Daily global emissions of CO2 fell by 17% at the peak of the shutdown because of measures taken by governments in response to Covid-19, say scientists.
The most comprehensive account yet published says that almost half the record decrease was due to fewer car journeys.
But the authors are worried that, as people return to work, car use will soar again.
They fear CO2 emissions could soon be higher than before the crisis.
They are urging politicians to grasp the moment and make real, durable changes on transport and personal mobility.