Scientists studying the effect of large volcanic eruptions on global climate
have long focused on the major quantities of carbon dioxide （C02), a gas known
to contribute to the greenhouse effect, produced by these eruptions. It is well
observed that such greenhouse gases trap heat radiated from the surface of the
（5) earth, thereby forming a type of insulation around the planet. The greenhouse
effect is essential for ecological equipoise because it maintains the temperature
of the planet within habitable parameters, but there is growing concern that
man-made production of gases such as CO[,2] from the burning of fossil fuels may
be threatening the system's tolerance, and have resulted in excessive warming
（10) on a global scale.
While volcanic eruptions indubitably metabolize and accumulate C0[,2] in the
atmosphere, it has been recently discovered that their impact is virtually trivial
compared to the quantity produced by human activities, especially heavy
industry. In reality, the more substantive climatic effect from volcanoes results
（15) from the production of atmospheric haze, whereby large eruption columns inject
ash particles and sulfur-rich gases into the troposphere and stratosphere, clouds
that circumscribe the globe within weeks of the volcanic activity. Ash and
aerosol clouds from large volcanic eruptions disseminate quickly through the
atmosphere, and the small ash particles decrease the amount of sunlight
（20) reaching the surface of the earth and lower average global temperatures, while
the sulfurous gases combine with water in the atmosphere to form. acidic
aerosols that also absorb incoming solar radiation and scatter it back out into
space.
There is evidence that volcanoes' stratospheric ash clouds has a lesser
（25) effect on global temperatures than aerosol clouds, given that the major Mt. St.
Helens eruption had lowered global temperatures by about 0.1 degree C, while
two years later the much smaller eruption of El Chico had, by contrast, three to
five times the global cooling effect worldwide. Despite its smaller ash cloud, El
Chico emitted more than 40 times the volume of sulfur-rich gases produced by
（30) Mt. St. Helens, revealing that the formation of atmospheric sulfur aerosols has
a more substantial effect on global temperatures than simply the volume of ash
produced during an eruption. Sulfate aerosols appear to necessitate several
years to settle out of the atmosphere, one of the reasons their effects are so I
widespread and enduring. This corroborates the opinion of those scientists who
（35) argue that without the cooling effect of major volcanic eruptions such as El
Chico, global warming effects caused by human activities would be far more
substantial. It should be noted that major volcanic eruptions have additional
climatic effects beyond global temperature decreases and acid rain, for ash and
aerosol particles suspended in the atmosphere scatter light of red wavelengths,
（40) often resulting in brilliantly colored sunsets and sunrises around the world.
According to the passage, scientists have decided that sulfur aerosols from volcanic eruptions affect global temperatures to a greater extent than ash clouds because
A．the greenhouse gas effect produced by human activities tends to have a much greater environmental effect than that of volcanic explosions
B．clouds of sulfur-rich gases tend to achieve greater heights in the stratosphere than do ash clouds
C．a particular explosion producing high sulfur and little ash had a greater environmental effect than the converse
D．the burning of fossil fuels tends to exacerbate the effects of sulfurous aerosols, but not those of ash clouds
E．global warming effects tend to minimize the impact of ash clouds, but not those of sulfurous aerosols