Climate Milestone: Earth’s CO2 Level Passes 400 ppm

Greenhouse gas highest since the Pliocene, when sea levels were higher and the Earth was warmer.

Two teams of scientists at the Mauna Loa Observatory in Hawaii have been measuring carbon dioxide concentration there for decades, and have watched the level inch toward a new milestone.

An instrument near the summit of Mauna Loa in Hawaii has recorded a long-awaited climate milestone: the amount of carbon dioxide in the atmosphere there has exceeded 400 parts per million (ppm) for the first time in 55 years of measurement—and probably more than 3 million years of Earth history.

The last time the concentration of Earth's main greenhouse gas reached this mark, horses and camels lived in the high Arctic. Seas were at least 30 feet higher—at a level that today would inundate major cities around the world.

The planet was about 2 to 3 degrees Celsius (3.6 to 5.4 degrees Fahrenheit) warmer. But the Earth then was in the final stage of a prolonged greenhouse epoch, and CO2 concentrations were on their way down. This time, 400 ppm is a milepost on a far more rapid uphill climb toward an uncertain climate future.

Two independent teams of scientists measure CO2 on Mauna Loa: one from the U.S. National Oceanic and Atmospheric Administration (NOAA), the other from the Scripps Institution of Oceanography. The NOAA team posted word on its web site this morning before dawn Hawaii time: The daily average for May 9 was 400.03 ppm. The Scripps team later confirmed the milestone had been crossed.

The Scripps team is led by Ralph Keeling, son of the late Charles David Keeling, who started the Mauna Loa measurements in 1958. Since then the "Keeling curve," showing the steady climb in CO2 levels caused primarily by burning fossil fuels, has become an icon of climate change.

When the elder Keeling started at Mauna Loa, the CO2 level was at 315 ppm. When he died in June 2005, it was at 382. Why did he keep at it for 47 years, fighting off periodic efforts to cut his funding? His father, he once wrote, had passed onto him a "faith that the world could be made better by devotion to just causes." Now his son and the NOAA team have taken over a measurement that captures, more than any other single number, the extent to which we are changing the world—for better or worse.

Setting the Record Straight

Since late April that number had been hovering above 399 ppm. The Scripps lab opened the vigil to the public by sending out daily tweets (under the handle @Keeling_curve) almost as soon as the data could be downloaded from Mauna Loa, at 5 a.m. Hawaii time. NOAA took to updating its website daily. The two labs' measurements typically agree within .2 ppm. Both measure the amount of CO2 in an air sample by measuring how much infrared radiation it absorbs—the same process by which CO2 in the atmosphere traps heat and warms the whole planet.


A chart from Scripps shows the carbon dioxide level hovering near 400 ppm in the first week of May.


The measurement NOAA reported for Thursday, May 9, 400.03 ppm, was for a single day. Each data point on the Keeling curve, however, is actually an average of all the measurements made at Mauna Loa over an entire month. The CO2 concentration at Mauna Loa is unlikely to surpass 400 ppm for the whole month of May.

It certainly won't exceed 400 for all of 2013. CO2 peaks in May every year. By June the level will begin falling, as spring kicks into high gear in the Northern Hemisphere, where most of the planet's land is concentrated, and plants draw CO2 out of the atmosphere to fuel their new growth. By November, the CO2 level will be 5 or 6 ppm lower than it is now.

Then the curve will turn upward again: In the winter, plants stop making new carbohydrates but continue to burn the old, respiring CO2 back into the atmosphere.

This seasonal sawtooth—think of it as the breath of northern forests—is the natural part of the Keeling curve. The man-made part is its steady upward climb from one year to the next. Both were discovered at Mauna Loa.

Dave Keeling, as he was known, chose the Hawaiian mountain for his measurements because, at over 11,000 feet and in the middle of the Pacific, it is far from forests or smokestacks that might put a local bias on the data. But even Mauna Loa is not perfectly representative of the whole planet.

NOAA also monitors CO2 at a global network of stations, and the global average consistently lags the Mauna Loa number by a few parts per million—for a simple reason.

"Mauna Loa is higher because most of the fossil fuel CO2 is emitted in the Northern Hemisphere," says NOAA scientist Pieter Tans. It takes about a year, he says, for northern pollution to spread through the Southern Hemisphere.

On the other hand, Mauna Loa lags the Arctic, where CO2 levels are higher. A year ago, NOAA reported that the average of its Arctic measurements had exceeded 400 ppm for the entire month of May, not just for a single day.

The rest of the planet will catch up soon enough. By 2015 or 2016, the whole atmosphere will be averaging 400 ppm for the whole year. What difference will that make?

Back to the Pliocene?

In a way, 400 ppm is an arbitrary milestone, like a .400 batting average in baseball. But the fact that no one has batted .400 since Ted Williams in 1941 still says something important about baseball. The same goes for CO2 in Earth's atmosphere.

Policymakers worldwide have been stymied in their effort to reach a global agreement on reducing fossil fuel emissions. Many scientists argue that the CO2 concentration must be stabilized at 450 ppm to avoid the worst impacts of climate change. Some activists argue for a more ambitious goal of 350 ppm. NOAA has not recorded an average monthly CO2 reading below 350 ppm at Mauna Loa since October 1988.  (See related story: "Obama Pledges U.S. Action on Climate Change, With or Without Congress.")

The last time the concentration of CO2 was as high as 400 ppm was probably in the Pliocene Epoch, between 2.6 and 5.3 million years ago. Until the 20th century, it certainly hadn't exceeded 300 ppm, let alone 400 ppm, for at least 800,000 years. That's how far back scientists have been able to measure CO2 directly in bubbles of ancient air trapped in Antarctic ice cores.

But tens of millions of years ago, CO2 must have been much higher than it is now—there's no other way to explain how warm the Earth was then. In the Eocene, some 50 million years ago, there were alligators and tapirs on Ellesmere Island, which lies off northern Greenland in the Canadian Arctic. They were living in swampy forests like those in the southeastern United States today. CO2 may have been anywhere from two to ten times higher in the Eocene than it is today. (See related: "Hothouse Earth.")


Carbon dioxide levels can be seen climbing steadily in Scripps data from the last 55 years.


Over the next 45 million years, most of it was converted to marine limestone, as CO2-laden rains dissolved the ingredients of limestone out of rocks on land and washed them down rivers to the sea. CO2-belching volcanoes failed to keep pace, so the atmospheric level of the gas slowly declined. Some time during the Pliocene, it probably crossed the 400 ppm mark, as it's doing now-but back then it was on its way down. As a result, at the end of the Pliocene, it became cold enough for continental ice sheets to start forming in the northern hemisphere. The Pliocene, says geologist Maureen Raymo of Columbia University's Lamont-Doherty Earth Observatory, "was the last gasp of warmth before the slow slide into the Ice Ages."

What was Earth like then? In Africa, grasslands were replacing forests and our ancestors were climbing down from the trees. (See related: "The Evolutionary Road.") On Ellesmere, there were no longer alligators and cypress trees, but there were beavers and larch trees and horses and giant camels—and not much ice. The planet was three to four degrees Celsius warmer than it was in the 19th century, before man-made global warming began.

If anything, those numbers understate how different the Pliocene climate was. The tropical sea surface was about as warm as it is now, says Alexey Fedorov of Yale University, but the temperature gradient between the tropics and the poles—which drives the jet streams in the mid-latitudes—was much smaller. The east-west gradient across the Pacific Ocean—which drives the El Niño-La Niña oscillation—was almost nonexistent. In effect, the ocean was locked in a permanent El Niño. Global weather patterns would have been completely different in the Pliocene.

And yet the two main drivers of climate—the level of CO2, and the parameters of Earth's orbit, which determine how much sunlight falls where and at what season—were essentially the same as today. Fedorov calls it the Pliocene Paradox.

Climate scientists are just beginning to crack it, he says. Maybe clouds outside the tropics were darker in the Pliocene, such that they bounced less sunlight back to space. Maybe the warm ocean was stirred by a lot more hurricanes.

Hanging over this academic research is a very nonacademic issue: Could our climate be capable of flipping to a completely different state? "That's the big question—whether CO2 can move us to the Pliocene," says Fedorov.

Rising Seas

Beavers and camels on Ellesmere Island, instead of glaciers, might not be so bad.  But there was a lot less ice in general in the Pliocene. That means there was a lot more water in the ocean, which means sea level was a lot higher—how high exactly, no one knows.

"The estimates have been all over the map," Raymo says. They've ranged from 10 meters (33 feet) to 40 meters (131 feet) higher than today. But even the conservative estimate, were it to recur today, would mean flooding land inhabited by a quarter of the U.S. population.

Raised Pliocene shorelines have been identified all over the world. One is the Orangeburg Scarp, a wave-cut terrace that parallels the Atlantic coast of the U.S. from Florida to Virginia. Typically it lies more than a hundred miles inland. In the Pliocene, the Gulf Stream flowed past that terrace, over what is now the coastal plain.

The question is: How much has the sea receded since then, and how much has the land risen? Raymo has been asking that question on Pliocene shores in the U.S., Africa, Antarctica.

Land can rise, she explains, because it is was once depressed by massive ice sheets and is now rebounding. It can also rise because the underlying mantle is a hot, viscous fluid that pushes it up—by different amounts in different places. In Virginia the Orangeburg Scarp rises around 70 meters (220 feet) but in Florida only 30 meters (100 feet) above the current sea level. Yet in the Pliocene it was right at sea level in both places. What was that sea level?

Raymo's best guess at the moment, to be confirmed by further fieldwork and modeling, is that the last time Earth had 400 ppm of CO2 in its atmosphere, sea level was somewhere between 10 meters (33 feet) and 20 meters (66 feet) higher than today. To raise sea level 10 meters today would require melting most of the ice in Greenland and West Antarctica. To raise it 20 meters would require melting both those ice sheets entirely and some of the giant East Antarctic ice sheet too.

Ice-Age Clues

Could that happen at 400 ppm? Evidence from the past half million years suggests it could, given enough time.

Since the Pliocene, glacial periods, during which ice sheets advanced over northern continents, have alternated with interglacial periods like the one we're in today. The timing has been set by orbital variations, but CO2 has amplified their effect. For the past 800,000 years at least, its atmospheric concentration has marched up and down in step with the ice, but in the opposite direction.

In the last interglacial period, around 120,000 years ago, sea level was as much as 8 meters (26 feet) higher than today, Raymo says. In an earlier interglacial known as Stage 11, around 400,000 years ago, "the evidence is very strong that sea level was at least 9 meters higher than today. The ice sheets didn't stick around."

In Stage 11, the sunlight distribution was a little less favorable to ice sheets than it is now. CO2 peaked then at 290 ppm.

"What everything is telling you is that the system is very sensitive," says Raymo. "The threshold for losing the Greenland and West Antarctic ice sheets is very close to where we are now. Everything in the geologic record says we're very close. You don't need a lot of CO2—you just need a little bit of warming, and it doesn't matter how you get it."

It took between a thousand and a few thousand years, at the end of Stage 11, to melt all or most of the Greenland and West Antarctic ice sheets. The whole interglacial lasted 30,000 years, nearly three times as long as ours has lasted so far. So the warming had a long time to build up. That's the good news.

But at 400 ppm, CO2 is much higher now, and it's still climbing fast. And even if we could stop that rise tomorrow, the planet's temperature would still climb for centuries.

"For me personally that's the scary thing," says Raymo. "We really don't know what we've already committed ourselves to."

Editor's Note: An earlier version of the story was unclear on when the 400 ppm mark was surpassed. NOAA reported today that the mark had been crossed on Thursday, May 10. Scripps originally reported variable data for the day, then confirmed the milestone had been crossed.

This story is part of a special series that explores energy issues. For more, visit The Great Energy Challenge.