The third most important contributor to global warming isn’t included in official tallies of greenhouse gases or mentioned in plans to cut them. And it’s time the world did something about it, according to a research analysis published Thursday in the journal Science.So-called indirect greenhouse gases are generated by combustion of fossil fuels or they waft skyward from industrial chemicals.

They include carbon monoxide, nitrogen oxides and non-methane volatile organic compounds (VOCs).These gases don’t themselves trap heat — but once airborne, they join chemical reactions that form heat-trapping gases, such as tropospheric or ground-level ozone, a toxic gas in the lower atmosphere. Together with heat-absorbing black carbon, they are responsible for about 15% of global warming to date, or nearly 0.3 degrees Celsius, the analysis finds.