The best estimate of global surface temperature change is a 0.6°C increase
since the late 19th century with a 95% confidence interval of 0.4 to 0.8°C.
The increase in temperature of 0.15°C compared to that assessed in the IPCC
WGI Second Assessment Report (IPCC, 1996) (hereafter SAR) is partly due to the
additional data for the last five years, together with improved methods of analysis
and the fact that the SAR decided not to update the value in the First Assessment
Report, despite slight additional warming. It is likely that there have been
real differences between the rate of warming in the troposphere and the surface
over the last twenty years, which are not fully understood. New palaeoclimate
analyses for the last 1,000 years over the Northern Hemisphere indicate that
the magnitude of 20th century warming is likely to have been the largest of
any century during this period. In addition, the 1990s are likely to have been
the warmest decade of the millennium. New analyses indicate that the global
ocean has warmed significantly since the late 1940s: more than half of the increase
in heat content has occurred in the upper 300 m, mainly since the late 1950s.
The warming is superimposed on strong global decadal variability. Night minimum
temperatures are continuing to increase, lengthening the freeze-free season
in many mid- and high latitude regions. There has been a reduction in the frequency
of extreme low temperatures, without an equivalent increase in the frequency
of extreme high temperatures. Over the last twenty-five years, it is likely
that atmospheric water vapour has increased over the Northern Hemisphere in
many regions. There has been quite a widespread reduction in daily and other
sub-monthly time-scales of temperature variability during the 20th century.
New evidence shows a decline in Arctic sea-ice extent, particularly in spring
and summer. Consistent with this finding are analyses showing a near 40% decrease
in the average thickness of summer Arctic sea ice over approximately the last
thirty years, though uncertainties are difficult to estimate and the influence
of multi-decadal variability cannot yet be assessed. Widespread increases are
likely to have occurred in the proportion of total precipitation derived from
heavy and extreme precipitation events over land in the mid- and high latitudes
of the Northern Hemisphere.
Changes in Temperature and Related Variables
Changes in near-surface temperature from the instrumental record
- Average global surface temperature has increased by approximately 0.6°C
since the late 19th century, with 95% confidence limits of close to 0.4 and
0.8°C. Most of this increase has occurred in two periods, from about 1910
to 1945 and since 1976, and the largest recent warming is in the winter extra-tropical
Northern Hemisphere. The warming rate since 1976, 0.17°C/decade, has been
slightly larger than the rate of warming during the 1910 to 1945 period (0.14°C/decade),
although the total increase in temperature is larger for the 1910 to 1945
period. The most recent warming period also has a faster rate of warming over
land compared with the oceans. The high global temperature associated with
the 1997/98 El Niño event stands out in both surface and tropospheric
temperatures as an extreme event, even after consideration of the recent rate
- Confidence in the magnitude of global warming since the late 19th century
has increased since the SAR due to new analyses, including model simulations
of land-surface temperature changes and new studies of the effect of urbanisation
on global land temperature trends. There is a high level of consistency between
changes in sea surface temperatures (SSTs) and near-surface land air temperatures
across the land-ocean boundary over the 20th century, despite independent
observing systems and independent bias correction factors for SSTs before
1942. The assessed warming is considerably larger than the total contributions
of the plausible sources of error.
- Twentieth century temperature trends show a broad pattern of tropical warming,
while extra-tropical trends have been more variable. Warming from 1910 to
1945 was initially concentrated in the North Atlantic and nearby regions.
The Northern Hemisphere shows cooling during the period 1946 to 1975 while
the Southern Hemisphere shows warming. The recent 1976 to 2000 warming was
largely globally synchronous, but emphasised in the Northern Hemisphere continents
during winter and spring, with year-round cooling in parts of the Southern
Hemisphere oceans and Antarctica. North Atlantic cooling between about 1960
and 1985 has recently reversed. Overall, warming over the Southern Hemisphere
has been more uniform during the instrumental record than that over the Northern
- The patterns of global temperature change since the 1970s are related in
part to the positive westerly phase of the North Atlantic/Arctic Oscillation
and possibly to decadal to multi-decadal variability in the Pacific.
- A multi-decadal fluctuation of SST in the North Atlantic has been in a rising
phase since about the mid-1980s. Warming in many regions of this ocean has
accelerated over the last five years and is likely to have contributed to
quite rapid parallel increases of near-surface air temperature in much of
- New analysis shows that the global ocean heat content has increased since
the late 1950s. This increase is superimposed on substantial global decadal
variability. More than half the heating is contained in the uppermost 300
m where it is equivalent to an average temperature increase of 0.037°C/decade.
- Analyses of mean daily maximum and minimum land surface air temperatures
continue to support a reduction in the diurnal temperature range in many parts
of the world, with, globally, minimum temperatures increasing at nearly twice
the rate of maximum temperatures between about 1950 and 1993. The rate of
temperature increase during this time has been 0.1°C and 0.2°C/decade
for the maximum and minimum, respectively. This is more than twice the rate
of temperature increase over the oceans during this time.
Changes in temperature-related variables
- Alpine and continental glaciers have extensively retreated in response to
20th century warming. Glaciers in a few maritime regions are advancing, mainly
due to increases in precipitation related to atmospheric circulation changes,
e.g., Norway, New Zealand.
- The duration of Northern Hemisphere lake-ice and river-ice cover over the
past century, or more, shows widespread decreases averaging to about two fewer
weeks of ice cover.
- There is a highly significant interannual (+0.6) and multi-decadal correlation
between increases in the Northern Hemisphere spring land temperature and a
reduction in the Northern Hemisphere spring snow cover since data have been
available (1966). Snow cover extent has decreased by about 10% since 1966.
- A 10 to 15% reduction in sea-ice extent in the Arctic spring and summer
since the 1950s is consistent with an increase in spring, and to a lesser
extent, summer temperatures in the high latitudes. There is little indication
of reduced Arctic sea-ice extent during winter when temperatures have increased
in the surrounding region.
- New data from submarines indicate that there has been about a 40% decline
in Arctic sea-ice thickness in summer or early autumn between the period 1958
to 1976 and the mid-1990s, an average of near 4 cm per year. Other independent
observations show a much slower decrease in winter sea-ice thickness of about
1 cm per year. The influence of substantial interannual and inter-decadal
variability on these changes cannot be assessed because of restricted sampling.
- By contrast, there is no readily apparent relationship between decadal changes
in Antarctic temperatures and sea-ice extent since 1973. Satellite data indicate
that after a possible initial decrease in the mid-1970s, Antarctic sea-ice
extent has stayed almost stable or even increased since 1978.
Changes in temperature above the surface layer
- Analysis of global temperature trends since 1958 in the low to mid-troposphere
from balloons shows a warming of about +0.1°C/decade, which is similar
to the average rate of warming at the surface. Since the early 1960s no significant
trends have been detected for the global mean temperature in the uppermost
- Satellites have only been available since 1979. Between 1979 and 2000, based
on satellites and balloons, the lower-tropospheric trend has been +0.04 ±
0.11°C/decade and 0.03 ± 0.10°C/decade, respectively. By contrast,
surface temperature trends for 1979 to 2000 were greater, at 0.16 ±
0.06°C/decade. The trend in the difference of the surface and lower-tropospheric
series of 0.13 ± 0.06°C/decade is clearly statistically significant.
This is in contrast to near zero surface temperature trends over 1958 to 1978
when the global lower-tropospheric temperature warmed by 0.03°C/decade
relative to the surface.
- It is very likely that these significant differences in trends between the
surface and lower troposphere are real and not solely an artefact of measurement
bias, though differences in spatial and temporal sampling are likely to contribute.
The differences are particularly apparent in many parts of the tropics and
sub-tropics where the surface has warmed faster than the lower troposphere.
In some other regions, e.g., North America, Europe and Australia, lower-tropospheric
and surface trends are very similar.
- Throughout the stratosphere, negative temperature trends have been observed
since 1979, ranging from a decrease of 0.5 or 0.6°C/decade in the lower
stratosphere to 2.5°C/decade in the upper stratosphere.