You might not have realized it, but last Friday something quite historical happened: For the first time since the Pliocene (about five to three million years ago) CO2 concentration in the atmosphere crossed the 400 parts-per-million limit.  The last time that much carbon dioxide was in the atmosphere, the Arctic was ice-free, the Sahara desert was a savanna and the sea level was 40 meters above today’s level.

Measuring this carbon level is kind of a family business: More than 50 years ago, Charles Keeling started measurements on the dead Mauna Loa volcano, where air quality is very good. Back then, in 1958, CO2 concentration was around 315 ppm – compared to 280 ppm on preindustrial level. Today, his son Ralph Keeling continues the measurements.

The overall trend is increasing, though CO2 levels fluctuate periodically from winter to summer season, when plants naturally fix more carbon dioxide due to leaf growth.  Scientists attribute this overall increase to anthropogenic CO2 emissions – mainly from burning fossil fuels.

Within the last ten years, CO2 concentration has risen by two ppm per year. If the world continues emitting carbon at this pace, the next important threshold is not far away (german link). CO2 concentration may not rise upon 445 ppm CO2 (445 molecules of carbon dioxide in one million molecules) if the two degree target is to be met, scientists of the Intergovernmental Panel on Climate Change (IPCC) say.