The Mauna Loa Observatory in Hawaii.

The Mauna Loa Observatory in Hawaii. Creative Commons: Abby Swann, 2008.

Data released today by the World Meteorological Organization (WMO) reveal that the heat-trapping greenhouse gases in our atmosphere topped unprecedented levels throughout the northern hemisphere in April.
The figures show that that concentrations of atmospheric carbon dioxide (CO2) averaged more than 400 parts per million (ppm) throughout April, the first time the planet’s monthly average has surpassed the milestone. The average value for the month—which measured 401.33ppm—was calculated from data collected at the Mauna Loa observation station in Hawaii.
The 400ppm threshold was initially reached in May 2013, creating conditions that were last seen three million years ago during the Pliocene Epoch. Estimates show that CO2 levels reached up to 415ppm during the Pliocene, when temperatures were 3-4°C higher than today (and as much as 10°C higher at the poles) and sea levels were as much as 40 meters higher than today.
In a statement, WMO Secretary-General Michel Jarraud said:

This should serve as yet another wakeup call about the constantly rising levels of greenhouse gases which are driving climate change. If we are to preserve our planet for future generations, we need urgent action to curb new emissions of these heat-trapping gases. Time is running out.

While crossing the milestone has much symbolic heft, scientists are more concerned by the fast pace of rising emissions levels. When the Mauna Loa Observatory began collecting data in 1958, CO2 levels were only 313ppm, and before the Industrial Revolution they were about 280ppm. Now, though, atmospheric CO2 levels are rising at an average of 2-3ppm per year, and recent data show that global CO2 concentration increased by a record amount in 2013.
According the Intergovernmental Panel on Climate Change (IPCC), atmospheric CO2 levels above 450ppm would correlate to an expected temperature increase of 2°C over pre-industrial levels. If this international redline is crossed, massive impacts from climate change are expected, including the complete deglaciation of West Antarctica, the collapse of the Atlantic Meridional Overturning Circulation (MOC), and the intensification of extreme weather events such as floods, hurricanes, droughts, and heatwaves around the globe.
According to the IPCC’s latest report, if action to limit greenhouse gas emissions is not taken, global temperatures will increase between 3.7 and 4.8°C by 2100.