Australia's bushfires are contributing to one of the biggest annual increases in the concentration of carbon dioxide in the Earth's atmosphere since record-keeping began more than 60 years ago, according to a forecast published by Britain's Met Office on Friday, Eurasia Diary reports citing Reuters.

While human-caused greenhouse gas emissions are responsible for the bulk of the increase in CO2 levels, Australia's bushfires have made the problem measurably worse, underscoring the impact of the catastrophe on the global climate system.

"A forecast of the atmospheric concentration of carbon dioxide shows that 2020 will witness one of the largest annual rises in concentration since measurements began at Mauna Loa, in Hawaii, 1958," the Met Office said in a statement.

Concentrations of CO2 in the Earth's atmosphere have already far surpassed what scientists consider to be safe limits.

At a climate summit in Madrid in December, UN Secretary-General Antonio Guterres warned that 400 ppm had once been considered "an unthinkable tipping point".

The last time there was a comparable concentration of CO2 in the atmosphere was between 3 and 5 million years ago when the temperature was between 2 and 3 degrees Celsius warmer and sea levels were 10 to 20 metres higher than today, scientists say.

Although the data series started in 1958 has always shown CO2 concentrations increasing year-on-year, driven by fossil fuel burning and deforestation, the rate of rise has not been perfectly even, the Met Office said.

Fluctuations in the amount of CO2 absorbed by tropical forests and other natural carbon sinks can affect overall levels of the gas in the atmosphere.

"Overall these are expected to be weaker than normal for a second year running," Richard Betts of the Met Office Hadley Centre and the University of Exeter said.