Within the next couple of weeks a major climate change marker is likely to be passed in a remote part of north-western Tasmania called Cape Grim. It's the co2 monitoring site jointly run by CSIRO and the Bureau of Meteorology and researchers predict its baseline reading of 400 ppm (parts per million) of carbon dioxide in the atmosphere is likely to be exceeded. Cape Grim's readings are significant because they capture the most accurate reading of atmospheric conditions in the southern hemisphere and have records going back 40 years. ?While the fraction may seem small, it's 0.04% of the atmosphere and it's symbolic. By comparison, a similar level of alcohol is near the legal driving limit of less than 0.5% in Australia.