Prepared By: Rob Quayle, Tom Peterson, and Tom Karl, National Climatic Data Center, Asheville, N.C.

[Note from the Webmaster: A more recent version of this document is available as Global Warming: Frequently Asked Questions, posted on the site maintained by the National Climatic Data Center]

Q. What is the greenhouse effect, and is it affecting our climate?

A. The greenhouse effect is unquestionably real, and is, in fact, essential for life on Earth. It is the result of heat absorption by certain gases in the atmosphere (called greenhouse gases because they absorb radiation) and emission downward (thus exerting a "trapping" influence). Water vapor is the most important greenhouse gas, followed by carbon dioxide and other trace gases. Without the greenhouse effect, the temperature of the Earth would be about zero degrees F (-18°C) instead of its present 57°F (14°C).

Q. Are greenhouse gases increasing?

A. Human activity has been increasing the concentration of greenhouse gases in the atmosphere (mostly carbon dioxide from combustion of coal, oil, and gas; plus a few other trace gases). There is no scientific debate on this point. At rates of increase observed over the past few decades, the concentration of carbon dioxide will be double that of pre-industrial levels by about 2050.

Q. Is the climate warming?

A. Global surface temperatures have increased about one degree F (0.3 to 0.6°C) since the late-19th century, and about one half degree F (0.2 to 0.3°C) over the past 40 years (the period with the most credible data). The warming has not been globally uniform. That some areas (including parts of the southeastern U.S.) have cooled is not unexpected due to the somewhat chaotic behavior of the climate when changes are still small. The recent warmth has been greatest over North America and Eurasia between 40 and 70°N. Warming, assisted by the record El Nino of 1997-1998, has continued right up to the present.

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 seem to 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 near zero trends. If one calculates trends beginning with the commencement of radiosonde data in the 1950s, the data suggest that there has been a slight warming. There are statistical and physical reasons (e.g., short record lengths, the transient differential effects of volcanic activity and El Nino, 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. Recent research suggests that the mid-tropospheric temperatures may actually be warming after additional corrections for satellite orbital decay.

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 30-35,000 ft.) since 1979 is shown by both satellite Microwave Sounding Unit and radiosonde data, but is larger (and probably exaggerated because of changes in instrumentation) 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 more than 40% 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, as a result of the 1991 eruption of Mt. Pinatubo. The surface warming reappeared in 1994. A dramatic global warming, at least partly associated with the record El Nino, began in mid-1997 and continues as this is written. This warming episode is evident 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 agreement with the more direct indicators of relatively rapid warming during this century.

Q. Are El Ninos and La Ninas related to Global Warming?

A. El Ninos and La Ninas are not caused by global warming. Clear evidence exists from a variety of sources (including archaeological studies) that these phenomena have been present for hundreds, and some indicators suggest maybe millions, of years. However, it has been hypothesized that warmer global sea surface temperatures can enhance the intensity of El Ninos and La Ninas, and it is also true that El Ninos have been more frequent and intense in recent decades.

Q. Is the climate becoming more variable or extreme?

A. On a global scale there are few indicators 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 drought or excessive precipitation usually accompanies an El Nino, these anomalies have been more frequent and intense in recent years. Other than areas with El Nino-related drought 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 because global observations are limited. Despite the occurrence in recent years of several regional-scale extreme floods, there is no clear evidence of wide-spread changes in flood frequency, but there is evidence of an increase in mean precipitation and extreme precipitation events in recent decades in many mid and high latitudes. This difference may reflect the limited number of flood frequency studies, definition problems, and/or difficulties in distinguishing the results of land use changes from meteorological effects.

There is some evidence of recent (last few decades) increases in extreme extratropical cyclones over the North Atlantic and North Pacific. 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. And there is some indication of a decrease in day-to-day temperature variability in recent decades.

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

A. For the Northern Hemisphere summer temperature, recent decades appear to be the warmest since at least about 1400AD, and the warming since the late 19th century is unprecedented over the last 600 years for which we have data. Earlier 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.

Based on the incomplete evidence available, the projected warming of 2 to 7°F (1-3.5°C) over the next century would be unprecedented in comparison with the best available records from the last several thousand years. In the past, 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).

*(This material was based on the 1995 Intergovernmental Panel on Climate Change report (available at http://www.ipcc.ch/pub/reports.htm) and subsequent material.)