models of future climate conditions, or General Circula-
tion Models (GCMs). As with all models, GCMs are based
on a range of variables, none of which can be known
with absolute certainty. Consequently, modelers typically
present ranges of expected outcomes, based on what
they know and what they consider most likely to be true.
Computer models of weather changes caused by global
climate change indicate that precipitation patterns
will change, causing some areas to have more frequent
droughts. At the same time, heavier snow and rainstorms
may cause more frequent flooding in other areas. These
changes could lower the availability and quality of fresh
water in many locations, particularly in areas that are
currently arid or semiarid, such as the Sahel region just
south of the Sahara Desert.
Global climate change will have mixed effects on
agriculture. The rise in sea level will inundate some
river deltas, which are fertile agricultural lands. Certain
agricultural pests and disease-causing organisms will
probably proliferate and reduce crop yields. Increased
frequency and duration of droughts will be a particu-
larly serious problem, and lack of water for drinking
and agriculture may force millions of people to relocate.
However, agricultural productivity may increase in some
areas.Effects of Global Climate Change
One clearly observed effect of climate change to date
has been melting land and ocean ice. The ice cap at
the North Pole, for example, has decreased consis-
tently over the past three decades (Figure 9.10). Other
observed and potential effects of global climate change
include s ea-level rise, changes in precipitation patterns,
and impacts on agriculture, human health, and other
organisms.
Two factors contribute to sea-level rise. First, as the
Antarctic ice cap and continental glaciers melt, the
amount of water in the ocean increases. Second, as water
heats up, it expands. During the 20th century, the sea level
rose 10 to 20 cm (4 to 8 in). Climate scientists estimate
an additional 20 to 50 cm (7.9 to 20 in) rise by 2100. As
sea level rises, small island nations such as the Maldives,
a low-lying chain of islands in the Indian Ocean, will be
increasingly vulnerable to saltwater intrusion and storm
surges (Figure 9.11).
Predicted sea-level rise, as with predicted tempera-
ture and precipitation changes, is based on computer
James L. Stanfield/National Geographic Stock ÜÞ}ÊÃ>`ÊUÊ}ÕÀiÊ°££Ê
Residents of this low-lying island, which is part of the Maldives (in the
Indian Ocean), may need to relocate as sea level continues to rise.Arctic Sea Ice Volume, 1979–2011
UÊ}ÕÀiÊ°£äÊ
This figure depicts seasonal (winter high and summer low) extent
of polar ice cap volume. Some models predict that the North Pole
will be clear of ice during summers as early as 2020.
35,000
30,00025,000
20,00015,00010,000
5,0000,000
1980 1985 1990 1995 2000 2005 2010 2015 2020Ice volume in 1000 km3
Year
Based on data from United States National Snow and Ice Data Center, reported in- Photo from Sue Flood/Stone/Getty Images
Interpreting Data
What is the overall trend of ice cap volume from 1980
to 2011? What causes the variability over the course of
each year?226 CHAPTER 9 Global Atmospheric Changes