Levels of air pollutants and warming gases over some cities and regions are showing significant drops as coronavirus impacts work and travel.
Researchers in New York told the BBC their early results showed carbon monoxide mainly from cars had been reduced by nearly 50% compared with last year.
Emissions of the planet-heating gas CO2 have also fallen sharply.
But there are warnings levels could rise rapidly after the pandemic.
With global economic activity ramping down as a result of the coronavirus pandemic, it is hardly surprising that emissions of a variety of gases related to energy and transport would be reduced.
Scientists say that by May, when CO2 emissions are at their peak thanks to the decomposition of leaves, the levels recorded might be the lowest since the financial crisis over a decade ago.
While it is early days, data collected in New York this week suggests that instructions to curb unnecessary travel are having a significant impact.
Traffic levels in the city were estimated to be down 35% compared with a year ago. Emissions of carbon monoxide, mainly due to cars, have fallen by around 50% for a couple of days this week according to researchers at Columbia University.
They have also found that there was a 5-10% drop in CO2 over New York and a solid drop in methane as well.