Highest in human history at 400ppm, carbon dioxide levels are likely to have risen due to the gigantic El Niño that swept through the Pacific Ocean in 2015.Carbon dioxide levels in the Earth’s atmosphere have hit the highest point in human history, scientists have announced.Measured at the top of the Mauna Loa observatory on top of a volcano in Hawaii, CO2 levels have broken the 400 parts-per-million (ppm) boundary for the first time, reports mirror-.co.uk.That means for every million molecules in the atmosphere, 400 of them are carbon dioxide. It might not sound like a lot — but experts are very worried about the implications.They say it’s highly unlikely that levels will go back under this point in our lifetime.Crossing the 400 ppm boundary was predicted — but it serves as a reminder of the growing threat from climate change.The UN's Intergovernmental Panel on Climate Change (IPCC) has predicted that once we go over the 450 ppm threshold, the results could be catastrophic. It would contribute to warming the planet by two degrees and cause unpredicted shifts in weather patterns on Earth.Current predictions are that if humans don’t cut emissions by 40-70 per cent, we’ll break the 450 ppm mark in about twenty years.Part of the reason we’ve broken through the 400 ppm level this year is due to the gigantic El Niño that swept through the Pacific Ocean over the course of 2015. It caused high air pressure around the tropics that limited the ability of forests to draw in CO2 from the atmosphere. There were also vast forest fires that spit out extra carbon into the air. The combined effect of the El Niño and our continued burning of fossil fuels has resulted in breaking the 400 ppm barrier sooner than expected.“It’s important to note that this year’s rise in CO2is bigger than the last El Niño, in 1997/8, because human emissions have gone up by 25 per cent since,” said professor Richard Betts at the Met office. “We could be passing above 450 ppm in roughly 20 years. If we start to reduce our global emissions now, we could delay that moment but it is still looking like a challenge to stay below it,” he added.