Frode Stordal

New estimates of radiative forcing due to well mixed greenhouse gases

We have performed new calculations of the radiative forcing due to changes in the concentrations of the most important well mixed greenhouse gases (WMGG) since pre-industrial time. Three radiative transfer models are used. The radiative forcing due to CO2, including shortwave absorption, is 15% lower than the previous IPCC estimate. The radiative forcing due to all the WMGG is calculated to 2.25 Wm−2, which we estimate to be accurate to within about 5%. The importance of the CFCs is increased by about 20% relative to the total effect of all WMGG compared to previous estimates. We present updates to simple forcing-concentration relationships previously used by IPCC.

Estimation of the direct radiative forcing due to sulfate and soot aerosols

The direct radiative forcings due to tropospheric sulfate and fossil fuel soot aerosols are calculated. The change in the atmospheric sulfate since preindustrial time is taken from a recent three-dimensional chemistry transport model calculation. A multistream radiative transfer code and observed atmospheric input data is used. The direct radiative forcing due to sulfate is calculated to −0.32 W/m2. Our results for global and annual mean radiative forcing have been compared with results from other model studies. We have assumed a linear relationship between the concentration of fossil fuel soot and sulfate aerosols. The resulting radiative forcing due to soot particles is 0.16 W/m2. Two types of mixtures of sulfate and soot are further assumed. The calculated single scattering albedo is compared to observations.

Boundary-layer ozone depletion as seen in the Norwegian Arctic in spring

Several years of measurements of ozone, hydrocarbons, sulphate and meteorological parameters from Spitsbergen in the Norwegian Arctic are presented. Most of the measurements were taken on the Zeppelin Mountain at an altitude of 474 m a.s.l. The focus is the episodes of ozone depletion in the lower troposphere in spring, which are studied in a climatological way. Episodes of very low ozone concentrations are a common feature on the Zeppelin Mountain in spring. The low ozone episodes were observed from late March to the beginning of June. When the effect of transport direction was subtracted, the frequenty of the low ozone episodes was found to peak in the beginning of May, possibly reflecting the seasonal cycle in the actual depletion process. Analyses based on trajectory calculations show that most of the episodes occurred when the air masses were transported from W-N. Ozone soundings show that the ozone depletion may extend from the surface and up to 3–4 km altitude. The episodes were associated with a cold boundary layer beneath a thermally stable layer, suppressing mixing with the free troposphere. The concentration of several individual hydrocarbons was much lower during episodes of low ozone than for the average conditions. The change in concentration ratio between the hydrocarbons was in qualitative agreement with oxidation of hydrocarbons by Br and Cl atoms rather than by OH radicals.

Effects of anthropogenic emissions on tropospheric ozone and its radiative forcing

Tropospheric ozone changes since preindustrial times due to changes in emissions have been calculated by the University of Oslo global three-dimensional photochemical model. The radiative forcing caused by the increase in ozone has been calculated by two independent radiative transfer models; the University of Reading model (Reading), and the University of Oslo/Norwegian Institute for Air Research model (OsloRad). Significant increases in upper tropospheric ozone concentrations are found at northern midlatitudes (15–40 ppbv, depending on season) at about 10 km altitude. In the tropical regions the largest increase (about 20 ppbv for all seasons) is found at about 15 km altitude. The increase is found to be caused mainly by enhanced in situ production due to transport of precursors from the boundary layer, with a smaller contribution from increased transport of ozone produced in the boundary layer. The lifetime of ozone in the troposphere decreased by about 35% as a result of enhanced concentrations of HO2. The calculated increase in surface ozone in Europe is found to be in good agreement with observations. The calculations of radiative forcing include the effect of clouds and allow for thermal adjustment in the stratosphere. The global and annual averaged radiative forcing at the tropopause from both models (0.28 W m−2 and 0.31 W m−2, for the Reading and OsloRad models, respectively) are in the lower part of the Intergovernmental Panel on Climate Change [1995] estimated range. The calculated radiative forcing is similar in magnitude to the negative radiative forcing by sulfate aerosols, but displaced southward in source regions at northern midlatitudes. The increase in tropospheric ozone is calculated to have cooled the lower stratosphere by up to 0.9 K, with possibly half of this cooling occurring in the past 2 to 3 decades.

Global sensitivity experiments of the radiative forcing due to mineral aerosols

The radiative effects of mineral dust in the atmosphere are uncertain. Further, the human contribution to the mineral aerosol concentration is difficult to quantify. We have performed several global sensitivity experiments to investigate the radiative forcing due to mineral dust. Two global data sets of mineral aerosol distribution are used. Radiative transfer schemes for thermal infrared and solar radiation are used in the calculations. We have investigated the sensitivity of the global radiative forcing to the spatial distribution of the aerosols, including the altitude, the size distribution, and optical parameters. Our strongest emphasis has been on the size distribution of the mineral aerosol, for which we have found a strong sensitivity. A range of −0.7 Wm−2 to 0.5 Wm−2 is estimated for the human influence on the global radiative forcing due to mineral aerosols. We find it almost as probable with a positive radiative forcing as with a negative forcing. Even if the global mean radiative forcing is small, there are large contributions of different sign in certain regions.

On the tradeoff of the solar and thermal infrared radiative impact of contrails

Global calculations of the radiative forcing due to contrails from aircraft are performed. A contrail distribution computed based on aviation fuel consumption and radiative transfer models for solar and thermal infrared radiation have been used. A substantially smaller net radiative forcing due to contrails (about 0.01 Wm -2 ) is estimated in comparison to former studies, emphasizing the strong sensitivity of this value to uncertainties in the longwave and shortwave contribution. The solar forcing is negative and the magnitude maximizes for high solar zenith angles. Altering the time for aircraft traffic has the potential for reducing the radiative forcing due to contrails, and under certain assumptions made in this paper giving a net zero forcing.

Historical evolution of radiative forcing of climate

Abstract We have compiled the evolution of the radiative forcing for several mechanisms based on our radiative transfer models using a variety of information sources to establish time histories. The anthropogenic forcing mechanisms considered are well-mixed greenhouse gases, ozone, and tropospheric aerosols (direct and indirect effect). The natural forcing mechanisms taken into account are the radiative effects of solar irradiance variation and particles of volcanic origin. In general there has been an increase in the radiative forcing during the 20th century. The exception is a decline in the radiative forcing in the 1945–1970 period. We have found that the evolution of anthropogenic particle emissions in the same period may have been a major cause of this decline in the forcing. We have discussed uncertainties in the various forcings and their evolution. The uncertainties are large for many forcing mechanisms, especially the impact of anthropogenic aerosols. In particular the indirect effect of aerosols on clouds is difficult to quantify. Several evolutions of their effect may have been possible, strongly influencing the evolution of the total anthropogenic radiative forcing.

Intercomparison of Satellite Retrieved Aerosol Optical Depth over the Ocean

For an 8-month period aerosol optical depth (AOD) is compared, derived over global oceans with five different retrieval algorithms applied to four satellite instruments flown on board three satellite platforms. The Advanced Very High Resolution Radiometer (AVHRR) was flown on board NOAA-14, the Ocean Color and Temperature Scanner (OCTS) and the Polarization and Directionality of the Earth’s Reflectances (POLDER) on board the Advanced Earth Observing Satellite(ADEOS), and the Total Ozone Mapping Spectrometer (TOMS) on board the Earth Probe satellites. The aerosol data are presented on the same format and converted to the same wavelength in the comparison and can therefore be a useful tool in validation of global aerosol models, in particular models that can be driven with meteorological data for the November 1996 to June 1997 period studied here. Large uncertainties in the global mean AOD are found. There is at least a factor of 2 difference between the AOD from the retrievals. The largest uncertainties are found in the Southern Hemisphere, and the smallest differences mostly near the continents in the Northern Hemisphere. The largest relative differences are probably caused by differences in cloud screening.

Modeling the solar radiative impact of aerosols from biomass burning during the Southern African Regional Science Initiative (SAFARI-2000) experiment

[1] In this study, we model the radiative impact of biomass burning aerosols with meteorological data for the Southern African Regional Science Initiative (SAFARI-2000) experiment campaign period. Satellite, ground-based, and aircraft observations are used in the validation of the modeled aerosol optical depth (AOD), vertical profiles, and radiative impact of the aerosols. The modeled pattern and magnitude of the AOD is generally in good agreement with the observations. The meteorological conditions are found to be important in determining the distribution of the aerosols. The modeled radiative impact of the biomass aerosols compares well to measurements. During September 2000, the modeled radiative impact of biomass aerosols reaches 50 W m 2 locally. INDEX TERMS: 0305 Atmospheric Composition and Structure: Aerosols and particles (0345, 4801); 0360 Atmospheric Composition and Structure: Transmission and scattering of radiation; 3359 Meteorology and Atmospheric Dynamics: Radiative processes; KEYWORDS: biomass burning, transport model, single scattering albedo, aerosol optical depth, aircraft measurements, radiative impact Citation: Myhre, G., T. K. Berntsen, J. M. Haywood, J. K. Sundet, B. N. Holben, M. Johnsrud, and F. Stordal, Modeling the solar radiative impact of aerosols from biomass burning during the Southern African Regional Science Initiative (SAFARI-2000) experiment, J. Geophys. Res., 108(D13), 8501, doi:10.1029/2002JD002313, 2003.

Comparison of the radiative properties and direct radiative effect of aerosols from a global aerosol model and remote sensing data over ocean

Measurements of C2–C8 non-methane hydrocarbons (NMHCs) have been made in situ at Halley Base, Antarctica (75◦35°S, 26◦19°W) from February 2004 to February 2005 as part of the Chemistry of the Antarctic Boundary Layer and the Interface with Snow (CHABLIS) experiment. The data show long- and short-term variabilities in NMHCs controlled by the seasonal and geographic dependence of emissions and variation in atmospheric removal rates and pathways. Ethane, propane, iso-butane, n-butane and acetylene abundances followed a general OH-dependent sinusoidal seasonal cycle. The yearly averages were 186, 31, 3.2, 4.9 and 19 pptV, respectively, lower than those which were reported in some previous studies. Superimposed on a seasonal cycle was shorter-term variability that could be attributed to both synoptic airmass variability and localized loss processes due to other radical species. Hydrocarbon variability during periods of hour-to-day-long surface O3 depletion in late winter/early spring indicated active halogen atom chemistry estimated to be in the range 1.7 × 103–3.4 × 104 atom cm−3 for Cl and 4.8 × 106–9.6 × 107 atom cm−3 for Br. Longer-term negative deviations from sinusoidal behaviour in the late August were indicative of NMHC reaction with a persistent [Cl] of 2.3×103 atom cm−3.Maximum ethene and propene of 157 and 179 pptV, respectively, were observed in the late February/early March, consistent with increased oceanic biogenic emissions; however, their presence was significant year-round (June–August concentrations of 17.1 ± 18.3 and 7.9 ± 20.0 pptV, respectively).