Climate Change 2001:
Synthesis Report
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Question 2

What is the evidence for, causes of, and consequences of changes in the Earth's climate since the pre-industrial era?

  1. Has the Earth's climate changed since the pre-industrial era at the regional and/or global scale? If so, what part, if any, of the observed changes can be attributed to human influence and what part, if any, can be attributed to natural phenomena? What is the basis for that attribution?
  2. What is known about the environmental, social, and economic consequences of climate changes since the pre-industrial era with an emphasis on the last 50 years?

The Earth's climate system has demonstrably changed on both global and regional scales since the pre-industrial era, with some of these changes attributable to human activities.



Human activities have increased the atmospheric concentrations of greenhouse gases and aerosols since the pre-industrial era. The atmospheric concentrations of key anthropogenic greenhouse gases (i.e., carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), and tropospheric ozone (O3)) reached their highest recorded levels in the 1990s, primarily due to the combustion of fossil fuels, agriculture, and land-use changes (see Table SPM-1). The radiative forcing from anthropogenic greenhouse gases is positive with a small uncertainty range; that from the direct aerosol effects is negative and smaller; whereas the negative forcing from the indirect effects of aerosols on clouds might be large but is not well quantified.


An increasing body of observations gives a collective picture of a warming world and other changes in the climate system (see Table SPM-1). Q2.6
Globally it is very likely that the 1990s was the warmest decade, and 1998 the warmest year, in the instrumental record (1861-2000) (see Box SPM-1). The increase in surface temperature over the 20th century for the Northern Hemisphere is likely to have been greater than that for any other century in the last thousand years (see Table SPM-1). Insufficient data are available prior to the year 1860 in the Southern Hemisphere to compare the recent warming with changes over the last 1,000 years. Temperature changes have not been uniform globally but have varied over regions and different parts of the lower atmosphere. Q2.7
Table SPM-1: 20th century changes in the Earth's atmosphere, climate, and biophysical system.a
Indicator Observed Changes
Concentration indicators  
Atmospheric concentration of CO2 280 ppm for the period 1000-1750 to 368 ppm in year 2000 (31±4% increase).
Terrestrial biospheric CO2 exchange Cumulative source of about 30 Gt C between the years 1800 and 2000; but during the 1990s, a net sink of about 14±7 Gt C.
Atmospheric concentration of CH4 700 ppb for the period 1000-1750 to 1,750 ppb in year 2000 (151±25% increase).
Atmospheric concentration of N2O 270 ppb for the period 1000-1750 to 316 ppb in year 2000 (17±5% increase).
Tropospheric concentration of O3 Increased by 35±15% from the years 1750 to 2000, varies with region.
Stratospheric concentration of O3 Decreased over the years 1970 to 2000, varies with altitude and latitude.
Atmospheric concentrations of HFCs, PFCs, and SF6 Increased globally over the last 50 years.
Weather indicators  
Global mean surface temperature Increased by 0.6±0.2°C over the 20th century; land areas warmed more than the oceans (very likely).
Northern Hemisphere surface
Increased over the 20th century greater than during any other century in the last 1,000 years; 1990s warmest decade of the millennium (likely).
Diurnal surface temperature range Decreased over the years 1950 to 2000 over land: nighttime minimum temperatures increased at twice the rate of daytime maximum temperatures (likely).
Hot days / heat index Increased (likely).
Cold / frost days Decreased for nearly all land areas during the 20th century (very likely).
Continental precipitation Increased by 5-10% over the 20th century in the Northern Hemisphere (very likely), although decreased in some regions (e.g., north and west Africa and parts of the Mediterranean).
Heavy precipitation events Increased at mid- and high northern latitudes (likely).
Frequency and severity of drought Increased summer drying and associated incidence of drought in a few areas (likely). In some regions, such as parts of Asia and Africa, the frequency and intensity of droughts have been observed to increase in recent decades.
Biological and physical indicators  
Global mean sea level Increased at an average annual rate of 1 to 2 mm during the 20th century.
Duration of ice cover of rivers and lakes Decreased by about 2 weeks over the 20th century in mid- and high latitudes of the Northern Hemisphere (very likely).
Arctic sea-ice extent and thickness Thinned by 40% in recent decades in late summer to early autumn (likely) and decreased in extent by 10-15% since the 1950s in spring and summer.
Non-polar glaciers Widespread retreat during the 20th century.
Snow cover Decreased in area by 10% since global observations became available from satellites in the 1960s (very likely).
Permafrost Thawed, warmed, and degraded in parts of the polar, sub-polar, and mountainous regions.
El Niño events Became more frequent, persistent, and intense during the last 20 to 30 years compared to the previous 100 years.
Growing season Lengthened by about 1 to 4 days per decade during the last 40 years in the Northern Hemisphere, especially at higher latitudes.
Plant and animal ranges Shifted poleward and up in elevation for plants, insects, birds, and fish.
Breeding, flowering, and migration Earlier plant flowering, earlier bird arrival, earlier dates of breeding season, and earlier emergence of insects in the Northern Hemisphere.
Coral reef bleaching Increased frequency, especially during El Niño events.
Economic indicators  
Weather-related economic losses Global inflation-adjusted losses rose an order of magnitude over the last 40 years (see Q2 Figure 2-7). Part of the observed upward trend is linked to socio-economic factors and part is linked to climatic factors.
a. This table provides examples of key observed changes and is not an exhaustive list. It includes both changes attributable to anthropogenic climate change and those that may be caused by natural variations or anthropogenic climate change. Confidence levels are reported where they are explicitly assessed by the relevant Working Group. An identical table in the Synthesis Report contains cross-references to the WGI and WGII reports.

Box SPM-1: Confidence and likelihood statements.
Where appropriate, the authors of the Third Assessment Report assigned confidence levels that represent their collective judgment in the validity of a conclusion based on observational evidence, modeling results, and theory that they have examined. The following words have been used throughout the text of the Synthesis Report to the TAR relating to WGI findings: virtually certain (greater than 99% chance that a result is true); very likely (90-99% chance); likely (66-90% chance); medium likelihood (33-66% chance); unlikely (10-33% chance); very unlikely (1-10% chance); and exceptionally unlikely (less than 1% chance). An explicit uncertainty range (±) is a likely range. Estimates of confidence relating to WGII findings are: very high (95% or greater), high (67-95%), medium (33-67%), low (5-33%), and very low (5% or less). No confidence levels were assigned in WGIII.
There is new and stronger evidence that most of the warming observed over the last 50 years is attributable to human activities. Detection and attribution studies consistently find evidence for an anthropogenic signal in the climate record of the last 35 to 50 years. These studies include uncertainties in forcing due to anthropogenic sulfate aerosols and natural factors (volcanoes and solar irradiance), but do not account for the effects of other types of anthropogenic aerosols and land-use changes. The sulfate and natural forcings are negative over this period and cannot explain the warming; 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. The best agreement between model simulations and observations over the last 140 years has been found when all the above anthropogenic and natural forcing factors are combined, as shown in Figure SPM-2. Q2.9-11
Changes in sea level, snow cover, ice extent, and precipitation are consistent with a warming climate near the Earth's surface. Examples of these include a more active hydrological cycle with more heavy precipitation events and shifts in precipitation, widespread retreat of non-polar glaciers, increases in sea level and ocean-heat content, and decreases in snow cover and sea-ice extent and thickness (see Table SPM-1). For instance, it is very likely that the 20th century warming has contributed significantly to the observed sea-level rise, through thermal expansion of seawater and widespread loss of land ice. Within present uncertainties, observations and models are both consistent with a lack of significant acceleration of sea-level rise during the 20th century. There are no demonstrated changes in overall Antarctic sea-ice extent from the years 1978 to 2000. In addition, there are conflicting analyses and insufficient data to assess changes in intensities of tropical and extra-tropical cyclones and severe local storm activity in the mid-latitudes. Some of the observed 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. Q2.12-19

Observed changes in regional climate have affected many physical and biological systems, and there are preliminary indications that social and economic systems have been affected.

Figure SPM-2: Simulating the Earth's temperature variations (°C) and comparing the results to the measured changes can provide insight to the underlying causes of the major changes. A climate model can be used to simulate the temperature changes that occur from both natural and anthropogenic causes. The simulations represented by the band in (a) were done with only natural forcings: solar variation and volcanic activity. Those encompassed by the band in (b) were done with anthropogenic forcings: greenhouse gases and an estimate of sulfate aerosols. And those encompassed by the band in (c) were done with both natural and anthropogenic forcings included. From (b), it can be seen that the inclusion of anthropogenic forcings provides a plausible explanation for a substantial part of the observed temperature changes over the past century, but the best match with observations is obtained in (c) when both natural and anthropogenic factors are included. These results show that the forcings included are sufficient to explain the observed changes, but do not exclude the possibility that other forcings may also have contributed.
Q2.20 & Q2.25
Q2 Figure 2-4
Recent regional changes in climate, particularly increases in temperature, have already affected hydrological systems and terrestrial and marine ecosystems in many parts of the world (see Table SPM-1). The observed changes in these systems1 are coherent across diverse localities and/or regions and are consistent in direction with the expected effects of regional changes in temperature. The probability that the observed changes in the expected direction (with no reference to magnitude) could occur by chance alone is negligible. Q2.21-24
The rising socio-economic costs related to weather damage and to regional variations in climate suggest increasing vulnerability to climate change. Preliminary indications suggest that some social and economic systems have been affected by recent increases in floods and droughts, with increases in economic losses for catastrophic weather events. However, because these systems are also affected by changes in socio-economic factors such as demographic shifts and land-use changes, quantifying the relative impact of climate change (either anthropogenic or natural) and socio-economic factors is difficult. Q2.25-26

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