Overview of the various models for ice ages in the recent past (3 MYBP to present) - Ice Ages and Interglacials: Measurements, Interpretation, and Models - page 213

from the Greenland ice cores have been much sharper and more sudden than
slowly varying solar input indicates that solar input variability to high latitudes
cannot be the sole cause of climate fluctuations. Nevertheless, when comparison is
made between solar input variability to high latitudes and estimates of past glacial
ice volume from ocean sediments, there are some parallels between the two sets of
data. However, the translation of solar input variability to high latitudes (which
can be calculated accurately) into estimated changes in glacial ice volume is far
from straightforward, and only rudimentary models have been put forth up to this
point. Testing of these models is obfuscated in many cases by the use of the same
models to interpret the chronology of the data (tuning), thus introducing circular
reasoning.
The astronomical theory is the most widely used, discussed, and accepted of
theories for ice ages (it is discussed in greater detail in Chapter 9).
8.4 VOLCANISM
Volcanic eruptions have had a significant but not sustained effect on climate
(Rapp, 2008). Various indices classify volcanoes by size. In regard to climate
change induced by volcanic eruptions, the most important factor is radiative
forcing which is due to the injection of sulfurous material into the stratosphere.
This is not always directly related to the explosiveness of the eruption. For
example, the Mt. St. Helens eruption (1980) ejected material from the side of the
mountain and relatively little was injected into the stratosphere, so it had only a
minor effect on global climate. The Volcano Explosivity Index (VEI) is measured
in units from 1 to 8, with 8 being the most powerful. Each increase of 1 unit in the
index results in an increase of about a factor of 10 in the magma volume emitted.
The largest volcano in the past 250 years was Tambora (VEI 7) in 1815.
Volcanic eruptions can inject tens of teragrams of chemically and micro-
physically active gases and solid aerosol particles into the stratosphere. Large
volcanic eruptions inject sulfur gases into the stratosphere which are converted to
sulfate aerosols. These concentrations gradually diminish, typically dropping by a
factor of about 1/3 each year. Large ash particles fall out much more quickly. The
radiative and chemical effects of this aerosol cloud affect the climate system. By
scattering some solar radiation back to space, the aerosols cool the surface, but by
absorbing both solar and terrestrial radiation the aerosol layer tends to heat the
stratosphere. Because sulfate aerosol particles have an effective radius of about the
wavelength of visible light, they interact more strongly with shortwave incident
solar radiation than the longwave radiation emitted by the surface and atmo-
sphere. Some incident sunlight is back-scattered and reflected to space, cooling the
planet. Some is forward-scattered, depleting direct beam downward radiation, but
increasing downward diffuse radiation. Figure 8.1 shows the impact of two recent
volcanic eruptions on direct and diffuse irradiance at Mauna Loa, Hawaii a
considerable distance from the volcanoes. The major impact on solar irradiance
occurs in the first year, extending somewhat through the second year. The increase