The level of carbon dioxide in the atmosphere has broken above a symbolic threshold, 400 parts per million (ppm), for the first time, US monitors have said, indicating a record level for greenhouse gases.

Climate scientists said the findings should serve as a call for action to reverse the damage caused by human activities and heavy use of polluting fossil fuels.

The Earth has not had these levels of carbon dioxide in millions of years, said Bob Ward, policy and communications director at the Grantham Research Institute on Climate Change and the Environment at London School of Economics and Political Science.

"We are creating a prehistoric climate in which human societies will face huge and potentially catastrophic risks," Ward said.

"Only by urgently reducing global emissions will we be able to bring carbon dioxide levels down and avoid the full consequences of turning back the climate clock."

'Abrupt increase'

Data showing that the daily average carbon dioxide level over the Pacific Ocean was 400.03 ppm as of May 9 was posted online by the National Oceanic and Atmospheric Administration's monitoring centre in Mauna Loa, Hawaii.

A separate monitor at the Scripps Institution of Oceanography in San Diego, California initially reported its May 9 data showing that atmospheric carbon dioxide was at 399.73 ppm, but later revised that to show 400.08 ppm.

The difference came down to the time zone, with NOAA using the universal time clock and Scripps reporting on Hawaii time. When Scripps adjusted its measurements to UTC time, it concurred with NOAA that 400 ppm threshold had been breached.

Michael Mann, climate change author and director of the Earth System Science Centre at Penn State, said the main concern was the speed with which the concentrations of CO2 were rising.

"There is no precedent in Earth's history for such an abrupt increase in greenhouse gas concentrations," Mann told AFP.

"While living things can adapt to slow changes that took place over tens of millions of years, there is no reason to believe that they, and we, can adapt to changes that are a million years faster than the natural background rates of change."

Global temperatures hotter

Mann said that the last time scientists were confident that carbon dioxide was sustained at the present levels was more than 10 million years ago, during the middle of the Miocene Period.

Global temperatures then were hotter, ice was sparse and sea levels were dozens of metres higher than today.

"It took nature hundreds of millions of years to change CO2 concentrations through natural processes such as natural carbon burial and volcanic outgassing," Mann said.

"We're unburying it and burning it over a timescale of 100 years, a million times faster."