CO2 data is 'wake-up call' for Paris climate deal
Carbon dioxide levels in the atmosphere grew more in past 12 months than at any time in the past 56 years.
Measurements at the Mauna Loa Observatory in Hawaii went up by more than three parts per million(ppm) in 2015.
Scientists say the spike is due to a combination of human activities and the El Niño weather pattern.
They argue that the data increases the pressure on global leaders to sign and ratify the Paris Climate Agreement.
Mauna Loa is the world's oldest continuous atmospheric measurement station, with records dating back to the later 1950s.
It is regarded as the most important site in the global monitoring network, recording the see-saw, rise and fall of carbon in the atmosphere over a year.
Plants and trees tend to absorb more CO2 during the spring and lose it as autumn approaches and leaves die off.
For the past decade the average increase in carbon dioxide at the station has been 2ppm. But in 2015 the level grew by 3.05ppm - In the year to February 2016, the level went up by 3.76ppm.
The global climate phenomenon, El Niño, is believed to have played a role in the rise.
Scientists at the World Meteorological Organisation (WMO) say that the previous biggest increase was in 1998, also an El Niño year.
The weather event drives drought in many parts of the tropics and in 2015 this led to forest fires in Indonesia and other locations which pumped large amounts of stored carbon into the atmosphere.
"The impact of El Niño on CO2 concentrations is a natural and relatively short-lived phenomenon," said Petteri Taalas from the WMO.
"But the main long-term driver is greenhouse gas emissions from human activities. We have the power and responsibility to cut these," he added.
Pressure to sign
The latest figures show that in January and February this year the levels of CO2 at Mauna Loa went through the symbolic 400ppm level.
Prior to 1800, say the US National Oceanic And Atmospheric Administration (Noaa), atmospheric levels were 280ppm.
"Carbon dioxide levels are increasing faster than they have in hundreds of thousands of years," said Pieter Tans, lead scientist of Noaa's Global Greenhouse Gas Reference Network.
"It's explosive compared to natural processes."
The scientists say the latest figures should encourage global leaders to make progress on the Paris Climate Agreement.
The UN is hoping that prime ministers and presidents will turn up in large numbers at a signing ceremony in New York in April, and that the treaty will become operational this year.
"This should serve as a wake-up call to governments about the need to sign the Paris Climate Agreement and to take urgent action to make the cuts in CO2 emissions necessary to keep global temperature rises to well below 2C," said the WMO's Petteri Taalas.
Scientists will be closely monitoring atmospheric levels this year to see if there is any decrease as El Niño fades over the next few months.