"The Earth's climate is changing. Scientists are confident that many of the observed changes in the climate can be linked to the increase in greenhouse gases in the atmosphere, caused largely by people burning fossil fuels to generate electricity, heat and cool buildings, and power vehicles (see 'The Greenhouse Effect' below to learn about how these gases trap heat). Current and future emissions will continue to increase the levels of these gases in our atmosphere for the foreseeable future. One way to track and communicate the causes and effects of climate change is through the use of indicators. An indicator, such as a record of Arctic sea ice extent, represents the state or trend of certain environmental conditions over a given area and a specified period of time. Scientists, analysts, decision-makers, and others use environmental indicators, including those related to climate, to help monitor environmental trends over time, track key factors that influence the environment, and identify effects on ecosystems and society."
Environmental Protection Agency: http://www.epa.gov/