THE BLOOMBERG CARBON CLOCK

In 800,000 years. Before industrialization, carbon dioxide hovered around 280 parts per million. 450 ppm is considered the danger zone that will be associated with extreme weather patterns. The forecast is that the 450 ppm level will be reached by 2040.

The monthly values reflect the cyclical or seasonal changes in an inexorably upward secular trend. Check out the graph on the webpage cited below.