evidence: Carbon dioxide (CO2) is an important heat-trapping gas, also known as a greenhouse gas, that comes from the extraction and burning of fossil fuels (such as coal, oil, and natural gas), from wildfires, and natural processes like volcanic eruptions. The first graph shows atmospheric CO2 levels measured by NOAA at Mauna Loa Observatory, Hawaii, since 1958. The second graph shows CO2 levels during Earth’s last three glacial cycles, as captured by air bubbles trapped in ice sheets and glaciers. [...] A closer look at the carbon dioxide measurements at Mauna Loa shows a series of wiggles in the data. Although total CO2 is increasing each year, there is also a short-term cycle visible within the larger trend. This annual rise and fall of CO2 levels is caused by seasonal cycles in photosynthesis on a massive scale. In Northern Hemisphere spring, plants come to life and draw in CO2 to fuel their growth. This begins the process of lowering the amount of CO2 in the atmosphere. In northern autumn, plant growth stops or slows down, and the whole process reverses itself. Much of the plant matter decomposes, releasing CO2 back to the atmosphere.