Since 1956, the Mauna Loa Observatory in Hawaii has been gathering data on how much carbon dioxide is in the atmosphere — a very basic measure of how humans are transforming the planet and setting the stage for future climate change.
Notice that the curve is jagged. As humans keep burning fossil fuels, the amount of carbon dioxide in the atmosphere has trended upward over time. But there are still seasonal fluctuations. When trees in the Northern Hemisphere bloom in the spring and summer, they absorb some of that carbon. When the leaves wilt in the winter, carbon returns to the air and readings spike. The curve is a record of the planet's breathing.
As such, even if Mauna Loa does register levels above 400 parts per million this May, it will prove temporary. The readings will drop again this summer by a few parts per million, and it will likely be a few years before levels remain above 400 ppm all year.
But in an important sense, this year's milestone is beside the point. For decades now, carbon dioxide levels in the atmosphere have higher than at any point in the last 800,000 years — a fact scientists discovered by analyzing ancient air bubbles trapped in ice cores:
In fact, even that's probably an understatement. Recent studies have estimated that current levels of atmospheric carbon dioxide are at their highest levels since the Pliocene, the geologic era between five million and three million years ago. Here's a description of that era from the University of California, San Diego's Scripps Institution of Oceanography:
Recent estimates suggest CO2 levels reached as much as 415 parts per million (ppm) during the Pliocene. With that came global average temperatures that eventually reached 3 or 4 degrees C (5.4-7.2 degrees F) higher than today’s and as much as 10 degrees C (18 degrees F) warmer at the poles. Sea level ranged between five and 40 meters (16 to 131 feet) higher than today.
The Pliocene is not strictly comparable to our current era, and there are still questions about why that period was as warm as it was, but it's thought to be a useful historical guide. Here's a little more detail about what conditions were like at the time:
As for what life was like then, scientists rely on fossil records to recreate where plants and animals lived and in what quantity. Pliocene fossil records show that the climate was generally warmer and wetter than today. ...The absence of significant ocean upwelling in the warmest part of the Pliocene would have suppressed fisheries along the west coasts of the Americas, and deprived seabirds and marine mammals of food supplies. Reef corals suffered a major extinction during the peak of Pliocene warmth but reefs themselves did not disappear.
Keep in mind, too, that the rise in atmospheric carbon dioxide during the Pliocene era was a very gradual process over many thousands of years, caused by subtle changes in the Earth's orbit. Today, carbon dioxide is rising much more rapidly, largely due to fossil-fuel burning and land-use changes. Climatologists argue that speed will make a big difference:
Richard Norris, a geologist at Scripps Institution of Oceanography, UC San Diego, said the concentration of CO2 is one means of comparison, but what is not comparable, and more significant, is the speed at which 400 ppm is being surpassed today.“I think it is likely that all these ecosystem changes could recur, even though the time scales for the Pliocene warmth are different than the present,” Norris said. “The main lagging indicator is likely to be sea level just because it takes a long time to heat the ocean and a long time to melt ice. But our dumping of heat and CO2 into the ocean is like making investments in a pollution ‘bank,’ since we can put heat and CO2 in the ocean, but we will only extract the results (more sea-level rise from thermal expansion and more acidification) over the next several thousand years. And we cannot easily withdraw either the heat or the CO2 from the ocean if we actually get our act together and try to limit our industrial pollution–the ocean keeps what we put in it.”
One final point: Humanity is all but certain to zoom past 400 parts per million. The big question is, how far past?
For a long time, many climate experts thought we should aim to stabilize atmospheric carbon to about 450 parts per million. That goal looks daunting now. At the current rate, the world will pass that mark within a few decades. Indeed, even the most optimistic analyses of current trends, like this one from the International Energy Agency, which predicts that natural gas will displace coal, see us hitting at least 650 parts per million without drastic changes.
--There are a bunch of great resources about the Keeling Curve over at the Scripps Institution website, including a chart that is updated daily. There's also a Twitter account reporting daily readings as soon as they come in.
--Back in 2010, Justin Gillis of the New York Times wrote a nice profile of the Mauna Loa Observatory and Charles David Keeling, the scientist who first began measuring atmospheric carbon. (His son, Ralph Keeling, now runs the measurement program.)
--Here's a more detailed look, from the International Energy Agency, of the changes the world would need to make to its energy system to stay under 450 parts per million. (And some scientists, like NASA's James Hansen, have argued that we need to go even further and get back down to 350 parts per million.)