THE HAGUE — If uncertainty runs rampant in the global-warming debate, it is in part because scientific data is often too complex to be well understood by anyone but climate scientists.
This month, however, the world is likely to reach a scientific milestone that appears impressively scary even to those with only a cursory knowledge of climate science.
For the first time in human history, atmospheric carbon dioxide levels will surpass 400 parts per million, according Scripps Institution of Oceanography, which has been measuring carbon dioxide in the atmosphere at the Mauna Loa volcano in Hawaii since 1958.
“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 of the Scripps Institution in a statement.
The level of carbon dioxide in the atmosphere is closely linked to global warming. The more carbon dioxide, the higher global average temperatures have climbed, according to climate science. (This graphic shows how global temperature and atmospheric carbon dioxide levels have been linked in the past 400,000 years)
When atmospheric carbon dioxide levels were first measured, they were in at 316 parts per million, according to a report in the scientific journal Nature. Pre-industrial revolution pollution levels were thought to be about 280 parts per million.
“Our addiction to fossil fuels has taken us over yet another scary indicator, to a place we’ve never been before in the human history,” said Kaisa Kosonen, a climate policy adviser with Greenpeace.
While the milestone is arbitrary (why is hitting 400 parts per million more alarming than a measurement of 399?), scientists say it’s an important reminder of how the levels continue to rise.
Even if the landmark 400 is reached this month, it’s unlikely stay there. As Ralph Keeling of the Scripps Institution points out, the levels of carbon dioxide and other greenhouse gases in our atmosphere fluctuates throughout the year, with springtime usually representing the highpoint of the cycle. Trees and other plants absorb carbon dioxide during the warmer months in the Northern Hemisphere, lowering the level.
“If CO2 levels don’t top 400 p.p.m. in May 2013, they almost certainly will next year,” the release quoted Dr. Keeling as saying, but in either case it will take several years before the atmospheric carbon dioxide levels remain above 400 parts per million year-round.
Carbon dioxide levels have become part of the broader discussion about global warming, in part because it’s a tangible signpost of change.
A group called 350.org has taken as its name what some scientists consider a safe level of carbon dioxide in the atmosphere. The group created a very visible campaign to urge American colleges and universities divest their holdings of stock in fossil fuel companies.
On its Web site, 350.org compares the planet’s plight and the high level of atmospheric carbon dioxide with an overweight patient with dangerously high levels of cholesterol.
“He doesn’t die immediately — but until he changes his lifestyle and gets back down to the safe zone, he’s at more risk for heart attack or stroke,” the Web site says.
According to one of Scripps’ recent Twitter posts, atmospheric carbon dioxide is still at 399, but the milestone could be reached any day.
“There will be no balloons or noisemakers to celebrate the event. Researchers who monitor greenhouse gases will regard it more as a disturbing marker of humanity’s power to alter the chemistry of the atmosphere and by extension, the climate of the planet,” wrote Richard Monastersky in Nature last week.
What do you think? Is reaching 400 parts per million an important milestone? Why or why not?