The phrase global warming refers to the documented historical warming of the Earth's surface based upon worldwide temperature records that have been maintained by humans since the 1880s. The term global warming is often used synonymously with the term climate change, but the two terms have distinct meanings. Global warming is the combined result of anthropogenic (human-caused) emissions of greenhouse gases and changes in solar irradiance, while climate change refers to any change in the state of the climate that can be identified by changes in the average and/or the variability of its properties (e.g., temperature, precipitation), and that persists for an extended period, typically decades or longer.
According to the World Meteorological Organization (WMO), the decade of 1998-2007 is the warmest on record. The global mean surface temperature for 2007 is currently estimated at 0.41°C/0.74°F above the 1961-1990 annual average of ...view middle of the document...
The NCDC report states that "during the past century, global surface temperatures have increased at a rate near 0.06°C/decade (0.11°F/decade) but this trend has increased to a rate approximately 0.18°C/decade (0.32°F/decade) during the past 25 to 30 years. There have been two sustained periods of warming, one beginning around 1910 and ending around 1945, and the most recent beginning about 1976."
The NCDC's Preliminary Annual Report on the Climate of 2007 (released December 13, 2007) states that:
* "the global annual temperature for combined land and ocean surfaces for 2007 is expected to be near 58.0°F and would be the fifth warmest since records began in 1880," and that
* "the year 2007 is on pace to become one of the 10 warmest years for the contiguous U.S., since national records began in 1895."
Causes of Global Warming
n the IPCC Fourth Assessment Report scientists conclude that "warming of the climate system is unequivocal, as is now evident from observations of increases in global average air and ocean temperatures, widespread melting of snow and ice, and rising global average sea level" and, furthermore, they conclude with "very high confidence (at least a 9 out of 10 chance of being correct) that the globally averaged net effect of human activities since 1750 has been one of warming" of the Earth's climate system.
As with every environmental variable, there are multiple factors that contribute to the "warmth" of the Earth. Humans measure warmth as temperature which is a measure of the amount of heat contained in a physical object. One can envision this concept by thinking of a pot on a stove. As heat is applied to the pot from a flame or heating element, the temperature of the pot will increase. But heat will also begin escaping the pot in the form of steam and also through radiative and convective cooling from the top and the sides of the pot. Eventually the rates of both heat loss (cooling) and heat gain (warming) may stabilize and the heat then contained within the pot at an instantaneous point of time would be reflected in an equilibrium temperature. This equilibrium temperature could be measured directly but it also could be calculated by determining all of the flux rates of heat entering (heating) and leaving (cooling) the pot.