Pandemic lockdowns slashed daily global emissions of carbon dioxide (CO2) by 17 per cent, a team of researchers said yesterday, in a detailed assessment of how the crisis affected the global economy.

By April this year, CO2 emissions had plunged to 2006 levels, the authors say, adding that the findings could help shape policies on cutting emissions to curb climate change.

Calls are growing globally for the trillions of dollars in stimulus spending to be directed to green programmes to speed up the switch to a less polluting global economy and create jobs in new industries.

The multinational group of scientists examined data from 69 nations responsible for 97 per cent of mankind's carbon emissions, and calculated the drop in CO2 compared with last year's levels.

Carbon dioxide is the main greenhouse gas heating up the planet and the United Nations says emissions need to fall quickly this decade to try to keep warming to well below 2 deg C above pre-industrial levels.

The study, published in Nature Climate Change, considered the impact of lockdowns, border closures and other restrictions on six sectors, including aviation, ground transport, power generation and households from January to April.

Transport, industry and power stations are the main sources of CO2 and air pollution, and emissions have been growing rapidly in recent decades.

Pre-pandemic, the global economy pumped out about 100 million tonnes of CO2 a day - far more than nature can absorb, leaving it accumulating in the atmosphere.

The authors found that nearly half the calculated drop in carbon emissions was the result of fewer vehicles plying roads worldwide.

Aviation emissions also plunged - by 60 per cent by early April. This made up 10 per cent of the overall drop in CO2 emissions globally.

Emissions from the power generation sector fell 7.4 per cent, while household emissions rose 2.8 per cent, reflecting the millions of people staying at home worldwide as part of anti-virus measures.

"Road transport was one of the most important sectors contributing to the fall in emissions, highlighting the large opportunities that would come from addressing transport as a key area for carbon mitigation," said the study's co-author, Dr Pep Canadell, who is executive director of the Global Carbon Project which closely monitors fossil fuel emission trends.

This could include speeding up electrification of transport or using green hydrogen as a fuel, plus more support for walking and cycling as part of a vision for low-carbon, cleaner cities post-pandemic.

It could also encourage more plans to boost cycling, walking and public transport usage post-pandemic to try to cut air pollution and preserve blue skies.

"We have also had an extraordinary opportunity to assess what essential travel is, both for business and personal, and learnt it is possible in many instances that remote interactions are as efficient or more efficient than doing the travelling," Dr Canadell told The Straits Times.

In total, daily CO2 emissions fell 17 million tonnes by early April.

That is the equivalent to the annual emissions of 3.7 million average cars, according to the US Environmental Protection Agency.

The depth of the decline varied across nations, depending on the stringency of their lockdowns and other policies curbing movement.

In China, the fall peaked at 24 per cent, India at 26 per cent, Indonesia 18 per cent and Italy 28 per cent.

With countries now trying to ease restrictions, the question is how much CO2 emissions will drop by the end of the year.

The authors estimated emissions will drop between 4.2 per cent and 7.5 per cent, depending on how long restrictions or confinements last and economic activity remains depressed.

The big concern is what world leaders and businesses decide in the coming months, the authors said. Will they step up and use the stimulus spending to achieve the goals of the UN's Paris Climate Agreement to keep temperatures to well below 2 deg C and net-zero CO2 emissions by 2050?

"The extent to which world leaders consider the net-zero emissions targets and the imperatives of climate change when planning their economic responses to Covid-19 is likely to influence the pathway of CO2 emissions for decades to come," the authors noted.