Radiative Forcing of Climate Change Expanding the Concept and Addressing Uncertainties (2005) / Chapter Skim
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2 State of Scientific Understanding
2 State of Scientific Understanding
Pages 28-62
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From page 28... ...
The greatest uncertainty in Figure 2-1 is associated with the direct and indirect radiative effects of aerosols. If the actual negative forcing from aerosols were at the high end (most negative)
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A somewhat broader perspective is applied in this chapter to include, in particular, volcanic aerosols, the effects of land-use changes and aerosols on precipitation, and the radiative forcing due to changes in ocean color.
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From page 30... ...
Although it would be unrealistic to expect a single figure to fully portray all of these aspects of radiative forcings, there are clearly opportunities to improve upon Figure 2-1 and to introduce new figures that address these limitations in the next IPCC report. WELL-MIXED GREENHOUSE GASES The radiative forcing due to CO2, CH4, N2O, and various halocarbons is due to absorption of infrared (IR)
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31 per yr parts = time.
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. The IPCC estimate for CH4 forcing includes an observation-based estimate for both the direct forcing of CH4 and the indirect forcing due to changes in the hydroxyl radical (OH)
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. Beyond this direct radiative forcing effect, tropospheric ozone also has an indirect effect as the primary precursor of the OH radical.
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. Another critical issue is to better quantify indirect radiative forcings, particularly the effect on stratospheric water vapor, which could double the effective forcing according to Stuber et al.
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The mixing state and relative humidity history of sulfate-nitrate-ammonium aerosols have important implications for their water content and hence their direct radiative effect (Martin et al., 2004)
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A portion of the direct solar beam is scattered back to space, which leads to a reduction in solar radiation reaching the surface. This reduction
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Long-term negative trends in surface solar irradiances have been observed by surface radiometers worldwide (Ohmura et al., 1998; Stanhill and Cohen, 2001; Liepert, 2002)
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The increased diffuse irradiance permits a greater penetration of photosynthetically active sunlight into the canopy. However, if there is a sufficient reduction of total solar irradiance received at the ground, vegetation growth could be stunted.
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From page 39... ...
Aerosols have several indirect effects on warm stratiform clouds. The Twomey effect, also known as the first indirect aerosol effect, refers to the enhanced reflection of solar radiation due to more but smaller cloud droplets in a cloud whose liquid water content remains constant (Twomey, 1959)
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From page 40... ...
precipitation efficiency, thereby prolonging cloud lifetime Semidirect effect All clouds Absorption of solar Positive radiation by soot leads to evaporation of cloud particles Glaciation indirect Mixed-phase An increase in ice Positive effect clouds nuclei increases the precipitation efficiency Thermodynamic effect Mixed-phase Smaller cloud droplets Unknown clouds inhibit freezing, causing supercooled droplets to extend to colder temperatures Surface energy All clouds The aerosol-induced Negative budget effect increase in cloud optical thickness decreases the amount of solar radiation reaching the surface, changing the surface energy budget ciency and therefore enhance cloud lifetime and, hence, cloud reflectivity, which is referred to as the second indirect aerosol or cloud lifetime effect (Albrecht, 1989)
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None of these techniques, however, enable estimates of the anthropogenic indirect aerosol effect globally. Climate model estimates of the first and second indirect aerosol effects and the semidirect aerosol effect are still very uncertain.
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From page 42... ...
For large-scale, mixed-phase clouds, Cloud albedo _ + + Cloud cover and lifetime _ + _ Precipitation + + + Cloud droplets Mixed particles Ice crystals + + Cloud nuclei Aerosol mass Ice nuclei + + + Anthropogenic emissions FIGURE 2-2 Schematic of the warm indirect aerosol effect (blue arrows) and glaciation indirect aerosol effect (red arrows)
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From page 43... ...
. Aerosols emitted by the aircraft may also cause indirect effects associated with an increase in ice nuclei in the upper troposphere.
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From page 44... ...
characterized steadily decreasing pan evaporation due to decreases in solar irradiance resulting from increasing cloud coverage and aerosol concentration. Increasing aerosol optical depth associated with scattering aerosols alone in otherwise clear skies has been shown to produce a larger fraction of diffuse radiation at the surface, which results in greater carbon assimilation into vegetation (and therefore greater transpiration)
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STATE OF SCIENTIFIC UNDERSTANDING 45 Year 1700 Year 1900 Year 1990 Intensive Cropland Temperate Deciduous Forest/Woodland Dense Shrubland Marginal Cropland/Used for Grazing Boreal Evergreen Forest/Woodland Open Shrubland Boreal Deciduous Forest/Woodland Tundra Tropical Evergreen Forest/Woodland Evergreen/Deciduous Forest/Woodland Hot Desert Tropical Deciduous Forest/Woodland Savanna Polar Desert/Rock/Ice Temperate Broadleaf Evergreen Forest/Woodland Grassland/Steppe Temperate Needle-Leaf Evergreen Forest/Woodland
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From page 46... ...
As a result, the atmospheric boundary layer that develops above such land is typically thin and moist. In contrast, when the soil is dry, there is no latent heat flux, the sensible heat flux is large, the land surface temperature is higher, and as a result, the longwave radiation emitted by the land surface is relatively high.
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From page 47... ...
The surface albedo is usually higher when vegetation is reduced, exposing some of the soil. Land-use and land-cover change can also have indirect effects by affecting fluxes of greenhouse gases from the land surface.
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Land type also affects the duration of snow on the ground. Partitioning Between Latent and Sensible Heat Fluxes on a Regional Scale Changes in the partitioning of net radiative fluxes into sensible and latent can substantially alter the atmospheric circulation.
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From page 49... ...
The blackbody temperature of the solar surface is about 5800 K, and as a result, the spectrum of solar radiation peaks at visible wavelengths. Solar irradiance is the electromagnetic radiation from the Sun incident at the top of the Earth's atmosphere at a distance of one astronomical unit (1 AU = 1.5 × 1011 m)
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2 Multiple TSI datasets have been combined into a composite time series of daily total solar irradiance from 1979 to the present. This requires the cross-calibration of measurements made by overlapping datasets to adjust
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Likewise, models of total solar irradiance variability that account for the influences of solar activity features -- dark sunspots and bright faculae -- do not predict a secular change in the past two decades. Knowledge of solar irradiance variations is rudimentary prior to the commencement of continuous space-based irradiance observations in 1979.
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From page 52... ...
Ionization and Production of Cloud Condensation Nuclei Galactic cosmic rays have one billionth of the total solar irradiance energy, but can reach the troposphere where they produce ions that may serve as nuclei for cloud condensation, with subsequent climatic impacts.
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The approximate 15 percent modulation of cosmic ray flux by solar activity produces an energy change less than one millionth of the energy change in the 0.1 percent total solar irradiance cycle. Cosmic rays also interact with air nuclei to produce isotopes such as 14C in tree-rings and 10Be in ice cores.
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From page 54... ...
The regional pattern of such indirect climate forcing is likely quite different from the effects of direct surface heating by solar radiation. The effect of solar cycle UV irradiance changes on stratospheric ozone are now relatively well established as a result of extensive space-based datasets that span more than two solar activity cycles (McCormack et al., 1997)
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From page 55... ...
. These depletion events, whose frequency and strength vary with solar activity, are superimposed on the more sustained solar UV radiation-induced ozone changes that occur during the 11-year solar cycle.
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From page 56... ...
. Since ozone absorbs electromagnetic radiation in the UV, visible, and IR spectral regions, changes in ozone concentration can affect Earth's radiative balance by altering both incoming solar radiation and outgoing terrestrial radiation.
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. Solar-induced indirect effects on climate may also involve altered modes of variability.
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Past histories of radiative forcing by explosive volcanic activity are typically constructed from sulfate aerosols contained in annual ice core layers (e.g., Robock, 2000)
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From page 59... ...
provide excellent reviews and critiques of various indices of past explosive volcanic activity. Ice core volcanic radiative forcing estimates have been developed for the past century to the past couple of millennia by numerous researchers (Robock and Free, 1995, 1996; Robertson et al., 1998; Robock, 2000; Crowley, 2000; Ammann et al., 2003; Crowley et al., 2003)
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(2003) used a global climate model to assess the role of ocean color in the sea surface temperature and other aspects of the climate system.
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From page 61... ...
shows that absorbing aerosols over South Asia and the North Indian Ocean can cause subsidence motions over most of the tropics, which would have a drying effect (Figure 2-10)
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From page 62... ...
in the upper panel. The solar heating by absorbing aerosols, mainly due to black carbon, is concentrated over South Asia and the North Indian Ocean (i.e., over the red shaded regions in the lower panel)
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