Carbon dioxide levels throughout the northern hemisphere hit 400 parts per million (ppm) for the first time in human history in April, an ominous threshold for climate change, the World Meteorological Organization said on Monday.

The 400 ppm level in the atmosphere, up 40 percent since wide use of fossil fuels began with the Industrial Revolution, is rapidly spreading southwards. First recorded in 2012 in the Arctic, it has since become the norm for the Arctic spring.

The WMO expects the global annual average carbon dioxide concentration to be above 400 ppm in 2015 or 2016. Rising concentrations of the heat-trapping gas raise risks of more heatwaves, droughts and rising sea levels.

"Time is running out," WMO Secretary-General Michel Jarraud said in a statement.

So, yeah. That was a beautiful, warm end to Memorial Day Weekend in Washington State. Now, back to reality.