheric particles and clouds, while nitrogen oxides (NOx) increase the levels of the greenhouse gas (GHG) ozone (O3) and reduce the GHG methane (CH4), causing warming and cooling, respectively. The result is a net global mean radiative forcing (RF) from the shipping sector that is strongly negative, leading to a global cooling effect today. However, new regulations of SO2 and NOx, while reducing air pollution and its harmful effects on health and water/soil acidification, will reduce this sector’s cooling effects. Given these reductions, shipping will, relative to present-day impacts, impart a “double warming” effect: one from CO2, and one from the reduction of SO2. Therefore, after some decades the net climate effect of shipping will shift from cooling to warming.

Climatology of Surface Meteorology, Surface Fluxes, Cloud Fraction, and Radiative Forcing over the Southeast Pacific from Buoy Observations

Abstract A 5-yr climatology of the meteorology, including boundary layer cloudiness, for the southeast Pacific region is presented using observations from a buoy located at 20°S, 85°W. The sea surface temperature and surface air temperature exhibit a sinusoidal seasonal cycle that is negatively correlated with surface pressure. The relative humidity, wind speed, and wind direction show little seasonal variability. But the advection of cold and dry air from the southeast varies seasonally and is highly correlated with the latent heat flux variations. A simple model was used to estimate the monthly cloud fraction using the observed surface downwelling longwave radiative flux and surface meteorological parameters. The annual cycle of cloud fraction is highly correlated to that of S. A. Klein: lower-tropospheric stability parameter (0.87), latent heat flux (−0.59), and temperature and moisture advection (0.60). The derived cloud fraction compares poorly with the International Satellite Cloud Climatology Project (ISCCP)-derived low-cloud cover but compares well (0.86 correlation) with ISCCP low- plus middle-cloud cover. The monthly averaged diurnal variations in cloud fraction show marked seasonal variability in the amplitude and temporal structure. The mean annual cloud fraction is lower than the mean annual nighttime cloud fraction by about 9%. Annual and diurnal cycles of surface longwave and shortwave cloud radiative forcing were also estimated. The longwave cloud radiative forcing is about 45 W m−2 year-round, but, because of highly negative shortwave cloud radiative forcing, the net cloud radiative forcing is always negative with an annual mean of −50 W m−2.