Carbon milestone a reminder of things to come

Posted: May 13, 2013

Written by

Jean Chemnick, E&E News PM

The Earth's atmospheric carbon dioxide levels passed an ominous milestone yesterday when a National Oceanic and Atmospheric Administration observatory on the Big Island in Hawaii registered 400 parts per million for the first time.

It's a concentration of CO2 that has not existed in the Earth's atmosphere for at least 3 million years, and it could be enough to trigger 2 degrees Celsius in warming compared with preindustrial levels (ClimateWire, May 10).

But many scientists say the threshold serves as a reminder that man-made CO2 emissions are continuing to rise and will push global temperatures up with them unless steps are taken to reverse that trend.

"It's a milestone," said Susan Joy Hassol of Climate Communication.

Many scientists believe that before the Industrial Revolution, atmospheric greenhouse gas levels were at 280 or 290 ppm and have risen steadily since due to increased combustion of fossil fuels. Emissions have accelerated in recent years.

"I remember when I started in this business, we were still talking about whether we could stabilize at 350 [ppm]," Hassol said.

It is unclear what 400 ppm means for warming. Policymakers around the world often express a goal of keeping atmospheric temperatures below the 2-degrees-Celsius mark, a threshold at which Arctic ice is projected to melt every summer and which would spur everything from more forest fires to bleaching of coral reefs.

But scientists say it is hard to be sure how much CO2 equals 2 degrees and instead offer a range of 370 ppm to 540 ppm. But the higher end of that range is just as likely to mean 3 degrees of warming, depending on a variety of factors.

"It's very difficult to pin down a number," said Melanie Fitzpatrick of the Union of Concerned Scientists. "What we do know is that the last time we were at 400 ppm, it was a couple of degrees higher," she said, referring to the period between 3 million and 5 million years ago.

But Fitzpatrick and other scientists said the 400 ppm mark, which was recorded at the observatory yesterday and announced today, was more of a wake-up call for policymakers than a scientific milestone.

"We're looking at 400 ppm as a reminder that we still need strong action," she said. "We really have a choice right now, because the emissions pathway we choose from here determines what our climate impacts are."

Another point scientists make is that once CO2 enters the Earth's atmosphere, it tends to stick around for millennia, making it progressively harder to manage climatic effects.

"We're going to be living with 400 at least for a very long time, barring some miraculous unheard-of technology that allows us to suck carbon dioxide out of the air," said Joe Casola, staff scientist at the Center for Climate and Energy Solutions.

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