The level of carbon dioxide in the atmosphere has broken above 400 parts per million for the first time in human history, US monitors announced Friday, indicating that climate change has reached a new high.

The data showing that the average C02 was 400.03 as of May 9 was posted online by the National Oceanic and Atmospheric Administration's monitoring center in Mauna Loa, Hawaii.

The air measurements represent conditions over the Pacific Ocean, and the data is still considered "preliminary," pending quality control and recalibration checks, NOAA said.

A separate monitor at the Scripps Institution of Oceanography in San Diego, California has not yet returned the same reading, with its latest data on May 9 showing that atmospheric carbon dioxide was at 399.73 parts per million.

The Earth has not seen these levels of C02 in millions of years, according to 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.

"The Earth has not experienced this level of carbon dioxide in the atmosphere for about three million years, when global average temperature was two to three centigrade degrees higher than pre-industrial levels, the polar ice caps were much smaller and global sea level was about 20 meters (yards) higher than today," Ward said.

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