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, U.S. scientists
Carbon dioxide was measured at 400 parts per million
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's a
"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, such
as coal for electricity and oil for gasoline, has
caused the overwhelming bulk of the man-made 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 80 parts per
million, 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, said
Pennsylvania State University climate scientist
Michael Mann. If carbon dioxide levels go up 100
parts per million over thousands or millions of
years, plants and animals can adapt. But that can't
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 (33
to 66 feet)."
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 315 parts per million.
Levels are now growing about 2 parts per million
per year. That's 100 times faster than at the end
of the Ice Age.
Before the Industrial Revolution, carbon dioxide
levels were around 280 ppm, and they were closer
to 200 during the Ice Age, which is when sea levels
shrank and polar places went from green to icy.
Some scientists and environmental groups promote
350 parts per million as a safe level for CO2,
but scientists acknowledge they don't really know
what levels would stop the effects of global warming.
"Physically, we are no worse off at 400 ppm
than we were at 399 ppm," Princeton University
climate scientist Michael Oppenheimer said. "But
as a symbol of the painfully slow pace of measures
to avoid a dangerous level of warming, it's somewhat
The world pumps on average 2.4 million pounds
of carbon dioxide into the air every second for
a total of 38.2 billion tons in 2011, according
international calculations published in a scientific
journal in December. China spews 10 billion tons
of carbon dioxide into the air per year, leading
all countries, and its emissions are growing about
10 percent annually. The U.S. at No. 2 is slowly
cutting emissions and is down to 5.9 billion tons
Environmental activists, such as former U.S. Vice
President Al Gore, seized on this week's milestone.
"This number is a reminder that for the last
150 years — and especially over the last
several decades — we have been recklessly
polluting the protective sheath of atmosphere that
surrounds the Earth and protects the conditions
that have fostered the flourishing of our civilization," Gore
said in a statement. "We are altering the
composition of our atmosphere at an unprecedented
There are natural ups and downs of the greenhouse
gas, which comes from volcanoes and decomposing
plants and animals. But that's not what has driven
current levels so high, Tans said. He said the
amount should be even higher, but the world's oceans
are absorbing quite a bit, keeping it out of the
Carbon dioxide traps heat just like in a greenhouse
and most of it stays in the air for about a century.
Some lasts for thousands of years, scientists say.
It accounts for three-quarters of the planet's
heat-trapping gases. There are others, such as
methane, which has a shorter life span but traps
heat more effectively. Both trigger temperatures
to rise over time, scientists say, which is causing
sea levels to rise and some weather patterns to
Last year, regional monitors briefly hit 400 ppm
in the Arctic, but those monitoring stations aren't
seen as a world mark like the one at Mauna Loa,
Generally carbon levels peak in May then fall
slightly, so the yearly average is usually a few
parts per million lower than May levels.
Associated Press writer Seth Borenstein contributed.