Worldwide levels of the greenhouse gas that plays the biggest role in global warming have reached their highest level in almost 2 million years — an amount never before encountered by humans, US scientists said on Friday.
Carbon dioxide was measured at 400 parts per million (ppm) on Thursday at the oldest monitoring station in Hawaii, which sets the global benchmark.
The number 400 has been anticipated by climate scientists and environmental activists for years as a notable indicator, in part because it is a round number.
“What we see today is 100 percent due to human activity,” said Pieter Tans, a senior scientist with the National Oceanic and Atmospheric Administration.
The burning of fossil fuels has caused the overwhelming bulk of the manmade increase in carbon in the air, scientists say.
At the end of the Ice Age, it took 7,000 years for carbon dioxide levels to rise by 80ppm, Tans said.
Because of the burning of fossil fuels, carbon dioxide levels have gone up by the same amount in just 55 years.
The speed of the change is the big worry, Pennsylvania State University climate scientist Michael Mann said. If carbon dioxide levels go up 100ppm over thousands or millions of years, plants and animals can adapt — but that cannot be done at the speed it is now happening.
The last time the worldwide carbon level was probably this high was about 2 million years ago, Tans said. That was during the Pleistocene Era.
“It was much warmer than it is today,” Tans said. “There were forests in Greenland. Sea level was higher, between 10 and 20 meters.”
Other scientists say it may have been 10 million years since Earth last encountered this level of carbon dioxide. The first modern humans only appeared in Africa about 200,000 years ago.
When measurements were first taken in 1958, carbon dioxide was measured at 315ppm. Levels are now growing about 2ppm per year. That is 100 times faster than at the end of the Ice Age.
Before the Industrial Revolution, carbon dioxide levels were about 280ppm, and they were closer to 200ppm during the Ice Age, which is when sea levels shrank and polar places went from green to icy.
Some scientists and environmental groups promote 350ppm as a safe level for carbon dioxide, but scientists acknowledge they do not really know what levels would stop the effects of global warming.