Summary for Policymakers
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
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
The Third Assessment Report
of Working Group I of the Intergovernmental Panel on Climate Change
(IPCC) builds upon past assessments and incorporates new results
from the past five years of research on climate change1.
Many hundreds of scientists2
from many countries participated in its preparation and review.
This Summary for Policymakers (SPM), which was approved by IPCC
member governments in Shanghai in January 20013,
describes the current state of understanding of the climate system
and provides estimates of its projected future evolution and their
uncertainties. Further details can be found in the underlying
report, and the appended Source Information provides cross references
to the report's chapters.
An increasing body of observations gives a collective picture
of a warming world and other changes in the climate system.
Since the release of the Second Assessment Report (SAR4),
additional data from new studies of current and palaeoclimates,
improved analysis of data sets, more rigorous evaluation of their
quality, and comparisons among data from different sources have
led to greater understanding of climate change.
The global average surface temperature has increased over
the 20th century by about 0.6°C.
- The global average surface temperature (the average of near
surface air temperature over land, and sea surface temperature)
has increased since 1861. Over the 20th century the increase
has been 0.6 ± 0.2°C5,
(Figure 1a). This value
is about 0.15°C larger than that estimated by the SAR for
the period up to 1994, owing to the relatively high temperatures
of the additional years (1995 to 2000) and improved methods
of processing the data. These numbers take into account various
adjustments, including urban heat island effects. The record
shows a great deal of variability; for example, most of the
warming occurred during the 20th century, during two periods,
1910 to 1945 and 1976 to 2000.
- Globally, it is very likely7
that the 1990s was the warmest decade and 1998 the warmest year
in the instrumental record, since 1861 (see Figure
- New analyses of proxy data for the Northern Hemisphere indicate
that the increase in temperature in the 20th century is likely7
to have been the largest of any century during the past 1,000
years. It is also likely7
that, in the Northern Hemisphere, the 1990s was the warmest
decade and 1998 the warmest year (Figure
1b). Because less data are available, less is known about
annual averages prior to 1,000 years before present and for
conditions prevailing in most of the Southern Hemisphere prior
- On average, between 1950 and 1993, night-time daily minimum
air temperatures over land increased by about 0.2°C per
decade. This is about twice the rate of increase in daytime
daily maximum air temperatures (0.1°C per decade). This
has lengthened the freeze-free season in many mid- and high
latitude regions. The increase in sea surface temperature over
this period is about half that of the mean land surface air
Temperatures have risen during the past four decades in the
lowest 8 kilometres of the atmosphere.
- Since the late 1950s (the period of adequate observations
from weather balloons), the overall global temperature increases
in the lowest 8 kilometres of the atmosphere and in surface
temperature have been similar at 0.1°C per decade.
- Since the start of the satellite record in 1979, both satellite
and weather balloon measurements show that the global average
temperature of the lowest 8 kilometres of the atmosphere has
changed by +0.05 ± 0.10°C per decade, but the global
average surface temperature has increased significantly by +0.15
± 0.05°C per decade. The difference in the warming
rates is statistically significant. This difference occurs primarily
over the tropical and sub-tropical regions.
- The lowest 8 kilometres of the atmosphere and the surface
are influenced differently by factors such as stratospheric
ozone depletion, atmospheric aerosols, and the El Niño
phenomenon. Hence, it is physically plausible to expect that
over a short time period (e.g., 20 years) there may be differences
in temperature trends. In addition, spatial sampling techniques
can also explain some of the differences in trends, but these
differences are not fully resolved.
Snow cover and ice extent have decreased.
- Satellite data show that there are very likely7
to have been decreases of about 10% in the extent of snow cover
since the late 1960s, and ground-based observations show that
there is very likely7
to have been a reduction of about two weeks in the annual duration
of lake and river ice cover in the mid- and high latitudes of
the Northern Hemisphere, over the 20th century.
- There has been a widespread retreat of mountain glaciers in
non-polar regions during the 20th century.
- Northern Hemisphere spring and summer sea-ice extent has decreased
by about 10 to 15% since the 1950s. It is likely7
that there has been about a 40% decline in Arctic sea-ice thickness
during late summer to early autumn in recent decades and a considerably
slower decline in winter sea-ice thickness.
Global average sea level has risen and ocean heat content has
- Tide gauge data show that global average sea level rose between
0.1 and 0.2 metres during the 20th century.
- Global ocean heat content has increased since the late 1950s,
the period for which adequate observations of sub-surface ocean
temperatures have been available.