"At the current rate of increase, the global annual average CO2 concentration is set to cross the 400 parts per million threshold in 2015 or 2016," the UN agency said in a statement.
Climate scientists last week measured 400.03 parts per million of CO2 at a Hawaii station considered the global benchmark site for atmospheric observations -- marking the first time in human history that the level has clearly surpassed the symbolic 400 ppm threshold.
Several other stations in the Arctic and on the Canary Islands have over the past year also reported daily mean values exceeding the 400 ppm mark, but the benchmark Hawaii measurement sparked particular alarm, with the UN's climate chief Christiana Figueres warning the world had "entered a new danger zone".
Before the industrial revolution, when man first started pumping carbon into the atmosphere by burning fossil fuels, CO2 levels were about 280 ppm -- rising steadily since records began in the 1950s.
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The 400 ppm symbolic threshold had been expected to be breached for some time, but campaigners say it should nevertheless serve as a wake-up call in efforts to curb greenhouse gas emissions and thus global warming.
The UN is targeting a maximum temperature rise of two degrees Celsius (3.6 degrees Fahrenheit) on pre-industrial levels for what scientists believe would be manageable climate change.
Many scientists however believe we are heading towards warming levels of between 3.0 and 4.0 degrees Celsius by the end of the century.