(NASA) – The global concentration of carbon dioxide in the atmosphere – the primary driver of recent climate change – has reached 400 parts per million (ppm) for the first time in recorded history, according to data from the Mauna Loa Observatory in Hawaii.
Since 1958, the Mauna Loa Observatory has been gathering data on how much carbon dioxide is in the atmosphere. Carbon dioxide has increased by about 24 percent since the beginning of this record. (Source: NOAA) We rounded up a few scientists here at NASA and asked them what passing 400 ppm means to them. 
CO2 concentrations haven’t been this high in millions of years. Even more alarming is the rate of increase in the last five decades and the fact that CO2 stays in the atmosphere for hundreds or thousands of years. This milestone is a wake up call that our actions in response to climate change need to match the persistent rise in CO2. Climate change is a threat to life on Earth and we can no longer afford to be spectators. – Dr. Erika Podest, carbon and water cycle research scientist 
In some ways, 400 ppm is just a number, another milestone that we are blasting past at a rate that is now exceeding 2 ppm per year. Over time, this number takes on greater weight. It brings home the fact that fossil fuel combustion, land use practices, and human activities have increased the CO2 concentration in Earth’s atmosphere by more the 20 percent since I was born. Wow! – Dr. David Crisp, Principal Investigator, Orbiting Carbon Observatory-2 satellite mission
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Global Climate Change: NASA scientists react to 400 ppm carbon milestone
