11.   Greenhouse gases combine with hydrocarbons to form smog.

12.   He is resigned to public indifference to the benefits of efficiency, as well as to the effects of greenhouse gases.

13.   International controls are needed to reduce emissions of greenhouse gases.

14.   Laws controlling the emission of greenhouse gases are not nearly tight enough.

15.   Many regions have abandoned coal-burning electricity plants, which give off carbon dioxide, a potent greenhouse gas.

16.   Negotiations on a treaty to limit this greenhouse gas begin in December.

17.   Of course, the remaining dissenters are mostly those who depend on pouring out yet more greenhouse gases.

18.   Over the past few months, governments have been considering action to cut back on emissions of greenhouse gases.

19.   Scientific and technological advances will help the world deal with greenhouse gases, but not for a hundred years or so.

20.   The cocktail of greenhouse gases includes chlorofluorocarbons, methane, nitrous oxide and carbon dioxide.