Learning from the past Evidence from tree rings and other temperature prox-
ies (Figure 3.1) suggests that during the previous 500
The direct influence of variability of the sun’s radiation at the years global temperatures were 1.0ºC cooler than those
Earth’s surface is the major influence on the Earth’s climate of the 20th century during a period roughly from 1300
over a scale of hundreds of thousands of years. Long-term to 1870 – known as the Little Ice Age. While overall
variation in temperatures and CO
2
are inferred from Ant- temperatures during the Little Ice Age were cooler than
arctic ice cores (see the timeline on the inside back cover). now, there was much year-to-year variability and some
The last 10 000 years have been a warm period in the Earth’s warm periods
2
. The coldest part of the Little Ice Age,
history. Before then were the ice ages, each lasting approxi- from 1645 to 1715, was also a time of minimum sun
mately 100 000 years, with interglacial warm periods. The spots, referred to as the Maunder minimum. Although
timing of the ice ages is set by changes in solar radiation, there is a correspondence in time, the causal connection
amplified by CO
2
and water vapour changes and by the po- between sun variability and Earth climate is a subject of
sition of continents and oceans. These solar changes over ongoing debate. It is clear, however, that the 20th centu-
glacial time periods are caused by changes in the Earth’s ry was recovering from the average colder temperatures
orbit, and by the tilt and orientation of the Earth’s axis. of the 19th century and earlier.
Temperature Temperature
anomaly (ºC) anomaly (°C)
+0.5
more uncertain more accurate +2
0
+1
0
-0.5
-1
-1.0
-2
Year 1000 1200 1400 1600 1800 2000
1880 1900 1920 1940 1960 1980 2000
Tree rings Glacier lengths
Observed 10-year running mean
Multiple proxies
Instrument record
temperatures
(combination) Borehole temperatures
Figure 3.1: Global mean surface temperatures over previous Figure 3.2: Changes in Arctic mean annual land temperatures
centuries from various proxy records. Temperature estimates from 1880 through 2006. The zero line represents the average
before 1500 are considered much less reliable. temperature for 1961–1990.
Source: based on NRC 2006
3
Source: M. Wang ; data from CRU 2007
4
32 GLOBAL OUTLOOK FOR ICE AND SNOW