Scientists have confirmed that the heat-trapping greenhouse gases in our atmosphere have reached unprecedented levels, unseen for more than 2 million years. The National Oceanic and Atmospheric Administration  shows that concentrations of atmospheric carbon dioxide (CO₂) have passed the ominous milestone of 400 parts per million (ppm).

When our planet was last exposed to equivalent levels of greenhouse gases, global temperatures were 3-4? hotter and sea levels were 5-40 meters higher than today. Scientists are particularly alarmed at the fast-pace of rising emissions levels. Some of them have expressed hope that this moment could provide leaders with a much-needed wake-up call. A range of high-profile voices are now urging decision-makers to tackle rocketing emissions and prepare for unavoidable climate impacts.

Crossing the 400ppm threshold is being heralded as a symbolic milestone. It is based on a record daily average reading from one – rather iconic – site, while another measuring station at the same location measured a daily average for the same date just below the mark. It will likely be another few years until global average concentrations of CO₂ in our atmosphere reach 400ppm .

The speed at which levels of CO₂ are rising is the fundamental threat revealed by the flurry of attention devoted to a symbolic moment in the state of our changing climate.The planet could reach the 1000ppm level in only 100 years if emissions continue to rise at their present pace, whereas an increase of just 10ppm might have taken 1000 years or more as part of a natural cycle, without the impacts of a high carbon economy.