A few years ago I went to Australia. Sister descendant of the British motherland that it is, although the eastern half (and probably the western half too) had been experiencing one of the most severe droughts in the country's history, still the Australians clung to that stately British idea of well-manicured grasses as ground covering for human-occupied spaces.
Seeing as the drought had caused the government to implement severe water restrictions (how socialist of them), that dream was clearly clung to with considerable delusion: most everybody's short (and not because they mow it) lawn was yellow-brown.
Quite a few of my Australian friends apologized to me for all the dead grass, some even profusely. As if I, hailing from one of North America's temperate rain forests, would find it some ghastly breach of etiquette, some indication of Australian society's inferiority due to lack of ability to control nature.
On the scale of things, I find lawns a somewhat absurd idea. They keep the wildness of Real Nature (which is considerable, in a temperate rain forest) from encroaching on your house, and they look nice--when you're living on a continent to which the species used are remotely adapted. But with things in mind like global food and water shortages, not to mention that Climate Change thing that gets you labeled as a no-good-hippie-alarmist if you mention it, dedicating sections of land to short green grass really does seem, well, extravagant and wasteful. Millions barely get by with subsistence farming on marginal soils, and here some folks dump nitrogen and all-valuable drinkable water into already-pretty-decent soil with no intention of using that space for food production. We go on and on about needing to cut our carbon emissions for the sake of our future, yet we fail to see the hilarity in using gas-powered engines once a week or so just to forestall the undaunted efforts of a plant to get a little taller.
According to 2000 census projections, in 2008 there were 112 million occupied American households. 63.2% of those were detached, which for simplicity will be my only area of focus--even though apartment building, mobile homes, government areas and businesses often have lawn areas as well--so we can estimate that there were 71million American lawns of some size or other. According to the American Housing Survey data tables, the median lot size of occupied housing in 2008 was 0.36 acres, and the median house size was 1800 square feet. Subtract house square footage from 0.36 acres * 43,560 square feet per acre, and you get a rough estimate of 0.32 acres of yard per household. Multiply that by 71 million occupied households, and you get 23 million acres of private lawn.
Lawn mower efficiency is a pretty difficult concept to pin down, because it varies based on size, type, and age, and isn't generally advertised or noticed. Because they are smaller, lighter, and have a less complicated cycle than car engines, one could argue that they are much more fuel efficient than cars; however, they burn a heck of a lot dirtier, not being as as regulated as car engines. For the sake of forging ahead, and at risk of making this whole thing even more arbitrary than it already is, I'm going to throw out a guess of one cup of gasoline per acre, based on minimal experience mowing a 1.5 acre yard with a tiny 1980s push mower. So that's 23 million cups of gasoline, burned once a week throughout warm months, we'll say half the year. 598 million cups of gasoline. According to the EPA, 1 gallon of gasoline yields 19.4 pounds of CO2--that varies too based on the efficiency of combustion, which for lawn mowers probably is not particularly great. But lacking a better number, we'll take that one, so that 598 million cups of gasoline * 16 cups to a gallon * 19.4 pounds of CO2 to a gallon gives you a grand and arbitrary total of...186 billion pounds of CO2 in a year. 84 million metric tons, if you like.
All based on not-well-pinned-down numbers, but we can't deny that is is a darn lot of CO2, thrown up there in the name of neatness and conformity. Considering the supposed scope and potential of human ingenuity, it is pretty darn silly.