Is the climate becoming more variable or extreme?

On a global scale there is little evidence of sustained trends in climate variability or extremes. This perhaps reflects inadequate data and a dearth of analyses. However, on regional scales, there is clear evidence of changes in variability or extremes.

In areas where a drought usually accompanies an El Niсo, droughts have been more frequent in recent years. Other than these areas and the few areas with longer term trends to lower rainfall (e.g., the Sahel), little evidence is available of changes in drought frequency or intensity.

In some areas there is evidence of increases in the intensity of extreme rainfall events, but no clear global pattern has emerged. Despite the occurrence in recent years of several regional-scale extreme floods there is no evidence of wide-spread changes in flood frequency. This may reflect the dearth of studies, definition problems, and/or difficulties in distinguishing the results of land use changes from meteorological effects.

There is some evidence of recent (since 1988) increases in extreme extratropical cyclones over the North Atlantic. Intense tropical cyclone activity in the Atlantic appears to have decreased over the past few decades. Elsewhere, changes in observing systems confound the detection of trends in the intensity or frequency of extreme synoptic systems.

There has been a clear trend to fewer extremely low minimum temperatures in several widely-separated areas in recent decades. Widespread significant changes in extreme high temperature events have not been observed.

There is some indication of a decrease in day-to-day temperature variability in recent decades.

How important are these changes in a longer-term context?

For the Northern Hemisphere summer temperature, recent decades appear to be the warmest since at least about 1000AD, and the warming since the late 19th century is unprecedented over the last 1000 years. Older data are insufficient to provide reliable hemispheric temperature estimates. Ice core data suggest that the 20th century has been warm in many parts of the globe, but also that the significance of the warming varies geographically, when viewed in the context of climate variations of the last millennium.

Large and rapid climatic changes affecting the atmospheric and oceanic circulation and temperature, and the hydrological cycle, occurred during the last ice age and during the transition towards the present Holocene period (which began about 10,000 years ago). Based on the incomplete evidence available, the projected change of 3 to 7°F (1.5 - 4°C) over the next century would be unprecedented in comparison with the best available records from the last several thousand years.

Is sea level rising?

Global mean sea level has been rising at an average rate of 1 to 2 mm/year over the past 100 years, which is significantly larger than the rate averaged over the last thousand years. Projected increase for the 21st century is about 0.5 meter, but estimates range widely.

Can the observed changes be explained by natural variability, including changes in solar output?

Some changes, particularly part of the pre-1960 temperature record, show some relationship with solar output, but the more recent warm era is not well correlated. The exact magnitude of purely natural global mean temperature variance is not known precisely, but model experiments excluding solar variation indicate that it is likely less than the variability observed during this century.

Global Warming or Global Cooling the Threat for the Future?

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 ."

GREENHOUSE WARMING

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.