Guardian: On 10 May 2013, the concentration of climate-warming carbon dioxide in the atmosphere passed the milestone level of 400 parts per million for the first time in human history. The last time so much greenhouse gas was in the air was several million years ago, when the Arctic was ice-free, savannah spread across the Sahara desert and sea level was up to 40 metres higher than today.
The milestone moment was a sobering reminder that the emission of globe-warming gases is continuing unabated, despite...