Page 330 - Earth's Climate Past and Future

306 PART V • Historical and Future Climate Change

deposited nearby. The geographic area over which the
ash is distributed may also provide clues about the
Sulfate aerosols in stratosphere forms only a small and variable fraction of the total vol-
power of the eruption and whether the volcanic parti-
cles might have reached the stratosphere. But sulfur
ume of erupted material, and the volume of ash cannot
directly be used to estimate the amount of sulfur
erupted. As a result, it is difficult to estimate the cli-
matic effects of ancient eruptions reliably.
Despite these obstacles, several attempts have been
made to estimate the effect of volcanic eruptions on the
northern hemisphere temperature trends plotted in
Figure 16–12, including the example shown in Figure
0 1 2 3 4 5
Years after eruption 16–16. Comparison of the volcanic and temperature
histories indicates that sequences of large eruptions
played a role in decadal-scale cooling.
Cooling due to sulfate aerosols 16-9 Greenhouse-Gas Effects on Climate
IN SUMMARY, the more frequent clusters of eruptions
after 1300 appear to have contributed to the small
cooling trend into the Little Ice Age interval.

Several high-resolution CO records from Antarctic
2
ice cores have been accurately dated based on the pres-
ence of numerous layers of volcanic ash of known age.
CO concentrations were relatively high (~283–284 ppm)
0 1 2 3 4 5 2
Years after eruption near 1000–1200 but had fallen to ~276–277 ppm by
1600–1800 (Figure 16–17). Some scientists have sug-
FIGURE 16-15 Volcanic explosions and cooling Large gested that solar-volcanic changes caused this 7–8 ppm
volcanic eruptions (top) launch sulfate aerosols into the CO drop by cooling the surface ocean (which increased
2
stratosphere and (bottom) cool climate for a few years. CO solubility) and by reducing the CO emitted from
2 2
(Adapted from R. S. Bradley, “The Explosive Volcanic Eruption the oxidation of litter on land. Carbon cycle models sug-
Signal in Northern Hemisphere Continental Temperature gest, however, that these natural explanations can account
Records,” Climate Change 12 [1988]: 221–43.) for only a small part (~2–3 ppm) of the CO decrease
2
without violating the small size of the ocean cooling.
Another proposed explanation of the CO drop is
2
that some or all of it was anthropogenic in origin. The

0

Solar reduction (W/m 2 ) –2 FIGURE 16-16 Volcanic

explosions and solar radiation An
estimate of the history of volcanic
–4
explosions during the last millennium
shows their decadal-scale (and
of incoming solar radiation. (Adapted
–6 longer) effect in reducing the intensity
from T. Crowley, et al., “Modeling
Ocean Heat Content Changes During
1000 1500 2000 the Last Millennium,” Geophysical
Year Research Letters 30 [2003]: GL017801).