THE level of carbon dioxide in the atmosphere has surpassed 400 parts per million for the first time in human history, US monitors say, sparking new calls for action to scale back greenhouse gases.

Climate scientists say the threshold is largely symbolic and has been expected for some time, but warn that it serves as an important message that people need to reverse the damage caused to the environment by the heavy use of fossil fuels.

The Earth has not seen these levels of CO2 in three to five million years, long before humans existed, in a time when temperatures were several degrees Celsius warmer and the sea level was 20-40 metres higher than today, experts say.

"We are creating a prehistoric climate in which human societies will face huge and potentially catastrophic risks," said Bob Ward, policy director at the Grantham Research Institute on Climate Change and the Environment at the London School of Economics and Political Science.

"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."

Data showing that the daily average CO2 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 concurred, with its measurements showing atmospheric carbon dioxide at 400.08 ppm.

Michael Mann, director of the Earth System Science Center at Penn State, said the main concern is the speed with which the concentrations of CO2 are rising.

"There is no precedent in Earth's history for such an abrupt increase in greenhouse gas concentrations," Mann, who has authored two books on climate change, told AFP.

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

"What we are doing is unburying it. But not over 100 million years. We're unburying it and burning it over a timescale of 100 years, a million times faster."

Experts have long expected the 400 ppm threshold would be passed.

"In itself the value 400 ppm of CO2 has no particular significance for the physics of the climate system," said Joanna Haigh, atmospheric physicist and head of the department of physics at Imperial College London.

"However, this does give us the chance to mark the ongoing increase in CO2 concentration and talk about why it's a problem for the climate."

Haigh said that unless swift action is taken, "the planet will warm by more than two degrees Celsius, which is the temperature threshold that scientists are worried about."

Pre-industrial measurements of CO2 were about 280 ppm. Greenhouse gases have risen steadily since records began in the 1950s, and are likely to soar by the end of the century, said the Grantham Institute's director Brian Hoskins.

"We'll certainly see them rise higher than they are now. Given current human activity, levels of CO2 could be near 800 ppm by end of century," he said.

"Unless as a society we devise ways to remove CO2 directly from atmosphere, such as through negative emissions technologies, we're going to be stuck with a very slow decrease of CO2 from peak levels, and everybody will have to deal with the implications of global warming."