Hawai (USA) - Carbon dioxide levels in the atmosphere have broken through a symbolic mark. Daily measurements of CO2 at the authoritative "Keeling lab" on Hawaii have topped 400 parts per million for the first time.
The station, which sits atop the Mauna Loa volcano, has the longest continuous measure of the concentration of the gas, stretching back to 1958.
The last time CO2 was regularly above 400ppm was about 3-5 million years ago - before modern humans existed.
The climate back then was also considerably warmer than it is today, according to scientists.
Carbon dioxide is regarded as the most important of the manmade greenhouse gases, a product principally of burning fossil fuels such as coal, oil and gas.
The usual trend seen at the volcano is for the CO2 concentration to rise in winter months and then to fall back as the northern hemisphere growing season kicks in and pulls some of the gas out of the atmosphere.
This means the number can be expected to decline by a few ppm below 400 in the coming weeks. But the long-term trend is upwards.
When the famous US researcher Charles Keeling began recording CO2 concentrations at the volcano in 1958, they were around 315ppm (that is 315 molecules of CO2 for every one million molecules in the air). Every year since then, the curve has squiggled resolutely higher.
To determine CO2 levels before the introduction of modern stations, scientists must use so-called proxy measurements.
These include studying the bubbles of air trapped in Antarctic ice. The longest of these records goes back 800,000 years, and suggests CO2 held steady over this longer period at between 200ppm and 300ppm.
British atmospheric physicist Prof Joanna Haigh commented: "In itself, the value 400ppm of CO2 has no particular significance for the physics of the climate system: concentration levels have been in the 300s for so long and now we've passed the 400 mark. However, this does give us the chance to mark the ongoing increase in CO2 concentration and talk about why it's a problem for the climate."
