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Although carbon dioxide levels have reached 400 parts per million in the past in isolated locations and times, 2015 was the first year that the global average levels for the entire year reached the 400 parts per million mark, according to the report.

The WMO predicts that the carbon dioxide concentrations will stay above this threshold for the entirety of 2016, \"and not dip below that level for many generations.\"

Taalas applauded the recent international agreement in Kigali, Rwanda, to phase out hydrofluorocarbons, which have been shown to eat away at the ozone layer in the atmosphere.

\"[B]ut the real elephant in the room is carbon dioxide, which remains in the atmosphere for thousands of years and in the oceans for even longer. Without tackling CO2 emissions, we cannot tackle climate change and keep temperature increases to below 2°C above the pre-industrial era,” Taalas said in a statement.

New era of climate reality. For 1st time, CO2 in atmosphere averaged 400 ppm for whole world and whole year in 2015. #climatechange. pic.twitter.com/67ESDODoyc

— WMO | OMM (@WMO) October 24, 2016

“It's just a milestone more than anything,\" Ed Dlugokencky, a researcher who monitors carbon dioxide and other atmospheric gases with the National Oceanic and Atmospheric Administration’s Earth System Research Laboratory, told ABC News today of passing the 400 parts per million threshold. \"The alarming thing is that CO2 keeps going up, and the rate of increase keeps accelerating.”

Pre-industrial levels of carbon dioxide in the atmosphere was about 280 parts per million, Dlugokencky noted.

If we continue on the same trajectory with carbon emissions, the results would be that \"it gets warmer, ocean get more acidic,\" Dlugokencky added.

\"The tipping point that we don’t want to reach is where ocean levels rise to the point that they would inundate major coastal cities,\" Dlugokencky said. \"The timescale for this is quite long, but to reduce emissions sufficiently we have to start acting soon.\"

“What we find is that approximately half the CO2 that is emitted into the atmosphere from fossil fuel combustion remains in the atmosphere, so as we emit more, the amount that stays in the atmosphere increases,” Dlugokencky added.

The carbon dioxide concentration in our atmosphere will most likely not drop below 400 parts per million in our lifetime, he noted.

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