Something happened on Earth Thursday that scientists believe the planet hasn't experienced in as many as three to five million years.
Atop the Mauna Loa Observatory on the Big Island of Hawaii, sensors tracking all the substances swirling around in Earth's atmospheric soup are -- for the first time -- recording carbon dioxide at levels higher than at any time in human history.
Carbon dioxide, shorthanded as "CO2," is the primary greenhouse gas emitted into the air when people burn fossil fuels. It traps heat in the atmosphere that would normally radiate back into space. Simply put, the more carbon dioxide, the hotter we get.
Scientists express the level of carbon dioxide according to how many molecules of CO2 are floating around in a million molecules of air -- the "parts per million" measurement, or "ppm."
On Thursday, CO2 in the atmosphere surpassed a new milestone of 400.03 ppm, according to the National Oceanic and Atmospheric Administration. The Scripps Institution of Oceanography, which also tracks the number, recorded a slightly lower number of 399.73.
The fact that CO2 levels have hit 400 ppm comes as no surprise to scientists who have watched the number steadily climb since around 1780, the start of the Industrial Revolution. Back then, CO2 levels were about 280 ppm. By 1958, when scientist Charles Keeling began measuring CO2 on Mauna Loa (known today as the "Keeling Curve"), the number had risen to 317 ppm.
Scientists have long predicted that an unprecedented period of human prosperity built on the use of oil and coal would come with dark side effects for the climate.
"This generation has altered the composition of the atmosphere on a global scale," President Lyndon Johnson told Congress in 1965, through "a steady increase in carbon dioxide from the burning of fossil fuels."
What worries scientists in 2013 is not only the amount of CO2 in the atmosphere, but how fast it continues to build up without showing any sign of slowing or even stabilizing.
Today's rate of carbon dioxide increase is more than 100 times faster than the increase that occurred when the last ice age ended, NOAA said today.
At this rate, even 400 ppm will soon vanish in the rearview mirror. Unless emissions are slowed, scientists tell us that babies being born today will enter their thirties as the CO2 level reaches 450 ppm.
Without a concerted worldwide effort to curb greenhouse gas emissions, scientists predict global warming will produce a cockeyed climate concoction laced with increasing numbers of heat waves, melting glaciers, higher sea levels and more extreme weather.