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 a record level for greenhouse gases.
Climate scientists say that the symbolic threshold should serve as a call for action to begin reversing the damage caused to the environment by human activities and heavy use of polluting fossil fuels.
The data showing that the daily average CO2 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 difference may partly reflect time zone differences," Ralph Keeling told AFP in an email.
"NOAA uses UTC, whereas we use local time in Hawaii to define the start and stop of a given day. We are checking to see if Scripps also would have reported over 400 if we had used the same definition of the day as NOAA."
The Earth has not seen these levels of CO2 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," said Ward.
"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."
According to prominent US scientist Michael Mann, director of the Earth System Science Center at Penn State, 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 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."
Mann, who has authored two books on climate change, said the last time scientists are confident that CO2 was sustained at the current levels was more than 10 million years ago, during the middle of the Miocene Period.
Back then, global temperatures were hotter, ice was sparse and sea level was dozens of meters higher than it is 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.
"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."