Page 340 - Earth's Climate Past and Future

316 PART V • Historical and Future Climate Change

may respond to climate at rates that range from about 1500
a decade to as much as several hundred years. The
response times of most mountain glaciers fall in the 1000
range of 10–40 years.
Despite this wide range of possible responses, histor-
ical observations of the lower limits of glaciers between Length relative to year 1950 (m) 500
1860 and 1900 show that 35 of 36 glaciers examined
were already in retreat. More recent studies of glacier
lengths have been supplemented in some cases by analy- 0
ses of glacier thickness that permit calculations of full
glacier volume (Figure 17–11). Between 1900 and 1980, –500
142 of 144 glaciers analyzed retreated (Figure 17–12). 1700 1800 Year 1900 2000
The average retreat of all glaciers between 1850 and
2000 was ~1750 m, or just over a mile. The energy used FIGURE 17-12 Retreat of mountain glaciers since the
to melt these ice sheets (and polar sea ice) also used up a 1800s Mountain glaciers around the world have retreated by
fraction of the excess heat generated during the indus- an average length of more than 1.5 km since the 1800s.
trial era, but it used far less than was stored in the deep (Adapted from J. Oerlemans, “Extracting a Climate Signal from
ocean. 169 Glacier Records,” Science 308 [2005]: 675–77.)
Exceptions to this general pattern of retreat exist.
Some glaciers in the mountains of Norway advanced
during the 1960s and 1970s during an interval of cool-
ing in the Norwegian Sea, but the prevailing trend dur- 1900s have been both global in scale and of the right
ing the twentieth century has been melting. In recent magnitude to explain the glacial melting.
decades, the rate of melting has accelerated for many
glaciers. All tropical mountain glaciers studied are in 17-5 Ground Temperature
retreat, and some have disappeared entirely. Heat probes inserted into soils or bedrock can measure
This pervasive, near-global retreat cannot be past changes in temperature that have slowly penetrated
explained by reduced precipitation. This explanation from the atmosphere and ground surface into subsur-
would require an average drop in precipitation of 25% face layers. These profiles are sensitive to longer-term
in many sites across the globe. Instrumental evidence (century-scale) temperature changes at the surface but
indicates that precipitation changes in most regions are much less so to shorter, decadal-scale variations. Sub-
much smaller than 25%, with increases in some glacier surface temperature records have been taken at hun-
areas and reductions in others. In contrast, the temper- dreds of stations in both hemispheres (Figure 17–13A),
ature increases observed during the late 1800s and the and most profiles show warmer temperatures in the
near-surface layers than a few tens of meters below. The
measurements indicate that a warming has occurred
at the surface in the last century or two and that it is
in the process of penetrating to deeper layers. This
warming of subsurface continental areas also used up a
small fraction of the excess heat generated during the
industrial era.
Models that simulate the penetration of the tempera-
ture anomalies beneath the surface indicate that the
warming during the last two centuries lies at the upper
end of the range of surface temperature reconstructions
based on climatic proxies (Figure 17–13B). This match in
part reflects the fact that both the ground temperature
profiles and the proxy reconstructions showing larger
variations tend to be based on sites in extratropical
latitudes, where temperature responses are larger than
the global average. Other complications with ground
Melting glaciers
temperature profiles include depth of snow, which may
FIGURE 17-11 Locations of retreating mountain glaciers shield the ground from extreme temperature changes in
Mountain glaciers in many regions retreated during the 1900s winter, and clearance of forests, which can cool local
(Adapted from M. F. Meier, “Contribution of Small Glaciers to Sea temperatures because of the higher albedo of open land
Level,” Science 226 [1984]: 1418–21.) surfaces.