The scientific consensus on global warming is becoming clearer every day. Changes in our climate are real and they are underway. There is no question that the Earth is getting hotter and fast. The real questions are: How much of the warming is our fault, and are we willing to slow the meltdown by curbing our insatiable appetite for fossil fuels?
= Human activities also can induce hazards through resource acquisition, urban growth, land use decisions and waste disposal. Such activities can accelerate many natural changes.
= Human beings live within the world's ecosystems. Increasingly, humans modify ecosystems as a result of population growth, technology, and consumption. Human destruction of habitats through direct harvesting, pollution, atmospheric changes and other factors is threatening current global stability, and if not addressed, ecosystems will be irreversibly affected.
The global average surface temperature has increased over the 20th century by about 0.6' C.
= The global average surface temperature (the average of near surface air temperature over land, and sea surface temperature) has
increased since 1861. Over the 20th century the increase has been 0.6 - 0.2'C. This value is about 0.15- C larger than that estimated for the period up to 1994, owing to the relatively high temperatures of the additional years (1995 to 2000) and improved methods of processing the data. These numbers take into account various adjustments, including urban heat island effects. The record shows a great deal of variability; for example, most of the warming occurred during the 20th century, during two periods, 1910 to 1945 and 1976 to 2000.
= Globally, it is very likely that the 1990s was the warmest decade and 1998 the warmest year in the instrumental record, since 1861.
= New analyses of proxy data for the Northern Hemisphere indicate
that the increase in temperature in the 20th century is likely to have been the largest of any century during the past 1,000 years. It is also likely that, in the Northern Hemisphere, the 1990s was the warmest decade and 1998 the warmest year. Because less data are available, less is known about annual averages prior to 1,000 years before present and for conditions prevailing in most of the Southern Hemisphere prior to 1861.
= On average, between 1950 and 1993, night-time daily minimum air temperatures over land increased by about 0.2- C per decade. This is about twice the rate of increase in daytime daily maximum air temperatures (0.1- C per decade). This has lengthened the freeze-free season in many mid- and high latitude regions. The increase in sea surface temperature over this period is about half that of the mean land surface air temperature.
Temperatures have risen during the past four decades in the lowest 8 km of the atmosphere.
= Since the late 1950s (the period of adequate observations from weather balloons), the overall global temperature increases in the lowest 8 km of the atmosphere and in surface temperatures have been similar at 0.1- C per decade.
Since the start of the satellite record in 1979, both satellite and weather balloon measurements show that the global average temperature of the lowest 8 km of the atmosphere has changed by + 0.05 - 0.10- C per decade, but the global average surface temperature has increased significantly by + 0.15 - 0.05- C per decade. The difference in the warming rates is statistically significant. This difference occurs primarily over the tropical and sub-tropical regions.
= The lowest 8 km of the atmosphere and the surface are influenced differently by factors such as stratospheric ozone depletion, atmospheric aerosols, and the El Ni-o phenomenon. Hence, it is physically plausible to expect that over a short time period (for example, 20 years) there may be differences in temperature trends. In addition, spatial sampling techniques can also explain some of the differences in trends, but these differences are not fully resolved. Also, snow cover and ice extent have decreased.
= Satellite data show that there are very likely to have been decreases of about 10 per cent in the extent of snow cover since the late 1960s, and ground-based observations show that there is very likely to have been a reduction of about two weeks in the annual duration of lake and river ice cover in the mid- and high latitudes of the Northern Hemisphere, over the 20th century.
= There has been a widespread retreat of mountain glaciers in non-polar regions during the 20th century.
= Northern Hemisphere spring and summer sea-ice extent has decreased by about 10 to 15 per cent since the 1950s. It is likely that there has been about a 40 per cent decline in Arctic sea ice thickness during late summer to early autumn in recent decades and a considerably slower decline in winter sea-ice thickness.
Global average sea level has risen and ocean heat content has increased.
= Tide gauge data show that global average sea level rose between 0.1 and 0.2 metre during the 20th century.
= Global ocean heat content has increased since the late 1950s, the period for which adequate observations of sub-surface ocean temperatures have been available.
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