Global warming is caused by increased levels of carbon dioxide and other greenhouse gases in the atmosphere. This increase is the result of human activity, such as the burning of coal to produce energy and the deforestation of land for agricultural purposes.
Atmospheric levels of carbon dioxide have risen from around 320 parts per million in the 1950s to 400 parts per million in 2015. This gas absorbs energy that would otherwise escape into space, thereby warming the climate. Since the 1950s, the global average temperature has increased by nearly 0.7 degrees Celsius.
Of the 10 warmest years since the 1880s, nine have occurred since 2000. The 20 warmest years have all occurred after 1981. The year 2014 was the warmest year on record, according to NASA. This warming trend has continued even though solar activity has decreased in the 21st century.
Sea level rise is another result of this warming trend. In the 20th century, the average sea level rose around 17 centimeters. In the 2010s, the rate of sea level rise has increased to approximately 2.6 millimeters per year. Arctic sea ice has also decreased significantly in recent years, from nearly 8 million square kilometers in 1980 to a record low of 3.6 million square kilometers in 2012.