The last hundredyears 57Departures in temperature(
°C)
from the 1961–1990 average
0.8
0.6
0.4
0.2
0.0
−0.2
−0.4
−0.6
−0.8
1880 1900 1920 1940 1960 1980 2000
Year
Data from thermometersGLOBAL TEMPERATURE, 1861–2003
Figure 4.1Variations of the globally averaged Earth’s surface temperature over
the last 140 years. The dark bars are the year by year averaged values; the grey
line is a smoothed annual curve to illustrate decadal variations. Uncertainties in
the data are also shown; the thin whiskers represent the 95% confidence range.
The graph is based on an improved analysis of all years since the original
publication in the Third IPCC Scientific Assessment. The changes are small.
varies from place to place, from season to season and from day to day by
many tens of degrees. But here we are not considering changes in local
temperature but in the average over the whole globe. A change of a few
tenths of a degree in that average is a large change.
First of all, just how is a change in global average temperature es-
timated from a combination of records of changes in the near-surface
temperature over land and changes in the temperature of the sea surface?
To estimate the changes over land, weather stations are chosen where
consistent observations have been taken from the same location over a
substantial proportion of the whole 130-year period. Changesin sea sur-
face temperature have been estimated by processing over sixty million
observations from ships – mostly merchant ships – over the same period.
All the observations, from land stations and from ships, are then located
within a grid of squares, say 1◦of latitude by 1◦of longitude, covering
the Earth’s surface. Observations within each square are averaged; the
global average is obtained by averaging (after weighting them by area)
over the averages for each of the squares.