400
We humans like round numbers. We don't celebrate the 23rd annual summer fete or the 49th anniversary of an organisation but the 25th or 50th respectively. This is why the quantity of carbon dioxide in the atmosphere has hit the headlines this week. The daily measurement at Mauna Loa on Hawaii has passed 400 ppm (parts per million) for the first time. Not so much a cause for celebration; more a cause for commiseration The actual measurement was 400.03 but was subsequently revised down to 399.89 (but what is fourteen hundredths of a part per million between friends). Breaching this threshold was not particularly unexpected but it is symbolic in human terms; it is a memorable number. But what are the implications of exceeding a CO2 concentration of 400 ppm? Some climate change skeptics will be quick to highlight the scientific evidence that shows the atmospheric CO2 has been at this level and higher on the past. They are absolutely correct. Somewhere in the region of three