The planet reached two important climate milestones this year. The globally averaged concentration of CO2 reached 400 parts per million, and the global average temperature climbed to more than 1°C (1.8°F) above pre-industrial levels.
For hundreds of millennia, the levels of CO2 in the atmosphere underwent slow fluctuations, which largely mirrored gradual cycles in the earth’s orbit and varying levels of planetary ice coverage. The levels remained below 300 ppm for more than 400,000 years. But in the last century, the burning of fossil fuels has rapidly driven atmospheric CO2 levels to new heights, overriding the natural cycle. While the monthly average levels of CO2 at Hawaii’s Mauna Loa observatory exceeded 400 ppm in 2014, the globally average levels surpassed 400 ppm for the first time this year, and the rate of CO2 emissions continues to increase.
Within each year, there is a small saw-tooth pattern to the atmospheric CO2 concentration. As vegetation blossoms in the Northern Hemisphere each spring, CO2 is taken in, but returned to the atmosphere during the fall. As a result, there was a brief drop below 400 ppm this past summer. However, with winter settling in, that level will be reached again soon. Additionally, the current strong El Niño likely means more drought in tropical regions, leading to an increase in forest fires and more CO2 in the atmosphere, further suggesting the concentration is unlikely go below 400 ppm for the foreseeable future.