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