For the first time, measurements of atmospheric carbon dioxide (CO2) taken at Mauna Loa Observatory in Hawaii exceeded 400 parts-per-million on an hourly basis. It's a symbolic benchmark, but an important one, suggesting that efforts to curb greenhouse gas emissions have not yet shown any significant effect.

The Sydney Morning Herald reports on the significance of the new readings.

The daily CO2 level, measured at the Mauna Loa Observatory in Hawaii, was 399.72 parts per million last Thursday, and a few hourly readings had risen to more than 400 parts per million. …

The last time CO2 reached the symbolic milestone of 400 parts per million in the atmosphere - in the Pliocene era - temperatures rose by between 3 and 4 degrees and sea levels were between five and 40 metres higher than today. Carbon dioxide levels have been rising steadily since constant measurements began at the Hawaiian observatory in 1958, when the level was about 317 parts per million.

We're hitting that mark sooner than we anticipated. Last month, Peter Gleick of the Pacific Institute wrote an article titled, "An Inevitable Headline in 2014: Planet’s CO2 level reaches 400 ppm for first time in human existence." Gleick was overly optimistic. Over the past two decades, the rate at which carbon dioxide has collected in the atmosphere has increased slightly.

Climate scientist James Hansen, formerly of NASA, argued in 2009 that any amount of CO2 above 350 ppm would result in a dangerous amount of warming. (Hansen's advocacy spurred the creation of 350.org, a climate change advocacy organization.) He outlined his case that year.

Achieving a limit of 350 ppm appears increasingly unlikely, depending on sudden, massive shifts in the world's fossil fuel consumption. Each second, the world produces 2.4 million pounds of additional carbon dioxide. Meaning that it's more likely that CO2 density will increase, not decrease. The Morning Herald quotes Ralph Keeling of the Scripps Institution of Oceanography, which operates Mauna Loa. "I wish it weren't true but it looks like the world is going to blow through the 400 ppm level without losing a beat," he told the paper. "At this pace we'll hit 450 ppm within a few decades."

A mark of 450 ppm is widely considered the highest level of atmospheric carbon dioxide that would prevent the worst effects of global warming, though some, like Hansen, think that figure is too high. We may soon find out.