Introduction

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.

Global Mean Temperature over Land and Ocean (Jan-Dec). (Source: NCDC/NESDIS/NOAA)
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 14.00°C/57.20°F. WMO states that among other remarkable global climatic events recorded in 2007, a record-low Arctic sea ice extent was observed which led to first recorded opening of the Canadian Northwest Passage.
The United States National Climatic Data Center (NCDC), found that in 2006 "Globally averaged land temperatures were +0.78°C (+1.40°F) and ocean temperatures +0.45°C (+0.81°F) above average, ranking 4th and 5th warmest, respectively. The land and ocean surface temperatures for the Northern and Southern Hemisphere ranked 2nd and 6th warmest, respectively," since global temperature record monitoring began in 1880. 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."

Global in Situ Temperature Anomalies and Trends, Surface and Mid Troposphere (Jan-Dec).
The NCDC 2006 report also described temperature trends aloft in the atmosphere measured over the past 50 to 60 years using balloon-borne instruments (radiosondes) and for the past 28 years using satellites. The report states that temperature data collected from approximately 5,000 to 30,000 feet above the surface indicate that 1958-2006 global temperature trends in the middle troposphere are similar to trends in surface temperature; 0.12°C/decade for surface and 0.15°C/decade for mid-troposphere.
On 2 February 2007, the Intergovernmental Panel on Climate Change (IPCC) released the Summary For Policymakers (SPM), an executive summary of the first volume of its 4th Assessment Report entitled, "The Physical Science Basis of Climate Change." The IPCC Report documents that not only do the records show a warming trend during the past half century in land-based temperature data but also in global ocean temperature measurements. The increases in ocean temperatures indicate global warming trends are not an artifact of urbanization or the so-called "heat-island" effect.