The forecast comes from the Met Office, the national meteorological service for the U.K., which used data collected at  Mauna Loa Observatory in Hawaii. Even though there was a slight decrease in global greenhouse gas emissions in 2020 because of the COVID-19 pandemic and the current La Niña event (a weather pattern in the Pacific that usually lowers global carbon emissions), it wasn't enough to offset previous increases.

"Since CO2 stays in the atmosphere for a very long time, each year's emissions add to those from previous years and cause the amount of CO2 in the atmosphere to keep increasing," Richard Betts, head of the climate impacts group at the Met Office and lead researcher for the forecast, said in a statement.

The concentration of CO2 in the atmosphere follows predictable seasonal variations. Levels peak in May and then decrease over the summer as plants grow across the Northern Hemisphere and suck in carbon (with photosynthesis), before rising again from September onward. 

Although the total amount of CO2 emitted worldwide in 2020 was down 7% from previous years, emissions have almost returned to pre-pandemic levels, according to the Met Office.