PRECIPITATION AND DROUGHT
Another factor in the climatic equation is precipitation and drought. Studies indicate that, "since about 1970 precipitation has tended to remain above the twentieth century mean, averaging about 5% higher than in the previous 70 years. Such an increase hints at a change in climate. Statistical analysis suggests that the change is unusual, but there is still about a 10% chance that such a change could arise from a stable or quasi-stationary climate without any real long-term changes."
While during the 1930's there was a sharp rise in temperature, there was a modest cooling trend from the 1950's to the 1970 when the temperature began to rise again. There has been a rise in temperature since the 1970's . The report states, " A straightforward statistical average of mean temperatures across the U.S. gives evidence of a rise through the century of about 0.3 to 0.4° C (0.6 to 0.8° F), although so crude a characterization of mean temperature change in the U.S. would be indeed a gross oversimplification."
"The increase in annual temperatures after the 1970s is mainly the result of significant increases of temperature during the first six months of the year (winter and spring). Temperatures during summer and autumn have changed little after dropping from conditions of the warm 1930s. Unusually high precipitation and cloud amount tend to cool the air, especially during the second half of the year. It is rare to find much above normal precipitation and cloud amount during these two seasons when temperatures are higher than normal.
"On a regional basis the West contributes most to the increase of annual average nation-wide temperatures. As with drought and excessive moisture, portions of the country can be extremely cold at the same time that others are unusually warm, leading to an average national temperature that is near-normal. Similarly, abnormally high daytime maximum temperatures can occur while nighttime temperatures remain below normal, or vice-versa, although these are not usually the case."
"Changes and variations of destructive storms are of particular interest because of their socio-economic and biophysical impact. Reliable records of the number and intensity of tropical hurricanes that reach the U.S. go back to at least 1900.Based on a commonly used classification of hurricane intensity, the studies indicates that the frequency of these violent storms that make landfall in the U.S. has been relatively low over the past few decades, as compared to the middle of the century. The decline is reflected in both the total number of hurricanes making landfall in the U.S. and in the occurrence of more destructive storms. It is difficult to discern any long-term trend however, since the frequency of hurricanes was also low in the early part of the century. Furthermore, recent studies indicate that even if significant greenhouse induced warming were to occur, it is doubtful whether increases in tropical storms would be detectable due to the large natural variability in these storms."
CHANGES IN CIRCULATION
Another factor the climatologists have studied are changes in circulation over the past few decades. Since the winter of 1976-77, the sea-surface temperatures in the central and eastern equatorial Pacific have remained anomalously warm. The report states:"Such events have been directly linked to increased precipitation in the southeastern U.S. and warmer than normal temperatures in the Pacific Northwest. During these same years a large-scale redistribution of atmospheric mass has taken place in the North Pacific, associated with a change of the upper-level steering winds over the North Pacific and North America. El Niñ o events (and their opposition phases, La Niñ a events) have been quantitatively linked to the 1988 drought, to increased precipitation in the South, and to other abnormal temperature conditions in the U.S. Variations in the circulation of the North Atlantic Ocean have also directly influenced the eastern U.S. climate in the form of stronger than normal winds over these regions that seem to oscillate on decadal time-scales. Such oscillations have been linked to colder than normal temperatures in the region."
CLIMATE CHANGE INDICES
"Most readers will by now agree that it is difficult to draw a simple picture that summarizes the many parameters and multidimensional aspects of observed climate change and variability, no matter how complete the record. One approach toward simplification might be to consider only long-term measurements of a few near-surface conditions: temperature and precipitation, for example, are two primary elements of climate that affect many aspects of our lives. But neither tells the whole story.
Реферат опубликован: 11/04/2010