Atmospheric concentrations of carbon dioxide will shatter the symbolic barrier of 400 parts per million this year and will not fall below it during the lifetimes of people alive today, according to a new UK Met Office study published in the journal Nature Climate Change.
Carbon dioxide measurements at the Mauna Loa observatory in Hawaii are forecast to increase by a record 3.1ppm this year - up from an annual average of 2.1ppm - due in large part to the cyclical El Niño weather event in the Pacific. The surge will be larger than during the last big El Niño in 1997/98, because manmade emissions have increased by 25% since then.
The Met Office also attributes around a fifth of the current El Niño's severity to forest fires, which were started by humans and exacerbated by drought.
The report's lead author, professor Richard Betts of the Met's Hadley Centre and Exeter University, said the fact that the 400 ppm threshold had been breached a year earlier than expected carried a warning for the future.
"Once you have passed that barrier, it takes a long time for carbon dioxide to be removed from the atmosphere by natural processes," he said. "Even if we cut emissions, we wouldn't see concentrations coming down for a long time, so we have said goodbye to measurements below 400ppm at Mauna Loa."
Exceeding the 400ppm evel will not change any climate change fundamentals. Rather, it marks a psychological barrier, a reminder of the ticking clock of global warming. The UN's Intergovernmental Panel on Climate Change (IPCC) says that CO2 concentrations must be stabilised at 450ppm to have a fair chance of avoiding global warming above 2C, which could carry catastrophic consequences. Achieving that that will require a 40-70% emissions cut by 2050, compared to 2010 levels, and zero emissions by the end of the century.
However, despite the Paris agreement last December and a boost in renewable energy that has at least temporarily checked the growth in global emissions, the world is on track to substantially overshoot the target.
"We could be passing above 450ppm in roughly 20 years," Betts said. "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 450ppm. If we carry on as we are going, we could pass 450ppm even sooner than 20 years, according to the IPCC scenarios."
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