Geoscience Reference
In-Depth Information
value. CH 4 (g) mixing ratios were about 0.35 ppmv, 20
percent of their current value.
Figure 12.23 shows temperature changes from ice
core data in Greenland and the Antarctic during the past
20,000 years. Deglaciation from the last Ice Age caused
temperature to rise over Antarctica between 17,000 y.a.
and 10,000 y.a., with a hiatus between 13,500 y.a.
and 12,000 y.a. Temperatures increased over Green-
land started around 15,500 y.a. Northern Hemisphere
deglaciation was slow at first, but 14,500 y.a. an abrupt
increase in temperature hastened it. Around 12,800 y.a.,
temperature began to drop into ice age conditions again
until 11,500 y.a. This period of strong cooling is called
the Younger Dryas period ,where Dryas is the name
of an Arctic wildflower. The Younger Dryas cooling
period followed a shorter Older Dryas cooling period
from 14,000 to 13,700 y.a.
The Younger Dryas cooling may have been due
to changes in atmospheric circulation resulting from
large water releases from melting glaciers. During
glacial retreat, bursts of meltwater flooded the Columbia
Plateau in eastern Washington, the largest flood in
known history. Up to forty similar bursts may have
occurred during this period (Waitt, 1985). Some of the
bursts may have caused freshwater to flow northward
overland through Canada into the Arctic Ocean, before
flowing to the North Atlantic Ocean.
Normally, warm Gulf Stream ocean water head-
ing northeastward from the Gulf of Mexico along the
Atlantic Coast sinks when it reaches the North Atlantic
Ocean. The reason is that high winds in the North
Atlantic evaporate water, increasing the saltiness and
density of remaining ocean water, causing it to sink
faster. Fast-sinking water in the North Atlantic pulls
more Gulf Stream water northeastward. The injec-
tion of fresh water into the North Atlantic during the
Younger Dryas had the opposite effect, decreasing the
density and inhibiting the sinking of ocean water, slow-
ing the Gulf Stream. Because the Gulf Stream current
is responsible for transporting enough warm water to
northern Europe to keep it warmer than the same lati-
tudes over North America, the weakening of the current
caused an average Northern Hemisphere cooling. The
ice core record does not show a similar cooling signal in
Antarctica, due to the distance of Antarctica from this
event.
Following the Younger Dryas period, temperatures
increased again. Most of the Northern Hemisphere ice
sheets disappeared by 9,000 y.a., although remnants
of the Laurentide ice sheet remained until 6,000 y.a. In
sum, the bulk of the two major ice sheets in the Northern
Hemisphere disappeared over a period of 5,000 years,
from 14,000 to 9,000 y.a.
12.3.2.12. The Holocene Epoch
The time between 11,700 y.a. and the present is the
Holocene epoch .From10,000 to 5,000 y.a., tem-
peratures were warm. During this warm period,
humans developed agriculture and domesticated ani-
mals (
8,000 y.a.). The mid-Holocene maximum in
temperatures occurred 6,000 to 5,000 y.a., during which
Northern Hemisphere temperatures were
1 Cwarmer
than at the end of the ice core record. Corresponding
mid-Holocene warming over the Antarctic is less obvi-
ous (Figure 12.23).
Following the mid-Holocene, Northern Hemisphere
temperatures declined. However, between 950 AD and
1250 AD, they increased again to their greatest peak
since the mid-Holocene but lower than those today.
During this relatively warm period, called the medieval
climate optimum ,Arctic ice retreated, Iceland and
Greenland were colonized by the Vikings, alpine passes
between Germany and Italy became ice free, and grapes
were harvested for wine production in England (Le Roy
Ladurie, 1971; Crowley and North, 1991).
Between 1450 and 1890 AD, temperatures in North
America, particularly in Europe, decreased, and mild
glaciation reappeared. This period is the Little Ice Age
(Matthes, 1939). The lowest temperatures during the
Little Ice Age were in the mid- to late 1600s, early
1800s, and late 1800s (Crowley and North, 1991). Dur-
ing the Little Ice Age, glaciers advanced in the European
Alps; Sierra Nevada Mountains; Rocky Mountains;
Himalayas; southern Andes Mountains; and mountains
in eastern Africa, New Guinea, and New Zealand. The
area of sea ice around Iceland also increased (Bergth-
orsson, 1969). In addition, from 1607 to 1814, the
River Thames froze every winter. In 1658, the Swedish
Army marched across the frozen Oresund to invade
Copenhagen. In 1780, New York Harbor froze, allow-
ing pedestrians to walk from Manhattan to Staten Island.
In 1794, the French Army took the Netherlands, whose
fleet of ships was locked in ice.
Although temperatures decreased over much of the
world, temperatures over the Antarctic warmed during
part of the Little Ice Age, as seen in Figure 12.24.
Temperature decreases during the Little Ice Age were
smaller in magnitude than those during the Younger
Dryas period.
From April 5 to 12, 1815, during the Little Ice Age,
the most deadly volcano in human history, Ta mbora ,
Indonesia,
erupted,
killing
an
estimated
92,000
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