Atmospheric CO2 typically peaks in May when measured at the commonly accepted standard of Mauna Loa in Hawai’i. May 2017 hit 409.65 ppm, exceeding last May’s record of 407.7ppm. The planet has warmed by 1.8C, just below the limit of 2C which is believed to be the threshold where radical and irrevocable climate change will occur. Some argue that the limit was at 1.5C. Because the planet is so complex, there may not be one simple number. Each system may have its limit. Whatever the limits are, atmospheric carbon dioxide continues to increase at dangerous rates.