ods were about 1.1°C (2.0°F) higher than today (Figure17.28). Notice that fairly small
temperature changes can have major effects on global climate. Over the last 900,000 years,
Earth’s average temperature has varied less than 5oC. Some scientists think that glaciers
will advance again, but not for thousands of years.
Since the end of the Pleistocene, the global average temperature has risen about 4°C (7°F).
Glaciers are retreating and sea level is rising. The climate has been relatively mild and stable
whencomparedwithmuchofEarth’shistory. Climatestabilityhasbeenbeneficialforhuman
civilization. Stability has allowed the expansion of agriculture and the development of towns
and cities. While climate is getting steadily warmer, there have been a few more extreme
warm and cool times in the last 10,000 years. The Medieval Warm Period from 900 to 1300
A.D. allowed Vikings to colonize Greenland and Great Britain to grow wine grapes. When
the climate cooled in the The Little Ice Age, from the 14thto 19thcenturies, the Vikings
were forced out of Greenland and humans had to plant crops further south.
Figure 17.28: The graph is a compilation of 10 reconstructions (the colored lines) of mean
temperature changes over the past 2,000 years, and one graph of instrumentally recorded
data of mean temperature changes (black). This illustrates the high temperatures of the
Medieval Warm Period, the lows of the Little Ice Age, and the very high (and climbing)
temperature of this decade. ( 15 )
Short-Term Climate Oscillations
Short-term changes in climate are common as conditions oscillate (or change) from one
state to another (Figure17.29). The largest and most important of these is the Southern
Oscillation betweenEl NiñoandLa Niñaconditions. This oscillation drives changes in
climate that are felt around the world about every two to seven years.