According to the National Climatic Data Center, a division of the National Oceanic and Atmospheric Administration, global surface temperatures have increased by about 0.6 degrees Celsius, or about 1 degree Fahrenheit, since the late 19th century.
This claim is little disputed among scientists. Scientists also do not dispute the fact that the concentration of carbon dioxide, a greenhouse gas, has increased in Earth's atmosphere in the last century.
What is disputed, among scientists, politicians, industry officials and environmental activists, is the cause and implications of the change.
Many scientists, including those at the U.S. Environmental Protection Agency and the National Oceanic and Atmospheric Administration, believe that the increased amount of carbon dioxide and other so-called "greenhouse gases" in the Earth's atmosphere are causing the rise in temperatures.
They believe these gases are produced by the burning of fossil fuels for energy.
The Cooler Heads Coalition, a division of the Association of Concerned Tax Payers, argues that concern over global warming is exaggerated. According to its Web site, most greenhouse gases are natural - only 2 percent are from human-made sources.
The coalition also says the largest increase in the Earth's surface temperature in the last century occurred before 1940, prior to the upsurge in greenhouse gas emissions.
To learn more about global climate change issues, review these Web sites:
The National Climatic Data Center
The Environmental Protection Agency
The Cooler Heads Coalition