Figure 1: Variations of
the Earth's surface temperature over the last 140 years and the last millennium.
(a) The Earth's surface temperature is shown year by year (red bars) and
approximately decade by decade (black line, a filtered annual curve suppressing
fluctuations below near decadal
time-scales). There are uncertainties in the annual data (thin black whisker bars
represent the 95% confidence range) due to data gaps, random instrumental errors
and uncertainties, uncertainties in bias corrections in the ocean surface temperature
data and also in adjustments for urbanisation over the land. Over both the last
140 years and 100 years, the best estimate is that the global average surface
temperature has increased by 0.6 ± 0.2°C.
(b) Additionally, the year by year (blue curve) and 50 year average (black curve)
variations of the average surface temperature of the Northern Hemisphere for the
past 1000 years have been reconstructed from "proxy" data calibrated
against thermometer data (see list of the main proxy data in the diagram). The
95% confidence range in the annual data is represented by the grey region. These
uncertainties increase in more distant times and are always much larger than in
the instrumental record due to the use of relatively sparse proxy data. Nevertheless
the rate and duration of warming of the 20th century has been much greater than
in any of the previous nine centuries. Similarly, it is likely7 that the 1990s
have been the warmest decade and 1998 the warmest year of the millennium.
[Based upon (a) Chapter 2, Figure 2.7c and (b) Chapter
2, Figure 2.20]