Our planet is set to reach a "grim milestone" this year: 2021 will see carbon dioxide (CO₂) in the atmosphere reach levels 50 percent higher than those before the Industrial Revolution.

The new forecast by the UK Met Office says that the concentrations of CO₂ will be around 2.29 parts per million (ppm) higher in 2021 than in 2020 at the Mauna Loa Observatory in Hawaii. This means atmospheric concentrations will exceed 417 ppm from April to June and be more than 50 percent higher than the level of 278 ppm seen in the late 18th century before the Industrial Revolution. 

The last time global carbon dioxide levels were consistently above 400 ppm was around 4 million years ago, a period when the world was about 3°C (5.4°F) hotter and sea levels were much higher than today.

“This is another grim milestone in the unrelenting rise of atmospheric CO₂ concentration," Dr Heather Graven, reader in Climate Physics at the Grantham Institute – Climate Change and the Environment, Imperial College London, commented.

Carbon dioxide concentrations briefly reached 417 ppm during the seasonal peak in 2020, but this new forecast indicates it will tip this record for several weeks in 2021. 

Carbon dioxide is the primary greenhouse gas that traps heat in the Earth’s atmosphere, resulting in warming global temperatures. Some is produced by natural processes, but it’s also pumped out in significant quantities from the burning of fossil fuels and deforestation. Compared to other greenhouse gases, such as methane, it’s very persistent and it can linger in the atmosphere for thousands of years. This is part of the reason why the recent dip in emissions from the COVID-19 lockdown is unlikely to have any real effect on the overall state of the planet. 

“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,” Professor Richard Betts MBE, who leads the production of the Met Office’s annual CO₂ forecast, said in a statement. 

“Although the Covid-19 pandemic meant that 7 percent less CO₂ was emitted worldwide in 2020 than in previous years, that still added to the ongoing build-up in the atmosphere,” Professor Betts added. “Emissions have now returned almost to pre-pandemic levels, but their effect this year will be partly dampened for a while by the stronger natural sinks due to the La Niña.” 

The Hawaii monitoring site is used as a global background reference for carbon dioxide in the atmosphere because of its altitude and remote location. Measurements of carbon dioxide concentration have been carried out here since 1958, making its record the longest unbroken measurement of carbon dioxide in the atmosphere. Throughout these six decades of work, the observatory has seen levels of carbon dioxide consistently climb year on year 

“Overall, this tells us that we continue to emit more CO₂ than the natural environment can absorb and that CO₂ concentrations (and therefore global warming) will continue to increase, even under favorable natural circumstances,” explained Professor Grant Allen, an atmospheric scientist at the University of Manchester, who was not involved in this new forecast.

“We urgently need to tip this balance," Allen said. "Emissions reductions policies must continue at speed.”