I hope someone does do a study to see what the impact really are. Shipping and jet travel account for as much CO2 as the country of Japan. With both down significantly, that has to be a lot of CO2 not being pumped into the air. Given that the northern hemisphere at the same time is experiencing greening as well, there are lots of plants around to suck up the smaller amount of CO2 being released.
NASA may do a short term study on this which I'd be interested in. They did for the long term.