Levels of the climate warming greenhouse gas carbon dioxide have reached 400 parts per million in the atmosphere for the first time in human history.
The last time the planet had atmospheric carbon dioxide levels this high was 3 to 5 million years ago in the Pliocene era. The world then was quite a different place, with temperatures 2 to 3C above those of today and sea levels 20m higher.
Geological records tell us this is where we are heading - is this the planet we want?
Data released this month shows the daily average atmospheric carbon dioxide concentration passed 400ppm for the first time in its half century of recording. The information is from two CO2 monitoring stations at 4000m above sea level on the Hawaiian volcano of Mauna Loa, run by the US National Oceanic and Atmospheric Administration and the Scripps Institution of Oceanography.
Professor Rajendra Pachauri, chair of the Intergovernmental Panel on Climate Change, which serves as science adviser to the world's governments, says this is a significant reminder of the rapid rate at which we have increased the concentration of greenhouse gases in the atmosphere.
We are well into the "Anthropocene" - an informal geologic term that serves to mark the evidence the global impact of human activities is having on the Earth's environment. The Anthropocene has no precise start date, but is generally taken to have started around 1750 when atmospheric emissions from fossil fuel burning from the Industrial Revolution began in earnest and the concentration of CO2 was just 280ppm.
Analysis of fossil air trapped in ancient ice and other data indicate that the current level has not been seen since the Pliocene, when reef corals suffered a major extinction, forests grew up to the northern edge of the Arctic Ocean and horses and camels lived in the high Arctic, a region which is today permanently frozen ground. This period has raised interest because the continental shape was similar to the present day and because the climate was warmer, which caused the sea level to rise by about 20m.
The Pliocene marked the end of the greenhouse epoch, when CO2 levels were much higher - there's no other way to explain how warm the Earth was then.
The CO2 concentrations were on their way down. This time 400ppm is a milestone on a far more rapid uphill climb toward an uncertain future.
The World Bank's report Turn down the heat - why a 4C world must be avoided, released late last year, concludes that without further commitments and action to reduce greenhouse gas emissions, the world is likely to warm by more than 3C above the pre-industrial climate.
With the current mitigation commitments and pledges fully implemented, there is roughly a 20 per cent likelihood of exceeding 4C by 2100. If they are not met, a warming of 4C could occur as early as 2060. Such warming and an associated sea-level rise of up to 1m by 2100 would not be the end point: A further warming to levels over 6C with several metres of sea-level rise would likely occur over the following centuries.
The World Bank concludes that "uncertainty remains about the full nature and scale of impacts. There is also no certainty that adaptation to a 4C world is possible. A 4C world is likely to be one in which communities, cities and countries would experience severe disruptions, damage, and dislocation, with many of these risks spread unequally".
Warming simply must not be allowed to occur. Only early, co-operative, international and national actions can make that happen.
Should this not occur, the geological record has painted very clearly the future of a hothouse earth.
Jim Salinger is an Auckland climate scientist who is the 2012 Lorrey Lokey Visiting Professor at Stanford University.