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The earth's temperature depends on the balance between energy entering and leaving the planet's system . When incoming energy from the sun is absorbed by the Earth system, Earth warms. When the sun's energy is reflected back into space, Earth avoids warming. When energy is released back into space, Earth cools. Many factors, both natural and human, can cause changes in Earth's energy balance, including
At right is a line graph from the EPA with a line that show the observed temperature increases, a blue band that show how the EPA believes temperature would have changed over the past century due to only natural forces, and a red band that shows the combined effects of natual and human forces. The blue band that shows natural forces starts and ends the 20th century just above 56 degrees Fahrenheit. The actual observed global average temperatures closely follows the model projections that use both human and natural forces - beginning in 1900 at just above 56 degrees Fahrenheit and ending in 2000 around 58 degrees Fahrenheit.
Models that account only for the effects of natural processes are not able to explain the warming over the past century. Models that also account for the greenhouse gases emitted by humans are able to explain this warming. Source: USGRCP (2009)
Scientists have pieced together a picture of Earth's climate, dating back hundreds of thousands of years, by analyzing a number of indirect measures of climate such as ice cores, tree rings, glacier lengths, pollen remains, and ocean sediments, and by studying changes in Earth's orbit around the sun.
The historical record shows that the climate system varies naturally over a wide range of time scales. In general, climate changes prior to the Industrial Revolution in the 1700s can be explained by natural causes, such as changes in solar energy, volcanic eruptions, and natural changes in greenhouse gas (GHG) concentrations.
The global average temperature increased by more than 1.4°F over the last century.  In fact, according to the National Oceanic and Atmospheric Administration (NOAA), the decade from 2000 to 2010 was the warmest on record, and 2010 was tied with 2005 as the warmest year on record.  Rising global temperatures have also been accompanied by other changes in weather and climate. Many places have experienced changes in rainfall resulting in more intense rain, as well as more frequent and severe heat waves. Note: You will see a few news stories and claims that these tables use data fabricated by computer models, but those allegations have been roundly refuted. The need to adjust the data was based on a number of technical changes. In the mid 1980s, the government settled on a list of about 1,200 stations across the country to track temperature trends. Around 1990, climatologists began delivering computer programs to factor in the artificial changes that systematically pushed the readings one way or the other. Over time, they accounted for the impacts of equipment, location, the time of day of measurements and urbanization (more asphalt leads to higher surface temperatures). See this Wikipedia article for the full story. and see US EPA, Climate Change Facts: Answers to Common Questions