Habits bad for the environment
So which of our habits is the worst enemy to the environment? For now, here's a look at some of our most common environmental transgressions, starting with the biggies.

HOME ENERGY

Consumption The average Chicago household uses 5,400 kilowatt hours of electricity per year, spending $600 on electric bills annually, and 1,400 therms per year in natural gas, spending $1,000 annually. Most of the energy goes to heating and cooling the home.

Environmental impactBecause Chicago gets so cold, two-thirds of the 10.7 tons of greenhouse gases emitted per home comes from natural gas (used for heating), while one-third comes from electricity.

Nationally, however, about 39 percent of the energy consumed in the U.S. is used to generate electricity, making electricity the country's No. 1 source of greenhouse gas emissions. Electricity accounts for 2.5 billion tons of carbon dioxide emissions in the U.S.—about 40 percent of the country's total CO2 emissions.

Saving grace Though Chicago continues to get most of its electricity from coal (the biggest carbon emitter) and nuclear power plants, wind energy is becoming more prevalent in the Midwest.

Alternatives Insulate your home and buy energy-efficient appliances (look for the EnergyStar label, which indicates the product meets efficiency guidelines set by the EPA and Department of Energy). Adjusting your air conditioner or thermostat a degree makes a big difference. And if every Chicagoan replaced four light bulbs with energy-efficient compact fluorescent bulbs, it would prevent 2.6 million tons of carbon dioxide emissions, the equivalent of removing 81,164 cars from the road.

Also, electronics on standby still use electricity, so turn off computers and power strips to avoid such "phantom loads."

Sources: Environmental Protection Agency, Chicago Department of the Environment, Center for Neighborhood Technology Energy

DRIVING/PARKING

Consumption Americans have bought 6.9 million cars and 7.8 million light trucks (including SUVs) so far this year, and they drive about 2.6 trillion miles annually. The more than 200 million cars on American roads account for 30 percent of all cars worldwide—though the U.S. represents less than 5 percent of the world's population.

Environmental impact Transportation contributes a quarter of the world's carbon dioxide emissions, emitting 1.5 billion tons of CO2 annually, mostly from automobiles. The average American passenger car, driven about 12,500 miles a year, spews 11,450 pounds of carbon dioxide in a year, while the average light truck emits some 16,000 pounds annually. Cars impact the environment even when they're not moving, as paved parking lots create rainwater runoff that pollute lakes and rivers and absorb solar heat, warming the area.

Saving grace Sales of cars that run on alternative fuels are increasing, and there are now 11.5 million such cars on American roads. There are 60 models of alternative- fuel autos on sale today, up from 12 in 2000.

Alternatives Walk, bike or take public transit when possible. If every Chicagoan replaced one car trip a month with another mode of transportation, it would decrease carbon dioxide emissions by 1.9 million tons per year. More drastically, you can use car-sharing services or buy a hybrid or a more fuel-efficient car.

Sources: Wall Street Journal, Environmental Defense, Fueleconomy.gov, Chicago Department of Environment, Alliance of Automobile Manufacturers

FLYING

Consumption U.S. airlines transported 744.6 million passengers in 2006, pulling in $163.8 billion in revenue. Added together, passengers flew 797.4 billion miles.

Environmental impact Air travel accounts for 2 percent of global greenhouse gas emissions, but with demand for flying rising, it is one of the fastest-growing contributors to global warming. Also, the fuel burned at high altitudes has a bigger global warming impact than fuel burned at ground level, and airplanes emit much more carbon than cars do on a per-passenger, per-mile basis.

Saving grace Airlines have doubled fuel efficiency between 1978 and 2006 and have pledged to improve fuel economy an additional 30 percent over the next 18 years. Efforts to modernize air traffic control operations would also cut down on trip lengths.