In the 1950s, Charles David Keeling began collecting air samples near the summit of Mauna Loa, Hawaii, and testing them for (among other things) the amount of carbon dioxide in the air, using a new and more accurate method than had been used previously. He collected those samples twice a day and soon had a monthly average: in March of 1958, the air at Mauna Loa contained 315.71 parts per million by volume of CO2.
Today Keeling's records have become the gold standard for the amount of CO2 in the air -- the "Keeling Curve" of background CO2 ("background" meaning, collected far from the major industrial CO2 sources in Europe, North America, and Asia).
This week, the hourly records on Mauna Loa went above 400 ppmv for the first time.
Here's the last month's worth of hourly data. The CO2 concentration edged above 400 on April 17th, and again on April 22nd.
What you can't see from this chart is that CO
2 has a strong seasonal pattern that peaks around May. Here are the last two years worth of data:
The drop-off starting in May happens because the springtime growth of land plants on Northern Hemisphere continents temporarily overtakes human fossil fuel use. Once that growth ceases around September, CO
2 rises again. That means that we can expect more days above 400 during the next few weeks before it begins falling again. It also means that it will probably be a year or two before we get an annual average above 400 ppmv. But the harbinger has arrived, and the caravan will arrive soon.
One other point: this spring has been unusually cool across Europe, northern Asia, and much of North America.
That suggests a late spring may be in store for much of the Northern Hemisphere, which implies that the annual CO
2 peak may be later than is typical -- meaning a somewhat longer run on the upside of the annual CO
2 wave.
Keeling's records of rising CO2 are today still being collected by his son, Ralph Keeling. For daily updates, the Scripps Institute of Oceanography has started a website to keep track:
http://keelingcurve.ucsd.edu/