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Common Questions about Climate Change
Published in 1997 by the United Nations Environment Programme - World Meteorological Organization




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See: Information from Encyclopædia Britannica about global warming


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Has the World Warmed?

The globally averaged temperature of the air at the Earth's surface has warmed between 0.3 and 0.6°C (about 0.5 and 1°F) since the late nineteenth century (Figure 2.1).

Figure 2.1

Measured global surface temperatures relative to the average for the 130-year period 1861 to 1990 (the horizontal line).









  The four warmest years on record since 1860 have all occurred since 1990. The warming has been greatest at night over land in the mid-to-high latitudes of the northern hemisphere. The warming during the northern winter and spring has been stronger than at other seasons. In some areas, primarily over continents, the warming has been several times greater than the global average. In a few areas, temperatures have actually cooled, e.g., over the southern Mississippi Valley in North America (Figure 2.2).

Figure 2.2

Trends in measured surface air temperature over the past century. Red circles represent warming; blue circles represent cooling. No data are available for the large areas with no circles.









Other evidence of global temperature increases since the nineteenth century includes the observed rise in sea level of 10 to 25 centimeters (about 4 to 10 inches), the shrinkage of mountain glaciers, a reduction of northern hemisphere snow cover (1973 to present), and increasing sub-surface ground temperatures. Data derived from measurements of tree rings, shallow ice cores, and corals, and from other methods of indirectly determining climate trends, suggest that global surface temperatures are now as warm as or warmer than at any time in the past 600 years.

Data from a few locations can be used to trace temperatures even further into the past. For example, deep ice cores and North Atlantic deep sea sediments suggest that the recent warming stands out against a record of relatively stable temperatures over the past ten thousand years, with century-to-century variations of temperature seldom approaching the observed increase of global mean temperatures of about 0.3 to 0.6°C (about 0.5 to 1°F) over the last century.

Satellite-based instruments have recently measured temperatures at higher altitudes (2 to 6 kilometers, or about 1 to 4 miles above the Earth's surface), rather than at the surface. These observations indicate that this portion of the atmosphere may have cooled slightly, by about 0.1°C (about 0.2°F), since 1979 when the measurements began. Although apparently at variance with the surface temperature measurements, they are not. Significant differences in short-term trends are to be expected between the surface and atmospheric temperatures at higher altitudes, because of the different factors affecting the variability and persistence of climate patterns at different altitudes. Furthermore, questions have recently arisen concerning the consistency of calibrations of the satellite-based instruments, suggesting that what was believed to be a small cooling may actually be a slight warming.

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