Given how Humankind keeps on spewing dozens of billion tonnes of carbon dioxide each year, this was expected : carbon dioxide concentration have reached 400 parts per million.
While this may sound mundane to most people, this is a real event, and one that could have dire consequences. Last time CO2 concentration were this high, temperatures were 8°C (or 14°F ) higher.
Additionally, sea level were significantly higher. Both higher temperatures and higher sea levels would be lethal threats to our civilization.
It is worth noting that before the Industrial Revolution, CO2 levels were of around 280 ppm. Such a high level of 400 ppm haven’t been seen on this Planet for as long as for a million year.
Scientists have stressed the fact that getting back to 350 ppm would be ideal for our civilization. This is why a global campaign have been started in the United States by Bill McKibben.
The United Nations Environment Program brings an interesting perspective :
Observed concentrations of carbon dioxide (CO2) in the atmosphere have exceeded the symbolic 400 parts per million (ppm) threshold at several stations of the World Meteorological Organization’s Global Atmosphere Watch network.
This is a wakeup call about the constantly rising levels of this greenhouse gas, which is released into the atmosphere by fossil fuel burning and other human activities and is the main driver of climate change. Carbon dioxide remains in the atmosphere for thousands of years, trapping heat and causing our planet to warm further, impacting on all aspects of life on earth.
Several other websites and newspapers have dedicated posts and articles to this.
For more, please check out the New York Times, the Washington Post, Grist, Treehugger and many others including National Geographic, On Earth and of course, Climate Progress.