Page 329 - Earth's Climate Past and Future

CHAPTER 16 • Climate Changes During the Last 1000 Years 305

irradiance during the last few centuries. In addition, 16-8 Volcanic Explosions
because these intervals of low sunspot activity persisted
for decades, the slow-responding parts of the climate Explosive eruptions of volcanoes cool climate over
system would have had time to respond to them more intervals of a few years. Volcanoes erupt sulfur dioxide
fully compared to the low values during the 11-year (SO ) gas, which mixes with water vapor in the air and
2
cycle. This link seemed particularly appealing because forms droplets and particles of sulfuric acid called sul-
the sunspot minima occurred at times when northern fate aerosols. Highly explosive eruptions can reach 20
hemisphere temperatures were considerably cooler than to 30 km into the stratosphere, where the aerosols block
they are today. As a result, some scientists suggested that some incoming solar radiation and keep it from reach-
changes in solar irradiance accounted for 50–75% of the ing the ground. With solar radiation reduced, Earth’s
0.6°C increase in temperature during the 1900s. surface cools.
Later astronomical observations failed to support this Latitude determines the geographic extent of the
claim. Archival images of Sunlike stars from several obser- impact of volcanic eruptions. Volcanoes that erupt pole-
vatories failed to show variations comparable to those pro- ward of about 25° produce particles that stay within the
posed for the Sun. No evidence for variations greater than hemisphere in which the eruption occurs, and the cool-
those at the 11-year cycle has been detected. Recent esti- ing impact is limited to that hemisphere. Explosions
mates place the contribution from solar irradiance that occur in the tropics are redistributed by Earth’s
changes since 1880 at less than 0.07°C, or about 10% of atmosphere to both hemispheres and have a global
the amount of the 0.7°C warming shown in Figure 16–12. impact on climate.
This finding has not ended the debate about possible Ocean-island volcanoes with iron- and magnesium-
solar effects on climate during this interval. Although rich compositions tend not to cause explosive eruptions
most of the Sun’s emissions arrive as visible or near-visi- but instead emit lava that flows easily across the land. Vol-
ble (ultraviolet and infrared) radiation, the Sun also sends canic particles sent into the air by these eruptions rarely
out a plasma or ionized gas called the “solar wind,” which reach the stratosphere but stay within the troposphere.
interacts with Earth’s stratosphere as it is deflected by Within a few days, the particles are brought back down to
Earth’s magnetic field. One possibility under considera- Earth by rain. With so brief a stay in the atmosphere, the
tion is that the solar wind affects the formation of ozone, particles cannot be widely enough distributed around the
which in turn alters the formation of clouds in the tro- planet to produce large-scale effects on climate.
posphere and thereby affects climate at Earth’s surface. In contrast, volcanoes along converging plate mar-
14
10
The similarity of C and Be trends during the last gins are fed by magmas richer in silica and other ele-
5000 years seems to point to a common origin from solar ments found in continental crust (Chapter 4). Their
changes (see Chapter 14). Scientists have looked for a cor- eruptions are more explosive because the natural resis-
relation between these isotopic trends and climatic proxies tance of this kind of molten magma to flow causes inter-
during recent millennia with only mixed success. Temper- nal pressures to build up to the point where volcanic par-
18
ature-sensitive changes in δ O within the last millennium ticles can be injected into the stratosphere, well above the
10
show a substantial correlation with Be variations, and the level where precipitation can wash them out. Because of
existence of this link would seem to imply a solar role. On the pull of gravity, slow settling of these fine particles
the other hand, the fact that no such relationship is evi- takes years, long enough for the particles to be distrib-
dent in previous millennia of the Holocene greatly weak- uted within a hemisphere or across the entire planet.
ens this case. The possibility also remains that the similar Sulfate aerosol concentrations in the stratosphere reach
changes in C and Be are both a response to changes their maximum regional distribution within months and
10
14
occurring within the climate system (changes in ocean cir- then begin to decrease as gravity removes the particles. The
culation, rate of ice accumulation, etc). decrease follows an exponential trend: each year about half
of the remaining particles settle out, and within two to
three years aerosol concentrations are much reduced (Fig-
IN SUMMARY, changes in solar irradiance featured
prominently in several initial attempts to explain ure 16–15 top). The effect of the aerosols on temperature
climatic trends during the last millennium, but follows the same trend, with a maximum initial cooling that
recent evidence suggests that irradiance changes soon fades away (Figure 16–15 bottom). If several explo-
were small enough to have had little impact on sions follow within an interval of a few years, their impact
climate during this interval. Still, the possibility on climate may be sustained for a decade or more.
exists that even very small changes in irradiance Climate scientists face difficulties in trying to recon-
might “excite” internal oscillations in naturally struct the effects of older volcanic explosions on cli-
varying parts of the climate system like ENSO or mate. They may get some idea about the magnitude of
the North Atlantic Oscillation (see Chapter 17). ancient eruptions from the sizes of the craters left by
the explosion or from the volume of volcanic ash