It’s official: Earth’s atmosphere is now in uncharted territory, at least since human beings evolved hundreds of thousands of years ago. Measurements of atmospheric carbon dioxide concentrations taken continuously at Mauna Loa in Hawaii since 1958 have shown a steady upward climb related to fossil fuel burning worldwide. The Mauna Loa measurements are considered to be some of the clearest evidence of human impact on the global climate.
Every single daily carbon dioxide measurement in April 2014 was above 400 parts per million. That hasn’t happened in nearly a million years, and perhaps much longer. Climate scientists have proven that the rise in human-produced greenhouse gases like carbon dioxide are “extremely likely” to be the dominant cause of global climate change. The likelihood of dangerous impacts—like sea level rise, hotter heat waves, and certain types of extreme weather—increases with each incremental annual rise in atmospheric carbon dioxide.
The data are even more striking when you take the long view (see figure).