Figure 1 Mean annual
temperature variations
over the northern
hemisphere for the last
1000 years
Table 1 Climate scenarios
for the 20th and 21st
centuries
Observations for the 20th century
• The global average surface temperature has increased by 0.6 °C
• Greater increases have been observed in night-time versus daytime
temperatures
• Global average sea level rose 0.1—0.2 m, largely due to thermal expansion
• The atmospheric concentrations of carbon dioxide (CO
) and methane (CH
)
have increased 31% and 151% since 1750
• Precipitation in the Northern Hemisphere (where data are reliable) increased
by 1% per decade, and the frequency of heavy precipitation events increased
by 2—4% in the century
• Warm episodes of the El Nin o—Southern Oscillation phenomenon have been
more frequent, persistent and intense since the 1970s compared to the previous
100 years
• Arctic sea-ice thickness during late summer to early autumn has decreased by
up to 40%. Global snow cover decreased by about 10% since 1960
Predictions for the 21st century
• Climate models predict that the mean annual global surface temperature will
increase by 1.4—5.8 °C over the period 1990—2100, a rate higher than the
warming rate experienced in the 20th century
• Warming is likely to be more pronounced over land areas, particularly in high
latitudes and in the cold season
• Most models predict a weakening but not a shutdown of the thermohaline
circulation by 2100. Shutdown may occur beyond 2100, perhaps irreversibly
• Thermal expansion and loss of ice mass suggest that sea level will rise between
0.09 and 0.88 m by 2100
• The Antarctic ice sheet is likely to gain mass due to greater precipitation,
while the Greenland ice mass will decrease because of increased run off
direct as well as indirect effects, and the overall response may often be unexpected,
and geographically uneven.
However, if global change research has been conclusive at all it has been to
acknowledge that the planet has a tremendous natural variability, and that
M. Barange
58
temperature variations
over the northern
hemisphere for the last
1000 years
Table 1 Climate scenarios
for the 20th and 21st
centuries
Observations for the 20th century
• The global average surface temperature has increased by 0.6 °C
• Greater increases have been observed in night-time versus daytime
temperatures
• Global average sea level rose 0.1—0.2 m, largely due to thermal expansion
• The atmospheric concentrations of carbon dioxide (CO
) and methane (CH
)
have increased 31% and 151% since 1750
• Precipitation in the Northern Hemisphere (where data are reliable) increased
by 1% per decade, and the frequency of heavy precipitation events increased
by 2—4% in the century
• Warm episodes of the El Nin o—Southern Oscillation phenomenon have been
more frequent, persistent and intense since the 1970s compared to the previous
100 years
• Arctic sea-ice thickness during late summer to early autumn has decreased by
up to 40%. Global snow cover decreased by about 10% since 1960
Predictions for the 21st century
• Climate models predict that the mean annual global surface temperature will
increase by 1.4—5.8 °C over the period 1990—2100, a rate higher than the
warming rate experienced in the 20th century
• Warming is likely to be more pronounced over land areas, particularly in high
latitudes and in the cold season
• Most models predict a weakening but not a shutdown of the thermohaline
circulation by 2100. Shutdown may occur beyond 2100, perhaps irreversibly
• Thermal expansion and loss of ice mass suggest that sea level will rise between
0.09 and 0.88 m by 2100
• The Antarctic ice sheet is likely to gain mass due to greater precipitation,
while the Greenland ice mass will decrease because of increased run off
direct as well as indirect effects, and the overall response may often be unexpected,
and geographically uneven.
However, if global change research has been conclusive at all it has been to
acknowledge that the planet has a tremendous natural variability, and that
M. Barange
58