Radiative Forcing of Climate Change Expanding the Concept and Addressing Uncertainties (2005) / Chapter Skim
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3 Radiative Forcing Over Earths History
3 Radiative Forcing Over Earths History
Pages 63-82
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From page 63... ...
timescales of tens to hundreds of thousands of years, over which Earth orbital and astronomical changes appear to dominate the variability of the climate system; (3) timescales of millennia to multimillennia as exemplified by evidence from our present interglacial climate, the Holocene; and (4)
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From page 64... ...
. Solar irradiance variations were likely irregular, with large dark spots dominating the surface.
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From page 65... ...
marine organisms Because silicate weathering rates vary with temperature (by a factor of about two for each 10°C) and area of exposed continental silicate rocks (Ruddiman, 2001)
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From page 66... ...
Higher temperatures and larger continents would have increased continental silicate weathering rates, thereby decreasing the CO2 greenhouse effect and cooling the Earth. Thus, many regard the silicate weathering-CO2 negative feedback as a thermostat that prevented permanent freezing of the early Earth and, later, prevented permanent temperatures too hot for life.
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From page 67... ...
Light blue lines between about 160 and 60 Ma are times of brief, geographically limited glaciations in high latitudes (Royer et al., 2004)
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From page 68... ...
Shaviv and Veizer (2003) have suggested that changes in cosmic ray fluxes reaching Earth may occur as the solar system passes through the Milky Way's spiral arms and may account, through the influence of cosmic rays on cloud formation, for the major variations in global mean temperature and continental glaciation during the Phanerozoic.
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(2003) have argued that if the revised estimate is correct and applies to the time of the Paleocene-Eocene transition, then ocean methane carbon released by any mechanism probably would have been insufficient to cause a recognizable perturbation of climate and the carbon cycle.
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relative to the timing of seasons. Currently, perihelion coincides approximately with the Northern Hemisphere winter solstice (favoring decreased seasonal changes in response to seasonal changes in insolation)
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From page 71... ...
. Recent modeling studies suggest that mid-Holocene global mean surface temperatures may actually have been cooler than those of the mid-twentieth century, even though extratropical summers were likely somewhat warmer (Kitoh and Murakami, 2002)
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From page 72... ...
The large, abrupt decreases in methane in the early Holocene coincide with abrupt coolings in at least the North Atlantic and western European regions. It has been argued recently that interpolar methane gradient data from ice cores is evidence of an abrupt switching on of a major Northern Hemisphere methane source, probably in Siberia, in the early Holocene between about 9,000 and 11,500 years ago (Smith et al., 2004)
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From page 73... ...
Solar Irradiance Estimates of solar irradiance variations during the pre-modern Holocene assume that cosmogenic isotope information recorded in tree rings (14C) and in ice cores (10Be)
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Even so, several recent studies document a relatively close connection at millennial timescales between regional climate proxies and nuclide variations (colored heavy lines in the smoothed records in Figure 3-5)
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. These results suggest either that the nuclidesolar irradiance connection is more direct and robust than models suggest, that there are amplified responses of the climate to solar irradiance variations, or both.
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From page 76... ...
using radiative forcing histories such as those shown in Figure 3-7. The forcing histories include nineteenth and twentieth century anthropogenic radiative forcing (greenhouse gas and sulfate aerosol forcing and, in some cases, land-use changes)
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From page 78... ...
Simulations that do not include land-use changes may exhibit an artificially cold pre-nineteenth century mean temperature relative to empirical estimates when, as in Figure 3-8, the model simulation results have been aligned vertically to have the same mean as the instrumental temperature record during the late nineteenth and twentieth centuries. The issue of the role of late twentieth century land-use changes in surface temperature measurements is currently being debated in the scientific literature (e.g., Kalnay and Cai, 2003; Marshall et al., 2004a; Trenberth, 2004; Vose et al., 2004)
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From page 79... ...
The simulations are based on varying radiative forcing histories, employing a hierarchy of models including one-dimensional energy-based models (Crowley, 2000) , two-dimensional reduced complexity models (Bertrand et al., 2002; Bauer et al., 2003; Gerber et al., 2003)
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From page 80... ...
Space-based observations are available for solar irradiance; volcanic aerosols; concentrations of ozone, CO2, other greenhouse gases, and CFCs; cloud cover and cloud properties; water vapor; land features, including snow cover, ice, and albedo; temperature of the ocean, land surface, and atmosphere; and other quantities relevant to radiative forcing. Indices are routinely produced, including for ENSO and the North Atlantic Oscillation, based on the extensive datasets and new analysis procedures that extract variability modes.
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From page 81... ...
Datasets spanning the past 25 years facilitate a comparison of empirical analysis and model simulations of radiative forcings and their effects on climate. Model experiments employing a combination of anthropogenic and volcanic radiative forcing best match the vertical pattern of temperature changes (Santer et al., 2000)
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From page 82... ...
Models are able to explain the cooling of the lower stratosphere in terms of stratospheric ozone loss; upper stratosphere cooling is due to a combination of ozone change and greenhouse gas increases.
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