National Geographic has reported that 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. Read More
In the UK this milestone appears to have been largely ignored, except by the Guardian:
To give some context here are C02 levels over the last 10,000 years graphed a short time back – you can see for yourself where the new reading goes: