Just two weeks into 2015, experts are expressing surprise and worry that the level of carbon dioxide in the atmosphere has already topped the 400 parts per million threshold several times—a troubling indication for the year ahead and an expression of humanity's continued failure to act on climate change.

"The new year has only just begun, but we’ve already recorded our first days with average carbon dioxide levels above 400 parts per million, potentially leading to many months in a row above this threshold," journalist Andrea Thompson wrote for Climate Central, an independent organization of scientists and reporters.

Thompson based her analysis on records from the Scripps Institution of Oceanography, showing that Jan. 1 was the first day of the new year above the 400 ppm concentration, followed by Jan. 3 and Jan. 7. Daily averages have continued at this level or higher through Jan. 9, "though they could continue to dance up and down around that mark due to day-to-day variations caused by weather systems," she wrote.

The world first hit the 400 ppm milestone in May, 2013.

This past summer, scientists observed that June was the third month in a row where, for the entire month, average levels of carbon dioxide were above 400 ppm—marking the longest time in recorded history that so much CO2 has been in the atmosphere. ThinkProgress reports:

The finding was troubling to climate scientists, several of whom told ThinkProgress at the time that the levels reminded them that humans are still emitting unsustainable amounts of greenhouse gases. If the trend continues, some worried that carbon levels might soon surpass 450 ppm—a level that many scientists worry would create a level of global warming that would be too difficult for some humans to adapt to.

"CO2 levels continue to increase, the amount of heat in the climate system continues to increase, ice continues to melt, and the seas continue to rise," said Ken Caldeira, an atmospheric scientist at the Carnegie Institution for Science’s Department of Global Ecology. "We will continue to break through threshold after threshold—unless we stop using the sky as a waste dump soon."

"The world’s plants can only pull so much CO2 out of the atmosphere in a given season, while human emissions keep rising," Thompson explained. "This is leaving an excess of about 2 ppm of CO2 in the atmosphere every year, meaning the 400 ppm mark will keep occurring earlier and earlier. In just a year or two, carbon dioxide levels will likely be about 400 ppm year-round."