Carbon dioxide levels in the atmosphere have been recorded at their highest level yet. Scientists on Friday announced that they had measured 400 parts per million (ppm) at their monitoring station in Hawaii.

"What we see today is 100 percent due to human activity," said Pieter Tans, a senior scientist with the National Oceanic and Atmospheric Administration.

Reaching the 400 ppm mark has long been anticipated and dreaded by climate experts and environmental activists. The rise, scientists say, is caused mainly by the burning of fossil fuels, such as coal and oil. The heat-trapping greenhouse gas is also emitted through natural activities, such as breathing.

Experts fear that the rate of the increase is too high for plants and animals to adapt.

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

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

Tracking levels

When measurements were first taken in 1958, the carbon dioxide level was 315 ppm. So a rise of more than 80 ppm has taken place in 55 years.  

To compare such climate change historically, Tans points to the end of the Ice Age, when it then took another 7,000 years for carbon dioxide levels to rise by 80 ppm.

He also noted that it was probably about 2 million years ago, during the Pleistocene Era, when the earth last endured such high levels of carbon dioxide.

The 400 ppm mark was hit briefly last year in the Arctic. But it is Mauna Loa, Hawaii, where the recent 400-level measurements have been taken. Mauna Loa, a volcanic mountain located on the Big Island of Hawaii, is considered the world's benchmark site for such measurements.  

Carbon levels usually peak worldwide in May before falling a bit, so the annual average may fall just short of the 400 ppm mark.

Scientists and activists have said that carbon concentrations should be pushed back to or below 350 ppm in order to keep the average temperature increase below 3.6 degrees Fahrenheit (2C) in the 21st Century.

tm/slk (AP, AFP, Reuters)