Susan Solomon

An observationally based energy balance for the Earth since 1950

[1]xa0We examine the Earths energy balance since 1950, identifying results that can be obtained without using global climate models. Important terms that can be constrained using only measurements and radiative transfer models are ocean heat content, radiative forcing by long-lived trace gases, and radiative forcing from volcanic eruptions. We explicitly consider the emission of energy by a warming Earth by using correlations between surface temperature and satellite radiant flux data and show that this term is already quite significant. About 20% of the integrated positive forcing by greenhouse gases and solar radiation since 1950 has been radiated to space. Only about 10% of the positive forcing (about 1/3 of the net forcing) has gone into heating the Earth, almost all into the oceans. About 20% of the positive forcing has been balanced by volcanic aerosols, and the remaining 50% is mainly attributable to tropospheric aerosols. After accounting for the measured terms, the residual forcing between 1970 and 2000 due to direct and indirect forcing by aerosols as well as semidirect forcing from greenhouse gases and any unknown mechanism can be estimated as −1.1 ± 0.4 W m−2 (1σ). This is consistent with the Intergovernmental Panel on Climate Changes best estimates but rules out very large negative forcings from aerosol indirect effects. Further, the data imply an increase from the 1950s to the 1980s followed by constant or slightly declining aerosol forcing into the 1990s, consistent with estimates of trends in global sulfate emissions. An apparent increase in residual forcing in the late 1990s is discussed.

Effects of ozone cooling in the tropical lower stratosphere and upper troposphere

[1]xa0In this paper, we examine the tropical lower stratosphere and upper troposphere and elucidate the key role of ozone changes in driving temperature trends in this region. We use a radiative fixed dynamical heating model to show that the effects of tropical ozone decreases at 70 hPa and lower pressures can lead to significant cooling not only at stratospheric levels, but also in the “sub-stratosphere/upper tropospheric” region around 150–70 hPa. The impact of stratospheric ozone depletion on upper tropospheric temperatures stems from reduced longwave emission from above. The results provide a possible explanation for the long-standing discrepancy between modeled and measured temperature trends in the uppermost tropical troposphere and can explain the latitudinal near-homogeneity of recent stratospheric temperature trends.

On the seasonal dependence of tropical lower-stratospheric temperature trends

Abstract. This study examines the seasonality of tropical lower-stratospheric temperature trends using the Microwave Sounding Unit lower-stratospheric channel (T4) for 1980–2008. We present evidence that this seasonality is largely a response to changes in the Brewer-Dobson circulation (BDC) driven by extratropical wave forcing. We show how the tropical T4 trend can be used as an indicator of changes in the BDC, and find that the BDC is strengthening for 1980–2008 in June–November related to the Southern Hemisphere (SH) and in December–February to the Northern Hemisphere (NH). In marked contrast, we find that the BDC is weakening in March–May, apparently because of a weakening of its northern cell. The novel observational evidence on the seasonal dependence of the BDC trends presented in this study has important implications for the understanding of climate change in the stratosphere as well as testing climate model simulations.

Indirect radiative forcing of the ozone layer during the 21st century

[1] The response of a coupled two-dimensional radiative-chemical-dynamical model to possible 21 st century changes of the greenhouse gasses (GHGs) carbon dioxide, nitrous oxide and methane are explored using a range of IPCC marker scenarios of GHG emissions. The changes to the ozone layer caused by these GHGs are found to be relatively large (e.g., up to 5% global mean column ozone changes and 30% local changes for CO 2 using the IPCC A2 scenario between 2000 and 2100) and the mechanisms for these changes are discussed. The ozone changes are compared to the recovery of ozone due to expected decreases in chlorine containing compounds. Since carbon dioxide, nitrous oxide, and methane affect ozone they induce an indirect radiative forcing in addition to their direct radiative forcing. These indirect radiative forcings are computed using a combination of accurate line-by-line and band radiative transfer models and are compared to the radiative forcing of ozone during the 1979-2000 time period. Although the changes in ozone are large at some altitudes over the 2000-2100 time horizon, the range of associated future indirect radiative forcings from ozone over the range of IPCC scenarios are found to be -0.1 to 0.1 W m -2 , which is small compared with the corresponding range of total direct radiative forcing of 2.2 to 6.2 W m -2 for these GHGs over this time horizon.

Changes in the polar vortex: Effects on Antarctic total ozone observations at various stations

October mean total column ozone data from four Antarctic stations form the basis for understanding the evolution of the ozone hole since 1960. While these stations show similar emergence of the ozone hole from 1960 to 1980, the records are divergent in the last two decades. The effects of long-term changes in vortex shape and location are considered by gridding the measurements by equivalent latitude. A clear eastward shift of the mean position of the vortex in October with time is revealed, which changes the fraction of ozone measurements taken inside/outside the vortex for stations in the vortex collar region. After including only those measurements made inside the vortex, ozone behavior in the last two decades at the four stations is very similar. This suggests that dynamical influence must be considered when interpreting and intercomparing ozone measurements from Antarctic stations for detecting ozone recovery and ozone-related changes in Antarctic climate.

Is Antarctic climate most sensitive to ozone depletion in the middle or lower stratosphere

[1]xa0Antarctic stratospheric ozone depletion has been associated with an observed downward trend in tropospheric geopotential height and temperature. Stratospheric ozone depletion peaks in October–November, whereas tropospheric trends are largest in December–January, concurrent with maximum ozone changes close to the tropopause. Surface temperatures are most sensitive to ozone loss near the tropopause, therefore it has been suggested that the observed tropospheric response is forced mainly by ozone depletion in the lower stratosphere. In this study the climate response to ozone depletion exclusively below 164 hPa is simulated using HadSM3-L64, and compared with simulations in which ozone depletion is prescribed exclusively above 164 hPa. Results indicate that the tropospheric response is dominated by ozone changes above 164 hPa, with ozone changes in the lowermost stratosphere playing an insignificant role. A tropospheric response is also seen in fall/winter which agrees well with observations and has not been found in modeling studies previously.

Atmospheric fate and greenhouse warming potentials of HFC 236fa and HFC 236ea

The rate coefficient for the reaction OH + CF3CH2CF3 (1,1,1,3,3,3-hexafluoropropane, HFC236fa) was measured between 269 and 413 K using the pulsed photolysis-laser induced fluorescence technique to be k1 = (1.60 ± 0.40) × 10−12 exp (−(2450 ± 150)/T) cm3 molecule−1 s−1. The rate coefficient, k2b, for the destruction of CF3CH2CF3 via reaction with O(1D) was measured to be (4.5 ± 1.9) × 10−12 cm3 molecule−1 s−1 using the laser photolysis-resonance fluorescence technique. From these data, along with previously published rate coefficients for OH reaction with HFC236ea, the atmospheric lifetimes of HFC236fa and HFC236ea were calculated to be 210 and 8.1 years, respectively. The room temperature infrared absorption cross sections for these two compounds were measured over the range 650 to 1350 cm−1. The global warming potentials (GWPs) of HFC236fa and HFC236ea, respectively, were calculated to be 5610 and 2200 for a 20-year horizon and 5160 and 220 for a 500-year horizon.

Sulfur dioxide emission flux measurements from point sources using airborne near ultraviolet spectroscopy during the New England Air Quality Study 2004

[1]xa0This work presents measurements of sulfur dioxide (SO2) emission fluxes from point sources using airborne near-ultraviolet (UV) spectroscopy. A Czerny-Turner spectrograph has been optimized to measure SO2 and the oxygen collision complex (O4) in the wavelength region of 286–408 nm from an aircraft platform. The spectrograph was deployed aboard the NOAA WP-3D Orion aircraft during the New England Air Quality Study during the summer of 2004. The spectrograph has zenith and nadir field of views, allowing for measurements of pollution plumes when the aircraft is in or above the planetary boundary layer. The near-UV spectra are analyzed using the differential optical absorption spectroscopy (DOAS) method to retrieve SO2 and O4 differential slant column densities (DSCDs). The SO2 DSCDs are used to identify point source plumes and are converted to vertical column densities (VCDs), which are needed to calculate emissions of SO2 from point sources. The conversion to VCDs requires knowledge of the photon optical path length or the air mass factor (AMF). We present a novel approach to calculate the AMF using observations of the absorption of solar radiation by O4. The SO2 VCDs, wind speed and direction, and aircraft speed are then used to obtain emission fluxes from power plants. The measured SO2 power plant emission fluxes are compared to the reported emissions from the power plants. The measured and reported SO2 emission fluxes are in good agreement.