Summary for policymakers Technical summary 1. The climate system - an overview 2. Observed climate variability and change 3. The carbon cycle and atmospheric CO2 4. Atmospheric chemistry and greenhouse gases 5. Aerosols, their direct and indirect effects 6. Radiative forcing of climate change 7. Physical climate processes and feedbacks 8. Model evaluation 9. Projections of future climate change 10. Regional climate simulation - evaluation and projections 11. Changes in sea level 12. Detection of climate change and attribution of causes 13. Climate scenario development 14. Advancing our understanding Glossary Index Appendix.

Climate change 2001: the scientific basis

The discrete-dipole approximation (DDA) for scattering calculations, including the relationship between the DDA and other methods, is reviewed. Computational considerations, i.e., the use of complex-conjugate gradient algorithms and fast-Fourier-transform methods, are discussed. We test the accuracy of the DDA by using the DDA to compute scattering and absorption by isolated, homogeneous spheres as well as by targets consisting of two contiguous spheres. It is shown that, for dielectric materials (|m| ≲ 2), the DDA permits calculations of scattering and absorption that are accurate to within a few percent.

Discrete-Dipole Approximation For Scattering Calculations

Aerosol radiative forcing is a critical, though variable and uncertain, component of the global climate. Yet climate models rely on sparse information of the aerosol optical properties. In situ measurements, though important in many respects, seldom provide measurements of the undisturbed aerosol in the entire atmospheric column. Here, 8 yr of worldwide distributed data from the AERONET network of ground-based radiometers were used to remotely sense the aerosol absorption and other optical properties in several key locations. Established procedures for maintaining and calibrating the global network of radiometers, cloud screening, and inversion techniques allow for a consistent retrieval of the optical properties of aerosol in locations with varying emission sources and conditions. The multiyear, multi-instrument observations show robust differentiation in both the magnitude and spectral dependence of the absorption—a property driving aerosol climate forcing, for desert dust, biomass burning, urban‐industrial, and marine aerosols. Moreover, significant variability of the absorption for the same aerosol type appearing due to different meteorological and source characteristics as well as different emission characteristics are observed. It is expected that this aerosol characterization will help refine aerosol optical models and reduce uncertainties in satellite observations of the global aerosol and in modeling aerosol impacts on climate.

/pdf/variability-of-absorption-and-optical-properties-of-key-56rawczexh.pdf

Variability of Absorption and Optical Properties of Key Aerosol Types Observed in Worldwide Locations

This paper reviews the many developments in estimates of the direct and indirect global annual mean radiative forcing due to present-day concentra- tions of anthropogenic tropospheric aerosols since Inter- governmental Panel on Climate Change (1996). The range of estimates of the global mean direct radiative forcing due to six distinct aerosol types is presented. Addition- ally, the indirect effect is split into two components corresponding to the radiative forcing due to modifica- tion of the radiative properties of clouds (cloud albedo effect) and the effects of anthropogenic aerosols upon the lifetime of clouds (cloud lifetime effect). The radia- tive forcing for anthropogenic sulphate aerosol ranges from 20.26 to 20.82 W m 22 . For fossil fuel black carbon the radiative forcing ranges from 10.16 W m 22 for an external mixture to 10.42 W m 22 for where the black carbon is modeled as internally mixed with sulphate aerosol. For fossil fuel organic carbon the two estimates of the likely weakest limit of the direct radiative forcing are 20.02 and 20.04 W m 22 . For biomass-burning sources of black carbon and organic carbon the com-

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/1999RG000078

Estimates of the direct and indirect radiative forcing due to tropospheric aerosols: A review

Daily distribution of the aerosol optical thickness and columnar mass concentration will be derived over the continents, from the EOS moderate resolution imaging spectroradiometer (MODIS) using dark land targets. Dark land covers are mainly vegetated areas and dark soils observed in the red and blue channels; therefore the method will be limited to the moist parts of the continents (excluding water and ice cover). After the launch of MODIS the distribution of elevated aerosol concentrations, for example, biomass burning in the tropics or urban industrial aerosol in the midlatitudes, will be continuously monitored. The algorithm takes advantage of the MODIS wide spectral range and high spatial resolution and the strong spectral dependence of the aerosol opacity for most aerosol types that result in low optical thickness in the mid-IR (2.1 and 3.8 pm). The main steps of the algorithm are (1) identification of dark pixels in the mid-IR; (2) estimation of their reflectance at 0.47 and 0.66 pm; and (3) derivation of the optical thickness and mass concentration of the accumulation mode from the detected radiance. To differentiate between dust and aerosol dominated by accumulation mode particles, for example, smoke or sulfates, ratios of the aerosol path radiance at 0.47 and 0.66 pm are used. New dynamic aerosol models for biomass burning aerosol, dust and aerosol from industrial/urban origin, are used to determine the aerosol optical properties used in the algorithm. The error in the retrieved aerosol optical thicknesses, r,, is expected to be AT, = 0.05 5 0.27,. Daily values are stored on a resolution of 10 X 10 pixels (1 km nadir resolution). Weighted and gridded 8-day and monthly composites of the optical thickness, the aerosol mass concentration and spectral radiative forcing are generated for selected scattering angles to increase the accuracy. The daily aerosol information over land and oceans (Tunr& et al., this issue), combined with continuous aerosol remote sensing from the ground, will be used to study aerosol climatology, to monitor the sources and sinks of specific aerosol types, and to study the interaction of aerosol with water vapor and clouds and their radiative forcing of climate. The aerosol information will also be used for atmospheric corrections of remotely sensed surface reflectance. In this paper, examples of applications and validations are provided.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/96JD03988

Robert A. West

Papers

Cassini UVIS Observation of Occultation of Sigma Leo by Jupiter

Methane Band Images of Saturn.

Jovian Stratospheric Circulation: Insights from Cassini Observations and Coupled Dynamical-Chemical Modeling

Aerosol properties in Titan's upper atmosphere

Photochemistry in Saturn’s Ring Shadowed Atmosphere: Production Rates of Key Atmospheric Molecules and Haze Observations