POLL: Is Global Warming Fact Or Fiction?

A report out Thursday states that the world's air has reached what scientists call a troubling new milestone.

Monitoring stations across the Arctic this spring are measuring more than 400 parts per million of carbon dioxide in the atmosphere; the gas is the main global warming pollutant, according to scientists.

The 350 ppm mark is what many scientists say is the highest safe level for carbon dioxide. The number now stands globally at 395, with the Arctic region at the 400 level.

"The fact that it's 400 is significant," Jim Butler, global monitoring director at the National Oceanic and Atmospheric Administration's Earth System Research Lab in Boulder, Colo., told the Associated Press. "It's just a reminder to everybody that we haven't fixed this and we're still in trouble."

It's been at least 800,000 years — probably more — since Earth saw carbon dioxide levels in the 400s, Butler and other climate scientists said.

The 400-level readings have come from Alaska, Greenland, Norway, Iceland and even Mongolia.

Globally, the average carbon dioxide level will pass the 400 mark within a few years, scientists said.

The Arctic is the leading indicator in global warming, both in carbon dioxide in the air and effects, Pieter Tans, a senior NOAA scientist told the Associated Press.

"This is the first time the entire Arctic is that high," he said.

Tans called reaching the 400 mark "depressing," and Butler said it was "a troubling milestone."

"It's an important threshold," Carnegie Institution ecologist Chris Field, a scientist who helps lead the Nobel Prize-winning Intergovernmental Panel on Climate Change, told the Associated Press. "It is an indication that we're in a different world."

Tans said the readings show how much the Earth's atmosphere and its climate are being affected by humans. Global carbon dioxide emissions from fossil fuels hit a record high of 34.8 billion tons in 2011, up 3.2 percent, the International Energy Agency announced last week.

"The news today, that some stations have measured concentrations above 400 ppm in the atmosphere, is further evidence that the world's political leaders — with a few honorable exceptions — are failing catastrophically to address the climate crisis," former Vice President Al Gore told the Associated Press. "History will not understand or forgive them."