The amount of carbon dioxide in the Earth's atmosphere has exceeded 402 parts per million (ppm) during the past two days of observations, which is higher than at any time in at least the past 800,000 years, according to readings from monitoring equipment on a mountaintop in Hawaii. Carbon dioxide, or CO2, is the most important long-lived greenhouse gas responsible for manmade global warming, and it is building up in the atmosphere due to the burning of fossil fuels such as coal, oil and natural gas.

Once emitted, a single molecule of carbon dioxide can remain aloft for hundreds of years, which means that the effects of today's industrial activities will be felt for the next several centuries, if not thousands of years. Carbon dioxide and other greenhouse gases, such as methane, warm the planet by absorbing and redirecting outgoing solar radiation that would otherwise escape back into space.

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In 2013, atmospheric levels of carbon dioxide briefly hit 400 ppm for the first time in mid-May, but this year that symbolic threshold has been crossed even earlier. This means it is more likely that the annual peak, which typically occurs in mid-to-late May, will climb further above 400 ppm for the first time.

Although crossing above 400 ppm is largely a symbolic milestone, scientific research indicates that the higher that carbon dioxide concentrations get, the more global temperatures will increase, resulting in a wide range of damaging effects. These impacts will range from global sea level rise to a heightened risk of heat waves, severe droughts and floods, according to a recently released comprehensive assessment of climate science produced by the U.N. Intergovernmental Panel on Climate Change (IPCC).

The 800,000 year history of atmospheric carbon dioxide concentrations, showing the huge spike in the past century. The bottom axis, is measured in thousands of years. Image: Scripps Institution of Oceanography

Modern carbon dioxide monitoring began in 1958 on the peak of Hawaii's Mauna Loa volcano, which is more than two miles high. At that time, carbon dioxide concentrations were at just 313 ppm. They have risen rapidly and steadily since then, both at Mauna Loa and at other observatories around the world. The chart documenting this rise is perhaps the most iconic in all of climate science, known as the "Keeling Curve" for Charles David Keeling, the Scripps Institution of Oceanography scientist who began and maintained the monitoring program. Keeling's son, Ralph, is now continuing his father's work, although he has encountered funding difficulties in recent months.

According to the Keeling Curve website, carbon dioxide concentrations spiked to 402.20 parts per million on April 7, whereas data from the National Oceanic and Atmospheric Administration (NOAA) showed a slightly lower level of 402.11 parts per million on the same day. Both data sets indicate that daily carbon dioxide measurements have been at or above 400 ppm since March 29, and the graph appears on course to stay above 400 ppm throughout the rest of the month and into the next.

“The underlying significance of all this is that the rate of carbon dioxide increase is higher than ever at the moment, year after year now it’s been more than 2 ppm per year,” said Pieter Tans, a senior scientist at the National Oceanic and Atmospheric Administration's Climate Monitoring and Diagnostics Laboratory in Boulder, Colo. “The increase is manmade … This is one of the known things about climate change.”

While studies show conflicting dates for when Earth's atmosphere last had carbon dioxide levels this high, estimates range from 800,000 years ago to 15 million years ago. These estimates resulted from studies of air bubbles recovered in ice cores from deep within Antarctica, Greenland and other glaciers, as well as chemical analyses of coral samples from beneath the sea. Air bubbles trapped in the ice and chemical clues about atmospheric carbon levels in corals have shown that there is now more carbon dioxide in the air than at any other point in the history of human civilization.

“Carbon dioxide is higher now in the atmosphere than it has been for millions of years and the rate of increase has not been seen, certainly not in the ice core record going back 800,000 years," Tans told Mashable. "This is truly exceptional compared to times when geology or the natural system prevailed.”

While the Earth's atmosphere has seen higher levels of carbon dioxide than it does now, as well as higher temperatures and far greater sea levels, those instances were due to natural drivers of climate change, such as periodic variations in the planet's orbit and in solar energy output. Scientists have studied and ruled out natural climate drivers as the main cause of global warming since the preindustrial era.