Arctic sea ice was at a record low wintertime maximum extent for the second straight year. At 5.607 million square miles, it is the lowest maximum extent in the satellite record, and 431,000 square miles below the 1981 to 2010 average maximum extent. Credit: NASA Goddard's Scientific Visualization Studio/C. Starr.
Carbon dioxide (CO2) is an important heat-trapping (greenhouse) gas, which is released through human activities such as deforestation and burning fossil fuels, as well as natural processes such as respiration and volcanic eruptions.