The Working Group I (WGI) TAR described an increasing body of observations that gave a collective picture of a warming world and other changes in the climate system (IPCC, 2001b). The WGII TAR documented methods of detecting observed changes in natural and managed systems, characterised the processes involved, and summarised the studies across multiple systems (see Sections 2.2,5.2.1 and 19.1) (IPCC, 2001a). In the TAR, about 60 studies considered about 500 data series in physical or biological systems.
Changes in physical systems:
• Sea ice: Arctic sea-ice extent had declined by about 10 to 15% since the 1950s. No significant trends in Antarctic sea-ice extent were apparent.
• Glaciers and permafrost: mountain glaciers were receding on all continents, and Northern Hemisphere permafrost was thawing.
• Snow cover: extent of snow cover in the Northern Hemisphere had decreased by about 10% since the late 1960s and 1970s.
• Snow melt and runoff: snowmelt and runoff had occurred increasingly earlier in Europe and western North America since the late 1940s.
• Lake and river ice: annual duration of lake- and river-ice cover in Northern Hemisphere mid- and high latitudes had been reduced by about 2 weeks and become more variable.
Changes in biological systems:
• Range: plant and animal ranges had shifted poleward and higher in elevation.
• Abundance: within the ranges of some plants and animals, population sizes had changed, increasing in some areas and declining in others.
• Phenology: timing of many life-cycle events, such as blooming, migration and insect emergence, had shifted earlier in the spring and often later in the autumn.
• Differential change: species changed at different speeds and in different directions, causing a decoupling of species interactions (e.g., predator-prey relationships).
Preliminary evidence for changes in human systems:
• Damages due to droughts and floods: changes in some socioeconomic systems had been related to persistent low rainfall in the Sahelian region of Africa and to increased precipitation extremes in North America. Most of the increase in damages is due to increased wealth and exposure. However, part of the increase in losses was attributed to climate change, in particular to more frequent and intense extreme weather events in some regions.
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