‘The Keeling Curve’ is the name given to the chart of rising atmospheric CO₂ levels, as measured at the Mauna Loa Observatory in Hawaii. So far, none of the action taken to tackle climate change has slowed the curve's inexorable rise. CO₂ has now reached the symbolic threshold of 400 parts per million (ppm).
The famous ‘saw-tooth’ line of the graph reflects the seasons: there are more trees and plants in the northern hemisphere than in the southern hemisphere, so globally the world’s land areas ‘breathe in’ during the northern spring/summer and out during the northern autumn/winter.
To see how rapid the build up of CO₂ has been since the industrial revolution, it’s necessary to zoom out. This graph shows the Keeling Curve (in red) in the context of the last two millennia. It shows how CO₂ levels were very flat until widespread fossil fuel use took off in the 1800s.
CO₂ levels will keep rising until carbon emissions are radically reduced. The dotted lines reflect the latest emissions scenarios used by the IPCC and others. So far emissions have roughly tracked the red dotted line, which could lead to almost 1000ppm by the end of the century.
Ice cores show that during the past 800,000 years CO₂ fluctuated only within narrow constraints. The current level of 400ppm was last seen millions of years ago, when the world looked radically different and was much hotter. Hence many scientists think today's level is extremely dangerous – and the rise will continue in future decades.