The atmospheric gas carbon dioxide, which scientists claim is mainly responsible for global warming, has exceeded 400 Parts Per Million (PPM) for the first time in months. Researchers say September 2016 will go down in history books as the month when the world crossed this detrimental milestone.

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Scientist Ralph Keeling told USA Today that for the first time in years, the weekly and daily values of the carbon dioxide levels have remained above 400 PPM. Keeling is responsible for monitoring the famed "Keeling Curve" put forward by his late father, Charles Keeling. The father and son duo have monitored the carbon dioxide levels since 1958 in Hawaii at the Mauna Loa Observatory.

"We won't be seeing a monthly value below 400PPM this year —or ever again for the indefinite future," Keeling wrote in his blog post. The increase in atmospheric gasses such as nitrous oxide, methane, and carbon dioxide have all contributed to climate change.

With carbon dioxide pollution on the increase since the age of the industrial revolution, scientists claim the levels had never indicated signs of decreasing. Instead, it was a matter of "when" rather than "if" the planet would cross the threshold, Climate Central reported.

Since the beginning of the industrial revolution, the atmosphere has warmed up to 1.8F. This year is set to be the warmest year on record with the planet warming up to the threshold of up to 2.7F. This threshold was a key figure in the Paris Climate Agreement adopted last year by countries across the globe.

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