What is the greenhouse effect, and is it affecting our climate?
The greenhouse effect is unquestionably real, and is essential for life on Earth. It is the result of heat absorption by certain gases in the atmosphere (called greenhouse gases because they trap heat) and re-radiation downward of a part of that heat. Water vapor is the most important greenhouse gas, followed by carbon dioxide and other trace gases. Without a natural greenhouse effect, the temperature of the Earth would be about zero degrees F (-18°C) instead of its present 57°F (14°C). However, the concern is not with the fact that we have a greenhouse effect, but it is with the question regarding whether human activities are leading to an enhancement of the greenhouse effect.
Are greenhouse gases increasing?
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. Pre-industrial levels of carbon dioxide (prior to the start of the Industrial Revolution) were about 280 parts per million by volume (ppmv), and current levels are about 370 ppmv. According to the IPCC "business as usual" scenario of carbon dioxide increase (IS92a) in the 21st century, we would expect to see a doubling of carbon dioxide over pre-industrial levels around the year 2065.
Is the climate warming?
Global surface temperatures have increased about 0.6°C (plus or minus 0.2°C) since the late-19th century, and about one half degree F (0.2 to 0.3°C) over the past 25 years (the period with the most credible data). The warming has not been globally uniform. Some areas (including parts of the southeastern U.S.) have cooled. The recent warmth has been greatest over N. America and Eurasia between 40 and 70°N. Warming, assisted by the record El Niсo of 1997-1998, has continued right up to the present.
Linear 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 may 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 slight warming trends. If one calculates trends beginning with the commencement of radiosonde data in the 1950s, there is a slight greater warming in the record due to increases in the 1970s. There are statistical and physical reasons (e.g., short record lengths, the transient differential effects of volcanic activity and El Niсo, 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 (see National Research Council report "Reconciling Observations of Global Temperature Change").
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 30-35,000ft.) since 1979 is shown by both satellite Microwave Sounding Unit and radiosonde data, but is larger 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 about 50% 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, following the 1991 eruption of Mt. Pinatubo. The warming reappeared in 1994. A dramatic global warming, at least partly associated with the record El Niсo, took place in 1998. This warming episode is reflected 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 substantial agreement with the more direct indicators of recent warmth.
Arctic sea ice has decreased since 1973, when satellite measurements began but Antarctic sea ice may have increased slightly.
Are El Ninos related to Global Warming?
El Ninos are not caused by global warming. Clear evidence exists from a variety of sources (including archaeological studies) that El Ninos 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 El Niсo phenomenon, and it is also true that El Ninos have been more frequent and intense in recent decades.Recent climate model results that simulate the 21st century with increased greenhouse gases (using the IPCC IS92a greenhouse gas increase scenario) suggest that El Niсos are likely to become more common in the future.
Is the hydrological cycle (evaporation and precipitation) changing?
There has probably been only a small (1%) increase in global precipitation over land during the 20th century. Precipitation has increased over land in high latitudes of the northern hemisphere, especially during the cold season, concomitant with temperature increases. A step-like decrease of precipitation occurred after the 1960s between the equator and about 35 degrees latitude, from Africa to Indonesia, as temperatures increased. These changes are consistent with observed changes in streamflow, lake levels, and soil moisture (where data are available and have been analyzed).
Northern Hemisphere snow cover extent has consistently remained below average since 1987.
Pan evaporation, a measure of potential evaporation, has decreased since 1951 over much of the former Soviet Union and the U.S. However, actual evaporation, which is dependant on available water, may have increased. Evaporation appears to have increased over the tropical oceans (although not everywhere). The evidence suggests an increase of atmospheric water vapor in the tropics, at least since 1973.
In general, cloud amount has increased both over land and ocean in recent decades. Over the ocean, increases in convective and middle- and high-level clouds have been reported.
Is the atmospheric/oceanic circulation changing?
A rather abrupt change in the El Niсo - Southern Oscillation behavior occurred around 1976/77 and the new regime has persisted. There have been relatively more frequent El Niсo episodes. This behavior is highly unusual in the last 120 years (the period of instrumental record). Changes in precipitation over the tropical Pacific are related to this change in the El Niсo - Southern Oscillation, which has also affected the pattern and magnitude of surface temperatures.
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 temperat