Scientific evidence of global warming comes from several areas of study, including analysis of ancient ice samples, observed retreating glaciers and melting ice sheets in the polar regions. Experts have also measured rising sea levels and global temperatures and the increased occurrence of extreme weather events, among others data.
Ice samples are analyzed to track how atmospheric carbon dioxide levels have changed over time. Scientists discovered in the 19th century that carbon dioxide traps heat. Analysis of Antarctic ice samples throughout the past 400,000 years shows that atmospheric carbon dioxide averaged about 250 parts per million (ppm) until the Industrial Revolution. Since then, CO2 has risen to 400 ppm and is still climbing, as of 2015.
Earth-orbiting satellites have photographed the breakup of polar ice sheets and shrinking glaciers. Both range and ice cover thickness is rapidly decreasing. For example, Antarctica lost 36 cubic miles of ice in only three years between 2002 and 2005.
Sea levels rose 6.7 inches in the century prior to 2015. In the previous decade, sea levels rose almost double that amount. Temperatures have risen steadily since the 1970s. The 10 warmest years in this period occurred within a 12-year frame.
In the United States alone, the occurrence of record high temperatures has increased since 1950, while the incidence of record low temperatures has decreased. Intense rainfall events such as hurricanes have also increased.