Possible Clues:

• Environmentalist's concern
• Greenhouse-gas emission measure
• The Greens are always going on about this trail in 8 across, 10 across and most of 12 across
• It's gas leaving Mark out in the snow and that's noted by The Greens!
• What an environmentalist wants to reduce in 20 across and 24 across and most of 19 down
• Walking on coals could lead to measure of environmental damage
• Term popularized by BP
• Length in additional copy needs reduction