Concentrations of CO2 in the Earth's atmosphere surged to a record high in 2016, according to the World Meteorological Organization (WMO).
Last year's increase was 50% higher than the average of the past 10 years.
Researchers say a combination of human activities and the El Niño weather phenomenon drove CO2 to a level not seen in 800,000 years.
Scientists say this risks making global temperature targets largely unattainable.
This year's greenhouse gas bulletin produced by the WMO is based on measurements taken in 51 countries. Research stations dotted around the globe measure concentrations of warming gases including carbon dioxide, methane and nitrous oxide.
The figures published by the WMO are what's left in the atmosphere after significant amounts are absorbed by the Earth's "sinks", which include the oceans and the biosphere.
2016 saw average concentrations of CO2 hit 403.3 parts per million, up from 400ppm in 2015.
"It is the largest increase we have ever seen in the 30 years we have had this network," Dr Oksana Tarasova, chief of WMO's global atmosphere watch programme, told BBC News.
"The largest increase was in the previous El Niño, in 1997-1998, and it was 2.7ppm; and now it is 3.3ppm. It is also 50% higher than the average of the last 10 years."
El Niño impacts the amount of carbon in the atmosphere by causing droughts that limit the uptake of CO2 by plants and trees.
Emissions from human sources have slowed down in the last couple of years according to research, but according to Dr Tarasova, it is the cumulative total in the atmosphere that really matters as CO2 stays aloft and active for centuries.
Over the past 70 years, says the report, the increase in CO2 in the atmosphere is nearly 100 times larger than it was at the end of the last ice age.
Rapidly increasing atmospheric levels of CO2 and other gases have the potential, according to the study, to "initiate unpredictable changes in the climate system... leading to severe ecological and economic disruptions".
The study notes that since 1990 there has been a 40% increase in total radiative forcing. That's the warming effect on our climate of all greenhouse gases.
"Geological-wise, it is like an injection of a huge amount of heat," said Dr Tarasova.
"The changes will not take 10,000 years, like they used to take before; they will happen fast. We don't have the knowledge of the system in this state; that is a bit worrisome!"
According to experts, the last time the Earth experienced a comparable concentration of CO2 was three to five million years ago, in the mid-Pliocene Epoch. The climate then was 2-3C warmer, and sea levels were 10-20m higher due to the melting of Greenland and the West Antarctic ice sheets.
The report has been issued just a week ahead of the next installment of UN climate talks, in Bonn. Despite the declaration by President Trump that he intends to take the US out of the deal, negotiators meeting in Germany will be aiming to advance and clarify the rulebook of the Paris agreement.