Yesterday, atmospheric carbon dioxide levels either broke 400 parts per million for the first time in recorded history, or came incredibly close to doing so.

The figure depends on who's doing the reporting, but either way, it's not a good milestone. Carbon dioxide from human activities, particularly fossil fuels, is driving global climate change. Leading climate scientists and activists say global levels need to be reduced to at least 350 ppm to avoid the worst impacts of climate change.

The National Oceanic and Atmospheric Administration's Mauna Loa observatory reported the 400 ppm reading. The reading is of symbolic importance more than it is practical; global atmospheric carbon dioxide levels have been approaching the mark for a while.

The University of California San Diego's Scripps Institute of Oceanography reported a 399.73 ppm figure. Scripps explains that the difference between what it and NOAA reported basically comes down to the fact that the two organizations measure daily levels using different time zones. Scripps has been posting daily carbon dioxide measurements to keep attention on the fact that global concentrations of the greenhouse gas were/are likely to soon exceed the 400 ppm mark.