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The latest human first has chilling consequences for our species, and all others: for the first time since scientists began tracking global carbon dioxide concentrations in our atmosphere, we have surpassed 400 parts per million worldwide.

The National Oceanic and Atmospheric Administration (NOAA) announced this dubious milestone. The agency measures and complies data from 40 locations around the world.

"It was only a matter of time that we would average 400 parts per million globally," lead scientist of NOAA's Global Greenhouse Gas Reference Network, Pieter Tans said in a statement on March 6. "We first reported 400 ppm when all of our Arctic sites reached that value in the spring of 2012. In 2013 the record at NOAA's Mauna Loa Observatory first crossed the 400 ppm threshold. Reaching 400 parts per million as a global average is a significant milestone."

"This marks the fact that humans burning fossil fuels have caused global carbon dioxide concentrations to rise more than 120 parts per million since pre-industrial times. Half of that rise has occurred since 1980."

The implications of these facts are sobering. Even if the rate of carbon emissions is somehow immediately stabilized, this will not prevent climate change. The NOAA has also announced that by 2040 summers in the Arctic will be ice-free-a fact that will have major, permanent implications for ecosystems around the globe starting with zooplankton in the area experiencing drastic loss of fat.

Climate scientist Dr. Ed Hawkins of the University of Reading told the Guardian: "This event is a milestone on a road to unprecedented climate change for the human race. The last time the Earth had this much carbon dioxide in the atmosphere was more than a million years ago, when modern humans hadn't even evolved yet."

The NOAA and partner agencies take these measurements to assist policymakers as well as the scientific community. The choice of the 40 global sites for sampling are in remote areas; this allows for more accurate results.

"We choose to sample at these sites because the atmosphere itself serves to average out gas concentrations that are being affected by human and natural forces. At these remote sites we get a better global average," NOAA scientist who manages the global network, Ed Dlugokencky says.

In March Dlugokencky predicted that the global average would stay above 400 ppm through May. This is significant because at this time of year natural atmospheric and plant growth cycles cause global carbon dioxide concentrations to peak. This prediction appears to be well on its way to being borne out.

What options will remain to us as we face our planet's growing list of troubles? Perhaps not as many as we'd like. NOAA's director of NOAA's Global Monitoring Division, James Butler, points out that the trend may not be easily reversible:

"Elimination of about 80 percent of fossil fuel emissions would essentially stop the rise in carbon dioxide in the atmosphere, but concentrations of carbon dioxide would not start decreasing until even further reductions are made and then it would only do so slowly."

How could an 80 percent reduction of fossil fuel emissions be achieved? The answer isn't completely clear, but at least one expert thinks the technology to manage this 80 percent reduction already exists. The question remains whether countries and people will do what would be necessary to make the change.