The only measure of our progress on climate change with any integrity is the record of atmospheric concentration of CO2. None more so than the series of readings that have been taken in the middle of the Pacific Ocean at the Mauna Loa Observatory in Hawaii which have been taken uninterrupted since 1958. This is the Keeling Curve, named after Charles Keeling who initiated the measurements. On May the 9th these readings passed a milestone – they reached 400 parts per million for the first time.
The last time atmospheric CO2 was at this level was some 3M years ago in the Pliocene geologic era. At that time the temperatures were 2-4 degrees C higher than today and sea levels were between 5 and 40 meters higher. The geological record gives us an insight into the long-term physical outcome of current CO2 concentrations. What is does not tell us is the impact on our biosphere, the countless living organisms on which we depend and which is facing an unprecedented rate of change in temperature.