Analysis - Roger Harrabin, Environment Analyst
Lockdown has raised questions about other pollutants, too. One of the UK's leading experts, Prof Frank Kelly, from King's College London, said he knew diesel cars were emitting far more pollution than advertised - fully two years before US authorities exposed the scandal.
He told Radio 4's The Life Scientific programme that his team discovered a huge mismatch between emissions declared by the car firms and real readings on the road.
Prof Kelly said he reported it to the Department for Environment, Food and Rural Affairs (Defra), but they didn't publish his findings. He said work undertaken subsequently in the US led to legal action against car makers that had installed "defeat devices" to fool regulators.
The government didn't deny the account. A spokesperson said: "We are taking urgent action to improve air quality and our Clean Air Strategy has been commended by the World Health Organization as an 'example for the rest of the world to follow'".
Meanwhile, on the Covid-19 crisis, he said levels of the pollutant NO2 had fallen by up to 60% in London since the fall in traffic under lockdown.
Levels of another pollutant, sooty particles, remained at harmful concentrations.
"A big worry that people will naturally want to go back to their cars to go to work, and that could rebound the emissions to the same level or even higher than before, once everybody goes back," said Prof Corinne Le Quéré from the University of East Anglia, who led the analysis.
The researchers say that fundamental, systemic change is needed if the emissions curve is to be flattened in a way that would limit the very worst impacts of climate change.
When it comes to transportation, there are huge opportunities, according to Prof Le Quéré.
She says that after the global financial crisis in 2008, some governments like China, US and Germany made significant investments in wind and solar energy and this drove down the prices of these renewables.
"Here now in 2020 we're very close to the same situation in electric mobility," she told BBC News.
"Battery prices have come down, we have lots of models and governments are going to try to boost their economies."
"So if these two things can align, then it could make a huge difference to the transportation of tomorrow."
Grabbing the opportunity that the virus has presented is also at the forefront of corporate thinking on climate change.
A letter signed by 155 major companies, representing $2.4 trillion (£1.96 trillion) in market capitalisation, calls for a net-zero emissions response to the covid crisis.
Corporations including Carlsberg, Iberdrola, EDF and Coca Cola Europe say they want governments to "prioritise a faster and fairer transition from a grey to a green economy".
The authors of the latest analysis on carbon emissions agree that now is the moment for action. They point to the fact that while emissions of CO2 may be temporarily reduced, all the while CO2 concentrations are lingering in the atmosphere, warming the planet.
It will take a a dramatic shift to change that.
"I think very much that we are at a crossroads. And at this point, like the UK prime minister Boris Johnson said, it could go either way."
"He was talking about his own health, but here we're talking about the health of the planet."
"It could go either way."