The
Temperature Is Rising! |
| Global surface temperatures
have increased about 0.6°C (plus or minus 0.2°C) since the late-19th century, and about
one half degree F (0.2 to 0.3°C) over the past 25 years (the period with the most
credible data). The warming has not been globally uniform. Some areas (including parts of
the southeastern U.S.) have cooled. The recent warmth has been greatest over N. America
and Eurasia between 40 and 70°N. Warming, assisted by the record El Niņo of 1997-1998,
has continued right up to the present. Linear trends can vary greatly depending on the period over which they are computed. Temperature trends in the lower troposphere (between about 2,500 and 18,000 ft.) from 1979 to the present, the period for which Satellite Microwave Sounding Unit data exist, are small and may be unrepresentative of longer term trends and trends closer to the surface. Furthermore, there are small unresolved differences between radiosonde and satellite observations of tropospheric temperatures, though both data sources show slight warming trends. If one calculates trends beginning with the commencement of radiosonde data in the 1950s, there is a slight greater warming in the record due to increases in the 1970s. There are statistical and physical reasons (e.g., short record lengths, the transient differential effects of volcanic activity and El Niņo, and boundary layer effects) for expecting differences between recent trends in surface and lower tropospheric temperatures, but the exact causes for the differences are still under investigation (see National Research Council report "Reconciling Observations of Global Temperature Change"). An enhanced greenhouse effect is expected to cause cooling in higher parts of the atmosphere because the increased "blanketing" effect in the lower atmosphere holds in more heat. Cooling of the lower stratosphere (about 30-35,000ft.) since 1979 is shown by both satellite Microwave Sounding Unit and radiosonde data, but is larger in the radiosonde data. There has been a general, but not global, tendency toward reduced diurnal temperature range (the difference between high and low daily temperatures) over about 50% of the global land mass since the middle of the 20th century. Cloud cover has increased in many of the areas with reduced diurnal temperature range. Relatively cool surface and tropospheric temperatures, and a relatively warmer lower stratosphere, were observed in 1992 and 1993, following the 1991 eruption of Mt. Pinatubo. The warming reappeared in 1994. A dramatic global warming, at least partly associated with the record El Niņo, took place in 1998. This warming episode is reflected from the surface to the top of the troposphere. Indirect indicators of warming such as borehole temperatures, snow cover, and glacier recession data, are in substantial agreement with the more direct indicators of recent warmth. Arctic sea ice has decreased since 1973, when satellite measurements began but Antarctic sea ice may have increased slightly.~National Oceanic and Atmospheric Administration~ |
| "A number of
studies have shown that 20th century Arctic and hemispheric warming are unprecedented
relative to the last six centuries. Now, high-resolution paleoclimate records stretching
back 1200 years confirm that the so-called Medieval Warm Period did not exist in the form
of a globally synchronous period as warm, or warmer, than today. Thus, recent record high
hemispheric temperatures are probably unprecedented in at least 1200 years". ~Jonathan
Overpeck N.O.A.A.~ Explorers in Antarctica Find Fresh Evidence of Global Warming |
| WMO Statement On The Status Of Global Climate In 2003. |
