Met Office scientists predict 2019 will see one of the largest rises on record of carbon dioxide levels in the atmosphere.
Each year natural “sinks” such as forests or grasslands absorb excess carbon produced by human activities.
But when there is hotter weather in the Pacific, like this year, they cannot absorb as much as plants grow less, experts said.
The Mauna Loa observatory in Hawaii has collected data on the concentration of carbon dioxide in the atmosphere since 1958.
Since recordings began, there has been a 30 per cent increase in the atmospheric concentration of CO2.
Professor Richard Betts of the Met Office Hadley Centre said: "This year we expect these carbon sinks to be relatively weak, so the impact of record high human-caused emissions will be larger than last year."
Average concentrations of carbon dioxide are forecast to be 411 parts per million (ppm) in 2019, reaching a peak of 414 ppm in May before dropping back to 408 ppm in September and rising again at the end of the year.
This compares to around 315 ppm when measurements began at Mauna Loa in 1958, and around 280 ppm before the Industrial Revolution based on ice core samples.
