For the first time in roughly 5 million years, the amount of carbon dioxide in the Earth's atmosphere could top 400 parts per million in the Northern Hemisphere next month.
Human ancestors were just learning how to walk on two feet about that time, in a world that was much warmer than the one we walk on today.
Carbon dioxide is the greenhouse gas that is responsible for 63% of the warming attributable to all greenhouse gases, according to the National Oceanic and Atmospheric Administration's Earth System Research Lab.
This latest report comes from the Scripps Institution of Oceanography, keepers of the famed "Keeling Curve," the longest continuous record of carbon dioxide measurements on the planet. The measurements were begun in 1958 by Scripps climate scientist Charles Keeling and taken near the top of Mauna Loa on the Big Island of Hawaii.
When Keeling first began his measurements, the amount of carbon dioxide (also known as CO2) was 316 parts per million (ppm). As of Tuesday, the reading was 398.44 ppm as measured at Mauna Loa.
While CO2 levels topped 400 at stations at high northern latitudes last May, the Mauna Loa record has never exceeded 400 in the monthly average, according to Ralph Keeling of Scripps. The monthly average could top 400 ppm in May, he says. Daily and weekly averages of 400 ppm are also possible in May for the first time. Ralph Keeling is the son of the late Charles Keeling.
Increasing amounts of carbon dioxide and other gases caused by the burning of the oil, gas and coal that power our world are enhancing the natural "greenhouse effect," causing the planet to warm to levels that climate scientists say can't be linked to natural forces.
Carbon dioxide levels were around 280 ppm prior to the Industrial Revolution, when we first began releasing large amounts into the atmosphere by the burning of fossil fuels.
For the past 800,000 years, CO2 levels never exceeded 300 parts per million, according to Scripps, which measures CO2 levels along with several other agencies, including NOAA. Records of past levels of CO2 are found in samples of old air preserved as bubbles in the Antarctic ice sheet, Scripps reports.
"The 400-ppm threshold is a sobering milestone, and should serve as a wake up call for all of us to support clean energy technology and reduce emissions of greenhouse gases, before it's too late for our children and grandchildren," said Tim Lueker, a Scripps oceanographer.