Has the climate of the United States changed significantly during the century that is about to end? In what ways and by how much? Have national trends emerged that agree--or perhaps disagree--with what is expected from projections of global greenhouse warming? These are questions addressed in a report entitled "Trends in U.S. Climate during the Twentieth Century," by Thomas R. Karl, Richard W. Knight, David R. Easterling, Robert G. Quayle who serve on the scientific staff of the National Oceanic and Atmospheric Administration's National Climatic Data Center (NCDC), in Asheville, North Carolina. Thomas "The challenge to the climatologist is to separate any meaningful signals from ever-present noise, and to discern, if possible, whether there is indeed at work the sometimes slow and subtle hand of significant change. The second task, which is even harder, is to identify, unequivocally, the cause," according to the scientists was the focus of their study.
"Before such questions can be answered, we need to remind ourselves that 'climate', as it is defined for a specific region and time, includes more than the simple average of weather conditions. Either random events or long-term persistent change, or more often combinations of them, can bring about significant swings in a variety of climate indicators from one time period to the next. Examples include a year dominated by severe drought and the next excessively wet; a series of bitterly cold winters followed by winters more mild; one scorching summer preceded by a summer pleasantly warm; years with numerous severe storms followed by years with few severe storms. The temptation at each time and place is often to attribute any of these temporal and sometimes local variations to a wider and more pervasive change in climate..."
In their assessment they noted that the so-called "greenhouse" gases "have all been markedly increasing in amount since about the time of the industrial revolution, that began in earnest some 150 years ago. The largest and best-known contributor is carbon dioxide, originating principally from the burning of wood and coal and petroleum derivatives. However, other climatic trends include "changes in the composition of the atmosphere in ways that act to cool the surface temperature. This includes the anthropogenic decrease of stratospheric ozone, and an increase in anthropogenic microscopic sulfate particles, often readily apparent during the warm season as smog. The effect of these additional atmospheric constituents on global climate is less certain than that of the better known greenhouse gases, but models suggest that in some areas they may have already acted to significantly retard greenhouse warming. It is important to note, however, that the global-scale warming predicted in climate modeling experiments from future greenhouse gas increases is substantially larger on a global average than the regional cooling expected from these other sources.
Measurements of past and current levels of carbon dioxide and other greenhouse gases indicate that we should have already increased the global greenhouse effect by man-made, or anthropogenic additions, by nearly 40% in the last 150 years. If these changes were the only process of importance, then the same mathematical climate models suggest that the average global surface temperature should have risen by about 1° C during this time. Available climate data suggest that the mean global temperature has indeed risen, but unsteadily and by only about half that amount.
"Confounding any search for anthropogenic effects are the natural changes and variations of climate that will constantly add to or subtract from the expected signal. Examples include changes in upper atmospheric steering winds (commonly known as the jet stream) due to ocean-atmosphere interactions; changes in the circulation of the ocean that can influence air temperatures; effects of major volcanic eruptions; feedbacks from changes in the land surface, as in soil moisture, snow cover, and plant cover; and changes in the energy received from the Sun.
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."
TEMPERATURE 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."
TROPICAL STORMS "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.
CONCLUSIONS "Several indicators stand out most conspicuously in the picture of surface climate variations and changes in the U.S. over the past century. These include the rather steady increase in precipitation derived from extreme 1-day precipitation events; the systematic decrease in the day-to-day variations of temperature; and the increased frequency of days with precipitation. Trends in other indicators of climate change are now neither sufficiently large nor persistent enough to be considered as strongly suggestive of systematic change, even though it remains a likely explanation. These include the increase of total precipitation and the related increase in cloud amount, as well as an overall increase in mean temperature. The area of the country that has experienced an increase in mean temperature has risen while the proportion of the country with much below normal mean minimum temperatures has decreased. Many of these indicators appear to have undergone significant change during the late 1970s and have more or less remained at these levels to the present. In contrast, other surface climate change indicators (such as the frequency of tropical cyclones) reflect the kind of climatic variability that is completely consistent with the premise of a stable or unchanging climate.
The increase in temperature across the U.S. in this century is slightly smaller, but of comparable magnitude to the increase of temperature that has characterized the world as a whole. The increase in minimum temperature and the related increase in area affected by much above normal minimum temperatures are also found in many other countries of the northern hemisphere. Worldwide precipitation over land has changed little through the twentieth century; increases noted in high latitudes have been balanced by low-latitude decreases. By comparison, the change in precipitation in the U.S. is still relatively moderate compared to some of the increases and decreases at other latitudes. Decreases in the day-to-day differences of temperature observed in the U.S. are also apparent in China and Russia, the only other large countries analyzed as of this date. The persistent increase in the proportion of precipitation derived from extremely heavy precipitation has not been detected in these other countries.