Climate Change 2001: Synthesis Report

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Table 2-1: 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 tO368 ppm in year 2000 (31±4% increase). [WGI TAR Chapter 3]
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. [ WG1 TAR Chapter 3 & SRLULUCF]
Atmospheric concentration of CH4 700 ppb for the period 1000-1750 to 1,750 ppb in year 2000 (151±25% increase). [WGI TAR Chapter 4]
Atmospheric concentration of N2O 270 ppb for the period 1000-1750 tO316 ppb in year 2000 (17±5% increase). [WGI TAR Chapter 4]
Tropospheric concentration of O3 Increased by 35±15% from the years 1750 to 2000, varies with region. [WGI TAR Chapter 4]
Stratospheric concentration of O3 Decreased over the years 1970 to 2000, varies with altitude and latitude. [ WGI TAR Chapters 4 & 6]
Atmospheric concentrations of HFCs, PFCs, and SF6 Increased globally over the last 50 years. [WGI TAR Chapter 4]
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). [WGI TAR Section]
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). [WGI TAR Chapter 2 ES & Section]
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). [WGI TAR Section]
Hot days / heat index Increased (likely). [WGI TAR Section]
Cold / frost days Decreased for nearly all land areas during the 20th century (very likely). [WGI TAR Section]
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). [ WGI TAR Chapter 2 ES & Section 2.5.2]
Heavy precipitation events Increased at mid- and high northern latitudes (likely). [WGI TAR Section]
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. [WGII TAR Sections 10.1.3 & 11.1.2]
Biological and physical indicators  
Global mean sea level Increased at an average annual rate of 1 to 2 mm during the 20th century. [WGI TAR Chapter 11]
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). [ WGI TAR Chapter 2 ES & Section, & WGII TAR Sections 5.7 &]
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. [WGI TAR Section & WGII TAR Section]
Non-polar glaciers Widespread retreat during the 20th century. [WGI TAR Section & WGII TAR Section 4.3.11]
Snow cover Decreased in area by 10% since global observations became available from satellites in the 1960s (very likely). [WGI TAR Section]
Permafrost Thawed, warmed, and degraded in parts of the polar, sub-polar, and mountainous regions. [ WGI TAR Sections & 11.2.5, & WGII TAR Section]
El Niño events Became more frequent, persistent, and intense during the last 20 to 30 years compared to the previous 100 years. [WGI TAR Section 7.6.5]
Growing season Lengthened by about 1 to 4 days per decade during the last 40 years in the Northern Hemisphere, especially at higher latitudes. [WGII TAR Section 5.2.1]
Plant and animal ranges Shifted poleward and up in elevation for plants, insects, birds, and fish. [WGII TAR Sections 5.2, 5.4, 5.9, &]
Breeding, flowering, and migration Earlier plant flowering, earlier bird arrival, earlier dates of breeding season, and earlier emergence of insects in the Northern Hemisphere. [WGII TAR Sections 5.2.1 & 5.4.3]
Coral reef bleaching Increased frequency, especially during El Niño events. [WGII TAR Section 6.3.8]
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. [WGII TAR Sections 8.2.1 & 8.2.2]
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.



The radiative forcing from the increase in anthropogenic greenhouse gases since the pre-industrial era is positive (warming) with a small uncertainty range; that from the direct effects of aerosols is negative (cooling) and smaller; whereas the negative forcing from the indirect effects of aerosols (on clouds and the hydrologic cycle) might be large but is not well quantified. Key anthropogenic and natural factors causing a change in radiative forcing from year 1750 to year 2000 are shown in Figure 2-2, where the factors whose radiative forcing can be quantified are marked by wide, colored bars. Only some of the aerosol effects are estimated here and denoted as ranges. Other factors besides atmospheric constituents -- solar irradiance and land-use change -- are also shown. Stratospheric aerosols from large volcanic eruptions have led to important, but brief-lived, negative forcings (particularly the periods 1880-1920 and 1960-1994), which are not important over the time scale since the pre-industrial era and not shown. The sum of quantified factors in Figure 2-2 (greenhouse gases, aerosols and clouds, land-use (albedo), and solar irradiance) is positive, but this does not include the potentially large, negative forcing from aerosol indirect effects. The total change in radiative forcing since the pre-industrial era continues to be a useful tool to estimate, to a first order, the global mean surface temperature response to human and natural perturbations; however, the sum of forcings is not necessarily an indicator of the detailed aspects of the potential climate responses such as regional climate change. For the last half of the 20th century (not shown), the positive forcing due to well-mixed greenhouse gases has increased rapidly over the past 4 decades, while in contrast the sum of natural forcings has been negative over the past 2 and possibly even 4 decades.

WGI TAR Chapter 5 & 6, & SRAGA Chapter 6

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