New data shows carbon dioxide (CO2) 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.
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.
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.
The global climate phenomenon, El Niño, is believed to have played a role in the rise.
BBC Radio reports scientists at the World Meteorological Organisation (WMO) said that the previous biggest increase was in 1998, also an El Niño year.
“The impact of El Niño on CO2 concentrations is a natural and relatively short-lived phenomenon,” said Professor 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.
Prior to 1800, said the US National Oceanic And Atmospheric Administration (NOAA), atmospheric levels were 280ppm.
“It’s explosive compared to natural processes.”
The scientists said 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 2.0°C,” said the WMO’s Professor 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.