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REPORTS - ASSESSMENT REPORTS |
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Synthesis Report - Question 2
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Climate Change 2001: Synthesis Report |
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Figure 2-3: The Earth's surface temperature
has increased by about 0.6 o C over the record of direct temperature measurements
(1860-2000, top panel) -- a rise that is unprecedented, at least
based on proxy temperature data for the Northern Hemisphere, over the last
millennium (bottom panel). In the top panel the global mean surfac
temperature is shown year-by-year (red bars with very likely ranges as
thin black whiskers)
and approximately decade-by-decade (continuous red line). Analyses take
into account data gaps, random instrumental errors and uncertainties, uncertainties
in bias corrections in the ocean
surface temperature data, and also in adjustments for urbanization over
the land. The lower panel merges proxy data (year-by-year blue line with
very likely ranges as grey band, 50-year-average purple line) and the direct
temperature measurements (red line) for the Northern Hemisphere. The proxy
data consist of tree rings, corals, ice cores, and historical records that
have been calibrated against thermometer data. Insufficient data are available
to assess such changes in the Southern Hemisphere.
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WGI TAR Figures SPM-1,
2-7c, & 2-20 |
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WGI TAR Figure 12-7 |
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There is new and stronger evidence that most of
the warming observed over the last 50 years is attributable to human
activities.
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| 2.10 |
The observed warming over
the 20th century is unlikely to be entirely natural in origin. The
increase in surface temperatures over the last 100 years is very unlikely
to be due to internal variability alone. Reconstructions of climate data
for the last 1,000 years also indicate that this 20th century warming was
unusual and unlikely to be the response to natural forcing alone: That is,
volcanic eruptions and variation in solar irradiance do not explain the
warming in the latter half of the 20th century (see Figure
2-4a), but they may have contributed to the observed warming in the
first half.
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WGI
TAR SPM & WGI
TAR Chapter 12 |
| 2.11 |
In the light of new evidence
and taking into account the remaining uncertainties, most of the observed
warming over the last 50 years is likely to have been due to the increase
in greenhouse gas concentrations. Detection and attribution studies
(including greenhouse gases and sulfate aerosols as anthropogenic forcing)
consistently find evidence for an anthropogenic signal in the climate record
of the last 35 to 50 years, despite uncertainties in forcing due to anthropogenic
sulfate aerosols and natural factors (volcanoes and solar irradiance). The
sulfate and natural forcings are negative over this period and cannot explain
the warming (see Figure 2-4a); whereas most of these
studies find that, over the last 50 years, the estimated rate and magnitude
of warming due to increasing greenhouse gases alone are comparable with,
or larger than, the observed warming (Figure 2-4b).
The best agreement for the 1860-2000 record is found when the above
anthropogenic and natural forcing factors are combined (see Figure
2-4c). This result does not exclude the possibility that other forcings
may also contribute, and some known anthopogenic factors (e.g., organic
carbon, black carbon (soot), biomass aerosols, and some changes in land
use) have not been used in these detection and attribution studies. Estimates
of the magnitude and geographic distribution of these additional anthropogenic
forcings vary considerably.
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WGI
TAR SPM & WGI
TAR Chapter 12 |
| 2.12 |
Changes in sea level, snow
cover, ice extent, and precipitation are consistent with a warming climate
near the Earth's surface (see Table 2-1). Some
of these changes are regional and some may be due to internal climate variations,
natural forcings, or regional human activities rather than attributed solely
to global human influence.
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WGI
TAR SPM & WGII
TAR Section 4.3.11 |
| 2.13 |
It is very likely that the
20th century warming has contributed significantly to the observed rise
in global average sea level and increase in ocean-heat content. Warming
drives sea-level rise through thermal expansion of seawater and widespread
loss of land ice. Based on tide gauge records, after correcting for land
movements, the average annual rise was between 1 and 2 mm during the 20th
century. The very few long records show that it was less during the 19th
century (see Figure 2-5). Within present
uncertainties, observations and models are both consistent with a lack of
significant acceleration of sea-level rise during the 20th century. The
observed rate of sea-level rise during the 20th century is consistent with
models. Global ocean-heat content has increased since the late 1950s, the
period with adequate observations of subsurface ocean temperatures.
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WGI TAR Sections 2.2.2.5,
11.2, & 11.3.2 |
| 2.14 |
Snow cover and ice extent
have decreased. It is very likely that the extent of snow cover has
decreased by about 10% on average in the Northern Hemisphere since the late
1960s (mainly through springtime changes over America and Eurasia) and that
the annual duration of lake- and river-ice cover in the mid- and high latitudes
of the Northern Hemisphere has been reduced by about 2 weeks over the 20th
century. There has also been a widespread retreat of mountain glaciers in
non-polar regions during the 20th century. It is likely that Northern Hemisphere
spring and summer sea-ice extent has decreased by about 10 to 15% from the
1950s to the year 2000 and that Arctic sea-ice thickness has declined by
about 40% during late summer and early autumn in the last 3 decades of the
20th century. While there is no change in overall Antarctic sea-ice extent
from 1978 to 2000 in parallel with global mean surface temperature increase,
regional warming in the Antarctic Peninsula coincided with the collapse
of the Prince Gustav and parts of the Larsen ice shelves during the 1990s,
but the loss of these ice shelves has had little direct impact.
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WGI
TAR Section 2.2.5 |
| 2.15 |
Precipitation has very likely
increased during the 20th century by 5 to 10% over most mid- and high latitudes
of the Northern Hemisphere continents, but in contrast, rainfall
has likely decreased by 3% on average over much of the subtropical land
areas (see Figure 2-6a). Increasing
global mean surface temperature is very likely to lead to changes in precipitation
and atmospheric moisture because of changes in atmospheric circulation,
a more active hydrologic cycle, and increases in the water-holding capacity
throughout the atmosphere. There has likely been a 2 to 4% increase in the
frequency of heavy precipitation events in the mid- and high latitudes of
the Northern Hemisphere over the latter half of the 20th century. There
were relatively small long-term increases over the 20th century in land
areas experiencing severe drought or severe wetness, but in many regions
these changes are dominated by inter-decadal and multi-decadal climate variability
with no significant trends evident over the 20th century. |
WGI TAR Sections 2.5, 2.7.2.2,
& 2.7.3 |
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