Global warming

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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.