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.