The UN's climate chief called for urgency Monday as she opened a new round of global talks amid warnings that Earth-warming carbon dioxide levels were approaching a symbolic threshold never seen in human history.
Data from the Mauna Loa Observatory in Hawaii have shown the concentration of CO2 in the atmosphere to be at 399.72 parts per million (ppm), Christiana Figueres told climate negotiators in Bonn.
"We are just about to cross the 400 ppm threshold," she said in a prepared speech that stressed "a heightened sense of urgency".
The Intergovernmental Panel on Climate Change (IPCC), which informs policy makers on the science of global warming, has said the atmospheric CO2 level must be limited to 400 ppm for Earth's average temperature rise to be contained at between two and 2.4 degrees Celsius.
The talks in Bonn are the first since negotiations in Qatar last December set down a two-track process for tackling greenhouse gas emissions.
The main goal is a new climate treaty that will be concluded by 2015 and take effect by 2020.
Pre-2020, countries agreed to seek ways of closing the growing gap between carbon emission targets and the actual curbs required to contain warming to a manageable two degrees Celsius (3.6 degrees Fahrenheit) from pre-industrial levels.
The US-based Scripps Institution of Oceanography, which keeps a record of the Mauna Loa figures, said last week that CO2 concentrations in the atmosphere could in May rise above 400 ppm for the first time in human history.
The observatory's record starts at 316 ppm in March 1958, rising every year.
Atmospheric levels of CO2, the main greenhouse gas, were probably last as high as 400 ppm in the Pliocene period, between 3.2 million and five million years ago when Earth was a warmer place, Scripps said in a statement.
The carbon concentration never exceeded 300 ppm for some 800,000 years, it added. Before the Industrial Revolution, when man first started pumping carbon into the atmosphere by burning fossil fuels, CO2 levels were at about 280 ppm.