PARIS • Scientists in the United States have detected the highest levels of planet-warming carbon dioxide in the earth's atmosphere since records began, sounding new alarm over the relentless rise of man-made greenhouse gas emissions.

The Mauna Loa Observatory in Hawaii, which has tracked atmospheric CO2 levels since the late 1950s, last Saturday detected 415.26 parts per million (ppm).

It was also the first time on record that the observatory measured a daily baseline above 415 ppm.

The last time earth's atmosphere contained this much CO2 was more than three million years ago, when global sea levels were several metres higher and parts of Antarctica were blanketed in forest.

"It shows that we are not on track with protecting the climate at all. The number keeps rising and it's getting higher year after year," Mr Wolfgang Lucht, from the Potsdam Institute for Climate Impact Research, told Agence France-Presse. "This number needs to stabilise."

But levels of CO2 - one of a trinity of greenhouse gases produced when fossil fuels are burnt - are climbing ever more rapidly.

Mr Ralph Keeling, director of Scripps Institution of Oceanography's CO2 Programme, said the trend would probably continue throughout 2019 - likely to be an El Nino year in which temperatures rise due to warmer ocean currents.

"The average growth rate is remaining on the high end. The increase from last year will probably be around 3 parts per million whereas the recent average has been 2.5 ppm," he said. "Likely we're seeing the effect of mild El Nino conditions on top of ongoing fossil fuel use."

The 2015 Paris Agreement calls on humanity to keep the rise in earth's temperature at "well below" 2 deg C compared with pre-industrial levels, and 1.5 deg C if possible.

The last four years were the four hottest on record and, in spite of the Paris deal and increasing public awareness of the problem, mankind continues to break its own emissions records, year on year.

Earth's average surface temperature has already risen by 1 deg C since pre-industrial times because of man-made emissions, the United Nations climate panel says.

"All of human history has been in a colder climate than now," said Mr Lucht. "Every time an engine runs, we emit CO2 and it has to go somewhere. It doesn't miraculously disappear, it stays in the atmosphere."

While there is some disagreement over what would constitute "safe" atmospheric CO2 levels, there is a broad consensus that 350 ppm - a level surpassed in the late 1980s - would stave off runaway global warming.

The 415 ppm threshold was first surpassed earlier this month and has already risen further.

"I'm old enough to remember when passing 400ppm was a big deal," Harvard University research associate Gernot Wagner said on Twitter. "Two years ago, we hit 410ppm for the first time. By now, it's 415ppm. And oh, the increase is increasing at an increasing rate."

AGENCE FRANCE-PRESSE