The global response to the Covid-19 pandemic has driven the biggest annual fall in CO2 emissions since World War Two, say researchers.
Their study indicates that emissions have declined by around 7% this year.
France and the UK saw the greatest falls, mainly due to severe shutdowns in response to a second wave of infections.
China, by contrast, has seen such a large rebound from coronavirus that overall emissions may grow this year.
The decline in carbon in 2020 has dwarfed all the previous big falls.
According to the Global Carbon Project team, this year saw carbon emissions decline by 2.4 billion tonnes.
In contrast, the fall recorded in 2009 during the global economic recession was just half a billion tonnes, while the ending of World War Two saw emissions fall by under one billion tonnes.
Across Europe and the US, the drop was around 12% over the year, but some individual countries declined by more.
France saw a fall of 15% and the UK went down by 13%, according to one analysis.
"The main reason is that these two countries had two waves of confinement that were really quite severe compared with other countries," said Prof Corinne Le Quéré, from the University of East Anglia, UK, who contributed to the study.
"The UK and France have a lot of their emissions come from the transport sector and generally have a bit less coming from industry and other sectors.
"This is even more true in France, because so much of their electricity production is from nuclear energy, so 40% of their emissions are from the transport sector."
Aviation around the world has been badly hit by restrictions and by the end of this year, it's expected that emissions from this sector will still be 40% below 2019 levels.
One country that may have bucked the trend is China.
Overall, the research team estimates that the country will experience a fall in emissions of 1.7% this year but some analysis suggests that the country has already rebounded enough from Covid-19 that the overall carbon output may have increased.
"All our datasets show that China experienced a big drop in emissions in February and March, but the datasets differ in the level of emissions towards the end of 2020," said Jan Ivar Korsbakken, a senior researcher at CICERO, who was involved in the study.
"In late 2020, China is at least close to having the same level of daily emissions as in 2019, and indeed some of our estimates suggest Chinese emissions may have actually increased for the year as a whole in 2020 relative to 2019, despite the pandemic," he added.
Researchers believe that dramatic drop experienced through the pandemic response might be hiding a longer term fall-off in carbon, more related to climate policies.
The annual growth in global CO2 emissions fell from around 3% in the early years of this century to around 0.9% in the 2010s. Much of this change was down to a move away from coal as an energy source.
"An emerging discussion pre-2020 was whether global fossil CO2 emissions were showing signs of peaking," said Glen Peters, research director at CICERO.
"Covid-19 has changed this narrative to one that involves avoiding a rebound in emissions and asking if emissions have already peaked," he said.
All the researchers involved in this project agree that a rebound of emissions in 2021 is almost certain.
To minimise the uptick in carbon, the scientists are urging a "green" rather than a "brown" response, meaning recovery funding should be spent on sustainable projects and not on fossil fuels.
They argue that efforts should also be made to boost walking and cycling in cities and to rapidly deploy electric vehicles.
While 2020's fall of over two billion tonnes of CO2 is welcome, the scientists say that meeting the goals of the Paris Climate Agreement will need cuts of up to two billion tonnes every year for the next decade.
"Although global emissions were not as high as last year, they still amounted to about 39 billion tonnes of CO2, and inevitably led to a further increase in CO2 in the atmosphere," said lead researcher Prof Pierre Friedlingstein from the University of Exeter, UK.
"The atmospheric CO2 level, and consequently the world's climate, will only stabilise when global CO2 emissions are near zero."
The study has been published in the journal Earth System Science Data.
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